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eric7, P59, 7000-671 vora, Portugal

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Article history:

2014 Elsevier B.V. All rights reserved.

meridge interactionss et al., 1994; Schilling,t al., 1976). The Azoresarge igneous provincef enhanced melt pro-

Lithos 210211 (2014) 2739

Contents lists available at ScienceDirect

Lith

.e lfers a unique alternative to overtake these limitations and to constrainthe interactions between mantle dynamics, regional deformation, meltproduction and migration in relation to plate boundary reconguration

duction centered on the Mid-Atlantic Ridge (MAR) between 20 and7Maago (e.g. Cannat et al., 1999; Gente et al., 2003). The plateau rough-ly has a triangular shape, and is delimited in the south by the prominenttion is generally inferred from available magnetic anomaly patterns ofthe seaoor. However, the mechanisms and the time over which aplate boundary changes its location remain poorly understood, especial-ly in areas where oceanic spreading occurs at very slow rates (e.g. Vogtand Jung, 2004). The study of volcanism occurring close to such TJs of-

nism in the Azores has long been attributed to plu(e.g. Cannat et al., 1999; Loureno et al., 1998; Lu1975; Searle, 1980; Vogt and Jung, 2004;White eplateau, especially, is generally interpreted as a l(LIP), formed during a plume-derived episode o1. Introduction

The evolution of triple junctions (TJs) is a topic of major interest forplate tectonics and geodynamic reconstructions (e.g. Georgen and Lin,2002; Georgen and Sankar, 2010). In oceanic settings, plate recongura-

in the North Atlantic is a target of particular interest for such purpose.The Azores archipelago comprises nine volcanic islands built duringthe Quaternary (except the easternmost Santa Maria Island) over anoceanic plateau encompassing the present-day TJ between America,Eurasia and Nubia lithospheric plates (Fig. 1). The origin of the volca-over short geological periods (~12Myr). The

Corresponding author at: Univ Paris-Sud, Laboratoir91405, France. Tel.: +33 1 69 15 67 42; fax: +33 1 69 15

E-mail address: anthony.hildenbrand@u-psud.fr (A. H

http://dx.doi.org/10.1016/j.lithos.2014.09.0090024-4937/ 2014 Elsevier B.V. All rights reserved.pre-existing MAR Fracture Zones. We propose that the sub-aerial volcanism over the last 1.3 Myr in the centralAzores records a sudden change in the conditions of melt generation, due to a major reconguration in regionaldeformation associated with the recent reorganization of the EurasiaNubia plate boundary.Received 30 March 2014Accepted 14 September 2014Available online xxxx

Keywords:Azores Triple JunctionGeochemistryK/Ar datingMelt productionMantle fertilityLithospheric deformationa b s t r a c t

The study of volcanism near unstable plate triple junctions (TJs) offers a unique opportunity to investigate theinteractions betweenmantle dynamics and lithospheric deformation in relation to short-term plate recongura-tion. From combined geochronological and geochemical analyses on Terceira, S. Jorge and Faial volcanic islands,we evidence contrasted modes of melt generation near the Azores Triple Junction during the last 1.3 Myr. Theoldest lavas (N800 ka) erupted along N150 elongated volcanic systems in S. Jorge and Faial have homogeneousisotopic compositions which partly overlap the compositional eld of MORBs from the adjacent Mid-AtlanticRidge (MAR). In contrast, the younger lavas (b750 ka) erupted along the N110 main structural direction onthe three islands are signicantly more enriched in LILE and LREE, and have more variable and generally moreradiogenic Sr, Pb, Nd and Hf isotopic ratios. Altogether, our data do not support the presence of an active mantleplume under the central Azores. Instead, they suggest that magma generation results from decompression melt-ing of a heterogeneously fertilized mantle (fossil plume?). The higher geochemical heterogeneity of the lavaserupted during the last 750 kyr likely reects low-degree partial melting promoted by recent reactivation ofUniversidade de Lisboa, Lisboa, Portugale Centro de Geofsica de vora and Dep. de Geocincias da Univ. de vora, R. Romo Ramalho,fRecent plate re-organization at the Azores Tcombined geochemical and geochronologiTerceira volcanic islands

Anthony Hildenbrand a,b,, Dominique Weis c, Pedro Ma Univ Paris-Sud, Laboratoire GEOPS, UMR8148, Orsay, F-91405, Franceb CNRS, Orsay, F-91405, Francec Pacic Center for Isotope and Geochemical Research, Department of Earth, Ocean and Atmosphd Estrutura de Misso para a Extenso da Plataforma Continental, R. Costa Pinto, 165, 2770-04

j ourna l homepage: wwwAzores volcanic province

e GEOPS, UMR8148, Orsay, F-48 91.ildenbrand).iple Junction: Evidence froml data on Faial, S. Jorge and

ureira d,e, Fernando Ornelas Marques f

Sciences, University of British Columbia, 2020-2207MainMall, Vancouver, BC V6T1Z4, Canadaao D'Arcos, Portugal

os

sev ie r .com/ locate / l i thosEast Azores Fracture Zone (EAFZ), a presently inactive transform zoneinterpreted as the fossil eastern branch of the Azores Triple Junction(ATJ). During the last few Myr, the plate boundary between Eurasiaand Nubia has jumped to the Terceira Rift (TR), considered an activehyper-slow spreading oceanic rift (e.g. Vogt and Jung, 2004). The

etws sh: Lo

28 A. Hildenbrand et al. / Lithos 210211 (2014) 2739western part of the plate boundary is presently the locus of diffuse de-formation distributed along a series of horsts and grabens extendingca. 150 km SWof the TR (e.g., Marques et al., 2013, 2014). Recent volca-nism has developed over the plateau, producing numerous elongatedsubmarine and sub-aerial volcanic ridges, with two main directions:N150 and N110N120 (e.g. Hildenbrand et al., 2008; Loureno et al.,1998). Recent geochronological studies on the emerged part of someof the ridges have led to signicant revisions regarding the timing andthe duration of sub-aerial volcanism in the central Azores, and showedthat part of the islands have experienced distinct stages of growthmost likely controlled by successive episodes of regional deformation(e.g. Hildenbrand et al., 2008, 2012; Silva et al., 2012).

Based on major and trace elements, and isotopic data, several

Fig. 1.Maingure: Location of the Azores volcanic archipelago near the triple junction (TJ) bdotted circles show the location of the present and the older TJ, respectively. Bold black lineEast Azores Fracture Zone. Background bathymetric data from Loureno et al. (1998). Insetauthors pointed out the existence of signicant geochemical heteroge-neity at the scale of the Azores Archipelago and also at intra-islandscale (Beier et al., 2008, 2010, 2012; Davies et al., 1989; Dupr et al.,1982; Elliott et al., 2007; Flower et al., 1976; Haase and Beier, 2003;Madureira et al., 2011; Mtrich et al., 2014; Millet et al., 2009; Moreiraet al., 1999; Turner et al., 1997; White et al., 1979; Widom et al.,1997). However, very scarce geochronologic data existed at the timeof these studies, and as a result, most of the geochemical analyseswere performed on lavas of unknown absolute age. It is important toevaluate the link between this heterogeneity and the distinct stages ofislands growth since individual events of melt production may haveoccurred at distinct loci within themantle, and/or possibly sampled dif-ferent sources. We here present a new study combining K/Ar dating,major element and high-precision trace element data, and Sr, Nd, Hfand Pb radiogenic analyses on three selected islands from the centralAzores: Terceira, S. Jorge and Faial (Fig. 1), which lie in the diffuse partof the EurasiaNubia plate boundary. K/Ar dating on fresh-separatedgroundmass and whole-rock geochemical analyses were systematicallycarried out on the same samples in order to constrain the evolution ofmagma geochemistry through time. This allows us to examine temporaland/or geographical variations in the mantle source composition andevaluate the links between TJ reconguration and melt production.

2. Sampling location and sample description

Volcanic rocks with no traces of alteration were sampled on the dif-ferent geological units of the three islands to encompass themain stagesof volcanism at the scale of the central Azores (Fig. 2). The location andthe characteristics of our samples fromS. Jorge and Faial are described indetail elsewhere (Hildenbrand et al., 2008, 2012; Silva et al., 2012).Fromeld relationships, petrographic examination, K/Ar geochronologyon separate phases, andwhole-rockmajor and trace element geochem-istry, twomain eruptive systems have been recognized at S. Jorge: (1) amainN150 volcanic ridge, partly preserved as a ca. 400m thick lava owsuccession in the SE part of the island, whichwas dated between 1.310.02 Ma and 1.21 0.02 Ma (samples AZ05-P, R, U, Z, and AB,Hildenbrand et al., 2008); (2) a more recent N110 volcanicridge, which experienced episodic short stages of growth over the last750 kyr (samples AZ05-AC to AZ05-AJ).

Similarly, the evolution of Faial has been controlled by successive

een the North American (NA), the Eurasian (EU) and theNubian (NU) plates. The bold andow theMid-Atlantic Ridge (MAR) and the Terceira Rift (TR), and the white line shows thecation of the Azores region at the Earth surface.short pulses of volcanic construction separated by considerable periodsof inactivity (Hildenbrand et al., 2012). The upper part of the older vol-canic system is exposed as a ca. 200 m-thick volcanic succession in theeastern end of the island, which has been extensively dated at about850 ka (samples AZ05-AL to AZ05-AR). Morphologic, structural,paleomagnetic and aerial magnetic data suggest that this old volcaniccenter had an elongated ridge-likemorphologywith a main orientationclose to theN150direction (Hildenbrand et al., 2012, 2013). After an ap-parent volcanic gap of ca. 450 kyr, a small edice developed on thenortheastern corner of the island, between 400 and 350 ka (samplesAZ05-AN and AZ05-AP), whereas subsequent activity yielded to therapid construction of the Central Volcano at about 120 ka (sampleAZ05-AO).

New sampling on Terceira Islandwas carried out on themain centralvolcanic systems (Calvert et al., 2006; Madureira et al., 2011), i.e. fromeast to west: (1) the Cinco Picos, (2) the Guilherme-Moniz, and(3) the Santa Brbara volcanoes. The morphology of Cinco Picos is cur-rently dominated by the largest caldera of the Azores Archipelago(~7 km in diameter) and by a graben (the Lajes Graben) bounded by ac-tive normal faults (with a trend N150) in NE Terceira. One of the lower-most accessible lava ows from this succession has been sampled at theshore level (AZ05-C), whereas the topmost lava ow (AZ05-F) has beencollected on the upper rim of the Lajes Graben at an altitude of ca.100m. A volcanic ow from the same succession has also been sampledwithin the southern caldera wall (AZ05-K). The GuilhermeMonizedice is dominantly composed of differentiated volcanic products, in-cluding lava ows and pyroclastic deposits possibly derived from

29A. Hildenbrand et al. / Lithos 210211 (2014) 2739multiple vertical caldera collapses (Calvert et al., 2006). A thick trachyticlava ow exposed on the eastern wall of the Gulherme Moniz caldeirawas sampled (AZ05-N). The Santa Brbara volcano is usually thoughtto be the youngest eruptive system on Terceira (Calvert et al., 2006).Its evolution includes both effusive and explosive eruptions. Part ofthe volcanic succession has been exposed on the western end of theisland by recent coastal erosion. Two lava ows were sampled at thebase and top of a western reduced cliff (samples AZ05-G and AZ05-J,

Fig. 2. Upper panels: Location of our samples from Terceira, S. Jorge and Faial (for eachsample, prex AZ05- not indicated). Background images are shaded-relief maps builtfrom SRTM topographic data. K/Ar ages are indicated into brackets. New ages measuredin this study (Terceira) are shown with normal characters; italic text show previous K/Arages obtained on our S. Jorge and Faial samples ((Hildenbrand et al., 2008, 2012; Silvaet al., 2012). Lower panel: Synthetic gure showing the age of volcanic construction inthe central Azores from unspiked K/Ar data on separated phases (Hildenbrand et al.,2008, 2012; Silva et al., 2012, and this study) and one Ar/Ar age (italic) on the Pico SE sub-marine volcanic ridge (Beier, 2006). Ellipses show the main elongation of the various vol-canic ridges. Background bathymetric data after Loureno et al. (1998).respectively). Most of the lavas collected at Terceira Island are sub-aphyric to slightly porphyric in character, and appear relatively basicin composition. Phenocrysts usually represent less than 10% in volume,and are dominantly composed of olivine and/or clinopyroxene andplagioclase. The microcrystalline groundmass additionally comprisesminor oxides. In contrast, the trachyte AZ05-N bears numerous sub-parallel alkali feldspar elongated crystals.

3. Methods and results

3.1. K/Ar dating

Six new K/Ar determinations on our Terceira samples are presentedin this study (Fig. 2), which together with 11 previous ages acquired onS. Jorge (Hildenbrand et al., 2008; Silva et al., 2012), and 15 on Faial(Hildenbrand et al., 2012), amounts to a total of 32 ages acquired withthe same protocol of sample preparation and the same analytical proce-dure on the three islands. After careful petrographic examination ofthin-sections, the micro-crystalline groundmass was selected for K/Aranalyses, except for the trachyte from Terceira (sample AZ05-N), fromwhich alkali feldspars have been extracted. In all cases, the rocks wererst crushed and sieved at 125250 m. Phenocrysts greater than125 m in size (olivine, pyroxene, oxides, and plagioclase) were re-moved with heavy liquids, because they may have crystallized earlierand deeper in the magma chamber. They are thus not representativeof the age of eruption at surface, andmay carry signicant inherited ex-cess argon. Narrow density spans have been achieved to select thefreshest part of the groundmass and eliminate the fraction with alower density, which is potentially affected by minor alteration andsecondary zeolitization. Potassium and argon were measured on twoseparate aliquots of the selected grains, the former by ame emission-spectrophotometry and the latter by mass spectrometry according tothe CassignolGillot unspiked technique (details on the analytical pro-cedure in Gillot et al., 2006). Both K and Ar were analyzed at leasttwice in order to obtain a reproducible value within the range of uncer-tainties. The used decay constants are from Steiger and Jger (1977).The results are presented in Table 1, where the uncertainties are quotedat the 1 level.

The new ages obtained on Terceira range between 401 6 ka and53 7 ka. The former has been measured on our sample AZ05-C fromthe base of the old volcanic succession exposed on the NE part of the is-land. The uppermost lava ow from the same succession (AZ05-F) isdated at 3706 ka,whereas our sample AZ05-K collected in themiddleof the same succession yields a coherent age of 391 6 ka (Table 1 andFig. 2). These new results are consistent with the ages recently obtainedon lava ows sampled at comparable stratigraphic positions and datedbetween 388 6 ka and 370 2 ka with the

40Ar/

39Ar step-heating

technique (Calvert et al., 2006). The trachyte AZ05-N sampled withinthe eastern rim of the GuilhermMoniz caldera in the central part ofthe island yields an age of 123 2 ka. This age is slightly older thanthe integrated age of 112 3 ka, but comparable to the isochron ageof 118 4 ka determined by furnace step-heating

40Ar/

39Ar on alkali

feldspars from a trachytic body sampled farther west in the upper partof the same succession (Calvert et al., 2006). The two samples fromthe young volcanic succession exposed on a seacliff on the westernend of Terceira yield ages of 61 12 ka and 53 7 ka, which areundistinguishable within the range of uncertainties. These results areconsistent with radiocarbon ages obtained on charcoal fragments cov-ered by younger lava ows in the same area (Calvert et al., 2006).

All together, our geochronological data on the three islands showthat sub-aerial volcanism in the central Azores has been active duringthe last 1.3 Myr. S. Jorge and Faial have both experienced old sub-aerial volcanic construction involving the development of N150 ridges,prior to 800 ka. The three islands have been active during the last 750kyr, and have experienced partly synchronous construction throughout

the last 400 kyr (Fig. 2).

mass extracted for K/Ar datingwas also powdered for isotopic analyses.

comparison of the few samples not analyzed in both laboratories.The three islands have overall similar patterns in extended trace

med

258

182

992

481

136

559

30 A. Hildenbrand et al. / Lithos 210211 (2014) 27393.2.1. Major elementsMajor elements have been analyzed by XRF and ICP-AES at ALS

Chemex (Canada) and Activation Laboratories (Actlabs, Canada),respectively (Table 2). Loss on ignition (LOI) reaches up to 2.2 wt.%(trachyte AZ05-N) but is generally lower than 0.5 wt.% for most of thesamples (Table 2), conrming the absence of signicant alteration. Allthe analyzed lavas are alkaline, ranging in composition from basalts totrachytes. The more primitive samples (MgO N 5 wt.%) have relativelyhigh TiO2, FeOT and P2O5 concentrations compared to typical N-MORB(e.g. Sridhar and Ray, 2003), but similar values compared to otherAtlantic OIBs. Most of our samples have Na O/K O ratios ranging3.2. Geochemistry

Newwhole-rock geochemical analyses have been performed on oursamples from the three islands. These include the six Terceira samplesdated in the present study (see previous section), and samples fromS. Jorge and Faial previously dated by Hildenbrand et al. (2008, 2012)and Silva et al. (2012). A fairly large amount of each sample (about500 g) was crushed with a hydraulic press and powdered in agatemills to obtain a representative and statistically homogeneous whole-rock powder. For our most porphyritic samples, the separated ground-

Table 1Results of the newK/Ar dating on our samples from Terceira. The analyses have been perforindicated in thousands of years (ka). The uncertainties are reported at the 1 level.

Sample Long. Lat. Phase K%

AZ05-C 27.0525 38.7334 groundmass 1.

AZ05-F 27.0569 38.7346 groundmass 1.

AZ05-K 27.1682 38.6746 groundmass 0.

AZ05-N 27.1945 38.7073 alkali feldspars 4.

AZ05-G 27.3580 38.6931 groundmass 1.

AZ05-J 27.3580 38.6931 groundmass 1.2 2

between 1.9 and 3.6. This is similar to values reported for the Azores(excluding S. Miguel) and other Atlantic islands such as Madeira, SaintHelens, Cape Verde and Canary Islands (e.g. Davies et al., 1989;Gurenko et al., 2006; Holm et al., 2006; Wilson, 1989).

Harker-like diagrams show slight but signicant variations at bothinter and intra-island scales (Fig. 3). Lavas from Terceira are character-ized bywell-dened and rather narrow trends inmost diagrams. In con-trast, samples from S. Jorge and Faial exhibit relatively large variationsfor most oxides at a given MgO. For both islands, compositional differ-ences are observed between lavas older than 800 ka and younger than750 ka, respectively. This is particularly clear for S. Jorge (Fig. 3),where the youngest samples have higher Na2O and K2O, and lowerFeOT and TiO2 than those from the old volcanic phase (1.31.2 Ma).Samples from the young volcanic phases on Faial (b400 ka) also appearto have lower FeOT and K2O than the older lavas, whereas no cleardifference is observed regarding the other elements. Most of thesamples from Faial, however, are more evolved in composition(MgO b 4 wt.%), and exhibit surprisingly high values for Al2O3 andCaO (cf. Table 2). Such characteristics are particularly evident for thesamples bearing signicant amounts of plagioclase phenocrysts/glomerocrysts (e.g. AZ05-AL, AM, AN and AO). Hence the variationselement diagrams normalized to primitive mantle (Fig. 4). All thelavas are highly enriched in incompatible elements, especially LILE(e.g., Ba, Rb), LREE (e.g., La, Ce, Nd), and several HFSE (e.g., Nb, Ta).The most evolved lavas (MgO b 4 wt.%) show a positive shift towardsobserved in these young Faial samples are to be considered withcaution, since they partly reect accumulation processes and/or incor-poration of xenocrysts.

3.2.2. Trace elementsTrace elements have been determined for all samples by ICP-MS.

Samples from S. Jorge (Hildenbrand et al., 2008) as well as a few sam-ples from Faial were measured previously in Actlabs (Canada). Most ofthem have been re-analyzed on a Thermonigan Element 2 at thePacic Center for Isotopic and Geochemical Research at the Universityof British Columbia (PCIGR, UBC), in Canada, following the procedurereported in Pretorius et al. (2006) and Carpentier et al. (2013). The re-sults are presented in Table 2. Values obtained on duplicate and repli-cate measurements on our sample AZ05-AF and on reference materialBHVO-2, BCR2 and Kil93 are shown additionally in Supplementary ma-terial S1. The results obtained at PCIGR and at Actlabs on several sam-ples from S. Jorge are generally consistent, allowing a meaningful

on fresh-separated groundmass, except for sample AZ05-N (alkali feldspars). The ages are

40Ar* (%)

40Ar* (1012 at/g) Age (ka) Unc. (ka)

15.0 0.5263 401 615.5 0.5267 401 6

mean 401 618.9 0.4609 373 625.3 0.4543 368 5

mean 370 611.1 0.4097 395 711.9 0.401 387 6

mean 391 610.8 0.5714 122 211 0.5815 124 2

mean 123 20.8 0.063 53 70.8 0.063 53 7

mean 53 70.5 0.0097 60 120.5 0.1156 71 14

mean 65 13higher concentrations for most elements. The trachytic sample fromTerceira (AZ05-N) is additionally marked by prominent negativeanomalies of Ba, Sr and Eu, most probably reecting fractionation andremoval of alkali feldspars and plagioclase.

All the analyzed lavas are enriched in REE relative to chondrite C1(McDonough and Sun, 1995). They show enrichment by more than afactor of 100 in light REE (La, Ce, Pr, Nd) and the typical trend observedfor alkaline OIB. Ratios between LREE and MREE (e.g. La/Sm) in macsamples (MgO N 4 wt.%) are signicantly higher than those reportedfor Atlantic N-MORB (e.g. Debaille et al., 2006), but comparable to thevalues from the adjacent part of the MAR between latitudes 39N and39.5N (Dosso et al., 1999). At the scale of the three islands, the lavaserupted prior to 800 ka and after 750 ka show signicant differences.The oldest lavas from both S. Jorge and Faial plot as a linear and relative-ly narrow trend in the (La/Yb)N vs. La diagram, whereas the young sam-ples from the three islands show higher values and a greater dispersion(Fig. 5).We note that the highest (La/Yb)N ratios do not simply correlatewith low MgO, and hence rule out a signicant artifact due to differen-tiation processes. Further differences between old and young volcanicsare also observed from incompatible element ratios, e.g. on the Zr/Nbvs. Rb/Th diagram, where twomain groups can be identied. The oldest

31A. Hildenbrand et al. / Lithos 210211 (2014) 2739lavas from both S. Jorge and Faial have relatively high Zr/Nb and Rb/Thcompared to the young volcanics from the three islands.

3.2.3. Radiogenic isotopesThe SrNdPbHf isotopic compositions of all our samples have

beenmeasured at PCIGR. Full details on the protocol of sample prepara-tion (including repeated leaching), analytical procedures, and data re-duction are provided elsewhere (Hanano et al., 2009; Nobre Silvaet al., 2009, 2010, 2013; Weis and Frey, 1991, 1996; Weis et al., 2005,2006, 2007), and summarized in Supplementary le S2. Measurementson BHVO-2 and BCR-2 referencematerial prepared alongwith our sam-ples (Table 2) yielded results within the range of values reported inWeis et al. (2005, 2006, 2007) for

87Sr/

86Sr,

143Nd/

144Nd,

206Pb/

204Pb,

207Pb/

204Pb,

208Pb/

204Pb and

176Hf/

177Hf. Duplicate measurements on distinct ali-

quots of sample AZ05-AF give consistent results for all isotopic ratioswithin the range of analytical uncertainties. Replicate measurementson the solution of a few samples also yield reproducible results, with arelative variability less than 100 ppm for all isotopic ratios (Supplemen-tary le S3). The isotopic composition measured on both whole-rockand groundmass for some of our samples is inmost cases indistinguish-able within the range of uncertainties (Table 3).

The new results obtained on the different samples from the threeislands are constrained within the range of values previously measuredon central Azores (e.g. Beier et al., 2012; Dupr et al., 1982; Elliott et al.,2007; Halliday et al., 1992; Madureira et al., 2011; Millet et al., 2009;Moreira et al., 1999; Oversby, 1971; Turner et al., 1997; White andHofmann, 1982; White et al., 1979). The Sr and Nd isotopic composi-tions dene a somewhat linear relationship (Fig. 6), partly overlappingthe eld of MORB data from the adjacent portion of the MAR (Agranieret al., 2005; Dosso et al., 1999). The three islands overall exhibit a limit-ed range of variations, except the trachyte AZ05-N, which shows a veryhigh

87Sr/

86Sr ratio (0.705523 0.000009). Lavas from S. Jorge generally

have a slightly higher143Nd/

144Nd than the other islands for comparable

Sr isotopic ratios (except sample AZ05-AH), whereas Faial is shifted to-wards higher

87Sr/

86Sr values. All the analyzed samples are characterized

by relatively high208Pb/

204Pb,

207Pb/

204Pb and

206Pb/

204Pb values, in agree-

ment with recently published values on the central Azores (e.g. Beieret al., 2008, 2012; Elliott et al., 2007; Madureira et al., 2011; Milletet al., 2009). In

208Pb/

204Pb vs.

206Pb/

204Pb and

207Pb/

204Pb vs.

206Pb/

204Pb dia-

grams (Fig. 7), most samples dene a linear trend, located below theNorthern Hemisphere Reference Line (NHRL, Hart, 1984). The oldestlavas from S. Jorge and Faial (age N 800 ka) exhibit a limited range ofcompositions partly overlapping the eld of local MORBs. The younglavas from the three islands (age b 750 ka) show more heterogeneity,and plot as two main groups: (1) one group comprises all the samplesfrom Terceira andmost of the young samples from S. Jorge, which large-ly overlap with respect to each other; (2) the other group includes thethree youngest samples from Faial (samples AZ05-AN, AZ05-AO, andAZ05-APg) and sample AZ05-AH from S. Jorge, which plot above theNHRL with a

206Pb/

204Pb signicantly lower than all the other lavas.

These peculiar samples are also characterized by87Sr/

86Sr ratios higher

than 0.70375 (see Table 3 and Fig. 6).The Hf isotopic composition of our samples has relatively limited

range of values constrained between 0.28296 and 0.28311 (2 uncer-tainties included). These are similar to the values reported on theyoung (b300 ka) island of Pico, but signicantly higher than thosemea-sured on samples from S.Miguel (Elliott et al., 2007; Snyder et al., 2004).In the HfNd and HfSr isotopic spaces (Fig. 8), our samples from thethree islands dene linear relationships sub-parallel to the trend de-ned by MORB data from the adjacent portion of the MAR (Agranieret al., 2005), but slightly shifted towards more radiogenic Sr and Nd ra-tios. Moreover, the Hf isotopic composition of the lavas does not appearto be correlated with Pb isotope systematics (Fig. 8).

In a plot of the isotopic composition of the samples as a function oftheir eruption age (Fig. 9), a clear distinction can be made between

the samples erupted prior to 800 ka and those after 750 ka. The samplesfrom S. Jorge and Faial N150 volcanic ridges (age N 800 ka) overall haverather homogeneous and weakly radiogenic compositions. In contrast,the lavas erupted over the last 750 kyr along the N110 direction showhigher

207Pb/

204Pb (up to 15.64) and a large range of variations in

208Pb/

204Pb and

206Pb/

204Pb, from weakly radiogenic to very radiogenic ratios.

4. Discussion

4.1. Source components and mantle heterogeneity

The new isotopic compositions measured in this study are generallyless dispersed than previous isotopic data obtained in the central Azoresislands (Figs. 68). This can be partly explained by the steadiness of theanalytical conditions at PCIGR (for a review see Weis et al., 2005, 2006,2007) and by the peculiar procedure of sample preparation hereadopted. Repeated leaching, especially, is particularly important formeasuring the pristine composition of volcanic products in such oceanicenvironments, and to remove eventual in-situ supercial contamina-tion by sea water, or secondary contamination during crushing of thesamples, as shown by recent systematic studies (e.g. Hanano et al.,2009; McDonough and Chauvel, 1991; Nobre Silva et al., 2009, 2010;Weis and Frey, 1991, 1996; Weis et al., 2005, 2006, 2007). We note,however, that leachingmay not have been sufcient to fully remove con-tamination by seawater in the case of our sample AZ05-N, which showsan extremely high

87Sr/

86Sr ratio. This sample also has a very low Sr ele-

mentary content (ca. 20 ppm in whole-rock). Therefore, a very small re-sidual contamination by seawater could be sufcient to shift the

87Sr/

86Sr

ratios, without signicantly affecting the other isotopic ratios. In contrast,values obtained on all the other samples are mutually consistent, and thehigh reproducibility amongst the various duplicates and replicates rulesout any signicant surcial contamination. Similarly, the isotopic ratiosobtained on both whole-rock and groundmass for the most porphyricsamples from Faial (AZ05-AL, AM, AN and AO) and S. Jorge (AZ05-AB,AZ05-AD) show that the isotopic ratiosmeasured onwhole-rock samplesare not biased by any eventual incorporation of inherited xenoliths orphenocrysts from earlier magma production events.

The narrow compositional trends observed in the NdSr, PbPb, HfNd and SrPb isotopic spaces in samples from the three islands are hereinterpreted as reectingmixing between distinct source components orend-members. From our new data, three components can be distin-guished and compared with end-members previously recognized inthe Azores (Figs. 68): (1) a weakly radiogenic component dominantlyexpressed in our oldest samples from both S. Jorge and Faial (Figs. 68).It has

87Sr/

86Sr,

143/144Nd,

208Pb/

204Pb,

206Pb/

204Pb and

176Hf/

177Hf ratios close to

the average composition ofMORBs from the adjacentMAR between lat-itudes 37N and 40N (e.g. Agranier et al., 2005; Dosso et al., 1999), rep-resented by a star in Fig. 7. We note that the

207Pb/

206Pb isotopic ratios of

our oldest samples are signicantly lower than the average regionalMORB composition, but remain close to the composition of some indi-vidual MAR samples. The weakly radiogenic component here inferredthus most likely reects the involvement of regional upper mantle sig-nature during melt generation. (2) A HIMU-type component is fairlyclear from the isotopic composition ofmost of our samples. It has valuessimilar to the local end-member recognized in Terceira and S. Jorgelavas (e.g. Madureira et al., 2011; Millet et al., 2009; Turner et al.,1997), and referred to as T in Fig. 7. The nature and the amount ofrecycled material possibly involved in the HIMU-like component hasbeen extensively discussed in earlier studies (e.g. Madureira et al.,2011; Millet et al., 2009), and is beyond the scope of this paper. Wenote, however, that

87Sr/

86Sr ratios measured here and in previous

works on the central Azores are somewhat higher than in typicalHIMU signatures recognized elsewhere, e.g. in the CookAustraleschain in the Pacic (Chauvel et al., 1992; Hofmann, 2003; Jackson andDasgupta, 2008), which suggests possible recycling of both highly al-tered oceanic crust and sediments by old subduction processes. (3) A

third component can be distinguished in the three young samples

Faia12)

* 1

32 A. Hildenbrand et al. / Lithos 210211 (2014) 2739Table 2Whole-rockmajor and trace element composition of our samples fromS. Jorge, Terceira andis indicated: *this study; **data from Hildenbrand et al. (2008); ***data from Silva et al. (20

Island Terceira Terceira Terceira Terceira Terceira Terceira

Sample AZ05-C AZ05-F AZ05-G AZ05-J AZ05-K AZ05-N

Long. (W) 27.0525 27.0569 27.358 27.358 27.168 27.1948Lat. (N) 38.7334 38.7346 38.6931 38.6931 38.6746 38.7073Age unc. (ka) 401 6* 370 6* 53 7* 65 13* 391 6* 122 2

Unnormalized major element oxides (wt.%)

Analytical technique XRF XRF ICP-OES ICP-OES XRF XRFfrom Faial and our young sample AZ05-AH from S. Jorge. Their positionto the left of theNHRL line in the Pb/Pb isotopic spaces points to a signif-icantly lower

206Pb/

204Pb, whereas Figs. 68 show a signicantly higher

87Sr/

86Sr, and lower

143Nd/

144Nd and

176Hf/

177Hf with respect to other

lavas. This suggests a source with compositional similarities to theEM-like component previously recognized or suspected in Faial, S.Jorge and Terceira (e.g. Madureira et al., 2011; Millet et al., 2009). Thethird component may thus represent small amounts of recycled re-sidual lithologies, e.g. delaminated sub-continental lithospheric frag-ments which, like the HIMU component, have been preferentiallyincorporated during recent melt generation over the last 750 kyr.

SiO2 47.26 47.8 48.21 50.96 47.27 64.09TiO2 3.8 4.02 3.7 2.96 4.4 0.59Al2O3 15.27 15.49 16.02 15.83 14.81 13.29Fe2O3* 13.3 13.46 12.07 11.28 13.77 7.46MnO 0.2 0.22 0.18 0.2 0.21 0.34MgO 4.73 4.02 4.58 3.5 4.83 0.7CaO 9.17 7.98 9.78 7.85 9.17 0.69Na2O 3.52 3.81 3.56 4.35 3.45 6.03K2O 1.39 1.38 1.22 1.74 1.25 4.43P2O5 0.65 0.76 0.63 0.83 0.86 0.05LOI 0.1 0.54 0.312 0.31 0.31 2.16Total 99.3 99.61 99.65 99.19 99.83 99.82

Trace elements (ppm)

Lab. PCIGR PCIGR PCIGR PCIGR PCIGR PCIGR

La 34.87 43.98 37.35 51.5 45.31 167.84Ce 78.7 81.9 80.8 109.1 90.7 321.3Pr 9.98 12.23 10.14 13.27 12.28 36.19Nd 42.4 51.9 41.8 54.1 51 130.1Sm 8.72 10.65 8.76 11.2 10.69 24.53Eu 2.92 3.7 2.92 3.61 3.62 3.53Gd 9.55 11.29 9.16 11.77 11.29 30.68Tb 1.42 1.66 1.33 1.72 1.63 4.17Dy 7.86 8.95 7.16 9.24 8.99 25Ho 1.33 1.46 1.15 1.51 1.43 4.38Er 3.72 4.07 3.19 4.19 3.91 13.19Tm 0.47 0.5 0.4 0.54 0.48 1.83Yb 3.02 3.18 2.57 3.32 3.05 12.21Lu 0.42 0.44 0.35 0.47 0.4 1.69Sc 20.38 19.33 21.38 17.18 24.7 4.17V 355 296 339 250 349 Cr 5 57 15 7 Co 37 31 36 25 39 1Ni 20 38 9 13 Cu 26 5 30 15 20 1Zn 112 123 110 127 129 312Ga 21 23 24 26 23 40Rb 29 29 25 39 28 119Sr 524 557 684 641 634 23Y 31.6 37.7 30.7 43.7 41.6 126.3Zr 258 262 295 388 258 1630Nb 52 51.3 55.5 73.1 52.9 284.3Cs 0.2 0.1 0.1 0.2 0.2 1.7Ba 393 525 316 431 428 74Hf 6.6 6.5 6.6 8.7 6 34.5Ta 3.39 3.25 3.17 3.93 2.98 14.52Pb 2.24 2.01 1.58 2.27 1.67 10.77Th 3.71 3.14 3.46 4.97 2.87 19.7U 1.09 0.95 1.21 1.69 0.91 6.65l. For each sample, the eruption agemeasured by unspikedK/Ar dating on separated phase; ****data from Hildenbrand et al. (2012).

Sao Jorge Sao Jorge Sao Jorge Sao Jorge Sao Jorge Sao Jorge

AZ05-P AZ05-R AZ05-U AZ05-Z AZ05-AB AZ05-AD

27.8662 27.8553 27.8559 27.8571 27.8517 27.968238.5601 38.547 38.5482 38.5503 38.5479 38.6388323 21** 1310 19** 1314 19*** 1267 18*** 1207 17** 736 12**

ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES4.2. Geodynamic controls on melt production

The genesis of the volcanism in the Azores has long been attributedeither to a plume-likemantle anomaly, whichwould carry temperatureexcess (e.g. Bourdon et al., 2005) and volatile-richmaterial to the uppermantle (Asimow et al., 2004; Beier et al., 2012) or to a hydrated uppermantle source (Bonatti, 1990). Recent data on melt and uid inclusionstrapped in young basaltic scoriae from Pico Island, especially, suggestthe presence of a wet spot (Mtrich et al., 2014). The HIMU and EM-type components recognized here and in some previous studies wouldreect the dual signature of the components incorporated in the Azores

43.28 47.26 47.28 52.9 48.09 45.634.43 3.49 3.43 2.24 2.78 2.9415.62 15.64 15.78 17 20.43 15.3614.18 12.23 12.39 9.7 9.76 11.720.18 0.17 0.18 0.17 0.13 0.165.49 4.44 4.34 2.85 3.28 8.989.93 8.42 8.35 6.04 9.98 10.352.65 3.52 3.68 5.1 3.31 2.880.88 1.49 1.63 2.44 1.17 0.870.57 0.74 0.82 0.89 0.53 0.461.52 2.04 1.52 0.03 0.35 0.2698.73 99.43 99.4 99.36 99.81 99.64

PCIGR PCIGR PCIGR PCIGR PCIGR PCIGR

28.44 37.19 39.95 60.69 24.55 27.0767 85.2 88.3 126 55.7 60.68.78 11.03 11.75 15.6 7.31 7.7437.6 46.4 48.8 63.6 31.4 32.28.52 10.24 10.85 13.05 6.86 6.612.83 3.37 3.61 4.03 2.5 2.258.97 10.84 11.52 13.52 7.49 7.371.32 1.62 1.68 2.03 1.1 1.047.21 8.75 9.03 11.13 6.01 5.521.17 1.45 1.49 1.81 0.96 0.913.19 3.9 4.02 5.14 2.66 2.470.4 0.51 0.51 0.65 0.33 0.322.52 3.14 3.27 4.29 2.17 2.050.34 0.42 0.44 0.57 0.29 0.2726.72 23.28 22.22 14.25 15.58 25.7394 276 254 132 199 25815 1 4 8 25044 30 29 17 24 5134 7 2 4 15 13526 24 18 11 18 24125 137 136 134 94 10224 26 26 28 25 2018 30 33 57 25 20622 580 599 536 795 68831.7 41.6 43.5 53 26.1 23.9261 361 375 580 248 22441.7 55.6 57.6 81.8 36.2 35.60.1 0.2 0.2 0.3 0.3 0.2243 315 339 510 310 2446.2 8.2 8.6 12 5.6 5.32.58 3.2 3.27 4.49 2.19 2.21.15 1.61 1.64 2.84 1.26 1.472.36 3.27 3.26 6.01 2.19 2.50.88 1.17 1.16 1.86 0.71 0.87

ira ant al.

***

33A. Hildenbrand et al. / Lithos 210211 (2014) 2739Table 2Whole-rockmajor and trace element composition of our samples from S. Jorge, Terce

phase is indicated: *this study; **data from Hildenbrand et al. (2008); ***data from Silva e

Island Sao Jorge Sao Jorge Sao Jorge Sao Jorge Faial

Sample AZ05-AFaverage

AZ05-AG AZ05-AH AZ05-AJ AZ05-AL

Long. (W) 27.9213 27.9399 28.0713 28.204 28.6094Lat. (N) 38.5773 38.591 38.6793 38.68162 38.5524Age unc. (ka) 729 11** 690 11** 368 6** 268 6** 846 12*plume (Turner et al., 1997),whereas primitive heliumandneon isotopiccompositions have been interpreted as evidence for sampling of arelatively undegassed reservoir attributed to the deep mantle (Jean-Baptiste et al., 2009; Madureira et al., 2005; Moreira et al., 2012). How-ever, the location, the size and the dynamics of the inferred plume re-main elusive. From seismic tomographic modeling, somewhatcontradictory conclusions have been reached. Global tomographicmodels argue for a low-velocity zone down to 1000 km depth,interpreted as evidence for a deep-seated plume rising from the lower

Unnormalized major element oxides (wt.%)

Analytical technique ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES

SiO2 45.24 48.26 45.1 47.04 49.14TiO2 3.9 3.22 3.97 3.48 2.32Al2O3 16.6 16.45 14.74 16.37 19.44Fe2O3* 12.6 11.87 12.31 12.18 9.46MnO 0.18 0.2 0.18 0.17 0.15MgO 5.6 4.17 8.27 6.04 3.5CaO 9.33 7.92 9.98 8.84 10.11Na2O 3.59 4.81 2.93 3.81 3.89K2O 1.11 1.5 1.54 1.41 1.49P2O5 0.66 1.21 0.67 0.78 0.45LOI 0.16 0.35 0.52 0.32 0.254Total 98.64 99.27 99.17 99.82 100.2

Trace elements (ppm)

Lab. PCIGR PCIGR PCIGR PCIGR Actlab

La 37.45 50.19 41.45 41.6 28.5Ce 82.1 109.4 92.3 90 60.6Pr 10.61 14.41 11.44 11.48 7.02Nd 44.6 61 45.3 47.6 28.6Sm 9.06 12.9 8.86 9.31 5.94Eu 3.01 4.24 2.85 3.09 2.29Gd 9.52 13.59 9.8 9.79 5.85Tb 1.37 1.98 1.28 1.4 0.95Dy 7.38 10.45 6.68 7.44 5.51Ho 1.19 1.71 1.04 1.17 1.04Er 3.28 4.49 2.87 3.25 2.9Tm 0.41 0.57 0.35 0.4 0.41Yb 2.62 3.65 2.3 2.61 2.54Lu 0.35 0.49 0.31 0.35 0.38Sc 21.86 16.43 27.08 19.82 16V 308 190 324 271 240Cr 44 5 277 111 20Co 39 24 47 38 23Ni 37 8 130 59 18Cu 21 11 26 26 24Zn 114 135 110 121 77Ga 23 24 22 25 23Rb 24 33 27 27 30Sr 792 797 725 740 638Y 30.7 47.3 27.2 31.1 28.4Zr 292 375 337 354 206Nb 55 66 61.8 63.4 35.1Cs 0.2 0.3 0.2 0.2 0.3Ba 294 411 320 325 348Hf 7 8.8 7.6 7.4 4.7Ta 3.34 3.66 3.69 3.66 2.72Pb 1.76 2.2 1.9 1.88 5Th 3.54 3.94 3.75 3.82 2.77U 1.21 1.42 1.18 1.22 0.67d Faial. For each sample, the eruption agemeasured by unspiked K/Ar dating on separated(2012); ****data from Hildenbrand et al. (2012).

Faial Faial Faial Faial Faial Faial

AZ05-AM AZ05-AN AZ05-AO AZ05-AR AZ05-AQ AZ05-AP

28.6073 28.6045 28.75 28.625 28.601538.5502 38.5963 38.6159 38.5528 38.5946

848 12**** 363 6**** 118 3**** 843 12**** 847 12**** 358 7****mantle (Montelli et al., 2004, 2006), whereas regional tomographicmodels (e.g. Pilidou et al., 2005; Silveira et al., 2006) emphasize thepresence of a main shallow anomaly (b200 km) under the Azoresplateau, possibly rooted at the transition between the lower andupper mantles a few tens of km NE of Terceira (Adam et al., 2013;Yang et al., 2006). Fromgeochemical data,most authors explicitly or im-plicitly consider the Azores plume as an active narrow feature, whichwould be either centered along the MAR (White et al., 1976), underTerceira (Bourdon et al., 2005), or even under Pico (Beier et al., 2012).

ICP-OES XRF XRF ICP-OES ICP-OES ICP-OES

48.93 46.7 49.77 50.09 49.16 47.622.46 3.09 2.41 3.02 2.78 3.2218.89 19.91 20.93 17 18.42 18.139.95 10.14 8.08 11.4 10.54 11.360.162 0.14 0.14 0.19 0.17 0.173.65 4.09 2.89 3.44 3.41 4.329.8 10.54 9.11 7.29 9 9.243.77 2.85 4.24 4.16 3.85 3.71.56 1.18 1.59 2.28 1.87 1.490.48 0.43 0.52 0.65 0.53 0.580.52 0.63 0.05 0.7 0.32 0.22100.2 99.83 99.87 100.2 100.05 100.04

PCIGR PCIGR PCIGR PCIGR Actlab Actlab

27.46 30.5 37.74 44.01 32.3 36.160.8 66.2 83.5 89.3 69.7 797.43 8.21 10.1 11.24 8.11 9.1830.3 32.8 40.5 44.8 33.1 37.46.18 6.8 7.81 9.27 7.03 7.752.12 2.39 2.77 3.11 2.65 2.936.84 7.35 8.42 10.07 6.98 7.361.02 1.1 1.22 1.51 1.16 1.235.75 5.93 6.67 8.31 6.46 6.70.99 0.97 1.08 1.43 1.21 1.232.84 2.74 3.04 4.07 3.39 3.380.38 0.35 0.39 0.54 0.48 0.452.49 2.21 2.48 3.51 2.91 2.680.34 0.3 0.35 0.49 0.4 0.38417.97 22.1 13.39 19.38 18 20273 257 176 202 256 26229 40 13 5 26 29 18 24 23 2821 22 9 4 13 1316 16 13 13 26 2392 92 86 138 82 9323 23 25 23 23 2430 23 35 45 35 24607 765 822 548 575 71026.3 25.3 27.2 37.2 32.9 33.1235 236 286 336 254 26040.5 36.4 49.8 55.1 38.5 41.30.2 0.1 0.3 0.3 0.2 0.3351 357 425 499 418 4075.2 5.6 6.8 8 5.6 5.92.38 2.15 3.05 3.34 3.08 3.282.1 1.94 2.8 2.48 4 2.87 2.97 4.18 4.44 2.99 3.270.85 0.76 1.31 1.35 0.9 1.14

Fig. 4. Spiderdiagrams showing the distribution of trace elements for all our various sam-ples (same symbols as in Fig. 3). The concentrations are normalized using the referencevalues given by McDonough and Sun (1995) for primitive mantle. Upper panel: the gryarea shows the overall range of variations for the three islands (undistinguished), exceptfor trachyte AZ05-N, which is shown separately (red curve). Lower 3 panels: spidergramsfor each of the three islandshere studied. In the case of Faial, a primitive picrite (whitecircles, data from Beier et al., 2010) is shown for comparison.

34 A. Hildenbrand et al. / Lithos 210211 (2014) 2739Nevertheless, the Azores Archipelago lacks a clear age progression asobserved on well-documented intra-plate linear chains of oceanicislands, e.g. in the Hawaiian-Emperor chain, or in French Polynesia(Bonneville et al., 2006; Hildenbrand et al., 2004). In contrast, recentunspiked K/Ar data and

40Ar/

39Ar ages acquired on separated ground-

mass and pure mineral phases on samples from both S. Miguel(Johnson et al., 1998) and in the central Azores (e.g. Calvert et al.,2006; Hildenbrand et al., 2008, 2012; Larrea et al., 2014; Sibrant et al.,2014; and this study) reveal short episodes of coeval volcanic construc-tion at the regional scale, e.g. around 850 ka, 700 ka, 400 ka, 200 ka,120 ka, 50 ka, and during the historical period. Such synchronous volca-nic construction cannot be explained simply as resulting from a station-ary active mantle anomaly, unless we consider the pulsative andsynchronous development of several small plumelets. It has been pro-posed recently that small P-wave velocity anomalies (Yang et al.,2006) could reect density anomalies, which would trigger local con-vection in the form of narrow plumelets (Adam et al., 2013). However,it appears unrealistic that such inferred active features would promoteshort-lived (b50 kyr) events of partial melting at the same time underthe various islands. Indeed, the development of small plumelets in theupper mantle, e.g., from a larger instability blocked at depth, typicallyoccurs over timescales of several Myr to several tens of Myr, asevidenced in the region of the South Central Pacic in Polynesia(Bonneville et al., 2006; Courtillot et al., 2003; Davaille, 1999). Further-

Fig. 3.Harker-like diagrams showing the compositional variations of our samples in Na2O,K2O, FeOT, and TiO2 as a function ofMgO (wt.%). Square symbols for S. Jorge,with dark andlight color for samples older than 800 ka and younger than 750 ka, respectively; circles forFaial, with dark and light color for samples older than 800 ka and younger than 750 ka,respectively; triangles for Terceira.more, the development of N150 and N110 volcanic ridges at the plateauscale is incompatible with present absolute plate movement towardsSSW over stationary narrow plumelets (Hildenbrand et al., 2008).

We propose an alternative model that reconciles available geophys-ical, geochemical, geochronological and geological data. In this model,the enriched signatures expressed along the MAR and the severalislands of the Azores, reect the presence of residual enriched mantlezones dispersed in the shallow upper mantle (b300 km) under theAzores region. These could either represent the remnants of a large,presently dormant, heterogeneous mantle plume responsible for theformation of the Azores plateau several million years ago (Cannatet al., 1999; Silveira et al., 2006), and/or shallow delaminated litho-spheric fragments inherited from the early stages of opening of theAtlantic (Moreira et al., 1999). Although recent trace element, Pb, Ndand Hf isotope data provide some evidence for the occurrence of litho-spheric delamination, noble gas data from the Azores and adjacentMAR support the plume hypothesis with the entrainment of recycledoceanic lithosphere and surrounding upper mantle (Madureira et al.,2011). However, we argue that current deep mantle upwelling is notthe main driving force responsible for the development of the recentvolcanism. We propose instead that melt generation in the Central

Fig. 5. Diagramms (La/Yb)N vs. La and (Zr/Nb) vs. (Rb/Th). The old lavas erupted on Faialand S. Jorge (age N800 ka) are distinguished. Same symbols as in Fig. 3.

e gromen

(

0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

35A. Hildenbrand et al. / Lithos 210211 (2014) 2739Table 3Isotopic composition of our samples from S. Jorge. Terceira and Faial. g:measurement on thand this study). Stars show the average composition after duplicate and replicate measure

Sample87Sr/

86Sr (2SE)

143Nd/

144Nd (2SE)

208Pb/

204Pb

AZ05-C 0.703610 0.000009 0.512955 0.000006 39.3541AZ05-F 0.703588 0.000008 0.512963 0.000007 39.3348AZ05-G 0.703565 0.000008 0.512957 0.000006 39.2226AZ05-J 0.703556 0.000008 0.512968 0.000005 39.2621AZ05-K 0.703652 0.000008 0.512956 0.000005 39.2114AZ05-N 0.705523 0.000009 0.512960 0.000005 39.2486AZ05-P 0.703538 0.000009 0.513004 0.000006 38.9103AZ05-Pg 0.703517 0.000009 0.513003 0.000007 38.9250AZ05-R 0.703644 0.000009 0.512987 0.000008 38.8454AZ05-U 0.703627 0.000008 0.512977 0.000006 38.8144*AZ05-Z 0.703608 0.000007 0.512967 0.000016* 38.9145AZ05-AB 0.703788 0.000007 0.512960 0.000006 38.7487AZ05-ABg 0.703778 0.000008 0.512958 0.000007 38.7440*AZ05-AD 0.703746 0.000007 0.512964 0.000006 39.0641AZ05-ADg 39.0866AZ05-AF 0.703643* 0.000006* 0.512984* 0.000007* 39.2752*AZ05-AG 0.703705 0.000007 0.512987 0.000006 39.1046AZ05-AH 0.703787 0.000008 0.512891 0.000006 39.3232*AZ05-AHg 0.703794 0.000007 0.512883* 0.000012* 39.3270*AZ05-AJ 0.703569 0.000008 0.512941 0.000005 39.3827AZ05-AJg 39.3888*AZ05-ALg 0.703728 0.000008 0.512934 0.000006 39.0294*AZ05-AM 0.703735 0.000007 0.512928 0.000006 39.0091*AZ05-AMg 0.703750 0.000009 0.512936 0.000007 38.9961*AZ05-AN 0.703874 0.000008 0.512898 0.000006 38.8634*AZ05-ANg 0.703905 0.000008 0.512905 0.000006 38.8614*AZ05-AO 0.703833 0.000009 0.512894 0.000006 39.2600*AZ05-AOg 0.703836 0.000008 0.512888 0.000007 39.2626*AZ05-APg 0.703850 0.000009 0.512919 0.000007 38.8969AZ05-AQg 0.703796 0.000007 0.512908 0.000007 38.8247AZ05-AR 0.703737 0.000009 0.512929 0.000007 38.8396Azores over the last 1.3 Myr has been mostly controlled by lithosphericdeformation. A similar process has been pointed out on S.Miguel, wherestrong isotopic differences between lavas erupted synchronously by ad-jacent volcanoes suggest that the tectonic structure of the lithospherelocally controls mantle upwelling and partial melting (Haase andBeier, 2003). In the Central Azores, the two successive generations ofvolcanic ridges reported here suggest a rapid change of regional stressesbetween 850 and 750 ka.We propose that such re-organization reectsa major change in regional deformation accompanying the recent evo-lution of the Azores Triple Junction. In this interpretation, signicantstretching of the lithosphere at the plateau scale rst yielded the crea-tion of new N150 oblique lithospheric structures in the whole area be-tween the EAFZ and the present TR, prior to 800 ka (Fig. 10, stage 1).This likely caused signicant decompression partial melting of the am-bient upper mantle at a relatively high-degree, leading to a dilute ex-pression of the enriched components, and yielding the production ofmelts with rather homogeneous trace element and isotopic composi-tion relatively similar to lavas erupted synchronously at the MAR. Un-fortunately, the use of classical thermo-barometers (e.g., Lee et al.,2009) is not possible for the vast majority of our samples, as most ofthem have a rather evolved composition. All our old samples fromboth S. Jorge and Faial, especially, have MgO b 6 wt.%. Fractionationmay thus have signicantly modied the initial composition of the pri-marymagma, precluding any robust estimation of the ambient temper-ature and pressure during partial melting. However, the most primitiveof the old samples (AZ05-P, MgO= 5.49 wt.%) has Tb/Yb and La/Yb ra-tios of 11.31 and 0.52, respectively, which support relatively deepmelt-ing of a mantle source within the garnet stability zone, and a degree ofpartial melting comprised between 5% and 8% (see Fig. 11 in Mtrichet al., 2014). Considering the higher incompatible behavior of La during

AZ05-ARg 0.703766 0.000007 0.512926 0.000006 38.8427 0.0BCR2 0.705004 0.000008 0.512645 0.000006 38.8343* 0.0BHVO-2 0.703466 0.000007 0.512986 0.000006 38.1929* 0.0undmass separated for K/Ar dating (after Hildenbrand et al., 2008, 2012; Silva et al., 2012;ts for some of ours samples (full data shown in Supplementary material S3).

2SE)207Pb/

204Pb (2SE)

206Pb/

204Pb (2SE)

176Hf/

177Hf (2SE)

020 15.6219 0.0008 20.0587 0.0009 0.283078 0.000006020 15.6172 0.0007 20.0206 0.0009 0.283066 0.000006022 15.6123 0.0007 19.9293 0.0008 0.283066 0.000004020 15.6166 0.0007 19.9960 0.0009 0.283073 0.000005025 15.6100 0.0010 19.8403 0.0009 0.283059 0.000005027 15.6091 0.0010 19.8677 0.0012 0.283068 0.000005028 15.5793 0.0010 19.7370 0.0012 0.283101* 0.000009*029 15.5848 0.0010 19.7453 0.0012 0.283094 0.000005019 15.5569 0.0006 19.5363 0.0008 0.283076 0.000005045* 15.5479* 0.0028* 19.4757* 0.0029* 0.283061 0.000003018 15.5569 0.0006 19.5365 0.0007 0.283055 0.000004025 15.5417 0.0009 19.3468 0.0007 0.283045 0.000005012* 15.5397* 0.0020* 19.3486* 0.0039* 0.283037 0.000004023 15.6244 0.0008 19.8015 0.0010 0.283058 0.000005026 15.6308 0.0008 19.8153 0.0013020* 15.6289* 0.0006* 20.0923* 0.0017* 0.283080* 0.000014*019 15.6199 0.0009 19.9007 0.0007 0.283080* 0.000021*043* 15.6310* 0.0056* 19.4947* 0.0018* 0.282966 0.000004033* 15.6301* 0.0007* 19.4968* 0.0016* 0.282963 0.000005027 15.6212 0.0010 20.0234 0.0011 0.283039 0.000004080* 15.6241* 0.0006* 20.0263* 0.0002*077* 15.5932* 0.0026* 19.6558* 0.0014* 0.282994 0.000005012* 15.5872* 0.0016* 19.6477* 0.0025* 0.282998 0.000005058* 15.5818* 0.0005* 19.6417* 0.0023* 0.282999 0.000005058* 15.6192* 0.0029* 19.0905* 0.0047* 0.283000 0.000005034* 15.6197* 0.0004* 19.0851* 0.0018* 0.283000 0.000006057* 15.6390* 0.0044* 19.5222* 0.0004 0.282986 0.000004033* 15.6399* 0.0019* 19.5237* 0.0044* 0.282987 0.000005024 15.6219 0.0009 19.1756 0.0010 0.283016 0.000005031 15.5593 0.0011 19.3622 0.0015 0.282965* 0.000010*022 15.5568 0.0009 19.3262 0.0010 0.282995 0.000006crystal fractionation in basaltic liquids, the initial La/Yb in the parentalmagma must have been signicantly lower, and the actual degree ofpartial melting is probably closer to 8%.

Thewidespreaddevelopment of N110 volcanic ridges during the last750 kyr marks a major geodynamic reconguration (Fig. 10, stage 2),which can coincide in time with the individualization of the westernpart of the TR (Vogt and Jung, 2004). We note that the TR orientationto thewest of Terceira, and themain strike of the several horsts and gra-bens presently active in the vicinity of S. Jorge and PicoFaial volcanicridges (Marques et al., 2013, 2014), are roughly paralell to FractureZones cutting the MAR between latitudes 38N and 40N. Accordingly,we interpret that the sudden re-organization of the EurasiaNubiaplate boundary involved reactivation of preexisting lithospheric struc-tures. The signicant enrichment in LILE (including LREE), the higher(La/Yb)N ratios and the variable but general more radiogenic isotopiccompositions in the young lavas (b750 kyr) support variable but overalllower degree of partial melting of the inferred fertile mantle sources.Such hypothesis is consistent with the recent work by Mtrich et al.(2014), which showed that recent melt generation under Pico can beexplained by decompression melting of a water-enriched mantle do-main. For the sake of comparison, the Tb/Yb and La/Yb values for oursample AZ05-AD (age of 736 12 ka, MgO = 8.9 wt.%) support rela-tively low degrees of partial meting (45%), in full agreement withwhat has been proposed in other recent studies conducted on youngvolcanics from the central Azores (e.g., Mtrich et al., 2014; Zanonet al., 2013). Therefore, episodic magma production in the centralAzores during the last 750 kyr can be explained by limited decom-pression melting associated with sporadic reactivation of inheritedN110 transform faults, e.g. promoted by far-eld stresses or MARridge-push.

025 15.5582 0.0009 19.3269 0.0011 0.282998 0.000004027 15.6261* 0.0011 18.8042* 0.0012 0.282866 0.000005296 15.4815* 0.0069 18.6354* 0.0102 0.283098 0.000005

4.3. Implications for plate re-organization near triple junctions (TJs)

Among the 16 distinct types of TJs theoretically possible, only theridgeridgeridge (R-R-R) type appears stable over a few millionyears (e.g. Georgen and Lin, 2002; McKenzie and Morgan, 1969).

The other types experience signicant geometric re-organizationthrough time, i.e. at least one of the branches may jump to reach amore stable conguration. The Azores Triple Junction (ATJ) appears to

Fig. 6. Diagramm143Nd/

144Nd vs.

87Sr/

86Sr showing the position of our samples from the

three islands (same symbols as in Fig. 3), along with previous data acquired on MORBsand in the Azores (full data source in the text). The DM, HIMU, EMI and EMII mantle com-ponents are drawn after Zindler and Hart (1986). Upper inset: zoom showing the limitedrange of variation of our samples.

Fig. 8. Variations in176Hf/

177Hf with respect to

87Sr/

86Sr,

143Nd/

144Nd, and

206Pb/

204Pb for our

samples (same symbols as in Fig. 6). Dark diamonds for MAR samples between latitudes33 and 37N. Previous data on Pico (diagonal crosses) and S. Miguel (straight crosses)from Snyder et al. (2004) and Elliott et al. (2007).

36 A. Hildenbrand et al. / Lithos 210211 (2014) 2739Fig. 7.Variations in208Pb/

204Pb,

207Pb/

204Pb and

87Sr/

86Srwith respect to

206Pb/

204Pb for our sam-

ples (same symbols as in Fig. 3). Diamonds show the composition of local MORBs dredgedbetween latitudes 37 N and 40 N, whereas the star provides the corresponding averagecomposition (data from Agranier et al., 2005; Dosso et al., 1999). The thin dashed lineshows the compositional eld of otherMORBS dredged along theMAR outside the Azoresarea. The light gray and dark gray elds show the range of variation previously measuredon samples from the central Azores and S. Miguel, respectively. Bold circles show localcomponents, such as distinguished in previous studies (Millet et al., 2009; Moreira et al.,1999): SM: S. Miguel component; T: Topo end-member (S. Jorge); F: Faial end-member.NHRL: Northern Hemisphere Reference Line.be a conspicuous illustration, as the eastern branch evolved fromamainlytransform behavior (RRF-type TJ) towards a rift-type (RRR-type TJ). Wenote that the TerceiraRift (TR) and thenorthernpart of theAzores plateauare presently the locus of signicant oblique extension, as shown bythe relationship between the TR orientation and plate velocities(e.g. DeMets et al., 2010) and focal mechanisms associated with recentmedium/high magnitude earthquakes (e.g. Borges et al., 2007). There-fore, the TR may not represent the nal stage of plate-reconguration,as proposed by Vogt and Jung (2004). The absence ofwell-denedmag-netic anomalies on the seaoor remains a main limitation, as it eithersuggests that no signicant oceanic accretion is occurring along theTR, or that oceanic accretion started too recently and at a too lowspreading rate to generate detectable anomalies.

The present study provides signicant new insights on the possibleshort-term behavior of plate re-conguration near such TJs. Our newdata show that the eastern branch of the ATJ, at least during recenttimes, has not experienced a continuous shift towards theN. In contrast,thewidespread development of oblique lithospheric structures inferred

the widespread creation of N150 oblique ridges prior to 850 ka; (2) at about 750 ka,sudden re-organization, with subsequent melt production/extraction controlled byre-activation of N110 FZ.

37A. Hildenbrand et al. / Lithos 210211 (2014) 2739here suggests that large-scale diffuse intra-plate deformationmay occurfor a signicant period (several Myr?) prior to sudden plate boundaryjump. We are fully aware that the old volcanic successions on S. Jorge

Fig. 9. Variations in207Pb/

204Pb through time. Same symbols as in Fig. 3.and Faialmay have only recorded the latest history precedingATJ recon-guration, as numerous other oblique volcanic ridges over the Azoresplateau (especially N of the East Azores Fracture Zone) remain to be in-vestigated. We cannot exclude, for instance, that large-scale intra-platedeformation and oblique volcanic ridge development occurred in a dia-chronic way, e.g. from S to N, as suggested by a recent study (Sibrantet al., 2013). However, our data in the central Azores document largechanges in melt generation and magma extraction around 800750 ka, which suggests that such plate reconguration can effectivelyoccur within only a few tens of thousands of years. The main questionis whether such short period is sufcient to yield the creation/re-activation of N110 lithospheric structures and trigger signicant chang-es in melt generation and extraction in a setting like the ATJ. Rubin andMacdougall (1988) constrained the combined duration of melting andmagma transport to less than a few hundred thousand years. They con-cluded that

226Ra excesses constrain the length of the magma transfer

times from the melting depth through eruption to less than about athousand years. Peate and Hawkesworth (2005) showed that the time-scales of magmatic processes are comparable to the half-lives of severalU series nuclides: 76 kyr for

230Th, 33 kyr for

231Pa, and 1.6 kyr for

226Ra.

This means that mantle melting and magma extraction to the surfaceshould take a few tens to a couple of hundreds of kyr, depending on anumber of regional factors, such as magma composition and viscosity,lithospheric thickness, tectonic setting, and regional stresses. Thestudied magmas in the central Azores are mostly low-viscosity basalts,and, according to Mtrich et al. (2014), some of them particularly richin water and volatiles. Although short-term magma ponding underthe oceanic lithosphere and/or in short-lived crustal magma reservoirhave episodically occurred (Zanon and Frezzotti, 2013; Zanon et al.,2013), the melts in the Azores thus can form and be extracted rapidly.

Ifmost of the oceanic lithosphere is characterized by a brittle behavior,the relevant equation for fracture nucleation is MohrCoulomb'sFig. 10. Two-stage conceptual model depicting how the EurasiaNubia boundaryreorganized from our new data: (1) diffuse stretching at the whole plateau scale yieldsequation, which is independent of time or strain rate: = tan() +C, where is the shear strength, is the normal stress, C is the interceptof the failure envelopewith the ordinate axis and is often called the cohe-sion, and is the angle of internal friction. This means that brittle rockyields by fracturewhen the differential stress reaches the brittle strength.However, in order for faults to grow to lithospheric scale, time is needed.If many faults form simultaneously, then a signicant amount of time isneeded, in the order of a fewMyr (e.g. Cowie et al., 2000), to reach litho-spheric scale dimension. However, master faults in the region under con-sideration are few. For instance, each side of the active TR is typicallybounded by one or at most two master faults (Searle, 1980). Given theopening rate of the TR (ca. 2.4 mm/year according to Marques et al.,2013, 2014), its average depth (ca. 2000 m), and the average dip of nor-mal faults (ca. 60), then the vertical displacement rate is ca. 2 mm/yearand the timeneeded to reach the TR's current depth is ca. 1Ma. The oldestphase of sub-aerial volcanic construction within the TR has beenconstrained in S. Miguel at about 0.88 Ma (Johnson et al., 1998). There,the maximum time needed to bring melts to the surface through a newfault system (of lithospheric scale, and under a new stress eld) is thusin the order of a hundred of kyr. The lithosphere under the central Azoresis less than 20 Ma in age and is relatively thinner than under S. Miguel(e.g. Beier et al., 2008; Lus et al., 1994). In such conditions, the time nec-essary to produce signicant changes in partial melting and magma ex-traction in the central Azores should not exceed a few tens of kyr. Itmay even be signicantly less, as reactivation of pre-existing lithosphericstructures is expected to occur faster than the creation of a new set oflithospheric faults. Therefore, it seems reasonable to infer that the severalvolcanic pulses here reported for the last 750 kyr result from successivebrief (b50 kyr) episodes of melt generation and extraction to the surface.As the three islands here studied closely follow the present area of diffusedeformation (Marques et al., 2013, 2014), it also seems reasonable to infer

38 A. Hildenbrand et al. / Lithos 210211 (2014) 2739that the current diffuse plate boundary near the ATJ has been settledaround 800750 ka ago. Our study shows that inherited lithosphericstructures such as fracture zones constitute preferential discontinuitiesfor plate boundary re-localization near such TJs.

5. Conclusions

The combined geochronological and geochemical study presentedhere brings new constraints regarding source heterogeneity and meltproduction in the central Azores throughout the last 1.3Myr. The distinc-tion of two periods of volcanism older than 800 ka and younger than750 ka especially provides an important basis for reinterpreting the com-positional variability previously reported in the absence of a reliable tem-poral framework, and to discuss the main geodynamic processes thatmay have governed melt production and extraction over time.

The lavas analyzed range in composition from alkaline basalts/basanites to trachytes, and overall exhibit a strong enrichment in highlyincompatible elements. The whole range of isotopic compositions wemeasured suggests the involvement of three components: (1) a weaklyradiogenic component reecting the source of regional MORBs, (2) amain HIMU-type component represented in the three islands, and(3) an additional component in Faial younger lavas, which appears sim-ilar to the EM type end-member previously recognized in other Azoreseruptive complexes. The geographical distribution of the enriched com-ponents and the synchronous construction of various islands at the re-gional scale rules out an origin from a single narrow active plume.They suggest in turn the presence of dispersed residual enrichedmantledomains, resulting from earlier fertilization of the upper mantle, possi-bly by themain plume event responsible for the formation of the Azoresplateau several million years ago. The lavas erupted in S. Jorge and Faialprior to 800 ka have very homogeneous isotopic ratios, which partlyoverlap the compositional eld of MORBs from the adjacent portion oftheMid-Atlantic Ridge (MAR). Their genesis can be explained by the re-gional development of N150 transtensive tectonic structures, whichpromoted signicant decompression melting of the upper mantle,with correlative dilute expression of the enriched components. In con-trast, the younger lavas (b750 ka) erupted along the N110 main struc-tural direction on the three islands are signicantly enriched in LILE,and generally have variable but more radiogenic isotopic compositions.Such characteristics suggest a lower-degree of partial melting andgreater incorporation of fertile residual mantle portions during passivetectonic reactivation of pre-existing transform faults. We propose thatthe sub-aerial volcanism over the last 1.3 Myr in the central Azores re-cord a sudden change in the conditions of melt generation, whichmost probably reveals a major reconguration of regional deformationaccompanying the recent geodynamic reorganization of the EurasiaNubia plate boundary.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2014.09.009.

Acknowledgments

We acknowledge P. Layer and an anonymous reviewer for theirvaluable comments, which helped to signicantly improve the manu-script. Thanks to J. Barling, E. Barnes,M. Carpentier, Y. Feng, R. Friedman,B. Kieffer, M. Kuga, V. Lai, I. Nobre Silva, A. Shiel and J. Scoates for theirhelp and fruitful discussions at PCIGR. This study was partially support-ed by FrenchCNRS (Dlgation Ile de France Sud, DR4), and by TEAMINT(POCTI/CTE/48137/2002) and EVOLV (PTDC/CTE-GIN/71838/2006)programs funded by FCT (Portugal). This is LGMT contribution 120.

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Recent plate re-organization at the Azores Triple Junction: Evidence from combined geochemical and geochronological data o...1. Introduction2. Sampling location and sample description3. Methods and results3.1. K/Ar dating3.2. Geochemistry3.2.1. Major elements3.2.2. Trace elements3.2.3. Radiogenic isotopes

4. Discussion4.1. Source components and mantle heterogeneity4.2. Geodynamic controls on melt production4.3. Implications for plate re-organization near triple junctions (TJs)

5. ConclusionsAcknowledgmentsReferences