COH Fluid Research Articles

CH4 Inclusions in High‐Pressure Metapelite: Revealing the Link Between Fluid Behavior and Redox Mechanisms for Subducting Sedimentary Carbon

Abstract(Ultra)high‐pressure metamorphic rocks provide valuable insights into the properties of slab‐derived fluids. Here, we report CH4‐rich fluid inclusions in garnet of a metapelite from the Zermatt‐Saas ophiolite, western Alps. Two types of metapelite, a CH4‐bearing pelitic schist and a calcareous pelitic schist, were investigated to unravel favorable P‐T‐fO2 conditions for preservation of CH4 in high‐pressure metapelite. In the CH4‐bearing pelitic schist, CH4‐rich fluid inclusions exclusively occur in the core of garnet (GrtI) rather than the rim (GrtII). GrtI records P‐T conditions of ∼2.85 GPa and ∼555°C, whereas GrtII records a prograde P‐T path from ∼1.75 GPa at 510°C to ∼2.0 GPa at 530°C. Compositional profile of garnet in the calcareous pelitic schist reflects a prograde metamorphic path from ∼1.9 GPa at 510°C to ∼2.12 GPa at 545°C. CH4‐rich fluid formation may primarily rise from graphite reduction at high‐pressure reduced conditions (ΔFMQ −3.5 to −4, 2.85 GPa, ∼550°C), while graphite and carbonates stabilize in a relatively oxidized environment (ΔFMQ ∼0, 2.12 GPa, 545°C). The initial redox budget of subducted sediments is primarily controlled by the amount of sedimentary carbonate and organic carbon, which plays the most important role in deciding the carbon speciation at different subduction depths. CH4 formation in COH fluids could primarily be attributed to the reduction of graphite. Subducted metasediments act as conduits for transporting non‐oxidized fluids to arc magmas, which provides crucial evidence to support the heterogeneity for slab‐derived COH fluids and offers new insights into the deep carbon cycle.

Crystallisation of trapped carbonate–silicate melts terminating at the carbonated solidus ledge: a record of carbon immobilisation mechanism in the lithospheric mantle

Orogenic peridotites in the crystalline basement of the northwestern Bohemian Massif contain multiphase solid inclusions (MSI), which are interpreted to be crystallisation products of trapped former carbonate–silicate melts metasomatizing their host rocks. We applied conventional thermobarometry and forward thermodynamic modelling to constrain the P–T evolution ranging from the peak metamorphic conditions of the investigated harzburgite and lherzolite, through entrapment of the melts in the outer parts of garnets, to the (re)-equilibration of the MSI assemblages. The peak conditions of c. 1100 °C/4.5–5.5 GPa are recorded by garnet cores and large pyroxene porphyroclasts. The melt entrapment, during which garnet outer parts grew, was associated with influx of the metasomatizing liquids and probably took place during the early stage of the exhumation. Thermodynamic model of amphibole-free MSI assemblage comprising kinoshitalite/Ba-rich phlogopite (approximated by phlogopite in the model), dolomite, magnesite, clinopyroxene, orthopyroxene, garnet and chromite provided robust estimate of P and T of its (re)-equilibration, c. 900–1000 °C, 1.8–2.2 GPa. Furthermore, the lack of olivine reflects co-existence of COH fluid with high X(CO2) = CO2/(CO2 + H2O) ≥ 0.6. Models employing identical P–T–X(CO2) parameters successfully reproduced the other two amphibole-bearing assemblages observed. The modelled stability fields show perfect alignment with a characteristic isobaric segment of the solidus curve of carbonated peridotite. This co-incidence implies that the (re)-equilibration corresponds to the termination of the melt crystallisation once the near-isothermal exhumation path intersected the solidus. Decreased solubility of silicates at the carbonated peridotite “solidus ledge”, inferred from the published experimental data, as well as concentric textures of some MSI indicates sequential crystallisation from the early silicates to late dolomite. The carbonated “solidus ledge” is a relatively narrow boundary in the lithospheric mantle capable of an abrupt immobilisation of fluxing or transported carbonated melts. The investigated rocks are estimated to store approximately 0.02 kg C/m3 (or 6 ppm C) occurring as carbonates in the MSI.

Methane genesis within olivine-hosted fluid inclusions in dolomitic marble of the Hida Belt, Japan

Abiotic synthesis of hydrocarbon-bearing fluids during geological processes has a significant impact on the evolution of both the Earth's biosphere and the solid Earth. Aqueous alteration of ultramafic rocks, i.e., serpentinization, which forms serpentinite, is one of the geological processes generating abiotic methane (CH4). However, abiotic CH4 generation is not limited to the serpentinization of mafic and ultramafic lithologies. Metasedimentary dolomitic marble from the Hida Belt, Japan, is characterized by the presence of forsterite-rich olivine (Fo~89–93), and olivine crystals contain abundant fluid inclusions (<1 to 10 μm in size). Raman spectroscopic analyses of olivine-hosted fluid inclusions found that both primary and secondary fluid inclusions contain CH4, lizardite/chrysotile, and brucite. This indicates that micro-scale interactions between COH fluid and host olivine produced CH4 through the reduction of CO2 by H2 released during local serpentinization within inclusions. Our observation implies that the dolomitic marble has the potential to be a key lithology for the synthesis and storage of abiotic CH4 in a shallower crustal portion of orogenic belts.

Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway)

Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (&lt;modality 39%), quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67–0.91; Ti &lt; 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En69–74) and/or clinopyroxene (En33–53Fs1–14Wo44–53); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl &lt; 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic δ13Cgraphite in the graphite schist is −38 to −17‰; δ13Ccalcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ13Ccalcite = −8.7‰ to −9.5‰ and heavier values for δ13Cgraphite = −11.5‰ to −8.9‰. δ18Ocalcite for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.

Corrigendum: Experimental dissolution of carbonaceous materials in water at 1 GPa and 550°C: assessing the role of carbon forms and redox state on COH fluid production and composition during forearc subduction of organic matter

Corrigendum: Experimental dissolution of carbonaceous materials in water at 1 GPa and 550°C: assessing the role of carbon forms and redox state on COH fluid production and composition during forearc subduction of organic matter

Experimental dissolution of carbonaceous materials in water at 1 GPa and 550°C: Assessing the role of carbon forms and redox state on COH fluid production and composition during forearc subduction of organic matter

Biogenic carbonaceous material (CM) is the main carrier of organic carbon in the subduction zone and contributes to COH fluid production and volcanic arc gaseous emissions. Here we investigated the effect of the structural, textural and chemical heterogeneity of CM on its reactivity and redox dissolution by conducting short-lived (1 h) experiments, where synthetic analogues of CM [ordered graphite, graphite oxide (GO), mesoporous carbon (MC), Vulcan® carbon (VC) and glass-like carbon (GC)], are reacted with water at p = 1GPa and T = 550°C–conditions typical of a warm forearc subduction–and fO2 buffered from ▵FMQ ≈ +4 to −7. We show that the amount of dissolved CM (CMdissolved) and the proportion of volatile carbon species (Cvolatile) in the fluid is related both to the structure and the peculiar surficial properties of the carbon forms, such as carbon sp2-and sp3-hybridization, amount of oxygen heteroatoms, presence of oxygenated functional groups (OFGs) and of active sites. MC and graphite (C(sp2) &amp;gt; 94 at%, O &amp;lt; 1 at%, OFGs &amp;lt; 2.2 at %, high proportion of active sites) are relatively inert (CMdissolved &amp;lt; 0.4 mol%) but the former reacts more extensively at extreme redox conditions (producing CO2 + CO and CO2 + CH4 Cvolatile mixtures at ▵FMQ ≈ +4 and −7, respectively), while the latter has a maximum of Cvolatile production (CO2 + CH4) at ▵FMQ ≈ 0, which is not observed in a 10-day long run; partly-ordered GO (C(sp3) ∼ 92 at%, O ∼31 at%, OFGs ∼41 at%) is the most reactive material at all redox conditions (CMdissolved &amp;lt; 2.6 mol%) and produces CO2 as the dominant Cvolatile species; disordered GC, and VC (C(sp3) &amp;lt; 18 at%, O &amp;lt; 8 at%, OFGs &amp;lt; 30 at%) are more reactive at ▵FMQ ≈ +4 (CMdissolved ∼ 1mol%) and ▵FMQ ≈ −7 (CMdissolved &amp;lt; 1 mol%), where Cvolatile is dominantly CO2 and CH4, respectively. Besides the significant deviations from thermodynamically predicted graphite-saturated COH fluid composition and speciation, our results suggests that: 1) immature CM [disordered, rich in C(sp3), O, OFGs] is preferentially dissolved under high fluid fluxes and may buffer fluids to rather oxidizing conditions; 2) a descending flux of oxygen (and hydrogen) bond to CM may exist.

The effect of COH fluids on partial melting of eclogite and lherzolite under moderately oxidizing and reducing conditions

Fluids rich in water and carbon (COH) are among the most reactive agents of mantle metasomatism and play a major role in the continuous degassing of the Earth. The addition of small amounts of H2O and CO2 depress the solidus of mantle peridotite by several hundred degrees, resulting in the common occurrence of highly mobile, volatile-rich melts, which are a main cause of heterogeneities in the Earth's mantle. Most experimental studies have investigated relatively oxidizing conditions in which COH fluids are mixtures of H2O + CO2, whereas experiments on the partial melting of mafic and ultramafic rocks under reducing conditions, in which CH4 + H2O mixtures occur, remain sparse. We have performed experiments on eclogite and lherzolite compositions at pressures from 1.8 to 6 GPa, and temperatures ranging from 800 to 1600 °C at different oxygen fugacity conditions (FMQ - 5.6 to FMQ - 0.3), either with 5 wt% CH4 + H2O or 5 wt% H2O added. Our results show that under reducing conditions (FMQ - 5.6 or IW - 0.7), the solidi of both eclogite and lherzolite with COH fluids are about 200 °C lower than the anhydrous solidi, but 100 °C higher than those under oxidizing conditions with 5 wt% H2O. The mineral chemistry of residual solid phases in both systems depends on oxygen fugacity (fO2). The stability of clinopyroxene at the solidus of lherzolite under reducing conditions is significantly greater than its stability under oxidizing conditions, whereas garnet is only stable during the partial melting of eclogite at low fO2. The composition of partial melts of lherzolite with 5 wt% CH4 + H2O under reducing conditions at low pressures is quartz normative rather than olivine normative. The higher-temperature solidi and phase relations under reducing conditions indicate that, if Archean mantle was more reduced than the modern mantle, komatiite could be formed under reducing conditions with no need for deep melting or the presence of abundant water. Further, our experiments indicate that low fO2 favors the precipitation of graphite or diamond, so a large amount of diamond or graphite could have accumulated in the Earth's deep mantle and the Martian mantle.

Carbon-saturated COH fluids in the upper mantle: a review of high-pressure and high-temperature ex situ experiments

Abstract. High-pressure COH fluids have a fundamental role in a variety of geological processes. Their composition in terms of volatile species can control the solidus temperature and carbonation/decarbonation reactions, as well as influence the amount of solutes generated during fluid–rock interaction at depth. Over the last decades, several systems have been experimentally investigated to unravel the effect of COH fluids at upper-mantle conditions. However, fluid composition is rarely tackled as a quantitative issue, and rather infrequently fluids are analyzed in the same way as the associated solid phases in the experimental assemblage. A comprehensive characterization of carbon-bearing aqueous fluids in terms of composition is hampered by experimental difficulties in synthetizing and analyzing high-pressure fluids without altering their composition upon quenching. Recently, improved techniques have been proposed for the analyses of experimental carbon-saturated COH fluids, leading to a significant advancement in synthetic fluid characterization. Here, we present a review of carbon-bearing aqueous fluid experiments conducted at lower-crust and upper-mantle P–T (pressure and temperature) conditions, in which fluids have been characterized quantitatively through ex situ techniques. We review the experimental background of the most commonly employed thermodynamic models for COH fluids, together with the techniques to synthetize them and analyze their composition when the fluid coexists with solid phases. We highlight how a quantitative approach to COH fluid analyses is a fundamental step to understand the effect of these fluids at upper-mantle conditions and to provide a strong experimental foundation to thermodynamic models to ultimately unravel the deep cycling of elements.

Diamond's depth distribution systematics

The thermobarometric analysis of inclusions in lithospheric diamonds has shown that these diamonds may originate from a wide range of depths, with a global mode at ~175 ± 15 km. Studies based on diamond depth distribution at global scale, however, cannot clarify if this mode reflects a real concentration of diamonds, preferential sampling of materials from this level by ascending kimberlites, or simply a statistical distribution within the hard limits imposed by diamond stability, lithosphere thickness and mantle adiabat under typical cratonic thermal regimes. We addressed this problem by comparing depth distributions for peridotitic diamonds from the three localities that have been the most prolific for diamond geobarometry (Cullinan, Kimberley and Voorspoed, South Africa) with those of mantle xenocrysts from the same kimberlite sources. The revised P–T estimates indicate that the diamonds were formed at T higher, equal or lower than the ambient geotherm recorded by the xenocrysts. These conditions may represent old mantle thermal regimes or local thermal perturbations related to infiltration of parental fluids or melts. Nonetheless, the studied diamonds show similar depth distributions for the different localities, with a distinct mode at ⁓180 ± 10 km. The similarity of these distributions with that calculated for peridotitic diamonds worldwide, as well as the lack of systematic correlation with kimberlite sampling efficiency as recorded by mantle xenocrysts, suggests that this mode has genetic significance. Based on observed depth distributions and thermodynamic modeling of COH fluids, diamond-forming processes are predicted to become less efficient with decreasing depth from at least ⁓165 km. In addition, diamond endowment near the base of the lithosphere may be negatively affected by infiltration of carbon-undersaturated melts or fluids after diamond formation. Considering the poor correlation between diamond and xenocryst depth distributions in single kimberlites or kimberlite clusters, even limited xenocryst records from diamond favorable depths (especially from the 160–190 km interval) may correspond to significant diamond potential.

Polymineralic inclusions as tracers of multistage metasomatism in a paleo mantle wedge

Paleosubduction zones are key areas where slab-derived fluids interact with the overlying mantle wedge. Slab fluids play therefore an important role in the deep element cycle and can metasomatize ultramafics showing particular mineral assemblages. The nature of slab fluids is usually investigated from fluid inclusions and/or multiphase solid inclusions, as well as from incompatible elements in mantle minerals. We have made a study on a garnet-peridotite from the Variscan Ulten Zone (Eastern Alps, Italy) in which peculiar polymineralic inclusions (PI) in garnet provide a wealth of information on the interaction between mantle wedge and slab fluids. The selected sample shows a fine-grained amphibole-olivine-orthopyroxene-spinel-clinopyroxene matrix and a fractured large garnet porphyroclast, surrounded by a kelyphitic corona. Garnet core and rim host randomly distributed PI, which may be as large as 284,000 μm2. The PI show irregular, fracture-controlled shapes and often contain unusual mineral associations. Many PI are directly associated with Mg-hornblende- or chlorite-sealed micro-fractures crosscutting garnet. The most abundant PI contain spinel and amphibole as the main minerals. Spinel-bearing PI consists of high-Cr spinel (Cr# = 0.31–0.41) surrounded by amphiboles (magnesio-hornblende), locally associated with dolomite, sapphirine, calcite, sulphides and kinoshitalite (i.e. a Ba-rich phlogopite). Amphibole-bearing PI contain zoned amphiboles (tschermakitic cores surrounded by Mg-hornblende, which is rimmed by tremolite and Mg-amphibole), dissakisite (an REE-bearing epidote), apatite, dolomite, calcite, spinel, sapphirine, chlorite, kinoshitalite, and sulfides. The retrieved mineral reaction history and the major and some incompatible elements (Cl, Ba, Sr) of PI minerals provide clues about the composition of metasomatic fluids. The studied PI formed in garnet fractures during exhumation by reactions between garnet and (1) slab-derived COH fluids and (2) late-stage saline brines. Metasomatic fluids/brines were rich in volatile elements and had a crustal signature (enrichment in LILE and LREE) with variable Cl contents and CO2/H2O ratios. Products of early garnet-fluid reactions became reactants of later reactions, thus generating complex textures, including replacement textures of sapphirine over spinel, calcite over dolomite and chlorite over Mg-hornblende. Textures and compositions of the PI testify to a multistage metasomatic overprint at peak P-T conditions above 2 GPa (M1), during the first exhumation stage at 775–1000 °C, below 2 GPa (M2) and finally at 775 °C and 1 GPa (M3). In this study we demonstrate that Gibbs free energy minimization may aid in distinguishing between multiphase solid inclusions and polymineralic inclusions; the latter do provide valuable information on the evolution of retrograde P-T paths in the paleosubduction channel, the relative chronology of metasomatic events and clues on the chemical signature of the fluids.

Investigation of Fluid-driven Carbonation of a Hydrated, Forearc Mantle Wedge using Serpentinite Cores in High-pressure Experiments

AbstractHigh-pressure experiments were performed to investigate the effectiveness, rate and mechanism of carbonation of serpentinites by a carbon-saturated COH fluid at 1·5–2·5 GPa and 375–700 °C. This allows a better understanding of the fate and redistribution of slab-derived carbonic fluids when they react with the partially hydrated mantle within and above the subducting slab under pressure and temperature conditions corresponding to the forearc mantle. Interactions between carbon-saturated CO2–H2O–CH4 fluids and serpentinite were investigated using natural serpentinite cylinders with natural grain sizes and shapes in piston-cylinder experiments. The volatile composition of post-run fluids was quantified by gas chromatography. Solid phases were examined by Raman spectroscopy, electron microscopy and laser ablation inductively coupled plasma mass spectrometry. Textures, porosity and phase abundances of recovered rock cores were visualized and quantified by three-dimensional, high-resolution computed tomography. We find that carbonation of serpentinites is efficient at sequestering CO2 from the interacting fluid into newly formed magnesite. Time-series experiments demonstrate that carbonation is completed within ∼96 h at 2 GPa and 600 °C. With decreasing CO2,aq antigorite is replaced first by magnesite + quartz followed by magnesite + talc + chlorite in distinct, metasomatic fronts. Above antigorite stability magnesite + enstatite + talc + chlorite occur additionally. The formation of fluid-permeable reaction zones enhances the reaction rate and efficiency of carbonation. Carbonation probably occurs via an interface-coupled replacement process, whereby interconnected porosity is present within reaction zones after the experiment. Consequently, carbonation of serpentinites is self-promoting and efficient even if fluid flow is channelized into veins. We conclude that significant amounts of carbonates may accumulate, over time, in the hydrated forearc mantle.

Primary CO2-bearing fluid inclusions in granulitic garnet usually do not survive

CO2-bearing fluid inclusions (FI) from granulites have been the main advocate for an active role of fluids during high-temperature metamorphism, yet the fluid regime of the deep crust and the role of fluids remain largely debated topics. In this dispute, one of the most controversial issues is the timing of FI entrapment relative to peak metamorphic conditions. Here we investigate three world-renowned high- to ultra-high temperature metamorphic terranes to evaluate the fate of primary CO2 and COH fluid inclusions certainly trapped during the prograde path. Fluid inclusions coexist along with nanogranitoids in peritectic garnet. Combination of cutting-edge techniques indicates that the FI are composed of fluid and aggregates of solid phases. The latter usually comprises siderite, ferroan magnesite, pyrophyllite, calcite, corundum, quartz, and in some cases kaolinite, dolomite, biotite and muscovite. In the fluid phase, low-density CO2 is the most common component and no free H2O has been detected. Methane and N2 may be also present. The high proportion of solids in the FI with carbonates and OH-bearing phases cannot have precipitated as daughter phases directly from a simple COH-fluid as daughter phases. These minerals require additional cations (e.g. Fe, Mg, Ca, Al, Si) which may be dissolved in very small amounts in crustal fluids and yet are very abundant in the host. The solids, therefore, imply that the initial composition of high-density carbonic inclusions must have been changed by the interaction with the host garnet during cooling. Thermodynamic modelling of such fluid-garnet interaction demonstrates that the assemblages found in FI are metastable and that, whatever retrograde path is followed by the host rock, primary C-bearing FI must change their nature to a multiphase assemblage of stepdaughter minerals as a natural consequence of cooling. It follows that supposed primary unmodified FI in garnet previously reported in the literature are, in most cases, secondary, retrograde features whose low temperature of entrapment inhibited fluid-host interaction. Our finding undermines the main pillar of the theory of carbonic fluid-assisted metamorphism (the presence of superdense CO2 inclusions in granulites), but at the same time it offers a novel perspective to identify prograde COH fluids in the deep hot crust.

In situ Sr isotope analysis of mantle carbonates: Constraints on the evolution and sources of metasomatic carbon-bearing fluids in a paleo-collisional setting

Carbonate-bearing wedge peridotites attest the mobilization of carbon (C) by slab fluids/melts circulating in a subduction setting. In general, COH fluids are thought to derive from the dehydration/partial melting of the crustal portions of slabs, especially during the exhumation of crust-mantle mélanges along continental subduction channels. In this study we combined textural observations with in-situ Sr isotope analyses of mantle carbonates occurring in different microstructural sites to test whether the fluids responsible for the carbonation of a mantle wedge are derived from the subducted continental crust or not. We focus on the Ulten Zone peridotites (Eastern Italian Alps) associated with high-grade felsic rocks, where carbonates occur mainly as dolomite and minor magnesite and calcite. In situ laser MC-ICP-MS analysis of peridotites representing different episodes of a complex metasomatic history, indicates that Sr isotopic variations can be linked to the different microstructural positions of carbonates. The C-metasomatism of the UZ peridotites is proposed to have occurred in two stages. The first stage is the HP‑carbonation at peak (eclogite-facies) conditions, with formation of interstitial matrix dolomite in textural equilibrium with hornblende to pargasite amphibole and Cl-apatite. This dolomite exhibits relatively unradiogenic 87Sr/86Sr present day values of 0.70487±0.00010, requiring different sources with respect to the associated migmatites and the overhanging mantle wedge. Carbonation continued during exhumation, with local injection of C-rich fluids forming a dolomite vein in association with tremolite and chlorite. The dolomite vein shows a wide range of 87Sr/86Sr (0.7036–0.7083), reflecting both the primary composition of carbonates and the consequent interaction with crustal fluids as expected in a crust-mantle mélange. The second stage is C-remobilization by dolomite dissolution and precipitation of brucite intergrowths with calcite during the final exhumation. This remobilization event has resulted in a similar Sr composition to the precursor dolomite.The mantle wedge is therefore capable of storing carbonates which have been shown to represent a complex metasomatic evolution from eclogite-facies conditions to very shallow structural levels. Therefore, fluids released from subducting slabs of continental lithosphere might be responsible for the crystallization of metasomatic minerals such as amphibole, phlogopite and zircon in the overlying ultramafic rocks. Conversely, the role of these metasomatic fluids on the carbonation of mantle wedge is likely overestimated. The combination of geochemical, isotopic and textural evidence suggests that dolomite inclusions and interstitial dolomite are derived in large part from a distinct source of C-bearing fluids that could be related to depleted mantle wedge sources and/or trondhjemitic igneous activity. In contrast, at the end of exhumation, residual COH-fluids released by the associated stromatic gneisses and orthogneisses resulted in late-stage dolomite veins having the highest Sr isotope values in the Ulten Zone peridotites.

Anatexis and fluid regime of the deep continental crust: New clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy)

AbstractWe investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO2, CH4, and N2, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO2 and reach the highest H2O contents (6.5–10 wt%). In the transition zone melts have intermediate H2O (4.8–8.5 wt%), CO2 (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO2 (up to 2,400 ppm), with H2O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe3+‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H2O contents of the melts throughout the three zones are higher than usually assumed for initial H2O contents of anatectic melts. The CO2 contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H2O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids.

Diamond destruction and growth during mantle metasomatism: An experimental study of diamond resorption features

Most diamonds found in kimberlites show complex patterns of growth and dissolution (resorption) surface features. Populations of diamonds from within single kimberlite bodies commonly contain a large diversity of diamond surface forms, some of which are a result of dissolution in kimberlite magma and others are inherited from the mantle. Morphological studies of natural diamonds differentiated features produced during dissolution in kimberlite magma and during mantle metasomatism. The former features were experimentally reproduced at 1–3 GPa and used to infer the presence and composition of magmatic fluid in different kimberlites. However, the mantle-derived resorption features have not been reproduced experimentally and the composition and origins of their formative solvents are unknown. Here we report the results of diamond dissolution experiments conducted in a multi-anvil apparatus at 6 GPa and 1200 to 1500°C in synthetic CaO–MgO–SiO2–CO2–H2O system. The experiments produced very different diamond resorption morphologies in COH fluid, in silicate-saturated fluid, and in silicate and carbonate melts. Dissolution in SiO2-free COH fluid developed rounded crystal forms with shallow negative trigons, striations and hillocks, which are commonly observed on natural diamonds and are similar in 6 GPa and in 1–3 GPa experiments. However, silicate-saturated fluid produced very different resorption features that are rarely observed on natural diamonds. This result confirms that natural, SiO2-poor fluid-induced resorption develops under the comparatively low-pressures of kimberlite ascent, because at mantle pressures the high content of SiO2 in fluids would produce features like those from the silicate-saturated experiments. Comparison of the experimental products from this study to natural diamond resorption features from the literature suggests that natural diamonds show no record of dissolution by fluids during mantle metasomatism. Diamond resorption morphologies developed in experiments with silicate–carbonate melts closely resemble many of the mantle-derived resorption features of natural diamonds, whose diversity can result from variable SiO2 concentration in carbonatitic melts and temperature variation. The experimental results imply that metasomatism by fluids does not dissolve diamond, whereas metasomatism by melts is diamond-destructive. The repetitive growth-dissolution patterns of natural diamonds could be due to diamond growth from fluids in harzburgitic lithologies followed by its dissolution in partial melts.

Redox processes and the role of carbon-bearing volatiles from the slab–mantle interface to the mantle wedge

The valence of carbon is governed by the oxidation state of the host system. The subducted oceanic lithosphere contains considerable amounts of iron so that Fe 3+ /Fe 2+ equilibria in mineral assemblages are able to buffer the (intensive) f O 2 and the valence of carbon. Alternatively, carbon itself can be a carrier of (extensive) ‘excess oxygen’ when transferred from the slab to the mantle, prompting the oxidation of the sub-arc mantle. Therefore, the correct use of intensive and extensive variables to define the slab-to-mantle oxidation by C-bearing fluids is of primary importance when considering different fluid/rock ratios. Fluid-mediated processes at the slab–mantle interface can also be investigated experimentally. The presence of CO 2 (or CH 4 at highly reduced conditions) in aqueous COH fluids in peridotitic systems affects the positions of carbonation or decarbonation reactions and of the solidus. Some methods to produce and analyse COH fluid-saturated experiments in model systems are introduced, together with the measurement of experimental COH fluids composition in terms of volatiles and dissolved solutes. The role of COH fluids in the stability of hydrous and carbonate minerals is discussed comparing experimental results with thermodynamic models and the message of nature.

Intra-slab COH fluid fluxes evidenced by fluid-mediated decarbonation of lawsonite eclogite-facies altered oceanic metabasalts

The interplay between the processes controlling the mobility of H2O and C-bearing species during subduction zone metamorphism exerts a critical control on plate tectonics and global volatile recycling. Here we present the first study on fresh, carbonate-bearing, lawsonite eclogite-facies metabasalts from Alpine Corsica, France, which reached the critical depths at which important devolatilization reactions occur in subducting slabs. The studied samples indicate that the evolution of oceanic crustal sequences subducted under present-day thermal regimes is dominated by localized fluid-rock interactions that are strongly controlled by the nature and extent of inherited (sub)seafloor hydrothermal processes, and by the possibility of deep fluids to be channelized along inherited or newly-formed discontinuities. Fluid channelization along inherited discontinuities controlled local rehydration and dehydration/decarbonation reactions and the stability of carbonate and silicate minerals at the blueschist-eclogite transition. Fluid-mediated decarbonation was driven by upward, up-temperature fluid flow in the inverted geothermal gradient of a subducting oceanic slab, a process that has not been documented in natural samples to date. We estimate that the observed fluid-rock reactions released 20–60 kg CO2 per m3 of rock (i.e. ~0.7–2.1 wt% CO2), which is in line with the values predicted from decarbonation of metabasalts in open systems at these depths. Conversely, the estimated time-integrated fluid fluxes (20–50 t/m2) indicate that the amount of carbon transported by channelized fluid flow within the volcanic part of subducting oceanic plates is potentially much higher than previous numerical estimates, testifying to the percolation of C-bearing fluids resulting from devolatilization/dissolution processes operative in large reservoirs.

Experimental determination of magnesia and silica solubilities in graphite-saturated and redox-buffered high-pressure COH fluids in equilibrium with forsterite + enstatite and magnesite + enstatite

We experimentally investigated the dissolution of forsterite, enstatite and magnesite in graphite-saturated COH fluids, synthesized using a rocking piston cylinder apparatus at pressures from 1.0 to 2.1 GPa and temperatures from 700 to 1200 °C. Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox conditions were buffered by Ni–NiO–H2O (ΔFMQ = − 0.21 to − 1.01), employing a double-capsule setting. Fluids, binary H2O–CO2 mixtures at the P, T, and fO2 conditions investigated, were generated from graphite, oxalic acid anhydrous (H2C2O4) and water. Their dissolved solute loads were analyzed through an improved version of the cryogenic technique, which takes into account the complexities associated with the presence of CO2-bearing fluids. The experimental data show that forsterite + enstatite solubility in H2O–CO2 fluids is higher compared to pure water, both in terms of dissolved silica (mSiO2 = 1.24 mol/kgH2O versus mSiO2 = 0.22 mol/kgH2O at P = 1 GPa, T = 800 °C) and magnesia (mMgO = 1.08 mol/kgH2O versus mMgO = 0.28 mol/kgH2O) probably due to the formation of organic C–Mg–Si complexes. Our experimental results show that at low temperature conditions, a graphite-saturated H2O–CO2 fluid interacting with a simplified model mantle composition, characterized by low MgO/SiO2 ratios, would lead to the formation of significant amounts of enstatite if solute concentrations are equal, while at higher temperatures these fluid, characterized by MgO/SiO2 ratios comparable with that of olivine, would be less effective in metasomatizing the surrounding rocks. However, the molality of COH fluids increases with pressure and temperature, and quintuplicates with respect to the carbon-free aqueous fluids. Therefore, the amount of fluid required to metasomatize the mantle decreases in the presence of carbon at high P–T conditions. COH fluids are thus effective carriers of C, Mg and Si in the mantle wedge up to the shallowest level of the upper mantle.

Silicate dissolution boosts the CO2 concentrations in subduction fluids

Estimates of dissolved CO2 in subduction-zone fluids are based on thermodynamic models, relying on a very sparse experimental data base. Here, we present experimental data at 1–3 GPa, 800 °C, and ∆FMQ ≈ −0.5 for the volatiles and solute contents of graphite-saturated fluids in the systems COH, SiO2–COH ( + quartz/coesite) and MgO–SiO2–COH ( + forsterite and enstatite). The CO2 content of fluids interacting with silicates exceeds the amounts measured in the pure COH system by up to 30 mol%, as a consequence of a decrease in water activity probably associated with the formation of organic complexes containing Si–O–C and Si–O–Mg bonds. The interaction of deep aqueous fluids with silicates is a novel mechanism for controlling the composition of subduction COH fluids, promoting the deep CO2 transfer from the slab–mantle interface to the overlying mantle wedge, in particular where fluids are stable over melts.

Dissolving dolomite in a stable UHP mineral assemblage: Evidence from Cal-Dol marbles of the Dora-Maira Massif (Italian Western Alps)

In deep and cold subduction such as that experienced by the UHP Units of the Western Alps, carbon dissolution is a relevant mechanism for carbon transfer from the slab into the mantle. The UHP impure Cal-Dol-marbles from the Dora-Maira Massif are studied to investigate the poorly known evolution of dolomite during deep subduction. Dolomite shows four stages of growth, from pre-Alpine to early-retrograde Alpine, coupled with chemical variations and distinct included mineral assemblages. To explain the evidence for growth and partial reabsorption of dolomite through HP prograde, UHP peak, and UHP early-retrograde Alpine metamorphism, a chemically simple marble (Cal, Dol, Di, Fo, and retrograde Atg, Tr, Mg-Chl) has been studied in detail. Microstructural relationships, coupled with mineral chemistry, indicate the growth of the assemblage dolomite+diopside+forsterite±aragonite during HP prograde, UHP peak, and UHP early-retrograde evolution. Mixed-volatile P-T projection modeled in the simple CaO-(FeO)-MgO-SiO 2 -H 2 O-CO 2 system and T - P - X CO 2 petrogenetic grids and pseudosections predict the prograde (1.7 GPa, 560 °C) growth of dolomite in equilibrium with diopside and forsterite through the breakdown of antigorite+aragonite. In a H 2 O-CO 2 -saturated system, the subsequent HP-UHP evolution is predicted in the Di+Fo+Dol+Arg stability field in equilibrium with a dominantly aqueous COH fluid [ 0 . 0003 X CO 2 0 . 0008 ], whose composition is internally buffered by the equilibrium assemblage. Thermodynamic modeling indicates that neither the consumption nor the growth of new dolomite generations at UHP conditions can have been induced by metamorphic reactions. The abundant primary H 2 O+Cal+Dol+Cl-rich Tr+Cl-rich Tlc±chloride fluid inclusions present in UHP Cpx indicate that a dominantly aqueous, saline (salinity >26.3 wt% of NaCl eq ) COH fluid, containing Ca, Mg, and Si as dissolved cations was present during the growth of the UHP assemblage Dol+Cpx+Ol+Arg. The complex zoning of dolomite is therefore interpreted as due to protracted episodes of dissolution and precipitation in saline aqueous fluids at HP/UHP conditions. Kinetics of dolomite dissolution in aqueous fluids is poorly known, and experimental and thermodynamic data under HP conditions are still lacking. Data on calcite indicate that dissolution at HP is enhanced by a prograde increase in both P and T , by high salinity in aqueous fluids, and/or low-pH conditions. In the studied marble, the P-T path and the occurrence of free high-saline fluids represent favorable conditions: (1) for the inferred dissolution-precipitation processes of the stable dolomite in a closed system, and (2) for possible migration of the dissolved carbonate, if the system would have been open during subduction.