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Journal ArticleDOI

Diamond formation due to a pH drop during fluid–rock interactions

03 Nov 2015-Nature Communications (Nature Publishing Group)-Vol. 6, Iss: 1, pp 8702-8702
TL;DR: It is shown that diamonds could form due to a drop in pH during water–rock interactions, and this results constitute a new quantitative theory of diamond formation as a consequence of the reaction of deep fluids with the rock types that they encounter during migration.
Abstract: Diamond formation has typically been attributed to redox reactions during precipitation from fluids or magmas. Either the oxidation of methane or the reduction of carbon dioxide has been suggested, based on simplistic models of deep fluids consisting of mixtures of dissolved neutral gas molecules without consideration of aqueous ions. The role of pH changes associated with water–silicate rock interactions during diamond formation is unknown. Here we show that diamonds could form due to a drop in pH during water–rock interactions. We use a recent theoretical model of deep fluids that includes ions, to show that fluid can react irreversibly with eclogite at 900 °C and 5.0 GPa, generating diamond and secondary minerals due to a decrease in pH at almost constant oxygen fugacity. Overall, our results constitute a new quantitative theory of diamond formation as a consequence of the reaction of deep fluids with the rock types that they encounter during migration. Diamond can form in the deep Earth during water–rock interactions without changes in oxidation state. The cause of diamond precipitation has previously been attributed to poorly understood redox changes at depth. Here, the authors propose that a drop in pH during water–rock interactions leads to diamond formation as a consequence of the migration of reactive fluids at elevated temperatures and pressures.

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Citations
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Journal ArticleDOI
TL;DR: The results show that the evolution of oxygen fugacity in serpentinite during subduction is sensitive to the amount of sulfides and potentially metal alloys in bulk rock, possibly producing redox heterogeneities in subducting slabs.
Abstract: Subduction zones facilitate chemical exchanges between Earth's deep interior and volcanism that affects habitability of the surface environment. Lavas erupted at subduction zones are oxidized and release volatile species. These features may reflect a modification of the oxidation state of the sub-arc mantle by hydrous, oxidizing sulfate and/or carbonate-bearing fluids derived from subducting slabs. But the reason that the fluids are oxidizing has been unclear. Here we use theoretical chemical mass transfer calculations to predict the redox state of fluids generated during serpentinite dehydration. Specifically, the breakdown of antigorite to olivine, enstatite, and chlorite generates fluids with high oxygen fugacities, close to the hematite-magnetite buffer, that can contain significant amounts of sulfate. The migration of these fluids from the slab to the mantle wedge could therefore provide the oxidized source for the genesis of primary arc magmas that release gases to the atmosphere during volcanism. Our results also show that the evolution of oxygen fugacity in serpentinite during subduction is sensitive to the amount of sulfides and potentially metal alloys in bulk rock, possibly producing redox heterogeneities in subducting slabs.

102 citations

Journal ArticleDOI
TL;DR: It is concluded that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver 34 S-enriched sulfur to produce the positive δ 34 S signature in arc settings.
Abstract: Sulfur belongs among H2O, CO2, and Cl as one of the key volatiles in Earth’s chemical cycles. High oxygen fugacity, sulfur concentration, and δ34S values in volcanic arc rocks have been attributed to significant sulfate addition by slab fluids. However, sulfur speciation, flux, and isotope composition in slab-dehydrated fluids remain unclear. Here, we use high-pressure rocks and enclosed veins to provide direct constraints on subduction zone sulfur recycling for a typical oceanic lithosphere. Textural and thermodynamic evidence indicates the predominance of reduced sulfur species in slab fluids; those derived from metasediments, altered oceanic crust, and serpentinite have δ34S values of approximately −8‰, −1‰, and +8‰, respectively. Mass-balance calculations demonstrate that 6.4% (up to 20% maximum) of total subducted sulfur is released between 30–230 km depth, and the predominant sulfur loss takes place at 70–100 km with a net δ34S composition of −2.5 ± 3‰. We conclude that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver 34S-enriched sulfur to produce the positive δ34S signature in arc settings. Most sulfur has negative δ34S and is subducted into the deep mantle, which could cause a long-term increase in the δ34S of Earth surface reservoirs. Sulfur is one of the key volatiles in Earth’s chemical cycles; however, sulfur speciation, isotopic composition, and flux during the subduction cycle remain unclear. Here, the authors provide direct constraints on subduction zone sulfur recycling from high-pressure rocks and explore implications for arc magmatism.

86 citations

Journal ArticleDOI
TL;DR: In this article, the authors found 32 microinclusions along the twinning planes in eight out of 30 diamonds and found that the major element compositions of the remaining 20 were similar to those of carbonate-bearing high density fluids (HDFs) found in fibrous diamonds.

78 citations

Journal ArticleDOI
TL;DR: In this paper, the authors use experimentally measured solubility data for multicomponent K-free eclogite, k-free peridotite and K-bearing peridotaite rocks at upper mantle conditions from the literature to construct aqueous speciation solubilities models that enabled calibration of the thermodynamic properties of ions and metal-complex species involving the elements Na, K, Mg, Ca, Fe, Al, Si, Si and C in an extended Deep Earth Water (DEW) model.

71 citations

Journal ArticleDOI
TL;DR: In this article, the number of solute species required to describe the thermodynamic behavior of electrolytic fluids is a hindrance to the incorporation of aqueous geochemistry in petrological Gibbs energy minimization procedures.

68 citations

References
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Journal ArticleDOI
TL;DR: The effects of the chemistry of ore-forming fluids on the sulfur and carbon isotopic compositions of hydrothermal minerals are quantitatively evaluated from available thermochemical data and isotopic fractionation factors as discussed by the authors.
Abstract: The effects of the chemistry of ore-forming fluids on the sulfur and carbon isotopic compositions of hydrothermal minerals are quantitatively evaluated from available thermochemical data and isotopic fractionation factors.The isotopic composition of both sulfur and carbon in hydrothermal minerals is strongly controlled by the f (sub O 2 ) and pH values of hydrothermal fluids as well as by the temperature and the isotopic composition of sulfur and carbon in the fluids (delta S 34 (sub Sigma S) and delta C 13 (sub Sigma C) values). For example, at 250 degrees C and within geologically important f (sub O 2 ) -pH regions, an increase in f (sub O 2 ) value by 1 log unit or in pH by 1 unit can cause a decrease in delta S 34 values of sulfur-bearing minerals by as much as 20 per mil. An increase in f (sub O 2 ) by 1 log unit or in pH by 2 units can cause a decrease of about 30 per mil in delta C 13 values of carbon-bearing minerals. Large variation in the delta S 34 values or in the delta C 13 values of hydrothermal minerals, which often have been interpreted as an indication of biogenic sulfur or carbon, could also be caused by slight variation in the f (sub O 2 ) and/or pH of ore-forming fluids during ore deposition.The concentrations in an ore solution of sulfur (or f (sub S 2 ) ) and of carbon (or f (sub CO 2 ) ) place limits on possible delta S 34 and delta C 13 values for hydrothermal minerals. Sulfur-bearing minerals and carbon-bearing minerals precipitating from sulfur- and carbon-rich solutions can have wider ranges of delta S 34 and delta C 13 values than those minerals precipitating from sulfur- and carbon-poor solutions.Sulfide minerals which precipitated in equilibrium with magnetite, hematite, or sulfate minerals, and carbonate minerals which precipitated in equilibrium with graphite, could exhibit isotopic compositions markedly different from those of the depositing fluids. Therefore, sulfides with delta S 34 values near zero per mil or carbonates with delta C 13 values near -6 per mil do not necessarily indicate a magmatic origin for the sulfur or the carbon.The mode of variation on the delta S 34 values of sulfide minerals and in the delta C 13 values of carbonate minerals in a given deposit may indicate the relative oxidation states of ore-forming fluids: variable delta S 34 + uniform delta C 13 values may suggest that the minerals were precipitated under relatively high f (sub O 2 ) conditions; uniform delta S 34 + uniform delta C 13 values, under intermediate f (sub O 2 ) conditions; and uniform delta S 34 + variable delta C 31 values suggesting deposition under relatively low f (sub O 2 ) conditions.Sulfur and carbon isotopic data, combined with geological and mineralogical data of ore deposits, may define the physico-chemical parameters (T, f (sub O 2 ) , f (sub S 2 ) , f (sub CO 2 ) , m (sub Sigma S) , m (sub Sigma C) ) and the origin (delta S 34 (sub Sigma S) and delta C 13 (sub Sigma C) values) of ore-forming fluids as well as the mechanisms of ore deposition.

1,212 citations

Journal ArticleDOI
29 Sep 2005-Nature
TL;DR: Measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth and temperatures of 700–1,200 °C constrain the recycling rates of key elements in subduction-zone arc volcanism.
Abstract: Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism1,2 The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown3,4,5,6,7 Here we report direct LA-ICPMS measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth of 120–180 km and temperatures of 700–1,200 °C The resultant liquid/mineral partition coefficients constrain the recycling rates of key elements The dichotomy of dehydration versus melting at 120 km depth is expressed through contrasting behaviour of many trace elements (U/Th, Sr, Ba, Be and the light rare-earth elements) At pressures equivalent to 180 km depth, however, a supercritical liquid with melt-like solubilities for the investigated trace elements is observed, even at low temperatures This mobilizes most of the key trace elements (except the heavy rare-earth elements, Y and Sc) and thus limits fluid-phase transfer of geochemical signatures in subduction zones to pressures less than 6 GPa

1,131 citations

Journal ArticleDOI
TL;DR: Estimates of the speciation of major, minor, and trace elements in hydrothermal and metamorphic fluids throughout most of the crust of the Earth are facilitated by correlations among experimentally determined standard partial molal thermodynamic properties.

939 citations

Journal ArticleDOI
TL;DR: The origin of cratonic diamonds is reviewed on the basis of nearly 5000 analyses of silicate, oxide and sulphide inclusions in diamonds as mentioned in this paper, and compositional fields are defined for common minerals of the peridotitic, eclogitic and websteritic inclusion suites.

468 citations