Diamonds and the Geology of Mantle Carbondeepcarbon.net/sites/dco.rpi.edu/files/diamonds.pdf ·...

Diamonds and the Geologyof Mantle Carbon

1Department of Terrestrial Magnetism,USA2Institut de Physique du Globe de Paris, France

3Bayerisches Geoinstitut, Germany 4Geosciences Montpellier, France5University of Padua, Italy 6University of Alberta, Canada

7Sobolev Institute of Geology and Mineralogy, Russia8University of Bristol, UK

Steven B. Shirey1 Pierre Cartigny2 Daniel J. Frost3 Shantanu Keshav4 Fabrizio Nestola5 Paolo Nimis5

D. Graham Pearson6 Nikolai V. Sobolev7 Michael J. Walter8

RiMG chapter of “Carbon in Earth”

Unique aspects of diamonds

• The flagship of carbon minerals in Earth

• Deepest samples (120 to 800 km)

• Old samples (100-3500 million years old)

• Most resistant to later geologic processes

• Unique potential to trace mantle processes

• Form under special conditions, (e.g. metasomatic)

Fronts for diamond research

• Element partitioning during diamond growth

• Inclusion paragenesis and diamond age

• Source of carbon and its geologic significance,

• Speciation of diamond-forming fluids and mantle fO2

• Deep diamonds and relation to geodynamic processes

• Experiments on diamond formation by fluids and melts

• Nanostructural characteristics of diamond

Components of Diamonds and the Mantle Geodynamics of Carbon (DMGC) project

CrystallographyP,T conditions for diamond crystallization and inclusion-host relationships

Experimental petrologymovement of C through the mantle with melt and fluid into diamond

Geochemistryage and geochemical constraints on C-bearing fluids in the mantle

Stable isotopesdetect primitive and recycled components and isotopic fractionation

Nanostructurediamond formation at the crystal chemical level

exposed Archaean crust

composite craton outline;Proterozoic amalgamation

well-defined cratons,part of composite cratons

craton correlations fromPangea

lithospheric

superdeep

alluvial

UHP crustal

impact

Diamond types

RiMG Chapter 12 of “Carbon in Earth”

Diamonds of the world

cratonupper m

antle

transition zo

ne

lower mantle

lithospheric

mantle

420 km

670 km

900 km

140 km GD

MgPv(FeAl) +

CaPv + CaFrt +

NAL + Sti

Maj + Cpx + CaTiPv

Maj + Sti ± CaTiPv ±

CaPv

GD

upper mantle

transition zone

lower mantle

Ol + Maj + Cpx

Wds/Rwd + Maj + CaPvMgPv + FePer + CaPv

LAB

subductionmid-ocean ridgevolcanic chain

Grt + Ol + Chr ± Cpx ± Opx ± Sf

Grt + Cpx + Sf

basalt/eclogite

lithospheric mantle

convecting mantle

kimberlite


Diamonds in relation to mantle keels and subduction

inclusions are uniform throughout multiple growth zones of fibrouscoats. One diamond (Sample 28) is zoned, with silicic inclusions in theinner part and carbonatitic in the outer (Fig. 1). All diamondsdemonstrate similar chemical trends plotted on Fig. 2 for individualinclusions in a representative subset of the studied diamonds, and alsoas compositions averaged by sample for all 20 stones. Analyses ofindividual fluid inclusions that plot away from the majority of otherinclusions in the same sample may represent microinclusions thattrapped as a solid phase or a mixed solid+fluid inclusion (cf. Izraeli etal., 2004) or inclusions with unusually large grains of one daughtermineral. In the studied diamonds, these are Fe oxides or hydroxides,halite, ilmenite, quartz, apatite, chromite, clinopyroxene, unidentifiedTi–Mg–Na minerals and Fe–Zn–Cu sulfides (Electronic SupplementaryTable 2). High–Si, Cl-bearing mica is also found as a discrete mineral inthe inclusions (Electronic Supplementary Table 2); its composition doesnot stand out but rather represents an end-member for the composi-tional spread of fluid inclusions.

Sample 79 differs from the rest of the samples due to its enrichmentin FeO (average 30.6 wt.% vs. 6–13 wt.% FeO). The FeO enrichment is

uniform in all fluid inclusions of this sample and truly is a characteristicof the fluid rather than controlled by a higher mode of trapped Fe-richminerals. Analysis of all correlations between elements suggeststhey group into 3 components, silicic (mainly SiO2+Al2O3), carbonatitic(mainly Na2O+MgO+FeO+CaO+P2O5+SrO) and saline (mainlyK+Cl). Silica strongly correlates with Al2O3 (Fig. 2B), but negativelycorrelates with CaO (Fig. 2A), P2O5, Na2O and MgO. Calcium oxidecorrelates with P2O5 (Fig. 2C) and SrO, whereas MgO correlates withNa2O (Fig. 2F) and FeO (Fig. 2E). All these elemental trends imply thatthe silicic component is strongly anticorrelated with the carbonatitic.The saline elements K and Cl correlatewith eachother (Fig. 2D) but varyindependently of carbonatitic and silicic components as indicatedby theabsence of correlations between K2O and CaO, or K2O and SiO2. BaOcorrelates with Cl and K2O. This may imply partitioning of BaO into thesaline component, or its incorporation inK and Cl-bearingmicawhich isthe main daughter mineral of the silicic component. Barian micakinoshitalite is found in the studied samples by XRD analysis as shownbelow.

A slope of the SiO2–Al2O3 (Fig. 2B) and FeO–MgO (Fig. 2E)correlations suggests their presence in high-Si mica (ElectronicSupplementary Table 2; Izraeli et al., 2004; Klein-BenDavid et al.,2006). Phlogopite or other sheet silicates cannot be major constitu-ents of the fluid as they do not plot on the observed compositionaltrends (Fig. 2B). The CaO–SiO2 trend can be ascribed to mixingbetween carbonates+apatite and high-Si mica (Fig. 2A). A strongapatite trend of correlated CaO and P2O5 is evident in 18 out of 20diamonds. Only samples 27 and 14 with the highest SiO2 content andthe lowest content of P2O5 do not show the apatite trend. It is absentin the silicic part of the zoned diamond, but is strong in the outercarbonatitic zone. Positive and variable intercepts of this trend withthe CaO axis (Fig. 2C) in different samples suggest that while apatite isthe main depository for P2O5, not all CaO is sequestered in apatite.

The composition of the carbonatitic component may be explainedby the presence of complex Ca–Mg–Fe carbonates (Fig. 2A, E) analyzedfrom fluid inclusions in diamonds (Klein-BenDavid et al., 2006;Logvinova et al., 2008) that also contain Na. It is suggested by abroad negative correlation between Na2O and K2O, the absence of theNa–Cl correlation and positive correlations of Na2OwithMgO, and CaOwith MgO and FeO.

Mica is the principal daughter mineral of the silicic component,and the most Si- and Al-rich inclusions consist entirely of the Si-richCl-bearing mica. The silicic component therefore incorporates someamounts of Fe, Mg, K, Cl, although the main budget for these oxidesare in other components and minerals. Correlations of BaO with K2Oand Cl and the detection of Ba mica kinoshitalite among daughterminerals of the studied fluid (Section 6) suggest that Ba may beresiding in the silicic component in the high-Si mica. Our study did notfind Ba carbonates reported in fibrous diamonds elsewhere (Walmseyand Lang, 1992; Logvinova et al., 2008) or correlations of Ba withother carbonate oxides.

The mineralogy of the saline component is the least understood.The approximate 3:1 ratio (Fig. 2D) of the K–Cl correlation indicatesthat the mineralogy of Cl-bearing phases in the diamond-formingfluid is complex and may not contain KCl alone. Besides Cl-bearingmica and possibly KCl, the saline component should include a K-bearing, Cl-free phase, as implied by a localization of most analysesabove the Cl-bearing mica-KCl tie-line on Fig. 2D. This phase couldbe KOH as reported in fluid inclusions from Siberian diamonds(Logvinova et al., 2008).

Overall, bulk compositions of fluid inclusions can be expressed as amixture of 3 groups ofminerals, 1) Cl-bearing Si-richmica, 2) Na–Mg–Fe–Ca carbonate material and apatite, and 3) K–Cl and K-bearingminerals or solutions. Other minerals associated with fluid inclusions,such as quartz, Fe sulfides/oxides/hydroxides, chromite, clinopyrox-ene, ilmenite, halite (Electronic Supplementary Table 2) and olivine,hydrous olivine, and garnet (Izraeli et al., 2004; Klein-BenDavid et al.,

Fig. 1. Scanning electron microphotographs of cathodoluminescence in two samples ofthe studied Congo diamonds. Dots indicate positions of analysed inclusions, scale bar is0.6 mm. Dashed line in sample 28 separates the inner fibrous coat with siliciccompositions of inclusions from the outer coat with carbonatitic inclusions.

128 M. Kopylova et al. / Earth and Planetary Science Letters 291 (2010) 126–137

diamond generation (Fig. 3b), are broken and resorbed

crystals (Fig. 3c) or display very complex growth patterns(Fig. 3d, e). For example, Fig. 3e shows the complex

growth structure of diamond J9. The core zone of an

octahedral/rounded shape likely formed in a regime ofoscillating growth and slight resorption, which is consistent

Fig. 2 Morphology of Collier 4diamonds. J1 chipped, resorbedand etched white octahedron, J2white octahedron/macle/intergrowth broken on one side,J9 pale brown resorbed, brokenoctahedron, J10 white resorbedoctahedron/dodecahedroid

Fig. 3 CL images revealing theinternal structure of Collier 4diamonds. Also shown on thediamond images are d13C values(% relative to PDB) asdetermined by SIMS analysis.a J2 diamond with weakoctahedral zonation and largechange in d13C from core to rim,b weakly zoned J3 stone withdeep system of internal cracks,c J6 broken and resorbedcrystal, d J8—broken, deformedand resorbed diamond, e J9diamond with ‘sheared’zonation described in the textand large change in d13C fromcore to rim

Contrib Mineral Petrol

123

RiMG Chapter 12 of “Carbon in Earth” (some original photos from Kopylova et al. (2010; Bulanova et al. 2010)

Internal textures of diamondscoated moncrystalline superdeep

2

FeMg

Ca

BotswanaPanda

Diavik

MgAPoorBYaku7an

MgARichBYaku7an Koffiefontein

Brazil

1

Si+Al

K+Na Ca+Mg+Fe Ca+Mg+FeK+Na

Si+Al(silicic)

(saline) (carbona77c)

RiMG Chapter 12 of “Carbon in Earth” (original from Kopylova et al. (2010)

Fluids in diamonds


Diamonds, fO2, fluid speciation, P, T

-20-25-30-35-40 -15 -10 -5 0

-20-25-30-35-40 -15 -10 -5 0

-15

Polycrystallinefrom kimberlites(n = 120)

Komatiiticfrom Dachine French Guyana(n=181)

-20-25-30-35-40 -15 -10 -5 0

-20-25-30-35-40 -15 -10 -5 0

Carbonados(n=54)

δ13C (‰)

F) Metamorphic diamonds(n = 120)

Recycled carbon

-20-25-30-35-40 -10 -5 0

Eclogiticfrom Jericho, Slave Craton, Canada (n=42)

-20-25-30-35-40 -15 -10 -5 0

lowest value

Jagersfontain(South Africa)

Sao Luiz(Brazil)

Kankan(Guinea)

highestvalue

Transition Zone (n = 31)

main mantle-range(fibrous diamonds, mid-ocean ridge basalts

carbonatites and kimberlites)

Peridotitic diamonds(n = 1357)

Eclogitic diamonds(n = 997)

Fibrous diamondsand diamond coats(n = 127)

Lower mantle diamonds(n = 78)

δ13C (‰)


Carbon isotopic compostions of diamonds

subcratonic lithospheric mantle. Diamonds with majoritic garnetinclusions (N6.12 cations Si per formula unit based on 24 [O]) aregrouped as deep peridotitic and eclogitic and account for 3.6% of thestudied population. A large portion of the deep peridotitic diamondsare likely derived from very deep lithospheric roots (Pokhilenko et al.,2001), although in some cases an origin from the peridotitic portion ofsubducting slabs has been proposed (Banas et al., 2007). Deep eclogiticdiamonds probably form in the crustal sections of slabs subductingthrough the asthenosphere and transition zone. Lower mantlediamonds are characterized by ferropericlase-bearing parageneses(e.g., Stachel et al., 2005). Because of strong scientific interest,sublithospheric diamonds from only a few localities have beenextensively studied and are clearly overrepresented in the data base.Adjusting for this bias, it is likely that lithospheric diamonds actuallyaccount for about 99% of worldwide production.

For southern Africa, Hawthorne et al. (1978) suggested a 3:1 ratioof peridotitic:eclogitic diamonds. Considering lithospheric diamondsonly (Fig. 6), 65% of diamonds in our worldwide database are derivedfrom peridotitic sources, 33% belong to the eclogitic and 2% to thewebsteritic suite. The lower peridotitic:eclogitic ratio (2:1) observedhere may, however, not be fully representative of run-of-mineproductions. In addition, the general restriction of inclusion studiesto diamonds from smaller sieve classes may introduce significant bias.In descriptive studies of run-of-mine diamonds from the Finsch andPremier mines (South Africa), variations of the principal inclusiongroups were determined as a function of diamond size (Table 3). ForFinsch, there is a systematic increase in the relative proportion ofdiamonds with visible eclogitic inclusions over five sieve sizes(Table 3). For Premier, a similar situation exists with the exception

of the largest diamonds (Table 3) where the trend of an increasingproportion of diamonds with eclogitic inclusions is reversed. Not allmines follow a general trend of an increasing proportion of eclogiticdiamonds towards larger stone sizes: at Venetia, for example,observational evidence indicates that diamonds with peridotiticinclusions remain equally common into the larger diamond sizes.Observed changes in the peridotitic:eclogitic ratio with diamond sizeindicate different size distributions for the various diamond suiteswithin individual mines.

The majority of peridotitic diamonds lack garnet and/or clinopyr-oxene inclusions and hence cannot be assigned to a specificparagenesis. 76% of the assigned diamonds belong to the harzburgiticparagenesis, 23% are lherzolitic and 0.9% wehrlitic. However, therelative proportion of peridotitic parageneses may be better assessedbased on garnet inclusion abundances (i.e., using only the sameinclusion phase for all three parageneses) resulting in 87% harzburgi-tic, 12% lherzolitic and 1% wehrlitic samples. To adjust for anoverrepresentation of De Beers Pool (Kimberley mines) in theperidotitic garnet data base, inclusions from this locality should beassigned only half the weight relative to other mines for thiscalculation resulting in 86% harzburgitic and 13% lherzolitic garnets(1% wehrlitic remains unchanged) which is essentially unchangedfrom the original study of Gurney (1984).

2.2. Inclusions

2.2.1. GarnetPerceived inclusion colour depends on sample thickness. For

peridotitic garnets, a colour range from pale pinkish to purple iscommonly observed, while green garnets are very rare. Eclogiticgarnets are pale orange to orange. The chemical composition ofperidotitic, eclogitic and websteritic garnet inclusions is summarizedin Table 4. Rare wehrlitic inclusions, generally very high in Ca (seeFig. 4), represent chemically exotic diamond sources likely related tomelt infiltration and are not further considered here.

Peridotitic garnet inclusions are Cr-pyropes with high Mg#:excluding 0.8% of the sample (four lherzolitic and three harzburgiticgarnets),Mg#Ca-corr areN83. Highly depleted low-Cagarnets (b1.8wt.%CaO; Grütter et al., 1999) have the highest Mg#Ca-corr (average 90.0),differences between “normal” harzburgitic and lherzolitic garnets aremore subtle (average of 88.0 for the former and 86.6 for the latter). Thistrend of decreasing Mg# is exaggerated by a strong dominance ofgenerally Mg-rich inclusions from De Beers Pool in the low-Ca garnetgroup. Harzburgitic and lherzolitic garnets show a large range in TiO2

content, up to 1.07 and 0.74 wt.%, respectively (Table 4). Fig. 7 showsthat both types of garnets split into low- (!0.04 wt.% TiO2) and high-Ti(N0.04 wt.% TiO2) groups. Harzburgitic garnets mainly (71%) fall into

Fig. 5. The relative abundance of diamond source regions in the Earth's mantle based on3145 inclusion bearing diamonds. “Deep” stands for diamonds containing garnet with adetectable majorite component. The deep websteritic paragenesis (4 diamonds, 0.1%) isomitted from the diagram.

Fig. 6. The relative abundance of various lithospheric diamond sources based on 2844inclusion-bearing diamonds. “Unspecified” refers to peridotitic diamonds that containneither garnet nor clinopyroxene. The wehrlitic paragenesis (7 diamonds, 0.2%) isomitted from the diagram.

Table 3Relative abundance of the principal inclusion groups (discernable during observationalstudies) as a function of diamond size

Sieve Class "6+5 "7+6 "9+7 "11+9 "13+12 "15+13

Aperture [mm] 1.83 2.16 2.46 2.88 4.09 4.52

FinschDiamonds studied 5929 841 432 464 394Peridotitic [%] 83.7 74.0 63.3 74.1 45.7Eclogitic [%] 2.8 7.7 6.9 15.9 24.1Sulphides [%] 13.5 18.2 29.9 10.0 30.2100 Ecl/(Ecl+Per) 3.2 9.5 9.8 17.7 34.5

PremierDiamonds studied 1549 319 179 200 3158Peridotitic [%] 38.8 28.9 21.2 16.5 31.5Eclogitic [%] 27.6 62.1 63.2 72.0 25.6Sulphides [%] 33.6 9.1 15.6 11.5 42.9100 Ecl/(Ecl+Per) 41.5 68.3 74.9 81.4 44.8

The diameter of the circular aperture of the lower screen is given for each sieve class.

9T. Stachel, J.W. Harris / Ore Geology Reviews 34 (2008) 5–32

Diamonds as characterized by their silicate inclusions

Stachel and Harris (2008) Ore Geology Reviews 34, 5-32.

subcratonic lithospheric mantle. Diamonds with majoritic garnetinclusions (N6.12 cations Si per formula unit based on 24 [O]) aregrouped as deep peridotitic and eclogitic and account for 3.6% of thestudied population. A large portion of the deep peridotitic diamondsare likely derived from very deep lithospheric roots (Pokhilenko et al.,2001), although in some cases an origin from the peridotitic portion ofsubducting slabs has been proposed (Banas et al., 2007). Deep eclogiticdiamonds probably form in the crustal sections of slabs subductingthrough the asthenosphere and transition zone. Lower mantlediamonds are characterized by ferropericlase-bearing parageneses(e.g., Stachel et al., 2005). Because of strong scientific interest,sublithospheric diamonds from only a few localities have beenextensively studied and are clearly overrepresented in the data base.Adjusting for this bias, it is likely that lithospheric diamonds actuallyaccount for about 99% of worldwide production.

For southern Africa, Hawthorne et al. (1978) suggested a 3:1 ratioof peridotitic:eclogitic diamonds. Considering lithospheric diamondsonly (Fig. 6), 65% of diamonds in our worldwide database are derivedfrom peridotitic sources, 33% belong to the eclogitic and 2% to thewebsteritic suite. The lower peridotitic:eclogitic ratio (2:1) observedhere may, however, not be fully representative of run-of-mineproductions. In addition, the general restriction of inclusion studiesto diamonds from smaller sieve classes may introduce significant bias.In descriptive studies of run-of-mine diamonds from the Finsch andPremier mines (South Africa), variations of the principal inclusiongroups were determined as a function of diamond size (Table 3). ForFinsch, there is a systematic increase in the relative proportion ofdiamonds with visible eclogitic inclusions over five sieve sizes(Table 3). For Premier, a similar situation exists with the exception

of the largest diamonds (Table 3) where the trend of an increasingproportion of diamonds with eclogitic inclusions is reversed. Not allmines follow a general trend of an increasing proportion of eclogiticdiamonds towards larger stone sizes: at Venetia, for example,observational evidence indicates that diamonds with peridotiticinclusions remain equally common into the larger diamond sizes.Observed changes in the peridotitic:eclogitic ratio with diamond sizeindicate different size distributions for the various diamond suiteswithin individual mines.

The majority of peridotitic diamonds lack garnet and/or clinopyr-oxene inclusions and hence cannot be assigned to a specificparagenesis. 76% of the assigned diamonds belong to the harzburgiticparagenesis, 23% are lherzolitic and 0.9% wehrlitic. However, therelative proportion of peridotitic parageneses may be better assessedbased on garnet inclusion abundances (i.e., using only the sameinclusion phase for all three parageneses) resulting in 87% harzburgi-tic, 12% lherzolitic and 1% wehrlitic samples. To adjust for anoverrepresentation of De Beers Pool (Kimberley mines) in theperidotitic garnet data base, inclusions from this locality should beassigned only half the weight relative to other mines for thiscalculation resulting in 86% harzburgitic and 13% lherzolitic garnets(1% wehrlitic remains unchanged) which is essentially unchangedfrom the original study of Gurney (1984).

2.2. Inclusions

2.2.1. GarnetPerceived inclusion colour depends on sample thickness. For

peridotitic garnets, a colour range from pale pinkish to purple iscommonly observed, while green garnets are very rare. Eclogiticgarnets are pale orange to orange. The chemical composition ofperidotitic, eclogitic and websteritic garnet inclusions is summarizedin Table 4. Rare wehrlitic inclusions, generally very high in Ca (seeFig. 4), represent chemically exotic diamond sources likely related tomelt infiltration and are not further considered here.

Peridotitic garnet inclusions are Cr-pyropes with high Mg#:excluding 0.8% of the sample (four lherzolitic and three harzburgiticgarnets),Mg#Ca-corr areN83. Highly depleted low-Cagarnets (b1.8wt.%CaO; Grütter et al., 1999) have the highest Mg#Ca-corr (average 90.0),differences between “normal” harzburgitic and lherzolitic garnets aremore subtle (average of 88.0 for the former and 86.6 for the latter). Thistrend of decreasing Mg# is exaggerated by a strong dominance ofgenerally Mg-rich inclusions from De Beers Pool in the low-Ca garnetgroup. Harzburgitic and lherzolitic garnets show a large range in TiO2

content, up to 1.07 and 0.74 wt.%, respectively (Table 4). Fig. 7 showsthat both types of garnets split into low- (!0.04 wt.% TiO2) and high-Ti(N0.04 wt.% TiO2) groups. Harzburgitic garnets mainly (71%) fall into

Fig. 5. The relative abundance of diamond source regions in the Earth's mantle based on3145 inclusion bearing diamonds. “Deep” stands for diamonds containing garnet with adetectable majorite component. The deep websteritic paragenesis (4 diamonds, 0.1%) isomitted from the diagram.

Fig. 6. The relative abundance of various lithospheric diamond sources based on 2844inclusion-bearing diamonds. “Unspecified” refers to peridotitic diamonds that containneither garnet nor clinopyroxene. The wehrlitic paragenesis (7 diamonds, 0.2%) isomitted from the diagram.

Table 3Relative abundance of the principal inclusion groups (discernable during observationalstudies) as a function of diamond size

Sieve Class "6+5 "7+6 "9+7 "11+9 "13+12 "15+13

Aperture [mm] 1.83 2.16 2.46 2.88 4.09 4.52

FinschDiamonds studied 5929 841 432 464 394Peridotitic [%] 83.7 74.0 63.3 74.1 45.7Eclogitic [%] 2.8 7.7 6.9 15.9 24.1Sulphides [%] 13.5 18.2 29.9 10.0 30.2100 Ecl/(Ecl+Per) 3.2 9.5 9.8 17.7 34.5

PremierDiamonds studied 1549 319 179 200 3158Peridotitic [%] 38.8 28.9 21.2 16.5 31.5Eclogitic [%] 27.6 62.1 63.2 72.0 25.6Sulphides [%] 33.6 9.1 15.6 11.5 42.9100 Ecl/(Ecl+Per) 41.5 68.3 74.9 81.4 44.8

The diameter of the circular aperture of the lower screen is given for each sieve class.

9T. Stachel, J.W. Harris / Ore Geology Reviews 34 (2008) 5–32

16

18

20

22

24

26

28

30

32

34

3614 16 18 20 22 24 26 28 30 32 34 36

150 km depth =

Longitude East

Latit

ude

Sout

h

-1 -0.5 0 0.5 1

P-wave velocity anomaly (%)

0.9

2.5P

2.5 BORDER OF 2.5 Ga PERIDOTITES

JW P

F

LE

O

V

RD

KO JAK

LZ

2.5

Shirey et al., Figure 1

P-TYPE SILICATE INCLUSIONS

E-TYPE SILICATE INCLUSIONS

M


Diamonds and mantle geology


Geothermobarometry on mineral inclusions in diamonds

Deep Mantle Cycling of Oceanic Crust:Evidence from Diamonds and TheirMineral InclusionsM. J. Walter,1* S.C. Kohn,1 D. Araujo,2 G. P. Bulanova,1 C. B. Smith,1 E. Gaillou,3

J. Wang,3 A. Steele,4 S. B. Shirey3

A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithosphericslabs into the mantle. Seismological studies extend this process to the lower mantle, andgeochemical observations indicate return of oceanic crust to the upper mantle in plumes. Therehas been no direct petrologic evidence, however, of the return of subducted oceanic crustalcomponents from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite,Brazil, which host inclusions with compositions comprising the entire phase assemblage expectedto crystallize from basalt under lower-mantle conditions. The inclusion mineralogies requireexhumation from the lower to upper mantle. Because the diamond hosts have carbon isotopesignatures consistent with surface-derived carbon, we conclude that the deep carbon cycleextends into the lower mantle.

Diamonds and the mineral inclusions thatthey trap during growth provide samplesof materials from deep within Earth. On

the basis of inclusionmineralogy, most diamondssampled at the surface originated in continentallithospheric mantle at depths of <200 km (1).Several localities, however, yield rare “superdeep”diamonds with inclusion compositions that re-quire a sublithospheric origin in the deep uppermantle and even the lower mantle (1, 2). Inclu-sions of majorite garnet that formed in the deepupper mantle (~200 to 500 km) commonly havecompositions linking them to basaltic oceaniccrust (1–8), and aluminous inclusions have beenidentified with compositions indicative of sili-ceous sediments (3). The diamonds that host theseinclusions have carbon isotopic compositionsthat are atypical of normal mantle (d13C ! –5‰),instead displaying a large isotopic range (~ "1 to–24‰) with a clear tendency toward isotopically“light” (< –10‰) compositions (1–3, 9). Althoughthere is debate regarding the origin of light car-bon in diamonds (10), a leading hypothesis isthe subduction of the isotopically light organiccarbon fraction of altered oceanic crust.

The rarest diamonds are those containing in-clusions with compositions indicating an originin the lower mantle (>660 km). Inclusions in-terpreted as representing the lower-mantle phasesMg-perovskite and Ca-perovskite have major el-ement compositions that indicate an origin inmantle peridotite (2, 11–13). No lower-mantle

inclusions have previously been identified withmajor element compositions consistent with anorigin in subducted basalt. Furthermore, the car-bon isotopic compositions of diamonds withlower-mantle inclusions are typically all mantle-like (~ –4 to –6‰) (2), which suggests thatsurface-derived carbon may not survive intothe lower mantle. Oceanic lithosphere clearly

enters the lower mantle (14, 15), so the rarityof lower-mantle diamonds with inclusions ofhigh-pressure phases that would occur in sub-ducted basalt suggests that once oceanic crustenters the lower mantle, it usually remains there,possibly as a result of intrinsic high density andnegative buoyancy (16–18).

Here, we describe a suite of mineral inclu-sions in low-nitrogen (type IIa) diamonds fromthe Juina-5 kimberlite pipe in the Juina kimberlitefield (92 to 95 million years old) (19) located inthe Proterozoic Rio Negro–Juruena Mobile Beltsouthwest of the Amazon Craton, Brazil (20). In-clusions in sublithospheric diamonds common-ly show mineralogical evidence of exsolutionfrom originally homogeneous silicate phases intocomposite assemblages, and these are interpretedto have formed during ascent in the mantle un-related to kimberlite eruption (2–5, 8, 12, 21, 22).Inclusion unmixing provides compelling evidencethat some superdeep diamonds were transportedupward by hundreds of kilometers in the uppermantle, presumably by upwelling of solid mate-rial (3, 4, 13). For example, the bulk composi-tions of composite garnet plus clinopyroxeneinclusions in diamonds from the Juina region in-dicate a deep upper-mantle origin asmajorite, withinclusion unmixing to garnet plus clinopyroxeneoccurring during transport to shallower levelsbeneath the lithosphere (2–4). Each of the in-clusions presented here is composed of a multi-phase mineral assemblage (Fig. 1). We interpret

RESEARCHARTICLE

1School of Earth Sciences, University of Bristol, Bristol BS8 1RJ,UK. 2Instituto de Geociências, Universidade de Brasília, CEP70910-900 Brasília, DF, Brazil. 3Department of Terrestrial Mag-netism, Carnegie Institution, Washington, DC 20015, USA.4Geophysical Laboratory, Carnegie Institution, Washington,DC 20015, USA.

*To whom correspondence should be addressed. E-mail:[email protected]

Fig. 1. Backscattered electron micrographs showing composite inclusions in diamonds from Juina-5. (A)An inclusion in diamond Ju5-20 composed of a mixture of spinel (Mg,Fe)Al2O4 (Sp) and nephelineNaAlSiO4 (Ne) (fig. S1 and table S5), together with a small sulfide (Sf) in one corner that we interpret as anoriginally distinct phase from the composite silicate; sulfide can participate in diamond crystallizationreactions as a melt phase that is immiscible in silicate (33). (B) An inclusion in diamond Ju5-67 that iscomposed of phases with the compositions of spinel and a nepheline-kalsilite (Ka) phase, (Na,K)AlSiO4(table S5). (C) An inclusion in diamond Ju5-89 containing spinel and a mixture of micrometer-sizedNa-rich (Na) and K-rich (K) silicate regions, with a bulk composition similar to Ju5-67 (fig. S2 and tableS5). (D) An inclusion in diamond Ju5-47 that consists of orthopyroxene (Opx), ulvospinel (Ulv), and olivine(Ol) (fig. S3 and table S5). (E) An inclusion in diamond Ju5-43 that consists of a complex mixture oforthopyroxene and a Ti-, Al-, and Fe-rich phase similar to tetragonal almandine pyrope phase (TAPP)(table S5). (F) An inclusion in diamond Ju5-104 composed of CaSiO3 plus micrometer-sized Ti-rich phases(e.g., CaTiO3) and a small sulfide (table S5).

7 OCTOBER 2011 VOL 334 SCIENCE www.sciencemag.org54

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diamonds (2, 11). The composite inclusion indiamond Ju5-104 has the bulk composition andstoichiometry of a CaSiO3 phase, but with a mod-erate Ti component [~3 weight % (wt %) TiO2]not previously observed in other CaSiO3 inclu-sions in sublithospheric diamonds. The inclusioncomposition is a close match to Ca-perovskitethat coexists with Mg-perovskite in experimentson basaltic compositions (Fig. 2C, Table 1, andtable S4).

Experimentally estimated changes in miner-alogy with depth for typical mid-ocean ridgebasalts (17, 18) indicate that, as a group, the in-clusions constitute a phase assemblage that co-exists in basaltic compositions at depths betweenabout 700 and 1400 km (Fig. 3A); an SiO2 in-clusion has been identified in our Juina-5 collec-tion as well, which likely originated as stishovite.We suggest that the inclusions must have orig-inated when diamond-forming fluids incorpo-rated basaltic components from oceanic lithospheresubducted into the lower mantle. To trap thesespecific inclusion compositions as homogeneousphases, the diamonds must have grown in theupper part of the lowermantle, and cannot simplybe diamonds derived from shallower depths inthe upper mantle but subducted into the lowermantle. For example, there are no known phasesstable in the upper mantle with the bulk com-positions of CF and NAL phases, and majoritegarnets included in diamond in the deep uppermantle have far more calcium (e.g., 5 to 15 wt %CaO) (3, 6) and less titanium than observed inMg-perovskite synthesized experimentally in ba-saltic compositions.

Diamond isotopic signatures and the originof carbon. If the above hypothesis is correct, thenthe carbon from which the diamonds formed

may have been deposited originally within oce-anic crust at the seafloor. We measured the car-bon isotopic composition (d13C values) of theJuina-5 diamonds and found a range from about–1 to –24‰, with four of the six diamonds hav-ing values less than –15‰ (Fig. 4 and Table 1).These “light” isotopic values possibly indicate arecycled organic source of carbon (10). In con-trast, all previously analyzed diamonds hostingultramafic inclusions of lower-mantle origin haveheavier, typical “mantle” carbon isotopic compo-sitions around –5‰ (2).

The origin of light carbon isotopic values(< ~ –10‰) in mantle-derived samples is a mat-ter of ongoing debate, with plausible explana-tions including intramantle isotopic fractionation,a primordial carbon reservoir with a light compo-nent, and subducted organic carbon (10, 24). Ray-leigh fractionation in an open system involvingphase separation can generate considerable iso-topic fractionation and may account for much ofthe variation in d13C in the range of –8 to –2‰seen in lithospheric diamonds (10). However, ithas yet to be demonstrated that this is a viableprocess for producing the isotopically light sig-natures (~ –10 to –25‰) commonly associatedwith sublithospheric diamonds of basaltic affin-ity, and an explanation would be needed for thecorrespondence of isotopically light carbon sig-natures with specific inclusion mineralogies. Aprimordial light carbon reservoir also appears un-likely, as the light component must have survivedvigorousmantle mixing in an early magma oceanand billions of years of solid-state mantle convec-tion to appear in the composition of these dia-monds (24). Juina superdeep diamonds are likelyonly about 100 million years old, on the basisof a dated sublithospheric inclusion from the

Collier-4 kimberlite 30 km to the north of Juina-5(3), and so these young diamonds are very un-likely to have formed from an ancient, isotopic-ally light primordial carbon component.

In contrast, a burgeoning body of evidencesupports a subducted carbon source for manysuperdeep diamonds. The Juina-5 composite in-clusions and many other inclusions in sublitho-spheric diamonds require an origin involvingoceanic crust and sediments, and these com-monly have light carbon isotopic compositions(2, 3, 6, 7, 9). Recent measurements of the carbonisotopic compositions in altered oceanic crustas deep as 2 km beneath the seafloor indicatemixing between an organic component (d13C !–27‰) and a carbonate component (d13C !0‰) (25). As a group, sublithospheric diamondswith inclusions showing affinity with subductedoceanic crustal materials have carbon isotopiccompositions that effectively span the entire iso-topic rangemeasured in altered oceanic crust. Ourresults suggest that subducted organic carbon canretain its isotopic signature even into the lowermantle. Experimental data indicate that subductedcarbon, regardless of its original form in carbo-nates or organic compounds, can become fixed aseither elemental carbon (graphite or diamond) orcarbonate at high pressures in oceanic crust, de-pending on the redox state (26). Because the in-clusions require that the diamonds grew in thelower mantle, we suggest that carbon was trans-ported as carbonate, some of which would havebeen isotopically light, having originated as or-ganic carbon (Fig. 4).

Implications for a deep carbon cycle. Thediamonds and their inclusions may have grown

Fig. 3. (A) Estimated modal mineralogy in subducted basaltic oceanic crust as a function of depth in themantle (17, 18). MgPv, Mg-perovskite; CaPv, Ca-perovskite; CF, CF phase; NAL, NAL phase; St, stishovite;Gt, garnet; Cpx, clinopyroxene. The inclusion mineralogy in diamonds from Juina-5, including MgPv,CaPv, CF phase, NAL phase, and stishovite, is stable at depths of ~700 to 1400 km in the lower mantle. (B)A schematic model for diamond formation and ascent beneath the Brazilian lithosphere. We suggest thatthe diamonds and inclusions initially formed from subducted oceanic crustal components in the upperpart of the lower mantle and were transported in an upwelling plume to the upper mantle, where theyunmixed into composite inclusions according to lower-pressure phase relations.

Fig. 4. Carbon isotopic compositions of diamonds inthis study compared to some possible carbon sources.White rectangles represent the range observed ineach diamond on the basis of multiple spot measure-ments in different growth zones revealed by cathodo-luminescence images (20). The isotopic compositionsof several possible carbon sources are drawn sche-matically on the basis of ranges given in (10). Organiccarbon denotes either biogenic or abiogenic noncar-bonate carbon originating in surface or near-surfaceenvironments; mantle carbon denotes a carbon com-ponent that is typical of primitive ultramafic mantlerocks and peridotitic lithospheric diamonds; car-bonate denotes surface-derived carbon depositedas carbonate from seawater.


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Basaltic minerals at high pressure in the deep mantle

Redrafted from Walter, Kohn, Araujo, Bulanova, Smith, Gaillou, Wang, Steele, and Shirey (2011). Science 334, 54–57.

diamonds (2, 11). The composite inclusion indiamond Ju5-104 has the bulk composition andstoichiometry of a CaSiO3 phase, but with a mod-erate Ti component [~3 weight % (wt %) TiO2]not previously observed in other CaSiO3 inclu-sions in sublithospheric diamonds. The inclusioncomposition is a close match to Ca-perovskitethat coexists with Mg-perovskite in experimentson basaltic compositions (Fig. 2C, Table 1, andtable S4).

Experimentally estimated changes in miner-alogy with depth for typical mid-ocean ridgebasalts (17, 18) indicate that, as a group, the in-clusions constitute a phase assemblage that co-exists in basaltic compositions at depths betweenabout 700 and 1400 km (Fig. 3A); an SiO2 in-clusion has been identified in our Juina-5 collec-tion as well, which likely originated as stishovite.We suggest that the inclusions must have orig-inated when diamond-forming fluids incorpo-rated basaltic components from oceanic lithospheresubducted into the lower mantle. To trap thesespecific inclusion compositions as homogeneousphases, the diamonds must have grown in theupper part of the lowermantle, and cannot simplybe diamonds derived from shallower depths inthe upper mantle but subducted into the lowermantle. For example, there are no known phasesstable in the upper mantle with the bulk com-positions of CF and NAL phases, and majoritegarnets included in diamond in the deep uppermantle have far more calcium (e.g., 5 to 15 wt %CaO) (3, 6) and less titanium than observed inMg-perovskite synthesized experimentally in ba-saltic compositions.

Diamond isotopic signatures and the originof carbon. If the above hypothesis is correct, thenthe carbon from which the diamonds formed

may have been deposited originally within oce-anic crust at the seafloor. We measured the car-bon isotopic composition (d13C values) of theJuina-5 diamonds and found a range from about–1 to –24‰, with four of the six diamonds hav-ing values less than –15‰ (Fig. 4 and Table 1).These “light” isotopic values possibly indicate arecycled organic source of carbon (10). In con-trast, all previously analyzed diamonds hostingultramafic inclusions of lower-mantle origin haveheavier, typical “mantle” carbon isotopic compo-sitions around –5‰ (2).

The origin of light carbon isotopic values(< ~ –10‰) in mantle-derived samples is a mat-ter of ongoing debate, with plausible explana-tions including intramantle isotopic fractionation,a primordial carbon reservoir with a light compo-nent, and subducted organic carbon (10, 24). Ray-leigh fractionation in an open system involvingphase separation can generate considerable iso-topic fractionation and may account for much ofthe variation in d13C in the range of –8 to –2‰seen in lithospheric diamonds (10). However, ithas yet to be demonstrated that this is a viableprocess for producing the isotopically light sig-natures (~ –10 to –25‰) commonly associatedwith sublithospheric diamonds of basaltic affin-ity, and an explanation would be needed for thecorrespondence of isotopically light carbon sig-natures with specific inclusion mineralogies. Aprimordial light carbon reservoir also appears un-likely, as the light component must have survivedvigorousmantle mixing in an early magma oceanand billions of years of solid-state mantle convec-tion to appear in the composition of these dia-monds (24). Juina superdeep diamonds are likelyonly about 100 million years old, on the basisof a dated sublithospheric inclusion from the

Collier-4 kimberlite 30 km to the north of Juina-5(3), and so these young diamonds are very un-likely to have formed from an ancient, isotopic-ally light primordial carbon component.

In contrast, a burgeoning body of evidencesupports a subducted carbon source for manysuperdeep diamonds. The Juina-5 composite in-clusions and many other inclusions in sublitho-spheric diamonds require an origin involvingoceanic crust and sediments, and these com-monly have light carbon isotopic compositions(2, 3, 6, 7, 9). Recent measurements of the carbonisotopic compositions in altered oceanic crustas deep as 2 km beneath the seafloor indicatemixing between an organic component (d13C !–27‰) and a carbonate component (d13C !0‰) (25). As a group, sublithospheric diamondswith inclusions showing affinity with subductedoceanic crustal materials have carbon isotopiccompositions that effectively span the entire iso-topic rangemeasured in altered oceanic crust. Ourresults suggest that subducted organic carbon canretain its isotopic signature even into the lowermantle. Experimental data indicate that subductedcarbon, regardless of its original form in carbo-nates or organic compounds, can become fixed aseither elemental carbon (graphite or diamond) orcarbonate at high pressures in oceanic crust, de-pending on the redox state (26). Because the in-clusions require that the diamonds grew in thelower mantle, we suggest that carbon was trans-ported as carbonate, some of which would havebeen isotopically light, having originated as or-ganic carbon (Fig. 4).

Implications for a deep carbon cycle. Thediamonds and their inclusions may have grown

Fig. 3. (A) Estimated modal mineralogy in subducted basaltic oceanic crust as a function of depth in themantle (17, 18). MgPv, Mg-perovskite; CaPv, Ca-perovskite; CF, CF phase; NAL, NAL phase; St, stishovite;Gt, garnet; Cpx, clinopyroxene. The inclusion mineralogy in diamonds from Juina-5, including MgPv,CaPv, CF phase, NAL phase, and stishovite, is stable at depths of ~700 to 1400 km in the lower mantle. (B)A schematic model for diamond formation and ascent beneath the Brazilian lithosphere. We suggest thatthe diamonds and inclusions initially formed from subducted oceanic crustal components in the upperpart of the lower mantle and were transported in an upwelling plume to the upper mantle, where theyunmixed into composite inclusions according to lower-pressure phase relations.

Fig. 4. Carbon isotopic compositions of diamonds inthis study compared to some possible carbon sources.White rectangles represent the range observed ineach diamond on the basis of multiple spot measure-ments in different growth zones revealed by cathodo-luminescence images (20). The isotopic compositionsof several possible carbon sources are drawn sche-matically on the basis of ranges given in (10). Organiccarbon denotes either biogenic or abiogenic noncar-bonate carbon originating in surface or near-surfaceenvironments; mantle carbon denotes a carbon com-ponent that is typical of primitive ultramafic mantlerocks and peridotitic lithospheric diamonds; car-bonate denotes surface-derived carbon depositedas carbonate from seawater.


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Walter, Kohn, Araujo, Bulanova, Smith, Gaillou, Wang, Steele, and Shirey (2011). Science 334, 54–57.

Diamond capture and inclusion unmixing

450

250

650

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diamond precipitationvia redox freezing

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melting of carbonated crust (hydrous?)

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redox crystallization from primordialand recycled !uids and melts rich in

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Diamond formation, deep to shallow

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Transcript of Diamonds and the Geology of Mantle Carbondeepcarbon.net/sites/dco.rpi.edu/files/diamonds.pdf ·...