Fayalite Magnetite Quartz Buffer Research Articles

The Redox State of the Asthenospheric Mantle and the Onset of Melting Beneath Mid‐Ocean Ridges

AbstractThe redox state of the convective asthenospheric mantle governs the speciation of volatile elements such as carbon and, therefore, influences the depth at which (redox) melting can occur, with implications for seismic signals. Geophysical observations suggest the potential presence of carbonatite melts at a depth of 200–250 km. However, thermodynamic models indicate that the onset of (redox) melting would occur at 100–150 km for a mantle with 3%–4% of Fe3+/∑Fe. Here, we present a new oxybarometer that is based on the V/Sc exchange coefficient between olivine and the melt, which is insensitive to surficial alteration, volatile degassing, electron exchange reactions and fractional crystallization. By applying this method to primary mid‐ocean ridge basalts (MORBs) from Southwest Indian Ridge and East Pacific Rise, we demonstrate that the average oxygen fugacity (fo2) of MORBs corrected for the depth of formation is 0.78 ± 0.26 (1σ) log units above the fayalite‐magnetite‐quartz (FMQ) buffer, which is slightly more oxidized than previously estimated (near FMQ buffer). Our findings indicate that the convective asthenospheric mantle exhibits higher oxygen fugacity than the continental lithospheric mantle. Along an adiabat, carbonatitic melts can form from a CO2‐bearing source at a depth of 200–250 km, explaining the asthenospheric mantle's electrical conductivity and seismic velocity anomaly.

Redox control of the partitioning of platinum and palladium into magmatic sulfide liquids

The partitioning behavior of platinum group elements in magmas is critical for their use as tracers of planetary accretion and in understanding magmatic sulfide deposits. Here we use laboratory experiments to determine sulfide liquid–silicate melt partition coefficients for platinum and palladium at 1.5 GPa, 1400 °C, and oxygen fugacity 1.5–2 log units above the fayalite–magnetite–quartz buffer. We find that the partitioning coefficients of these elements are 2.3 × 105 to 1.1 × 106 and are independent of the platinum and palladium concentration in the system. Combined with previous data obtained at oxygen fugacity below the fayalite–magnetite–quartz buffer, this indicates redox-controlled partitioning behavior whereby at oxidizing conditions platinum- and palladium-enrichments are achieved through their dissolution in sulfide liquids, while at reducing conditions the entrapment of platinum- and palladium-rich clusters in sulfide liquids is more critical. This redox-controlled partitioning behavior should be considered when studying crust–mantle differentiation and the formation of magmatic sulfide deposits.

Experimental determination of tin partitioning between titanite, ilmenite and granitic melts using improved capsule designs

Abstract Investigating mineral/melt Sn partitioning at high temperatures and pressures is a difficult task because Sn is a redox-sensitive multivalent element and easily alloys with noble metal sample capsules. To obtain accurate Sn partition coefficients between titanite, ilmenite, and granitic melts, we developed single capsule Pt or Au and double capsule Pt95Rh5 (or Au)–Re designs to avoid significant Sn loss at a controlled oxygen fugacity (ƒO2). With these new capsule designs, we performed piston-cylinder experiments of Sn partitioning between titanite, ilmenite, and granitic melts. The experimental P–T–ƒO2 conditions were 0.5–1.0 GPa, 850–1000 °C and ~QFM+8 to ~QFM–4 (QFM: Quartz–Fayalite–Magnetite buffer), with ƒO2 controlled by the solid buffers of Ru–RuO2, Re–ReO2, Co–CoO, graphite, and Fe–FeO. The obtained mineral/melt Sn partition coefficients (DSnmin/melt) are 0.48–184.75 for titanite and 0.03–69.45 for ilmenite at the experimental conditions. The DSnmin/melt values are largely dependent on ƒO2 although the effects of temperature and melt composition are also observed. DSnTtn/melt strongly decreases with decreasing ƒO2, from ~46–185 at the most oxidizing conditions (Ru–RuO2 buffer), to ~2–16 at moderately oxidizing to moderately reducing conditions (Re–ReO2 to Co–CoO and graphite buffers), to < 1 at the most reducing conditions (Fe–FeO buffer). DSnIlm/melt exhibits a variation trend similar to DSnTtn/melt, but is always lower than DSnTtn/melt at a given ƒO2. These DSnmin/melt values can be applied to quantitatively assess the mineralization potential of granitic magmas. Using DSnTtn/melt, we estimate that Sn contents are ~150–400 ppm in the premineralization magmas of the tin-mineralized Qitianling plutons (South China).

Oxidation state of Cu in silicate melts at upper mantle conditions

Beyond its economic value, copper (Cu) serves as a valuable tracer of deep magmatic processes due to its close relationship with magmatic sulfide evolution and sensitivity to oxygen fugacity (fO2). However, determining Cu’s oxidation state (+ 1 or + 2) in silicate melts, crucial for interpreting its behavior and reconstructing fO2 in the Earth’s interior, has long been a challenge. This study utilizes X-ray Absorption Near Edge Structure spectroscopy to investigate the Cu oxidation state in hydrous mafic silicate melts equilibrated under diverse fO2 (− 1.8 to 3.1 log units relative to the Fayalite–Magnetite–Quartz buffer), temperature (1150–1300 °C), and pressure (1.0–2.5 GPa) conditions. Our results reveal that Cu predominantly exists as Cu+ across all fO2 conditions, with a minor Cu2+ component. This dominance of Cu+ persists even in relatively oxidized melts, highlighting its limited sensitivity to fO2 under upper mantle conditions. This significantly constrains the utility of Cu as an oxybarometer in hydrous silicate melts in the deep Earth. However, our findings suggest that Cu isotopes primarily reflect the interplay of sulfide segregation/accumulation during magmatic differentiation, shedding light on these fundamental processes in Earth’s interior.

The role of high oxygen fugacity on genesis of the late Cenozoic Abaga basalts in Xilin Gol League, Inner Mongolia, China

ABSTRACTThe Abaga basalts, which erupted within an intraplate background in the late Cenozoic, are situated in eastern central Inner Mongolia. Previous studies have explored the genesis of these basalts, which were influenced by the subduction of slab. Although trace element ratios demonstrate that altered oceanic crust (AOC) and sediments have contributed to the genesis of the basalts, limited information about the detailed percentages of mixing among sediments, AOC, and depleted mantle (DM) based on Sr‒Nd‒Pb isotopic values has been reported. In this study, we modelled the possible mixing percentages among AOC, DM, and sediments. The calculated results illustrate that a proportion of 80% DM, 16% AOC, and 4% sediments is appropriate for interpreting the Sr‒Nd‒Pb isotopic variation of the basalts. In addition, the associated major and trace elements of olivine and the partition coefficient of V between olivine and melt are used to estimate the crystallization temperatures of the olivine phenocrysts and oxygen fugacity of the basalts. The calculated results display that the olivine mainly crystallized between 1248 and 1293°C (standard error estimate: 51°C), and the fO2 values of the Abaga basalts range from FMQ + 0.14 to FMQ + 1.46 (FMQ, fayalite–magnetite–quartz buffer). The estimated oxygen fugacity values plot in the range of island arc basalts (from FMQ to FMQ + 2) but are higher than those of both mid-ocean ridge basalt (MORB) (from FMQ-0.4 to FMQ + 0.5) and Abaga xenoliths (from FMQ-1.55 to FMQ + 0.53), which indicates that the basaltic melts were more oxidized than those of MORB and the Abaga lithospheric mantle. Furthermore, the primary magma and the corresponding mantle source were more oxidized than MORB. The Ba/Th and Ce/Pb ratios of the basalt present positive correlations with fO2 values, demonstrating that the addition of sediments and oceanic crust into the mantle source may have contributed to increasing the fO2 values of the basalts.

An Extended Calibration of the Olivine–Spinel Aluminum Exchange Thermometer: Application to the Melting Conditions and Mantle Lithologies of Large Igneous Provinces

Abstract The application of the olivine–spinel aluminum exchange thermometer to natural samples is limited by the restricted experimental data set on which it was calibrated. Here, we present a new data set of 46 high-temperature crystallization experiments and 21 reanalyzed published experiments, which we used to extend the calibration to higher and lower temperatures. The final calibration data set spans a range of conditions relevant to crustal and upper mantle processes: 1174–1606°C, 0.1–1350 MPa, QFM − 2.5 to QFM + 7.2 (oxygen fugacity, fO2, reported in log units relative to the quartz–fayalite–magnetite buffer, QFM), and 0–7.4 wt % H2Omelt. We propose three new models. The first is thermodynamically self-consistent, based on spinel Fe, Mg, Al, and Cr compositions and Al exchange between olivine and spinel. The second and third are empirical models that consider fewer elemental exchanges: the second uses only Al exchange and spinel compositions, whereas the third considers olivine–spinel Al and Cr exchange. All models include the modest effect of pressure on olivine-spinel equilibrium chemistry, whereas fO2 and water content have negligible effects. In general, as fewer elements are considered in the olivine–spinel exchange, the fit to experimental data worsens. Conversely, the associated decrease in model complexity improves their robustness against systematic errors when applied to natural crystal pairs: the thermodynamic model may underestimate crystallization temperatures in natural samples due to spinel subsolidus re-equilibration, whereas the empirical models (independent of Fe and Mg in spinel) are less sensitive to re-equilibration but yield temperatures with larger uncertainties. We applied a statistical test to select the most appropriate model for application to natural samples. When applied to lavas from mid-ocean ridges, Iceland, Skye, Emeishan, Etendeka, and Tortugal, our new temperature estimates are 30–100°C lower than previously proposed. The lower temperature estimates cause a lower mantle melting temperature and significant impacts on the mantle lithology constraints.

Experimental Equilibrium and Fractional Crystallization of a H2O, CO2, Cl and S-Bearing Potassic Mafic Magma at 1.0 GPa, With Implications for the Origin of Porphyry Cu (Au, Mo)-Forming Potassic Magmas

ABSTRACT The origin of intermediate to felsic potassic magmas is debated, and not much is known about the volatile content of potassic magmas associated with porphyry Cu (Au, Mo) deposits. To better understand the liquid line of decent of mafic potassic magmas and the behavior of volatiles during magma differentiation, we performed 19 experiments at 1.0 GPa and 1150 °C to 850 °C using piston cylinder presses. We developed a new experimental technique that involves a capsule liner made of single-crystal zircon to prevent the loss of Fe and S in the starting material to the noble metal capsule. The starting material is a high-Mg, basaltic trachyandesite (52 wt% SiO2, 12 wt% MgO, 1.9 wt% Na2O and 5.3 wt% K2O) from the Sanjiang region in southwestern China, doped with geologically realistic amounts of volatiles (i.e. 4.0 wt% H2O, 0.34 wt% CO2, 0.27 wt% Cl and 0.25 wt% S). The addition of 0.25 wt% S in the form of anhydrite internally buffered the experiments at an oxygen fugacity of 2.0 ± 0.5 log units above the fayalite–magnetite–quartz buffer, which is similar to the redox state of the Sanjiang variously evolved potassic magmas. The experimentally produced silicate melts match well with the Sanjiang intermediate to felsic magmas in terms of major, minor and trace element compositions, and also with regard to the S and Cl contents. The sequence of crystallizing minerals (olivine + clinopyroxene –> biotite ± orthopyroxene –> apatite –> K-feldspar) also fits with the one observed in the Sanjiang mafic to intermediate magmas. These results suggest that the Sanjiang intermediate to felsic magmas, including the porphyry Cu (Au, Mo)-forming magmas, can form solely by differentiation of potassic mafic magmas without any involvement of old crustal material. During experimental differentiation at 1.0 GPa, the S content of the evolving silicate melt first increased until ~57 wt% melt SiO2, and then decreased in response to precipitation of sulfides, sulfate melt and/or anhydrite, whereas the H2O and Cl contents of the evolving silicate melt increased exponentially until saturation in a CO2-rich fluid was reached at 60 to 65 wt% melt SiO2 and ~ 8 wt% melt H2O. During further magma differentiation the H2O and Cl contents of the evolving silicate melt remained constant until ~70 wt% melt SiO2, after which point the Cl content of the silicate melt decreased due to increased partitioning of Cl into the fluid phase ± increased fluid/melt ratio. Based on these experimental results and petrographic and geochemical evidence from natural samples, the Sanjiang porphyry Cu (Au, Mo)-forming magmas (65–70 wt% SiO2) are interpreted to have formed through differentiation of primitive, mantle-derived, potassic magmas in the lower crust (≥1.0 GPa), and to have ascended ±directly from the lower crust to shallow crustal levels. They likely contained 8 to 13 wt% H2O, 0.37 to 0.90 wt% Cl and 0.07–0.29 wt% S. This case study on the magma evolution in the Sanjiang region may have implications for the origin and nature of intermediate to felsic potassic magmas in various tectonic settings.

Petrological characterization of Fe–Ti oxides in metamafic rocks from the Nw borborema province, Ne Brazil

The coexistence of Fe–Ti oxide phases constitutes an important petrogenetic tool to understanding the evolution of metamafic rocks. In the NW part of the Borborema Province, NE Brazil, metamafic rocks occur in two distinct zones: the Cariré Granulite and the Forquilha Eclogite. The present work characterizes the sequence of crystallization involving Fe–Ti oxides of these lithotypes and constrains its redox conditions using the ilmenite-rutile oxybarometer. Multiphase association of Fe–Ti oxides of a mafic granulite from the Cariré Zone and a retrograded eclogite from the Forquilha Zone comprise ilmenite, magnetite, rutile, spinel, and hematite. Rutile occurs as inclusions, surrounded by vermicular, xenomorphic or coronitic titanite, within ilmenite crystals. The redox conditions set by the sequence of Fe–Ti oxides crystallization and the rutile-ilmenite oxybarometer are compared to the FMQ (fayalite-magnetite-quartz) buffer, indicating an increase in oxygen fugacity from peak to retrograde conditions. Our characterization stablishes (re)crystallization and oxyexsolution sequences for the Fe–Ti oxides and the rutile-ilmenite oxybarometer in the mafic granulite from the Cariré Zone suggests oxygen fugacity of FMQ – 3.6. The retrograded eclogite indicates a more reduced environment for the Forquilha Zone, in which the low Fe3+ content of ilmenite hampers the estimation of redox conditons. The constrained ΔFMQ is within the range of common redox compositions for subduction-related metamafic rocks, accordingly with the regional geochemical classification reported in the literature to the Cariré and Forquilha zones.

Redox species and oxygen fugacity of slab-derived fluids: Implications for mantle oxidation and deep carbon-sulfur cycling

The generation and migration of slab-derived fluids modulate subduction zone seismicity, arc magmatism, and deep volatile cycling. However, the redox species and oxygen fugacity (fO2) (hereafter expressed as log units relative to the fayalite–magnetite–quartz buffer, △FMQ) of slab-derived fluids are highly debated. Here we conducted phase equilibria modeling on altered oceanic crust (AOC) and serpentinites along typical subduction geotherms in the C-S-bearing system over a pressure range of 0.5–6 GPa. With the averaged compositions of AOC and serpentinite, our calculated results show that oxidized carbon-sulfur species dominate slab-derived fluids during slab subduction. As a result, slab-derived fluids are highly oxidized and at or above the typical △FMQ values of arc magmas at forearc to subarc depths. The predicted oxidized carbon and sulfur species are compatible with natural observations in fluid inclusions from many oceanic HP metamorphic rocks. More importantly, it is revealed that, the redox state of slab-derived fluids is primarily controlled by the redox budget (RB) of the slab prior to subduction. Subduction-zone thermal structure, however, only exerts a minor influence on the slab-derived fluid fO2, which is supported by the similar fO2 ranges in arc lavas from cold and hot subduction zones. Our models further show that, if an open system is assumed, most of carbon (>70%) and sulfur (>50%) in cold subducted AOC and serpentinite would be lost at subarc depths. Small amounts of carbon and sulfur could be transported into the deeper mantle via closed-system subduction and open-system cold subduction, supplying the source materials for volatile-rich intraplate magmas and superdeep diamonds.

An Experimental Study on Kinetics-Controlled Ca-Carbonate Aqueous Reduction into CH4 (1 and 2 GPa, 550°C): Implications for C Mobility in Subduction Zones

Abstract Abiotic methane (CH4) generation under subduction zone conditions has been experimentally investigated through aqueous reduction of pure C-bearing materials (e.g. carbonate minerals and organic matter). However, quantitative assessments of CH4 production in these experiments, as well as the potential effects of other components such as SiO2 on the reduction processes, have not yet been well established. Here, we performed experiments to quantitatively evaluate the time-resolved Ca-carbonate aqueous reduction into CH4 at P = 1 and 2 GPa and T = 550°C in the CaO + COH, CaO + SiO2 + COH, and CaO + SiO2 + MgO + COH systems, employing calcite + water ± quartz ± serpentine (synthetic chlorine (Cl)-bearing chrysotile and natural Fe−Al-bearing antigorite) as starting materials. Redox conditions of the experiments were buffered by iron−wüstite (IW) using a double capsule setting, corresponding to oxygen fugacity (fO2) values (expressed as log units relative to the fayalite–magnetite–quartz buffer, ΔFMQ) in the inner capsule of ΔFMQ ≈ −5.5 at 1 GPa and ΔFMQ ≈ −6.0 at 2 GPa. The solid products are mainly composed of portlandite ± larnite ± wollastonite ± brucite, while Ca-carbonate and/or silicate reactants commonly occur as relicts. Quadrupole mass spectrometric analysis shows that CH4 and H2O are the major COH molecular species in the fluid products, with molar ratios between CH4 and starting calcite representing the reaction progress ranging from ~0.13 to ~1.00. Comparisons of experimental run products with thermodynamically predicted phase assemblages, together with time-series experiments, indicate that the reduction processes are primarily controlled by reaction kinetics. At 1 GPa and 550°C, rate constants of 4.0 × 10−6 s−1, 7.4 × 10−6 s−1, and 2.6 × 10−6 s−1 were retrieved for reactions starting with calcite + quartz + water, calcite + synthetic Cl-bearing chrysotile + water, and calcite + natural Fe−Al-bearing antigorite + water, respectively, significantly higher than the constant of 0.8 × 10−6 s−1 for the silicate-absent reaction. Besides, an increase in pressures can also enhance the reduction efficiency of Ca-carbonates until reaching equilibrium with the fluids. Our data provide experimental evidence for kinetics-controlled Ca-carbonate aqueous reduction into CH4 in subduction zones, indicating that silicate involvement and/or pressure increase can accelerate the reaction rates through short-lived fluid–rock interactions, which may have important implications for deep C mobility.

Petrogenesis of a nepheline syenite from parts of the Chotanagpur Granite Gneissic Complex: implications for Neoproterozoic crustal extension in the East Indian Shield

AbstractThe North Purulia Shear Zone that dissects the granulite basement of the Chotanagpur Granite Gneissic Complex of the East Indian Shield exposes a deformed and metamorphosed nepheline syenite. The studied ‘foid-monzosyenite’ shows high abundances of large ion lithophile elements and high field strength elements with low abundances of compatible elements. Trace-element signatures show negative U, Th, Zr, Ti and Pb and positive Sr, Ba and Eu anomalies with respect to the primitive mantle. The chondrite-normalized diagram shows strongly fractionated rare earth element patterns ((La/Lu)N ∼23–87). Geochemical fingerprints suggest that the basanitic protolith was formed by low-degree partial melting of garnet peridotite in the sub-continental lithospheric mantle. The enriched large ion lithophile, high field strength element and light rare earth element concentrations (relative to primitive mantle) can be explained by a mixed mantle source with components from a previously deformed alkaline rock/carbonatite. Geochemical data do not support any significant crustal contamination and suggest variable fractionation of clinopyroxene, ilmenite, titanite and apatite from the parental melt. Petrological data are consistent with the view that the nepheline syenite magma was emplaced in a rift setting with a minimum temperature of 800–900°C, low fO2 conditions (below the fayalite–magnetite–quartz buffer) at a mid-crustal depth between 950 and 900 Ma. The continental rift zone, however, did not lead to the formation of an open ocean basin. Subsequently, the studied rock and its basement was deformed and metamorphosed in a continent–continent collisional setting at ∼900 Ma. Combining information from the other Indian occurrences with this study, it is demonstrated that the deformed alkaline rocks and carbonatite are potentially valuable for tracing the birth and demise of the palaeo-supercontinents.

Variable Modes of Formation for Tonalite–Trondhjemite–Granodiorite–Diorite (TTG)-related Porphyry-type Cu ± Au Deposits in the Neoarchean Southern Abitibi Subprovince (Canada): Evidence from Petrochronology and Oxybarometry

Abstract Most known porphyry Cu ± Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite–trondhjemite–granodiorite–diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ~2·7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu ± Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ~2·74 Ga, ~2·70 Ga, and ~ 2·69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t) ± SD values of 4·5 ± 0·3, 4·2 ± 0·6, and 4·3 ± 0·4, and δ18O ± SD values of 5·40 ± 0·11 ‰, 3·91 ± 0·13 ‰, and 4·83 ± 0·12 ‰, respectively. These isotopic signatures indicate that, although these magmas are mantle-sourced with minimal crustal contamination, for the St-Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids. Primary barometric minerals (i.e. zircon, amphibole, apatite, and magnetite–ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite–magnetite–quartz buffer as ΔFMQ, using zircon geochemistry indicates that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from ΔFMQ –0·3 ± 0·6 to ΔFMQ +0·8 ± 0·4 and to ΔFMQ +1·2 ± 0·4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of ΔFMQ +1·6 ± 0·3 and ΔFMQ +2·6 ± 0·1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate that amphibole may overestimate the fO2 of intrusive rocks by up to 1 log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e. S6+/ΣS) in primary apatite yielded ≥ΔFMQ −0·3 and ΔFMQ +1·4–1·8 for St-Jude and Clifford, respectively. The magnetite–ilmenite mineral pairs from the Clifford tonalite yielded ΔFMQ +3·3 ± 1·3 at equilibrium temperatures of 634 ± 21 °C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0·1 wt%) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results to indicate that the deposits at Côté Gold and Clifford formed from mildly (~ΔFMQ +0·8 ± 0·4) to moderately (~ΔFMQ +1·5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (~ΔFMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu–Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias.

Mechanism of formation of podiform chromitite: Insights from the oxidation states of podiform chromitites and host peridotites from the Luobusa ophiolite, southern Tibet

Although the mechanism of formation of podiform chromitite is debated, oxygen fugacity (fO2) is likely one of the controlling factors, although the detailed mechanisms are poorly understood. To provide insight into this issue, we determine the Fe3+/∑Fe ratio of chromite via Mӧssbauer spectrometry, and from this calculate the fO2 of chromitites, dunite envelopes, dunite lenses, and their host peridotites in the Luobusa ophiolite in southern Tibet. The fO2 (−1.49 to −0.52 log units relative to the fayalite–magnetite–quartz buffer) and mineral chemistry of the host peridotites are similar to those of both abyssal and forearc peridotites. Higher fO2 values of the dunites and chromitites (−0.65 to −0.33, 0.69 to 1.13, and −0.46 to 0.64 log units for the dunite lenses, dunite envelopes, and chromitites, respectively) relative to those of host peridotites must have been inherited from their parental magmas. Both chromite compositions and the fO2 of the dunites and chromitites are similar to those in boninite-like rocks. Importantly, there is a previously unrecognized but marked decrease in fO2 from the dunite envelopes to the chromitites. This decrease is intimately related to the chromitite-forming processes and preferably explained as the result of the exsolution of oxidized fluids from high-fO2 magmas during subduction initiation.

Cenozoic temporal variation of crustal thickness in the Urumieh-Dokhtar and Alborz magmatic belts, Iran

We present regional variations of whole-rock Sr/Y and (La/Yb)N ratios of magmatic rocks along the Cenozoic Urumieh-Dokhtar and Alborz magmatic belts, Iran. Both the magmatic belts are located at the north of the main Zagros Neo-Tethyan suture. The Urumieh-Dokhtar magmatic belt (UDMB), which trends NW-SE for 1000 km across Iran, was characterized by the intensive volcanism and plutonism, and defined the magmatic front (MF) of the Zagros orogenic belt. The Alborz magmatic belt (AMB) is situated to the north, and characterized by less intense magmatic activity. The Alborz magmatic belt was formed behind it in the rear-arc (RA) domain. A striking feature of the both magmatic belts is the transition from normal calc-alkaline arc magmatism during the Eocene–Oligocene to adakite-like calc-alkaline magmatism during the Middle to Late Miocene–Pliocene. The late-Cenozoic magmatism of the UDMB and AMB shows higher Sr/Y and (La/Yb)N. However, it should be noted that crustal thickening event is intensive in the UDMB than AMB during Late Cenozoic. Using the composition of the Lale-Zar zircons from the SE UDMB we determined the oxygen fugacity (fO2) during zircon crystallization to be between FMQ (fayalite–magnetite–quartz buffer) -0.69 to +2.41, whereas those of the Hashroud-Teckmdash-Gormolla zircons from NW AMB range from −1.22 to +5.99. The fO2 estimates suggest relatively more oxidized conditions for the Late Cenozoic igneous rocks of the AMB. Compiled data from the UDMB and AMB intrusions show an increase in average zircon crystallization temperatures with decreasing age. These outcomes have been interpreted in terms of variation of the crustal thickness, from 30 to 35 km during Eocene-Oligocene to 40–55 km during the middle-late Miocene. We propose the increase in crustal thickness is associated with the collision between the Arabian plate and Iran and subsequent convergence during the middle-late Miocene.

Textural and Geochemical Evidence for Magnetite Production upon Antigorite Breakdown During Subduction

AbstractMagnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in subducted ultramafic rocks. Here, we present a detailed petrological and geochemical study of magnetite in hydrous ultramafic rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite–hematite solid solution minerals, and sulfides coexist in both antigorite-serpentinite and chlorite-harzburgite at c. 670 °C and 1·6 GPa, displaying successive crystallization stages, each characterized by specific mineral compositions. In antigorite-serpentinite, magnetite inherited from seafloor hydration and recrystallized during subduction has moderate Cr (Cr2O3 < 10 wt%) and low Al and V concentrations. In chlorite-harzburgite, polygonal magnetite is in textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, and ilmenite–hematite solid solution minerals. The Cr2O3 contents of this magnetite are up to 19 wt%, higher than any magnetite data obtained for antigorite-serpentinite, along with higher Al and V, derived from antigorite breakdown, and lower Mn concentrations. This polygonal magnetite displays conspicuous core to rim zoning as recognized on elemental maps. Cr–V–Al–Fe3+ mass-balance calculations, assuming conservative behavior of total Fe3+ and Al, were employed to model magnetite compositions and modes in the partially dehydrated product chlorite-harzburgite starting from antigorite-serpentinite, as well as in the serpentinite protolith starting from the chlorite-harzburgite. The model results disagree with measured Cr and V compositions in magnetite from antigorite-serpentinites and chlorite-harzburgites. This indicates that these two rock types had different initial bulk compositions and thus cannot be directly compared. Our mass-balance analysis also reveals that new magnetite formation is required across the antigorite-breakdown reaction to account for the mass conservation of fluid-immobile elements such as Cr–V–Al–Fe3+. Complete recrystallization and formation of new magnetite in equilibrium with peak olivine (Mg# 89–91), chlorite (Mg# ∼95), orthopyroxene (Mg# 90–91), and pentlandite buffer the released fluid to redox conditions of ∼1 log unit above the quartz–fayalite–magnetite buffer. This is consistent with the observation that the Fe–Ti solid solution minerals (hemo-ilmenite and ilmeno-hematite) crystallized as homogeneous phases and exsolved upon exhumation and cooling. We conclude that antigorite-dehydration reaction fluids carry only a moderate redox budget and therefore may not be the only reason why the magmas are comparatively oxidized.

The Petrology of the Tarosero Volcanic Complex: Constraints on the Formation of Extrusive Agpaitic Rocks

Abstract The Quaternary Tarosero volcano is situated in the East African Rift of northern Tanzania and mainly consists of trachyte lavas and some trachytic tuffs. In addition, there are minor occurrences of extrusive basalts, andesites and latites, as well as peralkaline trachytes, olivine trachytes and phonolites. Some of the peralkaline phonolites contain interstitial eudialyte, making Tarosero one of the few known occurrences for extrusive agpaitic rocks. This study investigates the genetic relationships between the various rock types and focuses on the peculiar formation conditions of the extrusive agpaitic rocks using a combination of whole-rock geochemistry, mineral chemistry, petrography, thermodynamic calculations, and major and trace element modelling. The Tarosero rocks formed at redox conditions around or below the fayalite–magnetite–quartz buffer (FMQ). During multi-level magmatic fractionation at depths between ∼40 km and the shallow crust, temperature decreased from >1100 °C at near-liquidus conditions in the basalts to ∼700 °C in the peralkaline residue. Fractional crystallization models and trace element characteristics do not indicate a simple genetic relationship between the trachytes and the other rock types at Tarosero. However, the genetic relationships between the primitive basalts and the intermediate latites can be explained by high-pressure fractional crystallization of olivine + clinopyroxene + magnetite + plagioclase + apatite. Further fractionation of these mineral phases in addition to amphibole and minor ilmenite led to the evolution towards the peralkaline trachytes and phonolites. The eudialyte-bearing varieties of the peralkaline phonolites required additional low-pressure fractionation of alkali feldspar and minor magnetite, amphibole and apatite. In contrast to the peralkaline trachytes and phonolites, the peralkaline olivine trachytes contain olivine instead of amphibole, thus indicating a magma evolution at even lower pressure conditions. They can be modelled as a derivation from the latites by fractional crystallization of plagioclase, clinopyroxene, magnetite and olivine. In general, agpaitic magmas evolve under closed-system conditions, which impede the escape of volatile phases. In the case of the extrusive agpaitic rocks at Tarosero, the early exsolution of fluids and halogens was prevented by a low water activity. This resulted in high concentrations of rare earth elements (REE) and other high field strength elements (HFSE) and the formation of eudialyte in the most evolved peralkaline phonolites. Within the peralkaline rock suite, the peralkaline olivine trachytes contain the lowest HFSE and REE concentrations, consistent with mineralogical evidence for formation at a relatively high water activity. The lack of amphibole fractionation, which can act as a water buffer of the melt, as well as the evolution at relatively low-pressure conditions caused the early exsolution of fluids and loss of water-soluble elements. This prevented a strong enrichment of HFSE and REE before the magma was finally extruded.

Reduced amphibolite facies conditions in the Precambrian continental crust of the Siberian craton recorded by mafic granulite xenoliths from the Udachnaya kimberlite pipe, Yakutia

It is widely accepted that granulite xenoliths from kimberlites provide a record of granulite facies metamorphism at the basement of cratons worldwide. However, application of the phase equilibria modeling for seven representative samples of mafic granulites from xenoliths of the Udachnaya kimberlite pipe, Yakutia, revealed that a granulitic garnet + clinopyroxene + plagioclase ± orthopyroxene ± amphibole ± scapolite mineral assemblage was likely formed in the middle crust under amphibolite facies conditions (600–650 °C and 0.8–1.0 GPa) in a deficiency of fluid. Clinopyroxene in the rocks is characterized by elevated aegirine content (up to 10 mol.%) both in the earlier magmatic cores and in the later metamorphic rim zones of the grains. Nevertheless, the phase equilibrium modeling for all samples indicates surprisingly reduced conditions, i.e. oxygen fugacity 1.6–3.3 log units below the FMQ (Fayalite-Magnetite-Quartz) buffer. In contrast, the coexistence of Fe-Ti oxides indicates temperatures of 850–990 °C and oxygen fugacity about lg(FMQ) ± 0.5, conditions which correspond to earlier stages of rock evolution. Reduction of oxygen fugacity during cooling is discussed in the context of the evolution of a complex fluid. The reconstructed P-T conditions for the final equilibration in the mafic granulites indicate that temperatures were ~250 °C higher than those extrapolated from the continental conductive geotherm of 35–40 µW/m2 deduced from peridotite xenoliths of the Udachnaya pipe. Although the granulites resided in the crust for a period for at least 1.4 Ga, they did not re-equilibrate to the temperatures of the geotherm, likely due to the blocking of mineral reactions under relatively low temperatures and fluid-deficient conditions

Deserpentinization in Subduction Zones as a Source of Oxidation in Arcs: a Reality Check

AbstractPrevious studies have concluded that dehydration of serpentinites in subduction zones produces oxidizing fluids that are the cause of oxidized arc magmas. Here, observations of natural samples and settings are combined with thermodynamic models to explore some of the factors that complicate interpretation of the observations that form the basis of this conclusion. These factors include the variability of serpentinite protoliths, the roles of carbon and sulfur in serpentinite evolution, variability in serpentinization in different tectonic settings, changes in the bulk compositions of ultramafic rocks during serpentinization, fundamental differences between serpentinization and deserpentinization, and the absence of precise geothermobarometers for ultramafic rocks. The capacity of serpentinite-derived fluids to oxidize sub-arc magma is also examined. These fluids can transport redox budget as carbon-, sulfur-, and iron-bearing species. Iron- and carbon-bearing species might be present in sufficient concentrations to transport redox budget deep within subduction zones, but are not viable transporters of redox budget at the temperatures of antigorite breakdown, which produces the largest proportion of fluid released by serpentinite dehydration. Sulfur-bearing species can carry significant redox budget, and calculations using the Deep Earth Water (DEW) model show that these species might be stable during antigorite breakdown. However, oxygen fugacities of ∼ΔFMQ + 3 (where FMQ refers to the fayalite–magnetite–quartz buffer, and ΔFMQ is log fO2 – log fO2, FMQ), which is close to, or above, the hematite–magnetite buffer at the conditions of interest, are required to stabilize oxidized sulfur-bearing species. Pseudosection calculations indicate that these conditions might be attained at the conditions of antigorite breakdown if the starting serpentinites are sufficiently oxidized, but further work is required to assess the variability of serpentinite protoliths, metamorphic pressures and temperatures, and to confirm the relative positions of the mineral buffers with relation to changes in fluid speciation.

Olivine chemistry of the Quaternary Datong basalts of the Trans-North China Orogen: insights into mantle source lithology and redox–hydration state

AbstractCenozoic intraplate basalts are widespread above the Big Mantle Wedge (BMW) and its front in East Asia. While the mantle source lithology and redox-hydration state have been demonstrated to be crucial in the generation of basalts above the BMW, their nature and role in the basalts above the front of the BMW is poorly constrained. To address this, we report olivine compositions of the Quaternary Datong basalts. Datong basalts exhibit ocean island basalt-like trace-element compositions and depleted Sr–Nd isotopes with slightly enriched signatures (enriched mantle I, EMI) for tholeiitic basalts. Olivines of the Datong basalts show high Ni and Fe/Mn, and low Ca, Mn and Mn/Zn values, pointing to a pyroxenite source. Applying V and Ca partition coefficients between olivine and whole rock, respectively, the Datong basalts lie −0.44 to 0.64 log units above the fayalite–magnetite–quartz buffer for fO2, and contain 2.1–3.4 wt% H2O but highly variable H2O/Ce values (265–1498). Both fO2 and H2O/Ce in the basalts vary with whole-rock and olivine compositions, indicating that the source was the main control. Thus, there is a heterogeneous redox–hydration state in the source, the EMI component being relatively reduced but extremely wet and recycled oceanic crust being relatively oxidized but dry. The extremely wet EMI component was probably derived from the mantle transition zone. In the light of our findings, we propose a model in which mantle upwelling carried the recycled oceanic crust and EMI component from the mantle transitional zone to shallow mantle, owing to the Pacific slab stagnating in the mantle transitional zone, to form pyroxenite, which subsequently melted to generate Datong basalts.

The intrinsic nature of antigorite breakdown at 3 GPa: Experimental constraints on redox conditions of serpentinite dehydration in subduction zones

Antigorite is considered as the most important source of water in subduction zones, playing a key role during arc magma genesis. Although, these magmas seem more oxidized than mid-oceanic ridge basalts (MORB), the possible inherent link between the oxidation state of arc magmas and serpentinite-derived hydrous fluids is still not well established. Here, we have performed dehydration experiments of natural antigorite serpentinite containing 5 weight percent (wt%) magnetite at 3 GPa and in a temperature range from 600 to 900 °C using a multi-anvil apparatus. These experiments aim to reproduce the different stages of H2O release, forming chlorite, olivine and orthopyroxene and water. Our experimental set up permits to preserve the intrinsic high oxygen fugacity (fO2) of serpentinite during dehydration. The new olivine and orthopyroxene which formed in equilibrium with antigorite, chlorite and magnetite have high XMg numbers setting up the oxygen fugacity to high values, between 3.1 and 4.1 log units above QFM (Quartz–Fayalite–Magnetite buffer). Hematite is observed concomitantly with high XMg in olivine, of 0.94–0.97, generally at low temperatures, below 800 °C, in coexistence with chlorite. Once the magnetite is destabilized, upon chlorite breakdown which occurs above 800 °C, the oxygen fugacity decreases to 3.7 due to the decrease of the XMg of silicates. This study demonstrates the highly oxidizing nature of the fluids released from antigorite dehydration. Thus, at high pressure and high temperature conditions, fO2-sensitive elements such as carbon and sulfur are expected to be mobilized under their oxidized form, providing an oxidizing context for arc magmas genesis and assuming that they are not completely reduced by their percolation through meta-gabbro, meta-basalts and meta-sediments.