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Contrib Mineral Petrol (2016) 171:5 DOI 10.1007/s00410-015-1219-3

ORIGINAL PAPER

Serpentinization, element transfer, and the progressive development of zoning in veins: evidence from a partially serpentinized harzburgite

Esther M. Schwarzenbach1,2 · Mark J. Caddick1 · James S. Beard3 · Robert J. Bodnar1

Received: 15 July 2015 / Accepted: 25 November 2015 © Springer-Verlag Berlin Heidelberg 2015

during olivine hydration, where magnetite coexists with bru-cite Mg#96 and serpentine Mg#99, (3) chemical gradients in Si, Al, Cr, and Ca within and between orthopyroxene- and olivine-hosted veins, and 4) local (different) equilib-rium assemblages within different zones of veins. The stud-ied sample preserves rarely observed textures documenting continuous replacement of olivine, rather than individual vein generations and overprinting that is typically observed in more intensely serpentinized peridotites. Furthermore, the presence of a discrete sequence of vein textures and mineralogy allows direct comparison between mineral tex-tures and equilibrium thermodynamic models and permits new insights into mineral reactions during serpentinization.

Keywords Costa Rica · Olivine hydration · Peridotite · Serpentinization · Perple_X

Introduction

Serpentinization is a common alteration process that occurs when water interacts with the primary minerals olivine and pyroxene in ultramafic rocks to form a rock dominated by serpentine (Mg3Si2O5(OH)4). Serpentinites preserve some of the most reducing geological environments found on Earth—as oxidation of ferrous iron in olivine results in hydrogen formation (Frost 1985; Moody 1976; Neal and Stanger 1983)—and some of the lowest aqueous silica activities found in crustal rocks (e.g., Allen and Seyfried 2003; Frost 1985; Frost and Beard 2007; Palandri and Reed 2004; Peretti et al. 1992). The process of serpentinization has been studied for decades (e.g., Barnes and O’Neil 1978; Macdonald and Fyfe 1985; Mével 2003; Moody 1976; Wenner and Taylor 1971; Wicks and Whittaker 1977) and has gained widespread interest because serpentinization

Abstract Serpentinization is an important geochemical process that affects the chemistry and petrophysical prop-erties of the oceanic lithosphere and supports life through abiogenic formation of hydrogen. Here, we document through detailed mineralogical evidence and equilibrium thermodynamic models the importance of water (H2O) and silica (SiO2) activities on mineral assemblages produced during progressive serpentinization of a harzburgite. We describe a harzburgite from the Santa Elena Ophiolite in Costa Rica that is ~30 % serpentinized. Serpentine + bru-cite ± magnetite veins occur in olivine, Al-rich serpen-tine + talc veins occur in orthopyroxene, and Al-rich ser-pentine ± talc ± brucite veins occur at the boundary of orthopyroxene and olivine. Bulk vein chemistry and ele-ment distribution maps demonstrate distinct chemical zona-tions within veins and chemical gradients between orthopy-roxene- and olivine-dominated areas. Specifically, the sample records (1) varying brucite composition depending on whether or not it is associated with magnetite, (2) forma-tion of magnetite from Fe-rich brucite (±Fe-rich serpentine)

Communicated by Othmar Müntener.

Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1219-3) contains supplementary material, which is available to authorized users.

* Esther M. Schwarzenbach [email protected]

1 Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061, USA

2 Institut für Geologische Wissenschaften, Freie Universität Berlin, 12249 Berlin, Germany

3 Virginia Museum of Natural History, Martinsville 24112, VA, USA

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Contrib Mineral Petrol (2016) 171:5

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of ultramafic rocks impacts many large-scale tectonic pro-cesses and biogeochemical cycles. Specifically, the trans-formation of olivine and pyroxene to primarily serpentine and magnetite affects the rheology (Escartin et al. 1997, 2001), petrophysical, and seismic properties (Dyment et al. 1997; Miller and Christensen 1997) of the oceanic litho-sphere, while the chemical exchange between seawater and rock influences the chemical budget of the oceans (Früh-Green et al. 2004; Snow and Dick 1995) and the transport of elements and water into the mantle and, ultimately, to arc magmas (Alt et al. 2013; Hacker 2008; Hattori and Guillot 2007; Scambelluri et al. 1995, 2004; Ulmer and Trommsdorff 1995). Additionally, the microbial commu-nities that are found in association with rocks undergoing serpentinization have potentially important implications for the origin of life on Earth and other planets (Kelley et al. 2005; Martin et al. 2008; Martin and Russell 2007; McCol-lom 1999; Russell et al. 2010).

Serpentine is the dominant mineral in serpentinites; however, brucite, talc, and magnetite are important prod-ucts of serpentinization reactions and provide evidence for the temperature of water–rock interaction, water–rock ratios, and the chemical composition of the interacting fluid (e.g., Bach et al. 2004; Frost and Beard 2007; Klein et al. 2014). Additionally, the formation of magnetite during ser-pentinization is important for the production of hydrogen (e.g., Klein et al. 2009; McCollom and Bach 2009; Neal and Stanger 1983) and the interpretation of magnetization signatures in the oceanic lithosphere (Klein et al. 2014; Oufi et al. 2002). In the process of understanding serpen-tinization reactions, and specifically the mechanisms of magnetite formation, various authors have suggested that serpentinization is a two-stage process and that magnet-ite formation increases with degree of serpentinization, replacing Fe-rich brucite and/or serpentine (Bach et al. 2006; Oufi et al. 2002; Toft et al. 1990). Numerous studies (e.g., Beard et al. 2009; Frost et al. 2013; Katayama et al. 2010; Miyoshi et al. 2014) reported and identified several generations of vein formation associated with serpentini-zation and suggested that early veins form in a rock-dom-inated system, while later, magnetite-bearing veins form in a fluid-dominated, open system. In most of these studies, an increase in silica activities, for example derived from the decomposition of pyroxene, is interpreted to be a key factor for brucite to break down to form magnetite (e.g., Bach et al. 2006; Beard et al. 2009), while a more recent experimental study showed that magnetite is absent when silica activity is high (Ogasawara et al. 2013). In contrast to these studies, Evans (2008) suggests that magnetite is formed by the simple reaction of olivine + water = ser-pentine + magnetite (+iron alloys) and that high Mg# in serpentine is the result of an Fe–Mg exchange potential between olivine or orthopyroxene and serpentine.

Unraveling the detailed mineralogical processes and iden-tifying the factors that control the progress of serpentiniza-tion, specifically the formation of magnetite as a function of the degree of serpentinization, can significantly advance our ability to interpret seismic and magnetic signals from mid-ocean ridge to subduction zone settings and provide new insight into the extent of hydrogen production during serpen-tinization. Here, we present a study of a harzburgite sample from an ophiolite sequence that has been affected by low degrees of serpentinization, preserving detailed structures of vein formation during replacement of olivine and orthopy-roxene. Serpentinized peridotites generally preserve several generations of vein formation and reactivation of older veins, obscuring the primary vein textures. The sample studied here, however, is insufficiently serpentinized to have expe-rienced such overprinting, and records direct mineralogical evidence of the temporal evolution of the reactions that con-trol serpentinization and magnetite formation. Using petrog-raphy, Raman spectroscopy and mineral chemical data from numerous olivine- and orthopyroxene-hosted veins com-bined with simple thermodynamic models, we document the influence of water and silica activities on the mineralogical and chemical distributions within veins, tracking the pro-gression of serpentine, brucite, talc, and magnetite formation upon olivine and orthopyroxene hydration.

Analytical methods

Petrography

Mineral chemistry was determined by electron microprobe (EMP) analysis using a Cameca SX-50 electron microprobe at the Department of Geosciences at Virginia Tech. Accel-erating potential was 15 kV, and a 20 nA beam current and 1-μm spot size were used. Calibration was accomplished using natural and synthetic mineral standards. Element dis-tribution maps were produced using the EDS (energy-dis-persive spectrometers) and were run for 2–8 h at either 40 or 100 nA depending on run time. Analytical reproducibil-ity (1σ) for the major elements is <1 % and for the minor elements generally <4 %. Scanning electron microscope (SEM) images were carried out on a ZEISS SUPRATM 40 VP Ultra SEM at the Institute of Geological Sciences at the Freie Universität Berlin.

Raman spectroscopy

The mineralogy of serpentine and the possible presence of talc and brucite in veins were determined by Raman spectroscopy using a JY Horiba LabRam HR800 with 600 grooves/mm gratings. The slit width was set at 150 µm and the confocal aperture at 400 µm. Excitation was provided by a 514.53-nm


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(green) Ar+ laser, with an output of 50 mW at the source and <10 mW at the sample. The detector was an electronically cooled open electrode CCD. The laser spot size (analytical area) was on the order of 1 μm using a 100× objective.

Geological background

In this study, we examined a partly serpentinized harzbur-gite sample from the Santa Elena Ophiolite in Costa Rica. The ophiolite is located on the northwest coast of Costa Rica and comprises a 40-km-long and 16-km-wide com-plex including mafic and ultramafic lithologies that were emplaced during the Upper Cretaceous (Fig. 1; Baum-gartner and Denyer 2006; Denyer and Gazel 2009; Gazel et al. 2006). The Santa Elena Ophiolite comprises vari-ably serpentinized peridotites, dunites, locally layered gab-bros, and two generations of mafic dikes that intrude the peridotites: one generation preserving chilled margins and another without chilled margins, suggesting that the latter was emplaced into a hot mantle host preceding serpen-tinization (Gazel et al. 2006). The mafic sequences have been dated at ~110–125 Ma with geochemical evidence for melt formation associated with an ultra-slow to slow-spreading oceanic ridge and variable subduction input from a presumed underlying subduction zone (Baumgartner and Denyer 2006; Hauff et al. 2000; Madrigal et al. 2015). Additionally, some of the mafic lithologies contain a sec-ondary mineral assemblage that has been ascribed to ocean floor metasomatism (albite + epidote + actinolite + chlo-rite) and local rodingitization (the presence of tremolite, diopside, hydrogrossular, zoisite) (Gazel et al. 2006), while the sulfide mineral and metal assemblages in the peridotites indicate highly reducing conditions and low (<1) water–rock ratios during serpentinization (Schwarzenbach et al. 2014). The tectonic evolution of this ophiolite sequence is controversial and subject to current research (Madrigal et al. 2015; Schwarzenbach and Gazel 2013; Schwarzen-bach et al. 2014). Overall, there is increasing evidence that serpentinization of the peridotite occurred when sec-tions of the upper mantle were exposed to seawater along an ultra-slow to slow-spreading ridge, but that fluid influx was limited (Madrigal et al. 2015; Schwarzenbach et al. 2014). Precise temperatures of water–rock interaction are currently unknown, but a previous study of this ophiolite suggests serpentinization temperatures <250 °C based on mineralogical observations (Schwarzenbach et al. 2014).

Sample description

The peridotites of the Santa Elena Ophiolite are lherzolites, cpx-rich harzburgites, and dunites and have a degree of

serpentinization ranging between 30 and 100 %. Numerous samples were examined during this and a previous study (Schwarzenbach et al. 2014), with samples that are only partially serpentinized showing consistent textures (at the microscope scale) in the orthopyroxene- and olivine-hosted veins. We focus here on the least serpentinized sample studied, which also preserves the best-developed textures.

The studied sample is ~30 % serpentinized, with replacement occurring along veins in orthopyroxene and olivine, with a mesh texture developed around olivine grains (Fig. 2a). Replacement of olivine is significantly more advanced than replacement of orthopyroxene, while rare clinopyroxene has experienced little replacement presumably reflecting both thermodynamic stability and differential kinetics of the replacement reactions at this temperature (e.g., Bach et al., 2006). Olivine has a uni-form Mg# (=Mg/(Mg + Fe) × 100) of 90 with NiO con-tents of 0.26–0.43 wt% and traces of MnO (<0.24 wt%), TiO2 (<0.12 wt%), and Cr2O3 (<0.24 wt%) (Table 1).

Costa Rica

Santa Elenapeninsula

Pacific Ocean

Caribbean Sea

Panama

Nicaragua

Pacific Ocean

Santa Elena Nappe

Pillow and massive basaltsDolerite dikes Faults Santa Elena thrust

Dike swarm

Santa Rosa Accretionary Complex

5 km

10°55’

10°50’

85°50’85°60’

(a)

(b)

Fig. 1 a Location of the Santa Elena peninsula in Costa Rica and b geological map of the Santa Elena Ophiolite (rectangle in a) with the star indicating the sampling location of the studied harzburgite. After Gazel et al. (2006)


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Orthopyroxene has a Mg# of 90 and contains clinopyrox-ene exsolution lamellae that are resistant to serpentiniza-tion. Orthopyroxene typically contains 4.5–5.2 wt% Al2O3, <1.87 wt% CaO, and <0.83 wt% Cr2O3 (Table 1). Clino-pyroxene has a diopsidic composition with 2.4–5.2 wt% Al2O3, 0.74–1.34 wt% Cr2O3, 0.22–0.45 wt% TiO2, 0.18–0.57 wt% Na2O, and traces of MnO (<0.16 wt%). Spinel has a Cr# (=(Cr/Cr + Al) × 100) of 19.3 and Mg# of 72.4.

Several different types of serpentine ± brucite ± mag-netite ± talc veins can be observed in the studied sample, with the most common veins hosted in olivine or orthopy-roxene, while rare clinopyroxene contains few veins. Veins replacing olivine are dominated by serpentine, but a brownish color in some veins may suggest the presence of variable amounts of brucite, which was confirmed by Raman spectroscopy, as pure serpentine is typically color-less. Small veins (<10–20 μm) are optically homogene-ous (i.e., lack zonation). Veins >20 μm typically contain a brown center that is variably defined (i.e., a distinct line vs.

gradational) and locally contain magnetite in veins exceed-ing ~50 μm in width (Fig. 2b). These magnetite-bearing veins are characterized by a complex mineralogical zona-tion (Fig. 2b). The mineral zoning is consistent with chemi-cal variations determined by EMP analyses and chemical mapping, and is a major focus of this study. Veins replac-ing orthopyroxene are typically <80 μm wide, are domi-nated by greenish serpentine, and are most commonly cut oblique to the cleavage of the orthopyroxene (Fig. 2a, c). These veins are typically relatively homogeneous and only rarely show internal vein structures. Where orthopyroxene abuts olivine, the green serpentine veins in the orthopy-roxene extend into the olivine (e.g., Fig. 2c). These veins are later referred to as hybrid serpentinites. The opaque phases in the sample are mostly pentlandite, with traces of pyrrhotite, awaruite, native copper, and sugakiite. The typically observed intergrowth of pentlandite + magnet-ite + awaruite is attributed to desulfurization of primary pentlandite due to the highly reducing conditions during

Fig. 2 Thin section images (in transmitted light) of the studied har-zburgite. a Overview of partly replaced orthopyroxene and olivine, where replacement of olivine by serpentinite is more advanced than replacement of orthopyroxene. Rectangle A indicates the area shown in b and in Fig. 9, rectangle B indicates the area shown in c and in Fig. 10. White dashed lines outline areas dominated by hybrid ser-pentine veins, i.e., areas where olivine is replaced by serpentine in the vicinity of an olivine–orthopyroxene contact. b Vein formation during

hydration of olivine. Small veins (<10 μm) are optically homogene-ous and dominated by serpentine (subsequently termed stage 1 veins). Larger veins preserve a variably brucite-rich center reflected by brownish color (stage 2 and stage 3 veins). Veins >50 μm can contain opaque magnetite in the vein center (stage 4 veins). c Orthopyroxene and adjacent areas with greenish serpentine veins that cut oblique to the cleavage of the primary orthopyroxene


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serpentinization (Schwarzenbach et al. 2014). Otherwise, magnetite is rare; it only occurs in the centers of olivine-hosted veins that are >50 μm (Fig. 2b; see detailed discus-sion below) and is absent from orthopyroxene-hosted veins. Carbonate veins that are common in variably serpentinized peridotites were not observed in this sample.

Bulk vein chemistry

Overall chemical variability

EMP analyses were performed on numerous veins to reveal the chemical and mineralogical variability of different vein generations and veins hosted by either olivine or orthopy-roxene (Table 1; supplementary Table S1). Although EMP analyses of very fine-grained serpentinites (or other fine-grained materials) typically fail to resolve individual mineral phases (which form <1 μm intergrowths), large numbers of spot analyses taken together can help define the overall chemical variability of serpentinites based on knowledge of the phases that could be mixed in each analysis. EMP data indicate that there are three groups of

serpentinite in the Santa Elena sample (Table 1): one occur-ring as veins within protolith olivine, one as veins in proto-lith orthopyroxene and a hybrid serpentinite formed when olivine is replaced by serpentinite near an orthopyroxene grain boundary. The use of the term serpentinite is delib-erate and refers to a polymineralic assemblage commonly dominated by, but not necessarily consisting exclusively of, serpentine.

Note that the variations described here can, in most areas, be correlated with distinct patterns of margin-paral-lel zoning in individual veins. The immediate description, however, is restricted to overall host-dependent vein chem-istry and the implications for chemical fluxes.

Serpentinite derived from olivine

The serpentinites hosted by olivine are best described as mixtures of serpentine and brucite, with or without magnet-ite. The average composition of the veins is approximately that expected from simple hydration of the host olivine. For the majority of the magnetite-free veins, the most silicic compositions in the data set approximate stoichiometric serpentine, but the data set taken as a whole represents

Table 1 Averages and ranges of electron microprobe analyses of olivine, orthopyroxene, olivine- and orthopyroxene-hosted veins, and hybrid serpentinite

Averages are reported for analyses above detection limits

(in wt%) SiO2 TiO2 Al2O3 Cr2O3 MgO CaO MnO FeO Cl Total

Average olivine 41.4 0.03 0.04 0.02 48.99 0.03 0.14 9.7 0.04 100.31

SD 0.4 0.02 0.09 0.03 0.48 0.02 0.03 0.26 0.02 0.08

Average opx 55.55 0.09 3.62 0.55 33.04 0.78 0.14 6.64 0.03 100.42

SD 1.07 0.03 1.21 0.12 0.61 0.25 0.03 0.21 0.02 0.72

Mgt-free olivine-hosted veins (n = 603)

Average 35.92 0.03 0.07 0.04 41.97 0.08 0.1 6.94 0.1 85.08

Min 7.21 <l.o.d. <l.o.d. <l.o.d. 37.17 <l.o.d. <l.o.d. 1.46 <l.o.d. 75.92

Max 44.94 0.06 0.33 0.14 54.58 0.25 0.42 23.03 0.31 89.94

Mgt-bearing olivine-hosted veins (n = 295)

Average 33.68 0.02 0.09 0.04 42.31 0.07 0.13 9.29 0.07 85.54

Min 0.14 <l.o.d. <l.o.d. <l.o.d. 3.35 <l.o.d. <l.o.d. <l.o.d. <l.o.d. 74.12

Max 44.18 0.07 0.46 0.18 79.02 0.35 0.38 85.29 0.29 100.94

All olivine-hosted veins (n = 898)

Average 35.18 0.03 0.08 0.04 42.08 0.08 0.11 7.71 0.09 85.23

Min 0.14 <l.o.d. <l.o.d. <l.o.d. 3.35 <l.o.d. <l.o.d. <l.o.d. <l.o.d. 74.12

Max 44.94 0.07 0.46 0.18 79.02 0.35 0.42 85.29 0.31 100.94

Opx-hosted veins (n = 139)

Average 38.68 0.05 4.47 0.37 32.29 0.76 0.23 8.02 0.15 84.98

Min 32.26 <l.o.d. 1.82 <l.o.d. 23.35 0.17 0.1 4.93 0.04 76.72

Max 46.21 0.16 14.18 0.87 35.83 4.6 0.38 11.69 1.67 88.85

Hybrid serpentinites (n = 445)

Average 37.33 0.04 1.84 0.18 36.6 0.39 0.16 7.58 0.26 83.96

Min 15.74 <l.o.d. <l.o.d. <l.o.d. 24.48 <l.o.d. <l.o.d. 3.38 <l.o.d. 76.74

Max 43.84 0.17 6.08 0.93 48.74 4.6 0.33 14.52 1.2 89.71

Detection limits 0.02 0.01 0.02 0.02 0.05 0.02 0.05 0.05 0.02


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variable serpentine–brucite proportions (Fig. 3a). Regres-sion analysis of the whole data set for magnetite-free veins suggests that the serpentine end member is more magne-sian (Mg# = 94–95) than the host olivine (Fo 90), while the brucite end member is less magnesian (Mg# = 72–78), which is evident in the regression lines shown in Fig. 3a. These results are consistent with previous studies of par-tially serpentinized rocks (e.g., Beard et al. 2009; Frost et al. 2013). Also note the anti-correlation between Mg cations and Mg# in these veins (Fig. 3b), a result consistent with the presence of a Mg-rich, Si-poor phase, with brucite being the obvious candidate.

The less abundant magnetite-bearing veins differ from other veins in that some spot analyses are very Fe rich (i.e., magnetite rich; see trends in Fig. 3c, d). In the magnetite-bearing veins, serpentine-rich (i.e., silica-rich) analyses are similar to those from magnetite-free veins (Fig. 3d). More noteworthy is the presence of Mg-rich brucite (Mg# = 96)

in the magnetite-bearing veins (e.g., Fig. 3c). The diver-gence of brucite compositions in the two vein types is apparent from the deviation of low-Si, low-Fe, high-Mg analyses in the magnetite-bearing veins from the regression lines calculated for the magnetite-free veins (Fig. 3d). In this sample, the most magnesian brucite is restricted to the centers of magnetite-bearing veins.

Serpentinite derived from orthopyroxene

Serpentinites derived from orthopyroxene are chemically distinct from those derived from olivine. First, they are more silicic, with the only overlap in composition with oli-vine-derived serpentinites in the vicinity of stoichiometric serpentine (Fig. 4a). The average (and typical) orthopyrox-ene-derived serpentinite can be described as a mixture of serpentine and talc (Fig. 4a). Second, on average, orthopy-roxene-derived serpentines (average Mg# = 87.8 ± 1.8)

Mg/

(Mg+

Fe)

0 1 2 30.0

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/7 o

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Mg#

serp

entin

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Brucite interceptsMg Cations = Mg#78Mg# = Mg#72

Serpentine interceptsMg cations = Mg# 95Mg# = Mg#94

r2 = 0.95

r2 = 0.63

0 1 2 30

2

4

6

r2 = 0.84

magnetite trend

Si + Al cations/7 ox.Fe

Cat

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/7 o

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Mgt-free veinsMgt-bearing veins

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0 1 2 3 4 5 6 70.0

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Fig. 3 Bulk EMP analyses of serpentinite veins in olivine (due to submicron intergrowth structures, each analysis samples a mixture of mineral phases). a Variation of Mg with Si + Al in magnetite-free serpentinite veins. Brucite at Si = 0, serpentine at Si = 2. Serpentine Mg# (Mg/(Mg + Fe)) and Mg cations based on intercept value of 2.8 out of 3 Mg cations in the serpentine formula (intercept at 6.53 out of 7 on y-axis shown), for brucite based on 5.49 out of 7 Mg cati-ons in the brucite formula. Al concentration is negligible and included only for purposes of later comparison with high-Al serpentinites in orthopyroxene-hosted veins. b Mg# variation with Mg content. In magnetite-free veins, Mg content increases as Mg# decreases. In magnetite-bearing veins, two trends are observed: coupled Mg# and Mg content increase/decrease (attributable to a magnetite component)

and a trend of near constant Mg# with increasing Mg. This trend and that of the magnetite-free veins are most easily explained by a brucite component: Mg-rich brucite in the magnetite-bearing veins and more Fe-rich brucite in the magnetite-free veins. c Mg# variations in mag-netite-bearing veins. Low-Mg# analyses include a magnetite compo-nent. High-Mg# analyses are largely magnesian (Mg# 90+) brucite. Note the divergence at high Mg# between magnetite-bearing and magnetite-free veins in olivine. The field of magnetite-free vein com-positions and the regression line are from a. d Fe variation in olivine-hosted veins with the regression line through magnetite-free veins. Note the very Fe-poor, Si-poor compositions in magnetite-bearing veins, which reflect the presence of Mg-rich brucite coexisting with magnetite in the vein centers


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have lower Mg# than olivine-derived serpentinites (average Mg# = 90.8 ± 6.6), despite the orthopyroxene-derived ser-pentinites having a wider range in Mg# and, unlike the oli-vine-derived material, are slightly less magnesian than their host (Fig. 4b). Third, minor and trace elements that are enriched in orthopyroxene, specifically Cr, Al, Ca, and Ti, are enriched in orthopyroxene-derived serpentinite as well (Fig. 5a, b; Table S1), a characteristic that is typically used to identify serpentine after pyroxene (Dungan 1979). Note

that the average composition of serpentinite derived from orthopyroxene is substantially less silicic than would be expected from simple hydration of orthopyroxene (Fig. 4a, b), suggesting significant open-system behavior.

Hybrid serpentinites

Hybrid serpentinites are formed by hydration of olivine in the vicinity of an olivine–orthopyroxene contact, consist

Fig. 4 Major cations in magnetite-free olivine-hosted veins contrasted with veins in orthopyroxene. a Veins in orthopyroxene are more silicic than veins in olivine, with the only overlap in the vicinity of stoichiometric serpentine, and strongly depleted in silica relative to the protolith orthopyroxene. b Although there is substantial overlap, veins in orthopyroxene are on average less magnesian than those in olivine and slightly less magnesian than the protolith orthopyroxene. c, d Hybrid serpentinite veins have a bulk chemistry intermediate between serpentinite veins in olivine and orthopyroxene. Note that, for the most part, the hybrid veins form by replacement of olivine at and near the orthopyroxene–olivine grain boundaries Si+Al cations/7 ox.

Mg

catio

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ox.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

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7average vein serpentinites

hybrid serpentinitesOl vein field

Opx vein field

0.0 0.5 1.0 1.5 2.0 2.5 3.00.7

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Opx vein field

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average vein compositions

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serp

entin

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averageOpx

average Ol

averageOpx

average Ol

serp

entin

e

talc

talc

average protolith

Mg(

OH

) 2

cron

sted

tite

Fig. 5 Concentrations of minor cations in a and b orthopy-roxene- and olivine-hosted and c, d hybrid serpentinite veins. Al and Cr are elevated in host orthopyroxene and in veins in orthopyroxene. Hybrid serpentinites have intermediate compositions

0 1 2 30.00

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veins in Opxveins in Olaverages

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of variably Al-enriched serpentine ± brucite ± talc, and always lack magnetite. As the name implies, they have chemical characteristics suggesting input from both orthopyroxene and olivine during their formation. Both major (Fig. 4c, d) and minor (Fig. 5c, d) element com-positions of hybrid serpentinites straddle the boundaries between the olivine and orthopyroxene fields (Table 1). One interpretation is that individual analyses are recording protolith composition, i.e., the composition of the mineral that is replaced. However, this is clearly not the case since the mineral being replaced in all areas of hybrid serpent-inites is olivine. The hybrid serpentinites are characteristi-cally enriched in Cl (up to 1.19 ± 0.02 wt%), more so than any analysis from olivine- or orthopyroxene-hosted veins (Tables 1, S1).

Structural and mineralogical variations within veins

Olivine‑hosted veins

Petrographic observations show that olivine-hosted veins preserve various types of mineral and chemical zoning (Fig. 2b). Using a combination of petrography, EMP trav-erses through individual veins, and element distribution maps, we identified four stages of vein formation (i.e., oli-vine hydration) representing increasing reaction progress. These four stages were identified based on their distinct mineralogical, textural, and chemical characteristics, but they probably only represent snapshots at different stages of vein evolution. In other words, the initial fractures in the minerals developed at different times and then evolved from stage 1 toward stage 4. Fractures that developed later may only have evolved to stage 1 or 2 before the rock was exhumed to the surface, whereas fractures that developed earlier may have evolved through all four stages. Addition-ally, the different vein stages’ rare crosscutting relation-ships suggest that all veins evolved at a similar time. Given this, and the absence of data suggesting differently, we assume that evolution of stages 1–4 occurred at approxi-mately constant temperature. Locally, magnetite-bearing veins do crosscut older stages 1–3 veins, supporting our assertion that the magnetite-bearing veins represent the most evolved vein stage.

Because we associate increasing alteration of olivine with an increase in the vein width, we identify the thinnest (<10–20 μm), homogeneous, and unzoned veins (Fig. 2b) as representing the earliest stage of olivine replacement. EMP traverses across these stage 1 veins show that they consist of serpentine–brucite mixtures. The serpentine–brucite mixture has an average Mg# of ~92 and an Si/(Mg + Fe) of 0.51–0.53 (Fig. 6a), but is too fine grained

to permit separate analysis of each mineral. Increasing width of olivine-hosted veins is typically associated with the development of a brownish vein center (Fig. 2b). EMP analyses reveal that this central zone has a slightly higher brucite content (Mg# = 91–92; Si/(Mg + Fe) = 0.46–0.47) than the main part of the vein (zone 2: Mg# ~91; Si/(Mg + Fe) = 0.50–0.53; Fig. 6b). We identify these veins as stage 2 veins.

Petrographically, the next stage of vein formation is very similar to stage 2 and cannot be distinguished using microscopy alone. We identify stage 3 veins based on their more complex chemical zonation (Fig. 7a) and as a tran-sitional stage between stages 2 and 4. Stage 3 veins com-prise four zones between the boundary of the olivine grain and the vein center: At the contact with olivine, a thin serpentine-rich zone (zone 2*, Fig. 6c) can be identified locally, which is also seen in Fig. 7a (as Srp*). The main zone between olivine and the vein center (zone 2; Fig. 6c) consists of a serpentine–brucite mixture (Mg# = 90–92, Si/(Mg + Fe) = 0.52–0.56) that is chemically similar to zone 2 in stage 1 and stage 2 veins. A thin serpentine-rich zone generally forms between zone 2 and the center of the vein (Mg# = 93–94; Si/(Mg + Fe) = 0.60–0.64; Figs. 6c, 7a, Srp†). The center of the vein is enriched in Fe (Mg# ≈ 85; Si/(Mg + Fe) ≈ 0.25) and is brucite enriched (as magnetite is not identified by microscopy) compared to the center in stage 2 veins. This suggests the local presence of Fe-rich brucite, which is also confirmed by SEM analyses (Fig. 8 and supplementary material S.2).

We interpret the magnetite-bearing veins as represent-ing the latest (stage 4) and the most evolved veins for the following reasons: (1) the chemical zoning of stage 4 veins has significant similarities (except for the presence of magnetite) to the zoning of stage 3 veins (Fig. 6c, d), suggesting that stage 4 veins evolved from stage 3 veins, and (2) stage 4 veins locally crosscut stages 1–3 veins. A relatively complex zoning in magnetite-bearing veins is apparent from microscopic observations (Fig. 2b) but is pronounced in EMP traverses across individual veins (Fig. 7b), and in element distribution maps of Si, Mg, and Fe (Fig. 9). A serpentine-rich zone (Mg# = 91–93; Si/(Mg + Fe) = 0.58–0.65) forms at the immediate interface with olivine (Fig. 6d, zone 2*, and bright pink regions in Fig. 9e). The main part of the vein consists of a ser-pentine–brucite mixture (zone 2 in Fig. 6d; Fig. 7b) that ranges from very fine-grained to coarser intergrowths. These coarse-grained mixtures are visible in SEM images (Fig. 8d) and characterized by highly variable Mg# and Si/(Mg + Fe) (Mg# = 85–91; Si/(Mg + Fe) = 0.41–0.57) as shown by the ‘zig-zag’ pattern in the compositional pro-file shown in Fig. 7b. Toward the center of the vein, three distinct, thin zones can be distinguished: a brucite-rich zone (Mg# = 85–90; Si/(Mg + Fe) = 0.25–0.38) that


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may be absent in some areas (Fig. 9c), followed by almost pure serpentine with very high Mg# (Mg# = 96–99; Si/(Mg + Fe) = 0.65–0.69) and magnetite coexisting with almost pure brucite that has Mg# up to 96 (Fig. 7b). These coexisting phases are also distinguishable by SEM (Fig. 8d).

In summary, the development of veins during olivine alteration is characterized by an increase in vein width accompanied by an increase in the complexity of the vein

zoning. The most convincing evidence that stage 4 veins most likely evolve from stage 1 to stage 2 to stage 3 veins is that the main zone (zone 2 in Fig. 6a–d) generally shows relatively constant chemical composition (i.e., Mg# and Si/(Mg + Fe)), while the vein center experiences chemical and mineralogical changes as the vein interacts with fluid and evolves chemically and mineralogically. Evidence for increasing fluid flow in the center of the vein is discussed in detail below.

(d) 1 2* 2 3 4 2*234 1

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Fig. 6 Schematic description of the stages of vein formation within olivine showing the different zones determined by EDS mapping and average ranges of Mg/(Mg + Fe) (=Mg#/100) and Si/(Mg + Fe) for each zone from profile analyses of ~80 different veins. In all develop-mental stages, zone 1 contains olivine with Mg/(Mg + Fe) = 0.9 and Si/(Mg + Fe) = 0.5. The dashed gray line represents serpentine (Si/(Mg + Fe) = 0.67). a Stage 1 vein representing initial formation of serpentine + brucite in olivine. b Stage 2 vein contains a more bru-

cite-dominated (brc dom.) vein center. c Stage 3 vein has slightly ser-pentine-dominated (srp dom.) zone next to olivine and a distinct bru-cite-dominated vein center with lower Mg# than stage 2 vein centers. d Stage 4 veins with magnetite and almost pure brucite in the vein center (zones 5 + 6), next to a zone of almost pure serpentine that has Mg# of up to 99 (zone 4) and a slightly brucite-dominated zone (zone 3) next to it. See text for detailed discussion


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Orthopyroxene‑hosted veins

In the studied sample, orthopyroxene is less serpentinized than olivine, with serpentine veins generally cutting across the cleavage of the protolith orthopyroxene. Unlike in olivine, it is not possible to clearly distinguish different vein types.

Most veins in orthopyroxene have an Si/(Mg + Fe) of 0.66–0.73 (Fig. 7c) and have Mg# slightly lower (86–89) than the host orthopyroxene (Mg# = 90). An Si/(Mg + Fe) of >0.67 suggests the presence of traces of talc, as confirmed by Raman spectroscopy, most likely as a fine-grained intergrowth that cannot be resolved by EMP. A few veins have Si/(Mg + Fe) = 0.67 indicative of pure serpentine, while some veins contain thin, unusually Al-rich zones, where Al2O3 contents can reach 7.7–14.2 wt%. These Al enrichments are either at the edge or in the center of the serpentine vein. Where clinopyroxene exsolutions impinge on the veins, it is apparent that these exsolutions are less altered than the orthopyroxene host, consistent with the relative stability of clinopyroxene in this sample.

Distribution of hybrid serpentinites

Hybrid serpentinites show two modes of occurrence, both related to contacts between olivine and orthopyroxene: (1) In areas where substantial olivine is preserved near that contact, the hybrid serpentinites occur as flames or wedge-like structures replacing olivine. These structures occur at the point where serpentinite veins in the orthopy-roxene impinge on the olivine–orthopyroxene grain bound-ary (Figs. 2c, 10). (2) To the left of and above the large orthopyroxene grain in Fig. 2a, olivine is largely replaced by regions of extensive meshes of hybrid serpentinite and any relationship between specific veins in orthopyrox-ene and the areas of hybridization is obscured. In neither case is there any regular compositional variability in the hybrid serpentinite areas. Instead, wispy or irregular areas of element variability are especially evident for Si and Al (Fig. 10c, e). Although irregular, these variations are signif-icant as shown by EMP analyses (Figs. 4, 5). Also note in Fig. 10f the overall high concentrations of Cl in the hybrid serpentinite, which was also confirmed by EMP point analyses.

Temporal relationships

Element distribution maps collected at numerous locations along the contact of olivine and orthopyroxene suggest that the hybrid serpentinites are a relatively early-formed feature, specifically reflecting breakdown of orthopyrox-ene. The hybrid serpentinites are sometimes cut by oli-vine-hosted serpentinite veins, which typically have high Mg, low Al, significantly lower Fe, and low Cl contents (Fig. 10). Furthermore, there is evidence that a later stage fluid interacted with orthopyroxene to produce serpentinite veins that are variably enriched in Fe and Al. These veins are best distinguished from earlier serpentine veins associ-ated with orthopyroxene due to their distinct chemical com-position and because they crosscut the hybrid serpentinites

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Fig. 7 EMP analyses of three vein traverses (point analyses with constant analysis spacing of 2 μm) showing variations in Mg/(Mg + Fe) (black solid line) and Si/(Mg + Fe) (black dashed line). a Olivine-hosted stage 3 vein preserving slightly higher Si/(Mg + Fe) ratios directly next to a brucite-dominated vein center (Srp†). b Olivine-hosted and magnetite-bearing stage 4 vein show-ing distinct serpentine, brucite, and magnetite zoning. c Orthopy-roxene-hosted serpentine vein with Si/(Mg + Fe) of 0.70–0.73 sug-gesting fine-grained intergrowth of serpentine and traces of talc. Gray lines = Si/(Mg + Fe) of the standard minerals: orthopyrox-ene Si/(Mg + Fe) = 1; olivine Si/(Mg + Fe) = 0.5; serpentine Si/(Mg + Fe) = 0.67; and brucite and magnetite Si/(Mg + Fe) = 0 (not shown in the figure)


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(e.g., Fig. 10e). Overall, the element distribution maps suggest effectively concurrent alteration of olivine and orthopyroxene, possibly with a later stage fluid that mostly affected orthopyroxene.

Raman spectroscopy

Each of the vein types was analyzed using Raman spec-troscopy to confirm the mineralogy and to identify the dominant serpentine phases (Fig. 11; supplementary mate-rial S.3). We recognize that relative peak intensities can be affected by the orientation of the mineral relative to the polarization of the incident laser. Because the mineral phases are fine grained in most areas and many grains are included in the analytical volume, we can assume a random orientation of grains and use the variation in relative peak intensities as a first approximation of the relative abun-dances of different phases in different areas of the veins. Thus, by comparing the variation in relative peak intensi-ties of, for example, one of the dominant peaks for lizar-dite versus one of the dominant peaks for brucite, we can approximate the relative abundances (in a qualitative sense)

of these two phases in two different areas that were ana-lyzed. We emphasize, however, that the primary reason for Raman spectroscopy here is for phase identification.

Olivine‑hosted veins

Raman spectroscopy confirms that lizardite is a domi-nant mineral phase within olivine-hosted veins. Typical Raman bands of lizardite occur at 129–131, 231–235 cm−1 (assigned to vibrations of the O–H–O groups), 385–388 cm−1 (vibrational modes of the SiO4 tetrahedra) and 688–691 cm−1 (symmetric Si–O–Si stretching vibrations), with the OH-stretching bands at 3684–3692 and 3705 cm−1 (Groppo et al. 2006; Kloprogge et al. 1999; Rinaudo and Gastaldi 2003). The lizardite bands are most intense (rela-tive) in the early stage 1 veins and in the serpentine–brucite mixtures in stage 2 and stage 3 veins. Based on peak inten-sities, lizardite is a dominant phase along the vein contact with olivine, within the serpentine–brucite mixtures, and in the serpentine Mg#99 zone adjacent to the vein center. In many veins, distinct bands at 351 and 621 cm−1 indicate the presence of some chrysotile intergrown with lizardite (Kloprogge et al. 1999; Rinaudo and Gastaldi 2003). These

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Fig. 8 a SEM image of the area shown in Fig. 2b, where olivine is replaced by serpentine + brucite ± magnetite veins, with b, c, and d representing enlarged areas. b Variably Fe-rich brucite in the center of a magnetite-bearing vein. c Local Fe-rich brucite in the center of

a stage 3 vein. d Brucite (Mg#96) next to magnetite in the center of a stage 4 vein. The coarse-grained brucite–serpentine mixture of the stage 4 veins is also recognizable


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bands are most intense in serpentine Mg#99 in the center of magnetite-bearing veins.

Raman analyses in the center of the stage 4 veins show distinct bands at 279 and 444 cm−1 and pro-nounced OH-stretching bands at 3645 and 3650 cm−1 (Lutz et al. 1994), confirming coexistence of almost pure Mg–brucite (Mg#96) and magnetite (Fig. 11a, b).

Brucite was also confirmed within most of the serpen-tine–brucite mixtures and in the brucite-rich cores of stage 2 and stage 3 veins. These serpentine–brucite mix-tures usually show bands at 279, 444, and 3645 cm−1, but generally lack the stretching band at 3650 cm−1 that was observed in most analyses of pure brucite in mag-netite-bearing veins.

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Serpentine ± bruciteFe-brucite-dominated

Stage 4vein

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Fig. 9 Element distribution maps of olivine-hosted veins: a BSE image, b element distribution map of Mg, c Si, and d Fe. e Sche-matic description and enlargement of the area shown in a (white rectangle) showing a magnetite-bearing vein. The mineralogical and

chemical variations in Si, Mg, and Fe are indicated by different colors (explained on the right of panel e), mostly representing different mix-tures of serpentine, brucite, and magnetite. The pale blue arrows in e indicate fluid flow as discussed in the text


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Orthopyroxene‑hosted veins

Raman spectra of orthopyroxene-hosted serpentine veins are similar to those from veins hosted by olivine indicating a dominance of lizardite. Typical bands occur at 131, 231, 384–387, and 688–693 cm−1 with an OH-stretching band at

3681–3689 cm−1. In contrast to olivine-hosted serpentine veins, analyses consistently lack the OH-stretching band at ~3705 cm−1. Additional bands occur at 462–468, 536–549, and 1100–1105 cm−1. A band at 1102–1105 cm−1 can be ascribed to an antisymmetric stretching mode of the Si–O groups in chrysotile (Kloprogge et al. 1999; Rinaudo and

opx

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Fe

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olivineolivine

Fig. 10 Element distribution maps of an orthopyroxene–olivine grain boundary, with an initially orthopyroxene-hosted serpentine vein impinging into an olivine-dominated area. a BSE image, b element distribution map of Mg, c Si, d Fe, e Al, f Cl. The low resolutions of the Fe and Cl maps reflect the low concentrations. The element dis-tribution maps agree with the bulk vein chemistry, showing, e.g., that

orthopyroxene-hosted veins are depleted in Si, but slightly enriched in Mg compared to the host orthopyroxene. Note, Al enrichments and high Cl contents of hybrid serpentinite along the olivine–orthopyrox-ene grain boundary, and the relation between olivine-hosted veins and hybrid serpentinite in the Mg map. Light colors = high concentra-tions; dark colors = low concentrations


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Gastaldi 2003). Hence, traces of chrysotile are likely inter-grown with lizardite. The presence of talc is suggested by a distinct band at 196 cm−1 in several veins, which does not appear in the Raman spectrum of lizardite, chrysotile, or brucite. Raman spectra of talc typically show peaks at 362 and 675 cm−1, with the OH-stretching band at 3675 cm−1. These peaks are indicated by broad bands at 360, 688, and 3681 cm−1, respectively, but are largely obscured by over-lap with the lizardite peaks (Fig. 10c).

Al‑bearing serpentine

Serpentine with high Al contents observed in orthopy-roxene-hosted veins and hybrid serpentinites is identified by Raman spectroscopy because bands typically at 231 and 388 cm−1 are shifted to 220–227 and 377–383 cm−1, respectively, which was also observed by Groppo et al. (2006). Additionally, the chrysotile bands that occur at 350 cm−1, 620 cm−1, around 1100 cm−1 and the OH-stretching band at 3706 cm−1, are almost entirely lacking in these Al-rich serpentine phases, suggesting that chrysotile is relatively rare in these areas.

Discussion

Various studies have recently shown that hydration of oli-vine typically results in serpentine–brucite veins and that magnetite forms as a secondary product of olivine hydra-tion (Bach et al. 2006; Beard et al. 2009; Frost et al. 2013; Katayama et al. 2010). Importantly, most previous studies that look at several generations of vein formation describe textures that suggest reopening of older veins. In contrast, the textures of the harzburgite studied here imply continu-ous mineral evolution during vein formation, suggesting that individual veins were continuously open to fluid flow during this process, but that different veins were generated at different times. Hence, the results obtained here are con-sistent with previous studies, but due to the unusual preser-vation of mineralogical and chemical textures, we can give new insights into the processes during water–rock inter-action associated with the beginning of serpentinization. In particular: (1) The sample studied here records chemi-cal gradients between orthopyroxene and olivine imply-ing transfer of elements such as Si, Al, Cr, and Ca. (2) The chemistry of brucite in the studied sample varies depend-ing on whether it occurs with magnetite (brucite Mg#96) or without magnetite (brucite variably Fe rich). These vari-ations are linked to the mineral and chemical zoning within olivine-hosted veins that imply magnetite formation from Fe-rich brucite. (3) The mineral zoning in olivine-hosted veins records chemical gradients within individual veins, most likely due to focused fluid input into the vein center. These specific observations are discussed in detail below and are compared to thermodynamic models, also discuss-ing the implications of progressive olivine hydration and evolving fluid chemistry.

Implications for chemical fluxes

The geochemistry of the Santa Elena serpentinites and the element distribution maps demonstrate that serpentinization is an open-system process on the scale of mineral grains

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Fig. 11 Raman spectra of a brucite in the center of a magnetite-bear-ing, olivine-hosted vein, b a variably brucite-dominated zone in the center of a magnetite-bearing vein, showing brucite and lizardite, and c an orthopyroxene-hosted serpentine vein indicating the presence of serpentine and talc


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(<0.5–1 mm), allowing chemical transfer between orthopy-roxene-hosted and olivine-hosted veins. The most obvi-ously mobile element is Si, with the bulk vein chemistry indicating a net transfer of Si from orthopyroxene into the hybrid serpentinites that locally replace olivine (Fig. 4a). Si is transported up to ~200 μm from the orthopyroxene grain boundary into olivine-hosted veins (Fig. 10c), promoting in the hybrid serpentinite replacement of brucite by serpentine according to reaction (1):

As evinced by the element distribution maps (Fig. 10), the migration of Si is tracked by other elements such as Al, Ca, Cr, and Ti (Table 1; Fig. 5c,d) that were initially concen-trated in the protolith orthopyroxene and are transferred over approximately the same length scale. Taken together, this suggests a one-way element transport from the orthopyroxene toward the olivine during serpentinization to form hybrid ser-pentinites. This apparent one-way element transport suggests that gradients in silica activity between olivine and orthopy-roxene are the driving force for silica mobility, as has been suggested previously (e.g., Beard et al. 2009; Beard and Hop-kinson 2000; Frost and Beard 2007; Frost et al. 2008), and reflects the instability of the olivine hydration assemblage (serpentine–brucite) at the elevated silica activity imposed by the orthopyroxene hydration assemblages. However, it is evident that apart from Si, elements such as Al, Cr, Ca, and Ti develop similar chemical potential gradients between orthopyroxene and olivine. Serpentinization of olivine distal from the orthopyroxene grain boundary, on the other hand, appears to be approximately isochemical (Fig. 4a), with the obvious exception of water. Without exception, veins in oli-vine distal to the orthopyroxene contain brucite, as demanded by stoichiometry and thermodynamic constraints. The local brucite in limit is shown in Fig. 10c, marking the boundary where availability of SiO2(aq) is insufficient to allow com-plete conversion of brucite to serpentine.

Progressive olivine hydration and formation of magnetite

Progressive hydration of olivine has recently been studied with major focus on the formation of hydrogen and its cor-relation with magnetite formation and the partitioning of Fe into brucite and serpentine (Andreani et al. 2013; Klein et al. 2009, 2013, 2014; McCollom and Bach 2009). How-ever, the sequence of reactions during olivine hydration is still a matter of debate (e.g., Bach et al. 2006; Evans 2008; Frost and Beard 2007) as the olivine hydration reactions and the initial textures are rarely well preserved. In the following, we use simple thermodynamic models to track the role of water and silica activities during initial olivine hydration of the studied sample.

(1)3Mg(OH)2 + 2SiO2(aq) = Mg3Si2O5(OH)4 + H2O

The low degree of serpentinization of the harzburgite sample studied here preserves the relatively early stages of serpentinization (serpentinization degree 30 %) and records a critical stage in the initiation of magnetite forma-tion (Bach et al. 2006). Accordingly, the low abundance of magnetite in this sample agrees with studies that compare the magnetic susceptibility and rock density as a proxy for degree of serpentinization, showing that increasing degree of serpentinization and magnetite formation are initially decoupled (Bach et al. 2006; Beard et al. 2009; Oufi et al. 2002; Toft et al. 1990). This has previously been taken to imply that formation of magnetite during serpentinization is a two-step process, with initial formation of Fe-rich bru-cite, which is subsequently replaced by magnetite upon reaction with a silica-rich fluid. The sequence of vein for-mation within olivine described here clearly confirms this, but also emphasizes that vein evolution was continuous during interaction with a fluid, notably in the absence of an external silica source with preservation of local equilibria in the vein. In addition, the crosscutting nature of multiple veins at different stages of evolution (Fig. 2) implies that all stages also evolved at approximately the same temperature.

The initial reaction of olivine with water is preserved locally in the mineralogy of stage 1 to 3 veins, showing that serpentinization initiates by the formation of serpen-tine and brucite that is variably Fe rich, without forming magnetite. Mass balance calculations suggest that in these veins the Mg# of brucite varies between 65 and 91. Raman analyses of stage 1 and brucite-cored stage 2 and 3 veins confirm the presence of both serpentine (lizardite ± chry-sotile) and brucite. As neither Raman nor EMP analyses are able to resolve single grains, we suggest that serpen-tine and brucite are intergrown at the sub-micrometer scale. Mg# and Si/(Mg + Fe) in stage 1 veins are higher in the serpentine–brucite mixture than in the initial olivine, indi-cating that either some Fe is lost to the fluid during early reaction or that the composition of olivine along the reac-tion surface is changed. No clear change in composition is preserved within remnant olivine grains, which is also con-sistent with Evans (2010) for serpentinization temperatures below 300 °C.

Brucite is dominant in the center of stage 2 veins (Fig. 2b), but locally becomes less abundant toward the vein rim, particularly in more evolved veins (Fig. 6c, d). Assuming that the stages of vein formation presented above represent gradual reaction progress during replacement of olivine, we infer that an increase in water–rock ratio accompanies vein formation, with fluid flow in the vein center maintaining a high local water activity that dimin-ishes toward the vein margin. This is confirmed by model calculations: Modeling of simple dissolution of pure for-sterite into seawater shows that dissolution of Mg-bearing species substantially lowers aH2O at low fluid–rock ratios


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(Fig. 12a, b), but that log aSiO2 is buffered to ~−5.8 while forsterite remains in thermodynamic equilibrium (Fig. 12c). The serpentine-dominated outer margin of stage 3 and stage 4 veins (Fi.g 6c, d) thus implies protracted evolution dis-tal to the H2O source (i.e., olivine and fluid are physically separated by the early-formed serpentine–brucite ‘rind’), consistent with olivine-buffered equilibration at log aSiO2 above ~−6.2 and log aH2O below ~−0.8 (Fig. 12d).

This indicates that within a single vein, the progres-sion of the serpentinization reaction involves the migra-tion of H2O along a gradient from high aH2O in the vein center toward low aH2O at the olivine surface. Further fluid infiltration toward the contact with olivine is most likely

facilitated by grain boundaries of the finely intergrown ser-pentine–brucite mixture (Fig. 9e), which serve as suitable fluid pathways that eventually allow hydration of olivine at the vein wall. This occurs concurrently with reactions in the vein center that alter initial reaction products, eventu-ally resulting in the formation of magnetite from brucite, as preserved in the transition from stage 3 to stage 4 veins and expressed by the following simple reactions (Eqs. 2 and 3; with Mg# derived from EMP analyses and the thermody-namic calculations discussed below):

(2)Ol(Fo90)+ H2O = Mg-rich Srp(Mg# 93−95)

+ Fe-rich Brc(Mg# 82−84)

BruciteChrysotile

Forsterite

-7.00 -6.75 -6.50 -6.25 -6.00 -5.75 -5.50 -5.25 -5.00

log aSiO2 (aq)

log

aH2O

(aq

)

-2.00

-1.75

-1.50

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

Mg++

MgOH+

MgCl+

MgCl+

MgHCO3+

Mg4(OH4)++++

Mg4(OH4)++++

MgCO3 MgCO3

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Reaction progress upon additionof forsterite to seawater

1e-9

1e-8

1e-7

1e-6

1e-5

1e-4

1e-3

0.01

0.1

1

Mg-

bear

ing

solu

tion

spec

ies

(mol

/l)

SiO2(aq)

H3SiO4-

NaH3SiO4H2SiO4

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0


-9

-10

-11

-12

-8

-7

-6

-5

-4

-3

-2

-1

0

log

activ

ity o

f Si

-bea

ring

spe

cies

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0


0

10

20

30

40

50

60

70

80

90

phas

e ab

unda

nce

(vol

ume

%)

0.90

0.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

H2O

act

ivity

(a) (b)

(c) (d)

Brucite

abundance

Chrysotile

abundance

Fluid

abundance

H2O activity

Fig. 12 Water–olivine interaction, calculated with Geochemists Workbench at 200 °C. x-axes in panels a–c represent progressive dis-solution of 520 g of forsterite into 100 g of seawater with an initial composition of 19 mg/l Cl−, 1.26 mg/l Mg2+, 1 mg/l aqueous SiO2, 10 mg/l Na+, 400 mg/l Ca2+, 140 mg/l HCO3 and pH 8.0. ‘Reaction progress = 1’ refers to the point at which fluid saturation is lost: a

precipitated phases and the activity of H2O, b abundance of dissolved Mg-bearing species, c activity of SiO2 upon progressive reaction, and d forsterite and hydrated products as a function of H2O and SiO2 activities, consistent with previous observations that brucite is ther-modynamically more stable than serpentine only at high H2O activi-ties and low SiO2 activities (e.g., Frost and Beard 2007)


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This sequence of reactions also agrees with previ-ous studies of natural serpentinites (Beard et al. 2009; Frost et al. 2013; Klein et al. 2009; Miyoshi et al. 2014) and experimental studies (Lafay et al. 2012), implying that initial alteration of olivine produces serpentine with Mg# of 92–95 that coexists with less magnesian brucite (with reported Mg# as low as 65). However, temperature also likely affects the Mg#, particularly if Fe3+ is incorpo-rated into serpentine (Klein et al. 2014). Previous studies have attributed subsequent magnetite formation to either increasing or decreasing silica activity (destabilizing Fe-rich brucite or Fe-rich serpentine, respectively) (Andreani et al. 2013; Bach et al. 2006; Beard et al. 2009; Frost et al. 2013; Frost and Beard 2007; Miyoshi et al. 2014). We show below that, although changing Si activity may accompany magnetite formation, it is not a requirement, in accordance with magnetite formation in olivine-dominated (orthopy-roxene absent) domains.

Controls on the mineral assemblages in olivine‑hosted veins: further implications from thermodynamic modeling

As described above, there is strong evidence that the differ-ent stages of olivine alteration preserved in the Santa Elena peridotite generally represent a single phase of protracted vein evolution, with the reactions being controlled by fluid flow into the vein center. In addition, bulk vein chemis-try distal from orthopyroxene grains suggests that olivine hydration occurred in the absence of an external silica source (Fig. 4a, b). A better understanding of the role of water and silica abundance during serpentinization can be gained with further simplified thermodynamic calculations, assuming that a lack of overprinting in the studied sample preserves early-formed equilibrium assemblages.

We simulate here the equilibrium assemblages formed by simple interaction of Mg–Fe silicates with water, in a system in which oxidation of the Fe component is prohib-ited (Fig. 13, where ratios are expressed in molar terms). Fluid here is modeled as pure H2O, ignoring for simplic-ity all possible dissolved ions (such as the Mg-bearing species shown in Fig. 12b). Results accordingly indicate the final equilibrium state for a given composition, with-out necessarily revealing pathways by which this equi-librium is reached. For additional simplicity, and in the absence of unambiguous analysis of the extent of Fe3+ incorporation in serpentine, all Fe is fixed as FeO. This prohibits magnetite and H2 formation, which is beyond the scope of this contribution, though a subsidiary set of cal-culations were undertaken in which the reaction H2O +

(3)

Mg-rich Srp(Mg# 93−95)+ Fe-richBrc(Mg# 82−84)+ H2O

= Mgt+Mg-rich Srp(Mg# 99)+Mg-rich Brc(Mg#96)+ H2

Fe2+(in silicate) = H2 + Fe3+

(in magnetite) was permitted (avail-able in supplementary material S4). Possible incorporation of Fe3+ into serpentine has been suggested by several stud-ies (Klein et al. 2009, 2013; McCollom and Bach 2009), but suitable thermodynamic data and mixing models are as yet incomplete so we do not focus on this here (see discus-sion below). Minor components such as Al, Cr, Ti, Mn, and Cl were also excluded. x = 0, y = 0.5 in Fig. 13 thus rep-resents the composition Mg1.8Fe0.2SiO4 (olivine, or 1.8 mol MgO + 0.2 mol FeO + 1 mol SiO2). The x-axis represents addition of water to this, reflecting a composition of 2 mol H2O + 1 mol olivine (Mg1.8Fe0.2SiO4) at x = 1, y = 0.5. The y-axis represents changing Si/(Mg + Fe) ratio, from less silica rich than olivine below y = 0.5 to orthopyroxene (Mg1.8Fe0.2Si2O6) at y = 1. We show the resultant mineral stability fields as a function of molar ratios of water and sil-ica to olivine rather than water and silica activities because this allows us to predict the thermodynamic equilibrium mineral assemblages as a function of the actual local water abundance.

Figure 13 was calculated at 200 °C and 300 bar follow-ing a previous study that suggests serpentinization condi-tions of <250 °C for this sample (Schwarzenbach et al. 2014). We note that moderate changes in temperature and pressure would not notably affect the mineral stability fields within the boundaries where serpentinization is most likely to occur. We first calculated equilibrium stability fields for each mineral assemblage by free energy minimization with Perple_X (Connolly 2005). From this, we calculated the Mg# of brucite and serpentine in the equilibrium assem-blage and the volume proportions of olivine, brucite, and serpentine in their respective stability fields (normalized to exclude the proportion of coexisting fluid). The use of simi-lar thermodynamic models to study metasomatic alteration of ultramafic rocks has recently been evaluated in detail by Evans et al. (2013) who suggested that this type of ther-modynamic modeling is an ideal technique to simulate equilibrium thermodynamics on small length scales such as veins, where local equilibrium can be assumed.

The role of Fe3+ serpentine

Ferric iron in serpentine has been a focus of several recent studies, which presented reaction path modeling (Klein et al. 2009, 2013; McCollom and Bach 2009) or petro-graphic analyses of variably serpentinized peridotites (Andreani et al. 2013; Evans 2008), all concluding that incorporation of ferric iron can affect the amount of H2 pro-duced during serpentinization. Although we agree that Fe3+ incorporation into serpentine could control H2 formation, we do not include Fe3+ serpentine because its inclusion introduces several significant issues: (1) The most appro-priate end member for Fe3+-serpentine is unclear, with


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Fe23+Mg2SiO5(OH)4 (Evans et al. 2013), Fe2

3+Si2O5(OH)4 (Klein et al. 2009), or Fe2

2+Fe3+(Si,Fe3+)O5(OH)4 (cron-stedtite) being candidates. Testing shows that the choice of these end members strongly affects the thermodynamic calculations, as, for example, the replacement of Fe3+ for Si can prevent formation of brucite, due to the lower Si contents in cronstedtite compared to Fe-free serpentine. However, especially during initial serpentinization, brucite is clearly present, as shown here. We thus stress that the choice of ferric end member is crucially important, and an

inappropriate choice can substantially modify phase equi-libria. (2) Thermodynamic data for all Fe3+ serpentine end members are limited, and simple calculation of thermody-namic data, as suggested previously, as well as constructing suitable mixing models may not be sufficiently accurate to model the typically very low abundance of Fe3+ serpen-tine formed during initial stages of serpentinization (Evans 2008; Klein et al. 2009). (3) At temperatures ≤300 °C, Fe3+/ΣFe ratios in serpentine decrease with decreas-ing water–rock ratios (Evans 2008; Klein et al. 2009;

Fig. 13 a Phase diagram for the system MgO, FeO, SiO2, and H2O calculated at 300 bar and 200 °C as a function of the H2O/olivine ratio [addition of H2O toward the right] versus Si/(Mg + Fe) ratio. Calculated volume proportion of the solid phases of b olivine, c brucite, d serpentine, e calculated Mg# of brucite, and f Mg# of serpen-tine. Black dashed lines in b–f are the stability fields shown in a. ol olivine, opx orthopy-roxene, tlc talc, brc brucite, srp serpentine, per periclase, mgt magnetite

serpentine

serpentine

olivine

serpentine

olivineolivine

olivineolivine olivineolivine

olivine

1

2b

2a 311

2b2b

2a2a 33

11

2b2b

2a2a33

11

2b2b

2a2a 33

11

2b2b

2a2a 3311

2b2b

2a2a 33

olivineolivine

serpentine

ol + srp + brc

ol + srp + tlc

ol +

opx

+ tl

c serp + tlc + H2O

serp + brc + H2O

0.5

0.75

0.4 1.20.0

(a)

ol + brc + per

X(S

iO2):

Si/(

Mg+

Fe) r

atio

254540 35

5

5030

15

10

20

65

6055

8075

70

9085

95

254540 35

5

5030

15

10

20

65

6055

8075

70

9085

95

(b) olivine fraction

90

50

60

70

80

30

4010

20

90

50

60

70

80

30

4010

20

(d)

15

10

20

30

40

50

25

5

4535

15

10

20

30

40

50

25

5

4535

(c) brucite fraction

serpentine fraction

76

80

82

84

86

8890

92

9290

93

(e)

7876

80

82

84

86

8890

92

9290

93

7892

93

94

95

9697

98

9190

898887868584

8382 81

92

93

94

95

9697

98

9190

898887868584

8382 81(f)brucite Mg#

serpentine Mg#

X(H2O): H2O/(Mg1.8Fe0.2SiO4 ) ratio

0.25

1

2.0

X(S

iO2):

Si/(

Mg+

Fe) r

atio

X(S

iO2):

Si/(

Mg+

Fe) r

atio

0.8 1.6 0.4 1.20.0 2.00.8 1.6

X(H2O): H2O/(Mg1.8Fe0.2SiO4 ) ratio

0.5

0.75

0.25

1

0.5

0.75

0.00.25

1

0.5

0.75

0.00.25

1

0.5

0.75

0.00.25

1

0.5

0.75

0.00.25

10.4 1.2 2.00.8 1.60.4 1.2 2.00.8 1.6

0.4 1.2 2.00.8 1.6 0.4 1.2 2.00.8 1.6

olivine


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Marcaillou et al. 2011). At 200–250 °C and a water/rock ratio of <1 as suggested for this sample, Klein et al. (2009) calculated a Fe3+/ΣFe ratio in serpentine of <0.2. The only indication for Fe3+ serpentine in the studied sample is a weak compositional trend from end member serpentine (Mg3Si2O5(OH)4) toward cronstedtite (Fe2

2+Fe3+(Si,Fe3+)O5(OH)4) as shown in Fig. 4b. We suggest that only traces of Fe3+ serpentine may be present in some olivine-hosted stage 2 and stage 3 veins. Recognizing these many issues, we have neglected ferric iron in serpentine in our calculations.

Hydration of olivine by reaction with stoichiometric H2O

Calculations suggest that hydration of olivine without an external silica source eventually results in the equilibrium assemblage ol + srp + brc and srp + brc + H2O (Fig. 13). Without external silica, the serpentine:brucite volume ratio is constant at approximately 80:20 (Fig. 13c, d), which represents a Si/(Mg + Fe) of 0.53 that is consistent with the analyses of serpentine–brucite mixtures of stage 1 and stage 2 veins. This suggests that the observed mixtures rep-resent equilibrium assemblages and that initial hydration of olivine occurred in the absence of additional SiO2. An ana-lyzed Mg# of 92 for these zones further suggests that ser-pentine has a Mg# of ~94 and brucite a Mg# of ~82 (Point 1 in Fig. 13). At this stage of vein formation, equilibria are still to the left of water (fluid) saturation and the assem-blage is in equilibrium with olivine, implying that as water enters the system (fracture), it is completely consumed by hydration of olivine to produce a serpentine and brucite rind. However, in the sample the Mg# of the serpentine–brucite assemblage is slightly higher than that of the Mg# of the olivine protolith, implying that Mg is added, Fe is removed or the Mg# of the olivine is slightly altered (Mg# of 88, as predicted by the thermodynamic model) along the reaction surface, which was not observed by EMP analyses.

The composition of the center of the stage 2 veins indi-cates a higher brucite content with a serpentine:brucite ratio of 70:30 (Si/(Mg + Fe) ≈ 0.46). Comparison of Mg# and Si/(Mg + Fe) determined during the analyses with results shown in Fig. 13 suggests that the center of stage 2 veins formed in the presence of more H2O but at somewhat lower concentrations of SiO2 (point 2a in Fig. 13) than initial vein formation (point 1 in Fig. 13). At the same time, the serpen-tine-dominated zone along the outermost vein contact with olivine suggests a more silica-rich and H2O-poor environ-ment (point 2b in Fig. 13). This zone was only observed in stage 3 and stage 4 veins (Fig. 6), may not yet have evolved in the earlier vein stages, and is best explained by considering a locally low water activity (e.g., Fig. 12). The formation of a brucite-dominated center of the vein and a serpentine-dominated outer edge suggests the development

of a chemical gradient between the fluid present in the vein center and the olivine-buffered rim. Thus, these model results agree with the results shown in Fig. 12. High water–rock ratios and physical separation from olivine in the vein center finally move the system into the stability field of serp + brc + (fluid) H2O (area 3 in Fig. 13). In addition, the model calculations also suggest that the Mg# of the final serpentine and brucite is partly a function of the avail-ability of SiO2 in the fluid.

Overall implications for serpentinization reactions

The calculations performed in this study show that the observed silicate mineral assemblages can be produced by varying H2O and SiO2 activities and that the center of stage 2 and 3 veins record higher water–rock ratios than stage 1 veins. Furthermore, calculations can reproduce the observed mineral assemblages in the studied veins. In par-ticular, they show similar results in terms of abundance of phases produced (which is primarily constrained by the sto-ichiometry of olivine), though one model considers aque-ous species such as Mg2+ (Fig. 12) and the other considers solid solution mixing of FeO–MgO and addition of pure H2O (Fig. 13).

A subsidiary set of calculations that involve magnetite are discussed in the supplementary material S4 and over-all show that magnetite + serpentine (Mg# 99) + brucite (Mg# 96) is the stable equilibrium assemblage in the pres-ence of excess H2O. However, according to reaction 4, simple increase of H2O cannot drive the reaction of Fe–brucite to magnetite because equilibrium favors formation rather than consumption of brucite with increasing H2O. In contrast, increasing oxygen fugacity favors formation of magnetite. Because O2 fugacities are extremely low during early stages of serpentinization (e.g., Frost, 1985), one pos-sibility is that oxidation of Fe–brucite requires dissociation of H2O to provide O2, also producing H2:

This requires an open system for H2; otherwise, the high partial pressure of H2 in the fluid following initiation of reaction would prevent continued reaction progress. Gase-ous hydrogen has been observed venting from serpentiniz-ing systems (e.g., Kelley et al. 2001) and H2 can react with primary pentlandite to form native metals and metal alloys (Schwarzenbach et al. 2014). Consumption of O2 during oxidation of Fe–brucite and consumption or loss of H2 could thus drive reaction (5) to the right, constantly produc-ing O2 and H2, with the latter being released to the fluid, and allowing continuous reaction progress. Alternatively, the reaction 2FeO + 1H2O = 1Fe2O3 + 1H2 possibly

(4)6Fe (OH)2 + O2 = 2(FeO× Fe2O3)+ 6H2O

(5)2H2O = O2 + 2H2


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controls the equilibrium assemblages during oxidation of Fe2+ and formation of magnetite.

The trend toward higher water–rock ratios with progres-sive vein formation also agrees with Frost et al. (2013). These workers examined serpentinization at a much larger scale than studied here, but observed similar changes in mineral assemblages in different vein generations and suggested that these assemblages form during the transi-tion from a rock-dominated to a fluid-dominated system in which the infiltrating fluid is oxidizing. However, for the sample studied here, oxidizing conditions in the fluid are prevented due to the presence of abundant olivine, and fluid conditions probably remained highly reducing (Frost 1985). Thus, formation of magnetite has to be controlled by either dissociation of water or by equilibrium between FeO and Fe2O3.

As demonstrated in Fig. 13, the fraction of serpentine and brucite and the range of Mg# observed in early-formed veins in previously studied serpentinites (e.g., Beard et al. 2009; Frost et al. 2013; Katayama et al. 2010; Klein et al. 2009) is primarily controlled by the local silica abundances, which is influenced by orthopyroxene breakdown or an external silica source. In contrast, brucite occurring at the interface with olivine, as observed by Klein et al. (2009) and Frost et al. (2013), is likely the result of a disequilib-rium assemblage or overprinting by a secondary fluid and could, for example, be produced by a later fluid with lower silica activity (Fig. 13). Klein et al. (2009) suggest that this assemblage is the result of an arrested reaction.

Magnetite is either formed from brucite (Andreani et al. 2013; Frost and Beard 2007; this study) or from Fe-bearing serpentine (Eq. 6 after Frost and Beard 2007; Eq. 7 after Bach et al. 2006):

Our study clearly shows that the iron in magnetite is pri-marily derived from Fe-rich brucite, but with some input from serpentine revealed by the fact that both serpentine and brucite in the center of stage 4 veins are almost Fe free. In accordance with these reactions, the serpentine-rich zone observed near the vein center in stage 4 veins (zone 4 in Figs. 6d, 9c, e) could be the result of SiO2 release during reaction (6) causing breakdown of brucite to form serpen-tine [Eq. (1)].

Reaction (6) can further explain the absence of magnet-ite in hybrid serpentine veins and in orthopyroxene-hosted veins typically observed in partially serpentinized perido-tites (e.g., Frost et al. 2013) and in experiments (Ogasawara

(6)2Fe3Si2O5(OH)4 + O2 = 2Fe3O4 + 4SiO2 + 4H2O

(7)

57Mg0.75Fe0.25(OH)2 + 30SiO2(aq)

= 15Mg2.85Fe0.15Si2O5(OH)4

+ 23H2O+ 4Fe3O4 + 4H2

et al. 2013). In these areas, the assemblages are controlled by elevated silica activities, thus stabilizing Fe-rich serpen-tine (Frost and Beard 2007). Importantly, reactions (6) and (7) also suggest that magnetite can form as a result of either increasing or decreasing silica activity, depending on the protolith composition. This implies that silica activity plays an important role during magnetite formation, as suggested previously. Its particular role in orthopyroxene-hosted veins is still to be determined.

Summary and concluding remarks

This study provides new insights into processes during peridotite alteration. Detailed mineralogical investigations, chemical analyses, and thermodynamic modeling have led to the following major conclusions: (1) Element distribu-tion maps show a compositional gradient in the serpentini-zation products formed between orthopyroxene-dominated and olivine-dominated zones, (2) the composition of bru-cite varies based on whether it coexists with magnetite or not and is associated with the formation of magnetite through a two-step process, (3) thermodynamic modeling demonstrates the existence of a water and silica activity gradients within individual olivine-hosted veins, with fluid flow in the vein center, (4) hydration of olivine results in the equilibrium assemblage serpentine (Mg#99) + brucite (Mg#96) + magnetite, and (5) local thermodynamic equi-librium occurs within individual zones of olivine-hosted veins.

This study agrees with previous work on abyssal ser-pentinites and offers additional evidence that the mineral assemblages in serpentinites are imposed by the local insta-bility of the parent phases at the prevailing P–T conditions. Specifically, in the studied sample the chemical gradient between olivine- and orthopyroxene-dominated areas is strongly correlated with the low temperature of water–rock interaction (<250 °C), where olivine breakdown is favored over orthopyroxene breakdown and Si release into olivine-dominated areas is thus limited. This also allows magnet-ite formation in the absence of an external Si source to be observed. Further studies, however, are needed to explain the lack of magnetite formation during the very earliest stages of olivine hydration. It is likely that initial magnetite formation is kinetically prohibited and that the concentra-tion of free O2 in the early fluid is too low to allow substan-tial ferrous iron to be oxidized to form magnetite. Lastly, magnetite may indeed be present during initial stages but is not detectable with the analytical techniques used here.

In summary, the harzburgite sample from the Santa Elena Ophiolite gives insight into the early progress of olivine and orthopyroxene hydration in peridotites, where fluid input is limited and the mineral assemblages are


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controlled mostly by simple hydration and by chemical exchange with neighboring minerals. Limited fluid input and slow reaction kinetics also allowed preservation of local mineral equilibria within different zones of the veins.

Acknowledgments We would like to thank Luca Fedele, Bob Tracy, and Charles Farley with help during analytical work at Virginia Tech, Jan Evers at the Freie Universität Berlin, for SEM analyses, and Don Rimstidt for helpful discussions. We also thank O. Müntener, R. Frost, W. Bach, and two anonymous reviewers for helpful comments that greatly improved the manuscript. The sample was generously provided by Jonathan Snow, University of Houston. E.S. and M.C. gratefully acknowledge support from Virginia Tech Department of Geosciences.

References

Allen DE, Seyfried JWE (2003) Compositional controls on vent flu-ids from ultramafic-hosted hydrothermal systems at mid-ocean ridges: an experimental study at 400°C, 500 bars. Geochim Cos-mochim Acta 67(8):1531–1542

Alt JC, Schwarzenbach EM, Früh-Green GL, Shanks WC III, Bernas-coni SM, Garrido CJ, Crispini L, Gaggero L, Padrón-Navarta JA, Marchesi C (2013) The role of serpentinites in cycling of carbon and sulfur: seafloor serpentinization and subduction metamor-phism. Lithos 178:40–54. doi:10.1016/j.lithos.2012.12.006

Andreani M, Muñoz M, Marcaillou C, Delacour A (2013) μXANES study of iron redox state in serpentine during oceanic serpentini-zation. Lithos 178:70–83. doi:10.1016/j.lithos.2013.04.008

Bach W, Garrido CJ, Paulick H, Harvey J, Rosner M (2004) Seawater-peridotite interactions: first insights form ODP Leg 209, MAR 15°N. Geochem Geophys Geosyst 5(9):22. doi:10.1029/2004GC000744

Bach W, Paulick H, Garrido CJ, Ildefonse B, Meurer WP, Humphris SE (2006) Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274). Geophys Res Lett. doi:10.1029/2006GL025681

Barnes I, O’Neil JR (1978) Present day serpentinization in New Caledonia, Oman and Yugoslavia. Geochim Cosmochim Acta 42:144–145

Baumgartner PO, Denyer P (2006) Evidence for middle Cretaceous accretion at Santa Elena Peninsula (Santa Rosa Accretionary Complex). Costa Rica. Geologica Acta 4(1–2):179–191

Beard JS, Hopkinson L (2000) A fossil, serpentinization-related hydrothermal vent, Ocean Drilling Program Leg 173, Site 1068 (Iberia Abyssal Plain): some aspects of mineral and fluid chemis-try. J Geophys Res 105:16527–16539

Beard JS, Frost BR, Fryer P, McCaig AM, Searle RC, Ildefonse B, Zinin P, Sharma SK (2009) Onset and progression of serpenti-nization and magnetite formation in olivine-rich troctolite from IODP Hole U1309D. J Petrol 50(3):387–403

Connolly JAD (2005) Computation of phase equilibria by linear programming: a tool for geodynamic modeling and its appli-cation to subduction zone decarbonation. Earth Planet Sci Lett 236:524–541

Denyer P, Gazel E (2009) The Costa Rican Jurassic to Miocene oce-anic complexes: origin, tectonics and relations. J South Am Earth Sci 28(4):429–442. doi:10.1016/j.jsames.2009.04.010

Dungan MA (1979) Bastite pseudomorphs after orthopyroxene, clino-pyroxene and tremolite. Can Mineral 17:729–740

Dyment J, ArkaniHamed J, Ghods A (1997) Contribution of serpen-tinized ultramafics to marine magnetic anomalies at slow and

intermediate spreading centres: insights from the shape of the anomalies. Geophys J Int 129(3):691–701. doi:10.1111/j.1365-246X.1997.tb04504.x

Escartin J, Hirth G, Evans B (1997) Effects of serpentinization on the lithospheric strength and the style of normal faulting at slow-spreading ridges. Earth Planet Sci Lett 151(3–4):181–189. doi:10.1016/s0012-821x(97)81847-x

Escartin J, Hirth G, Evans B (2001) Strength of slightly ser-pentinized peridotites: implications for the tecton-ics of oceanic lithosphere. Geology 29(11):1023–1026. doi:10.1130/0091-7613(2001)029<1023:sosspi>2.0.co;2

Evans BW (2008) Control of the products of serpentinization by the Fe(2)Mg(1) exchange potential of olivine and orthopyroxene. J Petrol 49(10):1873–1887

Evans B (2010) Lizardite versus antigorite serpentinite: magnetite, hydrogen, and life. Geology 38(10):879–882

Evans KA, Powell R, Frost BR (2013) Using equilibrium thermo-dynamics in the study of metasomatic alteration, illustrated by an application to serpentinites. Lithos 168–169:67–84. doi:10.1016/j.lithos.2013.01.016

Frost BR (1985) On the stability of sulfides, oxides, and native metals in serpentinite. J Petrol 26(1):31–63

Frost BR, Beard JS (2007) On silica activity and serpentinization. J Petrol 48(7):1351–1368. doi:10.1093/petrology/egm021

Frost BR, Beard JS, McCaig A, Condliffe E (2008) The formation of micro-rodingites from IODP hole U1309D: key to understand-ing the process of serpentinization. J Petrol 49(9):1579–1588. doi:10.1093/petrology/egn038

Frost B, Evans KA, Swapp SM, Beard JS, Mothersole FE (2013) The process of serpentinization in dunite from New Caledonia. Lithos 178:24–39. doi:10.1016/j.lithos.2013.02.002

Früh-Green GL, Connolly JA, Plas A, Kelley DS, Grobéty B (2004) Serpentinization of oceanic peridotites: Implications for geo-chemical cycles and biological activity. In: the subseafloor bios-pere at Mid-Ocean Ridges, 2004. American Geophysical Union, Washington, DC

Gazel E, Denyer P, Baumgartner PO (2006) Magmatic and geotec-tonic significance of Santa Elena Peninsula, Costa Rica. Geol Acta 4(1–2):193–202

Groppo C, Rinaudo C, Cairo S, Gastaldi D, Compagnoni R (2006) Micro-Raman spectroscopy for a quick and reliable identifi-cation of serpentine minerals from ultramafics. Eur J Mineral 18(3):319–329

Hacker BR (2008) H2O subduction beyond arcs. Geochem Geophys Geosyst. doi:10.1029/2007GC001707

Hattori KH, Guillot S (2007) Geochemical character of serpentinites associated with high- to ultrahigh-pressure metamorphic rocks in the Alps, Cuba, and the Himalayas: recycling of elements in subduction zones. Geochem Geophys Geosyst. doi:10.1029/2007gc001594

Hauff F, Hoernle K, Van den Bogaard P, Alvarado GE, Garbe-Schön-berg D (2000) Age and geochemistry of basaltic complexes in western Costa Rica: Contributions to the geotectonic evolution of Central America. Geochem Geophys Geosyst 1:n/a

Katayama I, Kurosaki I, Hirauchi K (2010) Low silica activity for hydrogen generation during serpentinization: an example of natural serpentinites in the Mineoka ophiolite complex, central Japan. Earth Planet Sci Lett 298:199–204

Kelley SD, Karson JA, Blackman DK, Früh-Green GL, Butterfield DA, Lilley DM, Olson EJ, Schrenk MO, Roell KK, Lebon GT, Rivizzigno P, Party A-S (2001) An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30° N. Nature 412:145–149

Kelley DS, Karson JA, Früh-Green GL, Yoerger DR, Shank TM, But-terfield DA, Hayes JM, Schrenk MO, Olson EJ, Proskurowski G, Jakuba M, Bradley A, Larson B, Ludwig KA, Glickson D, Buck-man K, Bradley AS, Brazelton WJ, Roe K, Elend MJ, Delacour

http://dx.doi.org/10.1016/j.lithos.2012.12.006


http://dx.doi.org/10.1029/2004GC000744

http://dx.doi.org/10.1029/2004GC000744

http://dx.doi.org/10.1029/2006GL025681

http://dx.doi.org/10.1016/j.jsames.2009.04.010

http://dx.doi.org/10.1111/j.1365-246X.1997.tb04504.x

http://dx.doi.org/10.1111/j.1365-246X.1997.tb04504.x

http://dx.doi.org/10.1016/s0012-821x(97)81847-x

http://dx.doi.org/10.1130/0091-7613(2001)029%3c1023:sosspi%3e2.0.co;2


http://dx.doi.org/10.1093/petrology/egm021

http://dx.doi.org/10.1093/petrology/egn038


http://dx.doi.org/10.1029/2007GC001707

http://dx.doi.org/10.1029/2007gc001594

http://dx.doi.org/10.1029/2007gc001594


1 3

5 of 22

A, Bernasconi SM, Lilley DM, Baross JA, Summons RE, Sylva SP (2005) A serpentinite-hosted ecosystem: the lost city hydro-thermal field. Science 307:1428–1434

Klein F, Bach W, Jöns N, McCollom T, Moskowitz B, Berquo T (2009) Iron partitioning and hydrogen generation during serpen-tinization of abyssal peridotites from 15°N on the Mid-Atlantic Ridge. Geochim Cosmochim Acta 73:6868–6893

Klein F, Bach W, McCollom TM (2013) Compositional controls on hydrogen generation during serpentinization of ultramafic rocks. Lithos 178:55–69. doi:10.1016/j.lithos.2013.03.008

Klein F, Bach W, Humphris S, Kahl WA, Jöns N, Moskowitz B, Ber-quo TS (2014) Magnetite in seafloor serpentinite—some like it hot. Geology. doi:10.1130/G35068.1

Kloprogge JT, Frost RL, Rintoul L (1999) Single crystal Raman microscopic study of the asbestos mineral chrysotile. Phys Chem Chem Phys 1(10):2559–2564

Lafay R, Montes-Hernandez G, Janots E, Chiriac R, Findling N, Toche F (2012) Mineral replacement rate of olivine by chryso-tile and brucite under high alkaline conditions. J Cryst Growth 347(1):62–72. doi:10.1016/j.jcrysgro.2012.02.040

Lutz HD, Möller H, Schmidt M (1994) Lattice vibration spectra. Part LXXXII. Brucite-type hydroxides M(OH)2 (M = Ca, Mn Co, Fe, Cd)—IR and Raman spectra, neutron diffraction of Fe(OH)2. J Mol Struct 328:121–132

Macdonald AH, Fyfe WS (1985) Rate of serpentinization in seafloor environments. Tectonophysics 116(1–2):123–135

Madrigal P, Gazel E, Denyer P, Smith I, Jicha B, Flores K, Coleman D, Snow J (2015) A melt-focusing zone in the lithospheric man-tle preserved in the Santa Elena Ophiolite, Costa Rica. Lithos. doi:10.1016/j.lithos.2015.04.015

Marcaillou C, Munoz M, Vidal O, Parra T, Harfouche M (2011) Min-eralogical evidence for H2 degassing during serpentinization at 300 °C/300 bar. Earth Planet Sci Lett 303:281–290

Martin W, Russell MJ (2007) On the origin of biochemistry at an alkaline hydrothermal vent. Philos Trans R Soc B Biol Sci 362(1486):1887–1925. doi:10.1098/rstb.2006.1881

Martin W, Baross J, Kelley D, Russell M (2008) Hydrothermal vents and the origin of life. Nat Rev Microbiol 6:805–814

McCollom TM (1999) Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in hydrothermal systems on Europa. J Geophys Res 104(E12):30729–30742

McCollom TM, Bach W (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochim Cosmochim Acta 73:856–875. doi:10.106/j.gca.2008.10.032

Mével C (2003) Serpentinization of abyssal peridotites at mid-ocean ridges. C R Geosci 335(10–11):825–852

Miller DJ, Christensen NJ (1997) Seismic velocities of lower crustal and upper mantle rocks from the slow-spreading Mid-Atlantic Ridge, south of the Kane transform fault. Proc Ocean Drill Pro-gram Sci Results 153:437–454

Miyoshi A, Kogiso T, Ishikawa N, Mibe K (2014) Role of silica for the progress of serpentinization reactions: constraints

from successive changes in mineralogical textures of serpen-tinites from Iwanaidake ultramafic body, Japan. Am Miner 99:1035–1044

Moody JB (1976) Serpentinization—review. Lithos 9(2):125–138Neal C, Stanger G (1983) Hydrogen generation from mantle source

rocks in Oman. Earth Planet Sci Lett 66:315–320Ogasawara Y, Okamoto A, Hirano N, Tsuchiya N (2013) Coupled

reactions and silica diffusion during serpentinization. Geochim Cosmochim Acta 119:212–230

Oufi O, Cannat M, Horen H (2002) Magnetic properties of variably serpentinized abyssal peridotites. J Geophys Res 107(B5):2095

Palandri JL, Reed MH (2004) Geochemical models of metasomatism in ultramafic systems: serpentinization, rodingitization, and sea floor carbonate chimney precipitation. Geochim Cosmochim Acta 68(5):1115–1133

Peretti A, Dubessy J, Mullis J, Frost BR, Trommsdorff V (1992) Highly reducing conditions during alpine metamorphism of the Malenco peridotite (Sondrio, northern Italy) indicated by min-eral paragenesis and H2 in fluid inclusions. Contrib Mineral Pet-rol 112(2–3):329–340. doi:10.1007/bf00310464

Rinaudo C, Gastaldi D (2003) Characterization of chrysotile, antigor-ite and lizardite by FT-Raman spectroscopy. The Canadian Min-eralogist 41:883–890

Russell MJ, Hall AJ, Martin W (2010) Serpentinization as a source of energy at the origin of life. Geobiology 8(5):355–371

Scambelluri M, Müntener O, Hermann J, Piccardo GB, Trommsdorff V (1995) Subduction of water into the mantle: history of an Alpine peridotite. Geology 23(5):459–462

Scambelluri M, Müntener O, Ottolini L, Pettke TT, Vannucci R (2004) The fate of B, Cl and Li in the subducted oceanic man-tle and in the antigorite breakdown fluids. Earth Planet Sci Lett 222(1):217–234. doi:10.1016/j.epsl.2004.02.012

Schwarzenbach EM, Gazel E (2013) Serpentinization history of the Santa Elena complex peridotites, Costa Rica. Min Mag 77(5):2170

Schwarzenbach EM, Gazel E, Caddick MJ (2014) Hydrothermal pro-cesses in partially serpentinized peridotites from Costa Rica: evi-dence from native copper and complex sulfide assemblages. Con-trib Mineral Petrol 168:1079. doi:10.1007/s00410-014-1079-2

Snow JE, Dick HJB (1995) Pervasive magnesium loss by marine weathering of peridotite. Geochim Cosmochim Acta 59(20):4219–4235. doi:10.1016/0016-7037(95)00239-v

Toft PB, Arkani-Hamed J, Haggerty S (1990) The effects of serpenti-nization on density and magnetic susceptibility: a petrophysical model. Phys Earth Planet Inter 65:137–157

Ulmer P, Trommsdorff V (1995) Serpentine stability to mantle depths and subduction-related magmatism. Science 268(5212):858–861

Wenner DB, Taylor HP (1971) Temperatures of serpentinization of ultramafic rocks based on 18O/16O fractionation between coexist-ing serpentine and magnetite. Contrib Mineral Petrol 32:165–185

Wicks FJ, Whittaker EJW (1977) Serpentine textures and serpentini-zation. Can Mineral 15:459–488


http://dx.doi.org/10.1130/G35068.1

http://dx.doi.org/10.1016/j.jcrysgro.2012.02.040


http://dx.doi.org/10.1098/rstb.2006.1881

http://dx.doi.org/10.106/j.gca.2008.10.032

http://dx.doi.org/10.106/j.gca.2008.10.032

http://dx.doi.org/10.1007/bf00310464

http://dx.doi.org/10.1016/j.epsl.2004.02.012

http://dx.doi.org/10.1007/s00410-014-1079-2

http://dx.doi.org/10.1016/0016-7037(95)00239-v

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