Tracing carbonate-silicate interaction during subduction using magnesium and oxygen isotopes

Abstract

Subduction of carbonates and carbonated eclogites into the mantle plays an important role in transporting carbon into deep Earth. However, to what degree isotopic exchanges occur between carbonate and silicate during subduction remains unclear. Here we report Mg and O isotopic compositions for ultrahigh pressure metamorphic marbles and enclosed carbonated eclogites from China. These marbles include both calcite- and dolomite-rich examples and display similar O but distinct Mg isotopic signatures to their protoliths. Their δ²⁶ Mg values vary from -2.508 to -0.531[per thousand], and negatively correlate with MgO/CaO ratios, unforeseen in sedimentary carbonates. Carbonated eclogites have extremely heavy δ¹⁸ O (up to +21.1[per thousand]) and light δ²⁶ Mg values (down to -1.928[per thousand] in garnet and -0.980[per thousand] in pyroxene) compared with their protoliths. These unique Mg-O isotopic characteristics reflect differential isotopic exchange between eclogites and carbonates during subduction, making coupled Mg and O isotopic studies potential tools for tracing deep carbon recycling.

ARTICLE

Received 22 Jul 2014 | Accepted 19 Sep 2014 | Published 4 Nov 2014

Subduction of carbonates and carbonated eclogites into the mantle plays an important role in transporting carbon into deep Earth. However, to what degree isotopic exchanges occur between carbonate and silicate during subduction remains unclear. Here we report Mg and O isotopic compositions for ultrahigh pressure metamorphic marbles and enclosed carbonated eclogites from China. These marbles include both calcite- and dolomite-rich examples and display similar O but distinct Mg isotopic signatures to their protoliths. Their d26Mg values

vary from 2.508 to 0.531%, and negatively correlate with MgO/CaO ratios, unforeseen

in sedimentary carbonates. Carbonated eclogites have extremely heavy d18O (up to 21.1%)

and light d26Mg values (down to 1.928% in garnet and 0.980% in pyroxene) compared

with their protoliths. These unique MgO isotopic characteristics reect differential isotopic exchange between eclogites and carbonates during subduction, making coupled Mg and O isotopic studies potential tools for tracing deep carbon recycling.

DOI: 10.1038/ncomms6328

Tracing carbonatesilicate interaction during subduction using magnesium and oxygen isotopes

Shui-Jiong Wang1,2, Fang-Zhen Teng2 & Shu-Guang Li1,3

1 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China. 2 Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. 3 CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China. Correspondence and requests for materials should be addressed to S.-J.W. (email: mailto:[email protected]

Web End [email protected] ) or to F.-Z.T. (emai: mailto:[email protected]

Web End [email protected] ).

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Deep carbon recycling plays a key role in the global carbon cycle by modulating the CO2 budget of the Earths atmosphere over geologic timescales1,2. Among different

subducted lithologies, carbonates and carbonated eclogites are of particular importance in transporting carbon from Earths surface into its interior36. As a subducting slab descends, some fractions of the carbon carried by carbonate minerals are liberated back at the surface via volcanism7; however, most is delivered to great mantle depths together with silicates8,9. Carbonatesilicate interaction along the subduction pressuretemperature (PT) paths is particularly important because it not only inuences the long-term carbon ux in subduction zones10 but also controls the isotopic systematics of coexisting carbonate and silicate that are ultimately recycled into the mantle. The latter in turn contributes fundamentally to the chemical and physical properties of the mantle1113, for example, initiation of melting by lowering the mantle solidus, generation of alkalic magma in oceanic islands and carbonate metasomatism in cratonic mantle.

Magnesium (Mg) and oxygen (O) are two essential elements closely associated with carbon in carbonate minerals. Their isotopic compositions differ greatly from normal mantle rocks, that is, unaltered oceanic basalts and lherzolites have d26Mg of

0.250.07% (ref. 14) and d18O of 5.50.2% (ref. 15),

whereas sedimentary carbonates have extremely light d26Mg (down to 6%)1618 and heavy d18O (up to 28%)19,20. Thus,

MgO isotopic systematics can provide insight into carbonate silicate chemical interactions during the subduction of carbonates and carbonated eclogites.

Here we present Mg, O, C and Nd isotopic data for a suite of ultrahigh pressure metamorphic (UHPM) marbles and enclosed carbonated eclogites from the Sulu orogen, east China. For comparison, we also report Mg isotopic data for Sinian sedimentary carbonates from the South China Block, and eclogites hosted in UHPM gneisses (termed as normal eclogites hereafter) from the Sulu orogen. Our results show that the carbonated eclogites have anomalously light d26Mg and heavy d18O as compared with the normal eclogites; moreover, UHPM marbles display negative correlation between d26Mg and MgO/CaO. These MgO isotopic characteristics reect differential isotopic exchange between the marble and enclosed eclogite during subduction, and provide important constraints on deep carbon recycling.

ResultsGeologic setting and samples. Triassic subduction of the South China Block beneath the North China Block resulted in the UHPM belts of Dabie and Sulu regions (Supplementary Fig. 1a)21. Eclogites are widespread as pods, discontinuous layers or blocks in gneisses, ultramac massifs and marbles. Previous studies suggest that these eclogites were formed by UHP metamorphism of Neoproterozoic basaltic rocks during the Triassic subduction of the South China Block2124. Diamond and coesite inclusions present in eclogites and their country rocks indicate that they were in situ subducted and exhumed with peak metamorphic temperatures of 700880 C and pressures up to5.5 GPa (refs 2527). The marble masses in the Rongcheng area crop out as large-scale discontinuous lenses with lengths of more than 2,000 m and are surrounded by orthogneisses containing layers or lenses of eclogites, amphibolites and serpentinized peridotites (Supplementary Fig. 1b). Samples studied here were collected from local marble quarries where many eclogites are present as centimetre- to metre-sized lenticular or spherical blocks in host marbles (Supplementary Fig. 1c). These marble-hosted eclogites are termed as carbonated eclogites hereafter. Field observation shows that the margins of carbonated eclogite

blocks are invariably retrograded to amphibolites, while the cores are free of retrogression. The primary mineral assemblage of the carbonated eclogite is garnet, clinopyroxene, quartz, plagioclase and amphibole, with accessory minerals of calcite, dolomite, pyrrhotite, chalcopyrite and rutile (Supplementary Table 1). Garnet and pyroxene minerals in carbonated eclogites are enriched in Ca relative to those in normal eclogites because of the UHPM metasomatism by host marbles27. The marbles range from calcite-rich to dolomite-rich in composition, and consist mainly of calcite and dolomite with silicate mineral assemblage of

8

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700 C

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880 C

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Figure 1 | Magnesium isotopic compositions of carbonated and normal eclogites. (a) Histogram of whole-rock d26Mg of carbonated eclogites. The grey band represents the normal mantle value14; the red dashed and blue solid curves are kernel density estimates of d26Mg values of the normal eclogites from Sulu orogen and the Neoproterozoic basaltic protoliths from

South China Block30, respectively. d26Mg values of carbonated eclogites, represented by the histogram, are signicantly lighter than both normal eclogites and their protoliths; (b) d26Mg values of garnet (d26MgGrt) and pyroxene (d26MgCpx) separates from the normal and carbonated eclogites.

The green hexagon represents the mineral separates from a normal eclogite close to the UHPM marble in the Rongcheng area. The red squares are mineral separates from normal eclogites in Chinese Continental Scientic Drill (CCSD). The black circles represent the mineral separates from carbonated eclogites. Green dotted lines are the equilibrium garnet pyroxene Mg isotope fractionation lines51. Magnesium isotopic data are from Supplementary Tables 24.

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plagioclase, quartz and other minor phases such as pyroxene, amphibole, olivine and talc. Calcite contains signicant MgO up to 4.43 wt.%, and dolomite has MgO in the range of16.57B21.84 wt.% (refs 28,29).

Magnesium isotopes. The d26Mg values of normal eclogites vary from 0.440 to 0.092%, with an average value of 0.2240.048% (n 26; Fig. 1a and Supplementary Table 2),

similar to that of their Neoproterozoic basaltic protoliths (d26Mg 0.1960.044%; Fig. 1a)30. Garnets from these

normal eclogites are systemically lighter than coexisting pyroxenes (D26MgCpxGrt d26MgCpx d26MgGrt 0.8871.099%),

with d26Mg ranging from 0.943 to 0.622% in garnets and

from 0.151 to 0.438% in pyroxenes (Fig. 1b and

Supplementary Table 2). By contrast, carbonated eclogites are highly depleted in 26Mg as compared with the normal eclogites, with d26Mg varying from 1.928 to 0.648% (Fig. 1a and

Supplementary Table 3). These Mg isotopic ratios are the lightest known values in silicate rocks. The amphibolitic margin (11RC 2E) and eclogitic core (11RC 1) of an individual

retrograded eclogite block (with diameter of B30 cm) have d26Mg of 0.648% and 0.684% (Supplementary Table 3),

respectively. Garnet and pyroxene separates from carbonated eclogites also have the lightest d26Mg values (Fig. 1b and

Supplementary Table 4), that is, from 1.937 to 1.191% in

garnet and from 0.980 to 0.483% in pyroxene.

Sinian carbonates comprise limestone (MgO/CaO 0.02) and

dolostone (MgO/CaO 0.62B0.73), with d26Mg varying from 4.101 to 2.396% in limestone and from 2.210 to 1.009% in dolostone (Fig. 2a,b and Supplementary Table 3). The generally lighter d26Mg in limestone than in dolostone is

consistent with previous studies of normal sedimentary carbonates1618. By contrast, d26Mg values of the UHPM marbles vary from 2.508 to 0.531% and form a negative correlation with

the dolomite abundance and a positive correlation with the calcite abundance (Fig. 2c,d and Supplementary Table 3).

Oxygen and other isotopes. Two normal eclogites from Rong-cheng have d18O values of 5.65% and 3.96%, respectively

(Supplementary Table 3). The d18O of carbonated eclogites span a range of 15.9 to 21.1% (Supplementary Table 3), which is

consistent with previous studies of carbonated eclogites31. Neodymium isotope analyses of these carbonated eclogites yield initial eNd of 12.3 to 1.1 (Supplementary Table 6), falling in

the range of normal eclogites (eNd 16.7 to 0.1)32. The d18O

and d13C of the UHPM marbles are in the ranges 20.3 to 23.3% and 2.01 to 4.77% (Supplementary Table 3),

respectively, similar to those of their protoliths19,20.

DiscussionGarnet and pyroxene from both normal and carbonated eclogites reach Mg isotopic equilibrium at peak metamorphic temperatures of 700B880 C (Fig. 1b). The bulk carbonated eclogites and mineral separates have anomalously light d26Mg and heavy d18O values when compared with the normal eclogites and Neoproterozoic basaltic protoliths (Fig. 1a,b). Sedimentary limestones generally have lighter d26Mg than dolostones1618, whereas the UHPM marbles display the opposite trend, that is, isotopically heavy marbles have high CaO and low MgO contents (Fig. 2a,b). As such, the MgO isotopic signatures preserved in the carbonated eclogites and UHPM marbles do not reect their protolith heterogeneity, rather they reect carbonate

0

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60 100

3 0 20 40 80 60 100

0 20 40 80

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Figure 2 | Magnesium isotopic compositions of UHPM marbles and Sinian carbonates. (a) d26MgMgO variations of UHPM marbles and Sinian carbonates; (b) d26MgCaO variations of UHPM marbles and Sinian carbonates; (c) the variation of d26Mg against calcite abundance in UHPM marbles;

(d) the variation of d26Mg against dolomite abundance in UHPM marbles. The white circles represent the UHPM marbles and the green diamonds represent the Sinian carbonates. Magnesium isotopic and major elemental data are from Supplementary Table 3; mineral abundances are from

Supplementary Table 5.

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0

MgO/CaO = 0.03 0.28

Normal eclogite

Dolostone

Carbonate protolith

MgO/CaO = 0.31 0.64

Limestone

M/E = 0.1

M/E = 0.5

M/E = 9.5

Differential isotopic exchange

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MgO/CaO

Figure 3 | Magnesium and O isotopic exchange between UHPM marbles and enclosed eclogites. The UHPM marbles and carbonated eclogites are represented by the white and black circles, respectively. Two end members of marble protoliths, limestone and dolostone are consideredto have initial chemical compositions of Mg0.001Ca0.999CO3 and

Mg0.5Ca0.5CO3, respectively. Their O and Mg isotopic compositions are: d18O 24% and d26Mg 4.00% for limestone, and d18O 24%

and d26Mg 2.50% for dolostone. The eclogites are assumed to have

initial d18O of 5.5%, d26Mg of 0.25% and MgO 8 wt.%. The green

hexagons represent two normal eclogites in gneisses close to the UHPM marble in the Rongcheng area. Equations that govern the isotopic exchange model are given in Supplementary Note 1. The red and blue curves represent the isotopic exchange of normal eclogites with limestone and with dolostone, respectively. The solid curves represent the MgO isotopic evolution of eclogites and the dashed curves represent the MgO isotopic evolution of marbles by isotopic exchange. The d18O ranges of Sinian carbonates19 and normal eclogites23 are shown for comparison.

Magnesium and O isotopic data are from Supplementary Tables 2 and 3.

Figure 4 | Differential isotopic exchanges of eclogites with calcite-rich and dolomite-rich marbles. The eclogites are assumed to have initial d26Mg of 0.25% and MgO 8 wt.%. Equations that govern differential

isotopic exchange model are given in the Supplementary Note 2. At a given weighted ratio of carbonate to eclogite (M/E) during isotopic exchange, marbles with different compositions (limestone versus dolostone) shift their d26Mg in different degrees (shown by arrows). The red and blue curves assume that the initial d26Mg of carbonates are 4.00% and 2.50%, respectively. Different curves represent the isotopic exchange under different M/E ratios. The increment of M/E ratio is 0.1 for the red and1.0 for the blue. Data are from Supplementary Table 3.

eclogite interactions during subduction including carbonate eclogite mixing, metamorphic dehydration or rehydration, decarbonation and isotopic exchange.

Carbonates are known to have extremely heavy d18O and light d26Mg values1620; thus, carbonateeclogite mixing might explain the bulk MgO isotopic characteristics of carbonated eclogites. If we assume that eclogite lenses in marbles have initial d26Mg and d18O values similar to the normal eclogites, then a mixing calculation with carbonates indicates that B6090% contribution by carbonate is required to t the bulk MgO isotopic compositions. However, the low CO2 contents of carbonated eclogites (0.116.43 wt.%; Supplementary Table 3) imply that the calculated carbonate proportion is unrealistically high. In addition, carbonateeclogite mixing predicts lighter d26Mg values in those samples containing higher CO2 contents, which is not seen in the carbonated eclogites (Supplementary Fig. 2). Most importantly, garnet and pyroxene minerals in carbonated eclogites are equally enriched in 24Mg and 18O. This requires a chemical exchange rather than simple physical mixing between eclogite lenses and marbles.

The Mg and O isotope fractionations between carbonated eclogites and their protoliths are far greater than predicted by closed-system metamorphic dehydration30,33. In addition, the d26Mg of low MgO/CaO marbles are signicantly heavier than the values of their pre-metamorphosed counterpart, Sinian

limestones (Fig. 2a,b), which is opposite to the direction of isotope fractionation induced by metamorphic dehydration34. Thus, closed-system metamorphic dehydration is unlikely to account for the observed MgO isotopic variations in carbonated eclogites and UHPM marbles. Although the retrograded amphibolitic rim of an individual carbonated eclogite block has higher alkalies and CO2 relative to the eclogitic core, they have similar Mg isotopic compositions (Supplementary Table 3), ruling out the possibility of retrogression-induced Mg isotope fractionation.

Metamorphic decarbonation involves the decomposition of a substantial amount of carbonate minerals by incorporating Mg and Ca oxides into the silicates and releasing CO2 (ref. 35). This may explain the light d26Mg and heavy d18O of silicate minerals in carbonated eclogites. However, metamorphic decarbonation cannot signicantly fractionate Mg isotopes, neither can it form the d26MgCaO and d26MgMgO correlations in UHMP marbles (Fig. 2a,b). Furthermore, previous studies have revealed that the subduction PT trajectory of these carbonated eclogites and UHPM marbles does not intersect with experimentally determined carbonate-out boundaries (Supplementary Fig. 3)8, suggesting limited decarbonation of these UHPM marbles. This is further supported by the similar O and C isotopic compositions between Sinian carbonates and UHPM marbles (Supplementary Fig. 4), as large O and C isotope fractionations are expected during decarbonation36.

The most likely process causing the Mg and O isotopic variations in carbonated eclogites and UHPM marbles is thus through uid-mediated isotopic exchange. Previous studies revealed that Mg isotope fractionation between silicate and carbonate decreases dramatically from low temperatures (Z2%

at To300 C) to high temperatures (0.05B0.08% at T 600

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B800 C)3740. Similar is true for O isotopes33. Therefore, extensive isotopic exchanges between marbles and eclogites are expected during prograde metamorphism to reduce their Mg and O isotopic differences. Assuming that sedimentary limestone and dolostone have heavy d18O of 24% (ref. 19) but light d26Mg

around 4.00% and 2.50% (refs 1618), respectively, and

eclogite lenses in carbonates have initial Mg and O isotopic compositions similar to the normal eclogites (with average d18O 5.5% and d26Mg 0.25%), the eclogites would

become depleted in 26Mg and enriched in 18O after UHP isotopic exchange with the marbles (Fig. 3). Magnesium isotopic exchange also elevated the d26Mg values of UHPM marbles in various degrees (Fig. 3). As limestone contains signicantly lower Mg than dolostone, at a given weighted ratio between marble and eclogite the limestone is more susceptible than dolostone to Mg isotopic exchange with the eclogites (Fig. 3). Consequently, the calcite-rich carbonates exhibit a large shift in d26Mg towards heavier values, whereas the dolomite-rich carbonates tend to preserve their initial d26Mg values (Fig. 4). The negative covariation of d26Mg versus MgO/CaO for UHPM marbles (Fig. 4) is therefore produced by the differential Mg isotopic exchange of compositionally varied carbonates with enclosed eclogite lenses.

Sedimentary carbonates are the main light-d26Mg sink of Earths reservoirs18. Recycling of carbonates into the mantle has the potential to cause local mantle Mg isotope heterogeneity41. Knowledge of whether Mg isotopic compositions of sedimentary carbonates can be preserved during crustal subduction is fundamental to quantifying the amount of recycled carbonate in mantle sources using Mg isotopes. Our study suggests that calcite-rich carbonates are highly unlikely to retain their initial light d26Mg but become isotopically heavier towards the value of silicates, whereas dolomite-rich carbonates are more capable of preserving their initial values. On the other hand, the carbonated silicates (for example, eclogites) gain light d26Mg values that are signicantly different from other normal eclogites and ambient mantle. These silicates may produce signicant light-d26Mg components in the mantle, as sampled by cratonic eclogites42 and highly metasomatized peridotite xenoliths43.

Methods

Magnesium isotopic analyses. Magnesium isotopic analyses were carried out at the Isotope Laboratory of the University of Arkansas, Fayetteville, USA. Whole-rock powders of eclogites and carbonates, and the mineral separates were digested using Optima-grade mixed acid of HF-HNO3-HCl. After complete dissolution, dried residues from carbonate solutions were added 12 N HCl, and those from silicate solutions were taken up in 1 N HNO3 for ion column chemistry. Detail column chemistry procedures have been reported elsewhere44,45.

Chemical separation and purication of Mg were achieved by cation exchange chromatography with Bio-Rad AG50W-X8 resin in 1 N HNO3 media. Additional chromatographic step was processed for the carbonate to further separate Mg from Ca by using Bio-Rad 200400 mesh AG50W-X12 resin in 12 N HCl media. The same column procedure was performed twice in order to obtain the pure Mg recovery. The pure Mg solutions of silicate and carbonate were then dried down and re-dissolved in 3% HNO3 ready for mass spectrometry.

The Mg isotopic compositions were analysed by the sample-standard bracketing method using a Nu Plasma MC-ICPMS at low-resolution mode46. Each batch of sample analysis contains at least one well-characterized standard. Sample solution was repeated on ratio measurements for more than four times within a session. The long-term precision is better than 0.07% (2 s.d.) for the 26Mg/24Mg ratio.

Magnesium isotopic results are reported in the conventional d notation in per mil relative to DSM-3, d26Mg [(26Mg/24Mg)

sample/(26Mg/24Mg)DSM-3 1] 1,000.

Oxygencarbon and neodymium isotopic analyses. Oxygen and carbon isotopic analyses were conducted at the CAS Key Laboratory of Crust-mantle Materials and Environments in the University of Science and Technology of China, Hefei. Oxygen isotopes of eclogites were measured by the laser uorination technique. The O2 was extracted by a CO2 laser and transferred to a Finigan Delta mass

spectrometer for the measurement of 18O/16O and 17O/16O ratios. The O and C isotopes of carbonates were analysed by the GasBench II technique in the continuous ow mode and the extracted CO2 gases were measured on a Finnigan

MAT253 mass spectrometer. Detail procedures are described previously47. The

O and C isotopic results are reported in the d notation in per mil relative to Vienna Standard Mean Ocean Water and Vienna Peedee Belemnite, respectively.

Samarium-Nd isotopic analyses were performed using an IsoProbe-T thermal ionization mass spectrometer at the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences. Samarium isotopes were measured using single Ta lament. Neodymium isotopes were determined using single tungsten lament with TaF5 as an ionization activator. Detail column chemistry and mass spectrometer operation condition have been described previously48. 143Nd/144Nd ratios were corrected for mass fractionation using 146Nd/144Nd 0.7219. During the period of data collection, the

measured value for the JNdi-Nd standard was 0.51211710 (2 s.d.).

Whole-rock chemistry and mineral abundances. Major elements (except FeO, H2O and CO2) were analysed at the Hebei Institute of Regional Geology and

Mineral Resources, China, by wavelength dispersive X-Ray uorescence spectrometry49. The analytical uncertainties are better than 1%. The FeO contents of the samples were determined by conventional wet chemical method50. After the quantitative oxidation of Fe2 to Fe3 , the ferrous iron is regenerated by back-titration with ammonium ferrous sulfate. Then, the original Fe2 content can be calculated. H2O and CO2 were determined by gravimetric methods and potentiometry, respectively. The relative abundances of carbonate minerals were determined by X-ray diffraction using a Rigaku Smart lab (9 kW) at the China University of Geosciences, Beijing. The X-ray source was a Cu anode operated at 45 kV and 200 mA using CuKa1 radiation equipped with a diffracted beam graphite monochromator.

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Acknowledgements

We would like to thank Y.-F. Zheng for support on oxygen isotopic analysis and for thoughtful discussion, Z.-Z. Han, S.-C. An, H.-M. Zhang and J.-A. Hong for eld works, H.-O. Gu and W.-Y. Li for help in the clean lab. The work is supported by the National Science Foundation (EAR-0838227, EAR-1056713 and EAR1340160) toF.-Z.T. and the National Nature Scientic Foundation of China (No. 41230209, 41090372) to S.-G.L.

Author contributions

S.-J.W., F.-Z.T. and S.-G.L. designed the study, interpreted the data and wrote the paper.S.-J.W carried out all the laboratory work. All authors contributed equally to this work.

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How to cite this article: Wang, S.-J. et al. Tracing carbonatesilicate interaction during subduction using magnesium and oxygen isotopes. Nat. Commun. 5:5328 doi: 10.1038/ncomms6328 (2014).

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