Variable Oxygen Fugacity Research Articles

Mineral Geochemistry of Apatite in the Jiama Porphyry‐Skarn Deposit, Tibet and its Geological Significance

AbstractThe Jiama deposit, a significant porphyry‐skarn‐type copper polymetallic deposit located within the Gangdese metallogenic belt in Tibet, China, exemplifies a typical porphyry metallogenic system. However, the mineral chemistry of its accessory minerals remains under‐examined, posing challenges for resource assessment and ore prospecting. Utilizing electron microprobe analysis and LA‐ICP‐MS analysis, this study investigated the geochemical characteristics of apatite in ore‐bearing granite and monzogranite porphyries, as well as granodiorite, quartz diorite, and dark diorite porphyries in the deposit. It also delved into the diagenetic and metallogenic information from these geochemical signatures. Key findings include: (1) The SiO2 content, rare earth element (REE) contents, and REE partition coefficients of apatite indicate that the dark diorite porphyry possibly does not share a cogenetic magma source with the other four types of porphyries; (2) the volatile F and Cl contents in apatite, along with their ratio, indicate the Jiama deposit, formed in a collisional setting, demonstrates lower Cl/F ratios in apatite than the same type of deposits formed in a subduction environment; (3) compared to non‐ore‐bearing rock bodies in other deposits formed in a collisional setting, apatite in the Jiama deposit exhibits lower Ce and Ga contents. This might indicate that rock bodies in the Jiama deposit have higher oxygen fugacity. Nevertheless, the marginal variation in oxygen fugacity between ore‐bearing and non‐ore‐bearing rock bodies within the deposit suggests oxygen fugacity may not serve as the decisive factor in the ore‐hosting potential of rock bodies in the Jiama deposit.

Contribution discrepancy of two distinct wall rocks to wolframite mineralization from Dongshanwan quartz-vein tungsten deposit, Southern Great Xing’an Range, China

The Southern Great Xing’an Range is a significant polymetallic metallogenic belt in Northeast China. The ages of granite related to tungsten mineralization have been well quantified, however, the timing and mechanism of quartz vein-type wolframite deposition remain unclear. Therefore, our study focused on the Dongshanwan tungsten deposit, representing a typical wolframite-bearing quartz vein-type deposit in the Southern Great Xing’an Range. Quartz vein-type tungsten mineralization was mainly contained in the Early Cretaceous granite porphyry, with few exceptions in the Permian tuff. Representative wolframite samples were collected from intragranitic (Wol-I) and intra-tuff (Wol-II) quartz veins for wolframite U–Pb dating, electron probe microanalysis (EPMA), and trace elemental analysis. Wolframite U–Pb dating recognized an Early Cretaceous mineralization event at 139–141 Ma, consistent with the previous Dongshanwan granite’s ages (∼142 Ma) but much younger than tuff (∼260 Ma). All the wolframite samples belonged to the ferberite series (FeO/MnO = 4.0–6.9), which indicates a link to the highly fractionated granite. Wol-I showed high contents of Nb, Ta, Sc, and HREEs and a strongly negative Eu anomaly (0.02–0.29), while these elements in Wol-II were approximately one order of magnitude lower than in Wol-I and the Eu anomaly was highly variable (0.17–1.02). The redox characteristics reflected by the Eu anomaly of wolframite may correspond to the variations in oxygen fugacity. Based on petrology, geochemistry, and element mass migration, this study suggests that the granite porphyry and tuff in the Dongshanwan deposit contributed differently to the wolframite precipitation even if the ore-forming fluids were of a common magmatic-hydrothermal origin. The exsolved fluid reserved in the closed system interacts with granite to achieve continuous enrichment in Nb, Ta, Sn, and REEs and accepts external Fe supply from granite alteration to precipitate Wol-I. In contrast, the fluids in relatively open and non-granitic systems have lower Nb, Ta, Sn, and REEs than the former, and Fe supply depends on the alteration of tuff. The abundance of fluorite and the non-CHARAC (CHArge-and-RAdius-Controlled) behavior of Y/Ho and Zr/Hf in the wolframite suggested that fluorine plays an important role in the migration and enrichment of tungsten and other elements during tungsten mineralization.

Greenalite-Chamosite composition, geothermometry and oxygen fugacity variations in pisolitic ironstone and carbonates of the Chilpi Group: implication on Paleoproterozoic seawater chemistry

Greenalite-Chamosite composition, geothermometry and oxygen fugacity variations in pisolitic ironstone and carbonates of the Chilpi Group: implication on Paleoproterozoic seawater chemistry

Al and H incorporation and Al-diffusion in natural rutile and its high-pressure polymorph TiO 2 (II)

Abstract Rutile is an important accessory mineral in metamorphic rocks and is used as a geothermobarometer or geochronometer. This study aims to bridge the gap between diffusion studies in simplified and complex natural systems by investigating the incorporation and mobility of Al in natural rutile and its high-pressure polymorph TiO 2 (II). Experiments were performed at 0.1 MPa to 7 GPa, 1223–1373 K, at buffered μ(Al 2 O 3 ) and with f O 2 constrained to ≤CCO, which is the equilibrium between graphite and a CO-CO 2 gas phase. Based on electron probe microanalysis, secondary ion mass spectrometry and Fourier transform infrared analyses, we suggest a complex combination of mechanisms to explain the incorporation of Al and H in natural rutile and TiO 2 (II). This includes: (1) the incorporation of Al 3+ on octahedral Ti-sites charge balanced by the formation of oxygen vacancies; and (2) the incorporation of oxygen in interstitial positions charge balanced by hydrogen interstitials. Determined Al-diffusivities in natural TiO 2 are approximately eight to nine orders of magnitude faster compared to previously published data. A possible explanation includes a significantly enhanced rate of ionic diffusion through the combined effect of hydrolytic weakening, enhanced Al-diffusion through extended defects and to a minor extent oxygen fugacity variations. Consequently, results of this study question that the inferred high closure temperatures for the Al-in-rutile geothermobarometer can be applied to all natural systems.

Extreme Differentiation along Multiple Liquid Lines of Descent in Strongly Peralkaline Magma Series at Pantelleria (Italy)

AbstractThe liquid line of descent from trachyte to pantellerite is controlled primarily by fractional crystallization of alkali feldspar, with whole-rock compositions following a fractionation path along the ‘thermal valley’ in the peralkaline haplogranite system Qz-Ab-Or-Ac-Ns and terminating at a minimum on the feldspar-quartz cotectic. Although whole-rock compositions for different pantelleritic suites follow nearly identical paths in a Qz-Ab-Or projection that terminate near the experimental minimum (Qz40.5Or34.5Ab25 at 100 MPa, projected from Ac-Ns), matrix glass from samples with near-minimum compositions record extreme differentiation and form a ‘cotectic delta’ beyond the terminus of the ‘thermal valley’. Although each glass trend shows a continuing increase in Zr to >3000 μg/g, the most evolved compositions in each suite differ in peralkalinity (mol [Na + K] / Al) and in the proportions of FeOT, Qz, Ab, Or, and other components, which are related to subtle variations in the mafic phases controlled mainly by differences in oxygen fugacity (fO2) and pressure (P). To determine the controls over mafic mineral crystallization in pantelleritic magmas and the various paths these suites take beyond the apparent (whole-rock) minimum, amphibole-phyric suites from the ~159 ka Cala dell’Altura and Cala Gadir volcanic centres and the ~8–10 ka Cuddia Mida volcanic centre on Pantelleria have been analyzed and compared with each other and with the well-characterized and amphibole-free, compositionally zoned Green Tuff, the ~46 ka caldera-forming ignimbrite of the Cinque Denti caldera. Differences between the extended fractionation trends may be ultimately attributed to variations in oxygen fugacity, depth of emplacement, and water saturation. Shallower (lower pressure) magma reservoirs such as the one for the Green Tuff are water-saturated and undergo degassing, which leads to an increase in relative oxygen fugacity. Deeper (higher pressure) magma reservoirs remain water-undersaturated and retain water in the melt, which both maintains lower relative oxygen fugacities and enables the crystallization of amphibole. Amphibole formation appears to require melt water contents >4 wt%, low oxygen fugacity (<ΔNNO-1.5), and low temperatures (<700°C), although fluorine may stabilize it at higher temperatures in some rocks.

Petrology of Palaeoarchaean mafic–ultramafic rock suites of the western Iron Ore Group, Singhbhum Craton, eastern India, using the chemistry of minerals

AbstractThe petrogenesis of Palaeoarchaean mafic–ultramafic rock suites from the western Iron Ore Group, Singhbhum Craton, eastern India, has been evaluated based on the chemistry of constituent mineral phases. The rock suites include basaltic rocks and mafic (gabbro) to ultramafic (serpentinized peridotite) intrusive rocks, which occur in host rocks covering phyllite, ferruginous shale, banded haematite quartzite and jasper. The constituent clinopyroxene shows dominant uralitization, whereas plagioclase grains are generally saussuritized, being marked by relatively tiny granular aggregates of albite, chlorite, epidote and K-feldspar. The ultramafic intrusive rocks are overwhelmingly serpentinized. Clinopyroxene compositions are augitic, whereas relict plagioclase is typically bytownite. Amphiboles of the investigated rock suites are divisible into the ‘uralite’ type (occurring peripherally to clinopyroxene) and the ‘completely changed-over amphibole’ type (with no traces of initial clinopyroxene). Both the amphibole types belong to the ‘calcic group’, showing a compositional spectrum from actinolite–magnesiohornblende–ferrohornblende–ferroactinolite. Opaque minerals include magnetite (both Cr-magnetite and Al-magnetite), chromite (Al-chromite) and ilmenite, whereas serpentine (belonging to the ultramafic intrusive rocks) corresponds to lizardite. Looking at the mineral compositions of the pyroxene, glass and amphibole, the studied rock suites show a wide equilibration temperature–pressure domain range (∼750 °C to ∼1400 °C at ∼0.26 kbar to ∼21 kbar), which corresponds to an ascending magma that underwent a ‘hydration event’ at a shallow level. The assessment of the clinopyroxene and spinel chemistry characteristically suggests an arc setting for the parent magma that has undergone both equilibrium and fractional crystallization in the course of magmatic evolution. During differentiation, the magmatic density remains almost constant, with variable oxygen fugacity.

Copper isotope fractionation in magmatic Ni–Cu mineralization systems associated with the variation of oxygen fugacity in silicate magmas

Significant variations of oxygen fugacity (fO2) in silicate magmas are widely suggested to play an important role in formation of magmatic Ni–Cu deposits in convergent margin settings. Copper isotopes have potential to track redox change of magmatic systems, but the linkage of silicate magma oxygen fugacity with Cu isotope fractionation in magmatic Ni–Cu mineralization systems is still uncertain. A combined study of Cu isotopes and estimated oxygen fugacities can provide novel and direct insights into this issue. In this study, four magmatic Ni–Cu deposits in Eastern Tianshan (NW China) were investigated in order to reveal the fO2 variations in silicate magmas as a function of redox-induced Cu isotope fractionation. The silicate magma fO2 of the four deposits widely varies from ∼QFM –2.2 to + 0.6, from ∼QFM –0.7 to 0.0, from ∼QFM –1.4 to –0.2, and from ∼QFM –1.2 to –0.6 for the Tulaergen, Huangshandong, Huangshannan, and Hulu deposits, respectively. Models using olivine compositions, Cu/Pd ratios, and estimated silicate magma fO2 suggest that the silicate magmas gradually became more oxidized as Ni–Cu mineralization proceeded. The closed-system R factor equation and the Rayleigh equation were used to model mineralization processes and related Cu isotope fractionation. Modelling using δ65Cu values and the estimated fO2 indicates that the evolved silicate magmas with high fO2 tend to incorporate heavier Cu isotopes. Our studies indicate that the fO2 variation of silicate magmas is the key factor governing Cu isotope fractionation in magmatic Ni–Cu mineralization systems in convergent tectonic settings, which is potentially applicable for other magmatic Ni–Cu deposits worldwide.

The influence of variable oxygen fugacity on the source depths of lunar high-titanium ultramafic glasses

We present the results of high pressure, high temperature multiple saturation experiments at variable oxygen fugacity (fO2) conditions (IW+1.5 and IW−2.1) on three lunar high titanium ultramafic glasses: the Apollo 17 Orange glass (A17O, 9.1 wt% TiO2), the Apollo 15 Red glass (A15R, 13.8 wt% TiO2), and the Apollo 14 Black glass (A14B, 16.4 wt% TiO2). We performed experiments in graphite (fO2 = IW+1.5) and iron (fO2 = IW−2.1) capsules. The experimentally determined multiple saturation points (MSPs) in graphite capsules are 2.5 GPa and ∼1530 °C (A17O), 1.3 GPa and ∼1350 °C (A15R), and 1.55 GPa and ∼1425 °C (A14B). In iron, we found MSPs of 3.3 GPa and ∼1565 °C (A17O), 2.8 GPa and ∼1490 °C (A15R), and 4.0 GPa and ∼1540 °C (A14B). These results, when combined with previous experiments on the lunar ultramafic glasses, indicate that the increase in the pressure of multiple saturation is linearly proportional to the TiO2 content of the melt (ΔMSPGPa=0.14×TiO2(wt%)-0.15, R2 = 0.93, RMSE = 0.2 GPa). The high depths of melting correlated with the lowest fO2 conditions are hard to reconcile with buoyancy constraints on these iron and titanium rich magmas. In addition, measurements of fO2 on the orange glass as well as the presence of iron blebs in the glasses suggest that the glasses were reduced during eruption. To reconcile buoyancy constraints with fO2 estimates, we present a model in which the high titanium magmas experienced higher fO2 conditions at their source, but underwent subsequent reduction at shallow depths (4–52 km) just prior to their eruption. In this model, we can then further bracket the depth of melting to be from the minimum multiple saturation pressure in graphite to the deepest depth at which the magmas are buoyant: assuming the Hess and Parmentier (1995) post overturn cumulate mantle, the depths of melting range from ∼550–770 km for the A17O glass, ∼260–490 km for the A15R glass, and ∼320–350 km for the A14B glass.

Melt Percolation, Melt-Rock Reaction and Oxygen Fugacity in Supra-Subduction Zone Mantle and Lower Crust from the Leka Ophiolite Complex, Norway

Abstract Samples of peridotites and pyroxenites from the mantle and lower crustal sections of the Leka Ophiolite Complex (LOC; Norway) are examined to investigate the effects of melt-rock reaction and oxygen fugacity variations in the sub-arc oceanic lithosphere. The LOC is considered to represent supra-subduction zone (SSZ) oceanic lithosphere, but also preserves evidence of pre-SSZ magmatic processes. Here we combine field and microstructural observations with mineral chemical and structural analyses of different minerals from the major lithologies of the LOC. Wehrlite and websterite bodies in both the mantle and lower crust contain clinopyroxene likely formed at a pre-SSZ stage, characterised by high Al, high Cr, low Mg crystal cores. These clinopyroxenes also exhibit low Al, low Cr, high Mg outer rims and intracrystalline dissolution surfaces, indicative of reactive melt percolation during intrusion and disruption of these lithologies by later, SSZ-related, dunite-forming magmas. Chromian-spinel compositional variations correlate with lithology; dunite-chromitite Cr-spinels are characterised by relatively uniform and high TiO2 and Al2O3, indicating formation by melt-rock reaction associated with SSZ processes. Harzburgite Cr-spinel compositions are more variable but preserve a relatively high Al2O3, low TiO2 endmember that may reflect crystallisation in a pre-SSZ oceanic spreading centre setting. An important finding of this study is that the LOC potentially preserves the petrological signature of a transition between oceanic spreading centre processes and subsequent SSZ magmatism. Single crystal Cr-spinel Fe3+/ΣFe ratios calculated on the basis of stoichiometry (from electron microprobe [EPMA] and crystal structural [X-ray diffraction; XRD] measurements) correlate variably with those calculated by point-source (single crystal) Mössbauer spectroscopy. Average sample EPMA Fe3+/ΣFe ratios overestimate or underestimate the Mössbauer-derived values for harzburgites, and always overestimate the Mössbauer Fe3+/ΣFe ratios for dunites and chromitites. The highest Fe3+/ΣFe ratios, irrespective of method of measurement, are therefore generally associated with dunites and chromitites, and yield calculated log(fO2)FMQ values of up to ~ + 1.8. While this lends support to the formation of the dunites and chromitites during SSZ-related melt percolation in the lower part of the LOC, it also suggests that these melts were not highly oxidised, compared to typical arc basalts (fO2FMQ of > + 2). This may in turn reflect the early (forearc) stage of subduction zone activity preserved by the LOC and implies that some of the arc tholeiitic and boninitic lava compositions preserved in the upper portion of the ophiolite are not genetically related to the mantle and lower crustal rocks, against which they exhibit tectonic contacts. Our new data also have implications for the use of ophiolite chromitites as recorders of mantle oxidation state through time; a global comparison suggests that the Fe3+/ΣFe signatures of ophiolite chromitites are likely to have more to do with local environmental petrogenetic conditions in sub-arc systems than large length-scale mantle chemical evolution

In situ quantification of the nitrogen content of olivine-hosted melt inclusions from Klyuchevskoy volcano (Kamchatka): Implications for nitrogen recycling at subduction zones

Assessing the N content of arc magmas and their mantle source remains a challenge because the volatile element composition of melts and gases can be modified during magma ascent, storage, and eruption. Given that melt inclusions (MIs) in Mg-rich olivine represent the best proxies for primary arc melts, we applied, for the first time, an in situ high-resolution secondary ion mass spectrometry (SIMS) method to determine the N concentration in olivine-hosted MIs from Klyuchevskoy volcano in Kamchatka. To reverse the effects of post-entrapment modification processes (i.e., exsolution of volatiles into a fluid bubble), the MIs were partially to completely homogenized at high temperatures (1150–1400 °C) and pressures ranging from 0.1 to 500 MPa under dry to H2O-saturated conditions at variable oxygen fugacities (CCO to QFM + 3.3). After the experiments, N concentrations in water-rich MI glasses correlate positively with H2O and CO2 contents as well as with N/CO2 ratios, and negatively with the volume of the remaining fluid bubble. Glasses of completely homogenized (fluid bubble-free) MIs contain up to 25.7 ± 0.5 ppm N, whereas glasses of three unheated (natural, bubble-bearing) MIs have significantly lower N concentrations of ~1 ± 0.3 ppm. The N-CO2-Nb characteristics of completely homogenized MIs indicate that melts feeding Klyuchevskoy volcano have high absolute concentrations of both N and CO2, as well as large excess of these volatiles relative to Nb, compared to primary mid-ocean ridge melts. This implies that large amounts of N and CO2 in Klyuchevskoy melts and their mantle source are derived from the subducting slab, and that these subducted volatiles are (partially) returned to the crust and atmosphere by arc-related magmatism.

High sulfur solubility in subducted sediment melt under both reduced and oxidized conditions: With implications for S recycling in subduction zone settings

The relative enrichment of sulfur (S) observed in arc magmas when compared to MORB, reflects the addition of slab-derived S to the mantle wedge source region. However, the mechanisms and efficiency of such S recycling remain poorly constrained. In this study, sediment melting experiments have been conducted using a synthetic pelite starting composition containing ∼7 wt% H2O and ∼1.9 wt% S, at 3 GPa, 1050 °C and variable oxygen fugacity (fO2), to investigate the effect of fO2 on S solubility in sediment melts. To assess temperature and concentration effects, selected experiments were repeated either at lower temperatures of 950 °C and 1000 °C, or with a higher bulk S content of ∼4 wt%. All experiments produced hydrous rhyolitic melts, saturated with either pyrrhotite under reduced conditions or anhydrite under oxidized conditions. For 3 GPa, 1050 °C experiments, the sulfur content at sulfide saturation (SCSS) in melt is found to increase with decreasing fO2, from ∼200 ppm at FMQ-0.5 to ∼1900 ppm at FMQ-7.5. The highest S solubility is achieved at FMQ + 1.6 where melt is saturated with both anhydrite and pyrrhotite. The sulfur content at sulfate saturation (SCAS) decreases from ∼2600 ppm at FMQ + 1.6 to ∼2000 ppm at FMQ + 7. Increasing either bulk S content or temperature produces a positive effect on SCSS and SCAS. Raman spectra of our experimental melts show that S exists as H2S/HS− under reduced conditions and as SO42− under oxidized conditions. The solubility minimum, i.e., the onset of transition from S2− to S6+ is estimated to occur at ∼FMQ, with full transition to S6+ by ∼FMQ + 2. While the SCAS values are in good agreement with previous reports, the distinct increase of SCSS with decreasing fO2 (when fO2 < FMQ) has not been observed in previous slab melting experiments. Furthermore, we report for the first time that dissolution of H2S and SO2 in hydrous rhyolitic melt follows the Fincham-Richardson relationship; and propose the definition of a hydro-sulfide capacity as CHS = [HS]*(fO2/fS2)1/2; where [HS] is the concentration of S in melt (in ppm) dissolved as HS− and H2S. SCSS for H2S dissolution can therefore be modeled using CHS in an analogous fashion to modeling SCSS for anhydrous melts using the sulfide capacity (CS2−), with the relation ln[HS]SCSS=-ΔGFes - FeO°/RT+lnCHS-lnaFeOmelt+lnaFeSsulfide. As predicted by such a model framework, we indeed observe a linear correlation between logSCSS and logXFeO (the mole fraction of FeO in melt) with a slope close to −1, i.e., SCSS experiences a sharp increase when FeO in melt falls below 1 wt%. Therefore, both our experimental results and model predictions suggest hydrous low-Fe rhyolitic melt produced by sediment melting under reduced conditions has the required S solubility to account for the relative enrichment of S observed in arc magmas.

Oxidation state and metasomatism of the lithospheric mantle beneath the Rae Craton, Canada: strong gradients reflect craton formation and evolution

We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~ 100 km depth could have locally destabilised any pre-existing diamond or graphite.

Chemical composition of magnetite and chlorite from the stringer zone of the Nudeh volcanogenic massive sulfide (VMS) deposit, Iran: geological implications

The southwest Sabzevar basin situated in the Sabzevar zone is considered to be an attractive metallogenic province in Iran that hosts both volcanogenic massive sulfide (VMS) and stratiform manganese deposits. The Nudeh Besshi-type VMS deposit is located in the Lower Late Cretaceous volcano-sedimentary sequence. The ore mineralization in this deposit is hosted in the alkali olivine basalt flow and tuffaceous silty sandstone rocks. The Nudeh VMS deposit consists of 2 million metric tons of Cu-Zn massive sulfide overlying a Cu-Fe-rich stringer. The massive sulfide orebody consists dominantly of pyrite, chalcopyrite, friedrichite, magnetite, and sphalerite, together with minor quartz, chlorite, and sericite. Chloritization, silicification, and sericitization are the main wall-rock alteration types; alteration intensity increases towards the stringer zone. Chloritized footwall rocks extend up to 20 m below the stringer zone. The quartz-bearing stringer veins also contain pyrite, chalcopyrite, magnetite, and bornite. Magnetite crystals from the stringer ores show variable contents of many elements, such as MgO (0.05 wt%), Al2O3 (0.63 wt%), TiO2 (0.07 wt%), V2O3 (0.045 wt%), SiO2 (0.65 wt%), CoO (0.10 wt%), NiO (0.009 wt%), ZnO (0.023 wt%), and CaO (0.03 wt%). The moderate to high V contents are interpreted to result from relatively reduced, seafloor hydrothermal activiy. Compositional variations of magnetite are possibly related to variations in oxygen fugacity, temperature, and water/rock interaction. Within the stringer zone, chlorite 2 (Chl-2) in the vein-veinlets and chlorite 1 (Chl-1) in the chloritized alkali olivine basalt rock are chemically indistinguishable, with 26.92–34.67 wt% FeO and 5.99–14.01 wt% MgO. Chlorite geothermometer studies indicate crystallization formation temperatures of 414 °C (Chl-1) and 303 °C (Chl-2), respectively.

Oxidation State of the Lithospheric Mantle Beneath Komsomolskaya–Magnitnaya Kimberlite Pipe, Upper Muna Field, Siberian Craton

The oxidation state of the mantle plays an important role in many chemical and physical processes, including magma genesis, the speciation of volatiles, metasomatism and the evolution of the Earth’s atmosphere. We report the first data on the redox state of the subcontinental lithospheric mantle (SCLM) beneath the Komsomolskaya–Magnitnaya kimberlite pipe (KM), Upper Muna field, central Siberian craton. The oxygen fugacity of the KM peridotites ranges from −2.6 to 0.3 logarithmic units relative to the fayalite–magnetite–quartz buffer (∆logfO2 (FMQ)) at depths of 120–220 km. The enriched KM peridotites are more oxidized (−1.0–0.3 ∆logfO2 (FMQ)) than the depleted ones (from −1.4 to −2.6 ∆logfO2 (FMQ)). The oxygen fugacity of some enriched samples may reflect equilibrium with carbonate or carbonate-bearing melts at depths &gt;170 km. A comparison of well-studied coeval Udachnaya and KM peridotites revealed similar redox conditions in the SCLM of the Siberian craton beneath these pipes. Nevertheless, Udachnaya peridotites show wider variations in oxygen fugacity (−4.95–0.23 ∆logfO2 (FMQ)). This indicates the presence of more reduced mantle domains in the Udachnaya SCLM. In turn, the established difference in the redox conditions is a good explanation for the lower amounts of resorbed diamonds in the Udachnaya pipe (12%) in comparison with the KM kimberlites (33%). The obtained results advocate a lateral heterogeneity in the oxidation state of the Siberian SCLM.

Magmatic expression of tectonic transition from oceanic subduction to continental collision: Insights from the Middle Triassic rhyolites of the North Qiangtang Block

The tectonic transition from subduction to collision is a fundamental process during orogenesis, yet the magmatic expression of this transition and related deep geodynamic processes remain unclear. This study focuses on a newly identified volcanic belt within the Moyun–Zaduo–Sulu area of the North Qiangtang Block and presents new zircon U-Pb data that indicate that this belt formed during the Middle Triassic (247–241 Ma), a time characterized by a regional transition from subduction to collisional tectonism. The volcanic belt is located to the south of a Permian to Early Triassic arc and is dominated by high-K calc-alkaline and peraluminous rhyolites. These rhyolites have low Mg#, Nb/Ta, and δEu values, contain low contents of Sr, have high Rb/Sr and whole-rock εNd(t) values, and show positive zircon εHf(t) values, all of which suggest that they formed from magmas generated by the dehydration melting of juvenile crustal material. The migration of Middle Triassic volcanism in this region was most likely caused by rollback of the subducting Longmucuo–Shuanghu Tethyan oceanic slab. Combining our new data with previously published results of numerical modeling of subduction–collisional processes and regional data from north-central Tibet yields insights into the magmatic expressions and related deep geodynamics of the transition from oceanic subduction to continental collision. This combination of data also suggests that variations in oxygen fugacity can be used as a proxy for the discrimination of magmatism related to subduction, the transition from subduction to collision, and collisional tectonism.

LA-ICP-MS trace element analysis of magnetite and pyrite from the Hetaoping Fe-Zn-Pb skarn deposit in Baoshan block, SW China: Implications for ore-forming processes

The Hetaoping deposit is one of the largest Fe-Zn-Pb skarn deposit in SW China, which is characterized by Zn-Pb mineralization in the upper part and the Fe mineralization in the deeper part. The Fe mineralization is dominated by magnetite and pyrite. Magnetite can be subdivided into four types: primary banded magnetite samples in clinopyroxene-actinolite skarn (Mt-1), primary disseminated magnetite in garnet skarn (Mt-2), primary disseminated magnetite in clinopyroxene-actinolite skarn (Mt-3), and altered magnetite in clinopyroxene-actinolite skarn (Mt-4). Pyrite can be subdivided into three types: pyrite in oxide-ore stage (Py-1), pyrite in early sulfide-ore stage (Py-2), and pyrite in late sulfide-ore stage (Py-3). The flat time-resolved signals of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) imply that trace elements exist mainly in the form of isomorphism in magnetite and pyrite with the exception some incompatible trace elements (e.g., Ca, K and Na in magnetite and Pb, Bi and Ag in pyrite). Trace element concentrations in magnetite and pyrite demonstrate that the ore-forming fluid in Hetaoping is of magmatic origin. Furthermore, compared to porphyry, IOCG, Kinuna and BIF type magnetite, the magnetite from Hetaoping has relatively low Ti, V and Ni concentrations but high Al, Mn and Ca concentrations, implying a typical skarn genesis. The variation of Ti concentrations in magnetite is an indication of the formation temperature and shows that banded magnetite (Mt-1) precipitated in a relatively high-temperature environment compared with disseminated magnetite (Mt-2 and Mt-3). Compared to Mt-1 and Mt-3, Mt-2 has a higher Si, Al, and W contents and a lower Mg and Mn contents. The Mn content increases from Py-1 to Py-2, and decreases from Py-2 to Py-3, suggesting that the fluid-rock interaction increased from the oxide-ore stage to the sulfide-ore stage, and decreased from sulfide-ore stage to post-ore stage. The variation of the V concentration in magnetite grains indicates a relatively higher oxygen fugacity of Mt-2 compared to Mt-1 and Mt-3, implying that the oxygen fugacity of the ore-forming fluid in the garnet skarn zone is higher than that in clinopyroxene-actinolite skarn zone. The variable oxygen fugacity probably caused spatial zoning of mineralization in Hetaoping Fe-Zn-Pb skarn deposit. The temperature and oxygen fugacity of the ore-forming fluid, and the extent of fluid-rock interaction, controlled the temporal order and spatial zonation of magnetite and sulfide precipitation, which led to the formation of the Hetaoping Fe-Zn-Pb skarn deposit.

Geochemistry and iron isotope systematics of coexisting Fe-bearing minerals in magmatic Fe[sbnd]Ti deposits: A case study of the Damiao titanomagnetite ore deposit, North China Craton

Geochemical and iron isotopic compositions of magnetite, ilmenite and pyrite separates from the FeTi oxide ores hosted in the Damiao anorthosite-type FeTi ore deposit were analyzed to investigate sub-solidus cooling history of the titanomagnetite. The FeTi oxides form two series of solid solutions, namely, ulvöspinel-magnetite (Usp-Mtss) and hematite-ilmenite (Hem-Ilmss) solid solutions. The magnetite separates have 14–27 mol% ulvöspinel, while the ilmenite separates have 5–8 mol% hematite. Major element compositions of the mineral separates suggest that the ilmenites were mainly exsolved from the Usp-Mtss by oxidation of ulvöspinel in the temperature range of ~820–600 °C and experienced inter-oxide re-equilibration with the magnetites. Associated with the exsolution is the substantial inter-mineral iron isotope fractionation. The magnetite separates are characterized by high δ57Fe (+0.27 − +0.65‰), whereas the ilmenite separates have lower δ57Fe (−0.65 to −0.28‰). Two types of pyrite are petrographically observed, each of which has a distinctive iron isotope fingerprint. Type I pyrite (pyriteI) with higher δ57Fe (δ57Fe = +0.63 − +0.95‰) is consistent with magmatic origin, and type II pyrite (pyriteII) with lower δ57Fe (δ57Fe = −0.90 to −0.11‰) was likely to have precipitated from fluids. Iron isotopic fingerprints of the pyriteI probably indicate variations of oxygen fugacity, whereas those of the pyriteII may result from fluid activities. The iron isotopic fractionation between the magnetite and ilmenite is the net result of sub-solidus processes (including ulvöspinel oxidation and inter-oxide re-equilibration) without needing varying oxygen fugacity albeit its presence. Although varying composition of magnetite-ilmenite pairs reflects variations of oxygen fugacity, inter-oxide iron isotopic fractionation does not.

Is seawater geochemical composition recorded in marine carbonate? Evidence from iron and manganese contents in Late Devonian carbonate rocks

Iron and manganese are the important redox-sensitive elements in the ocean. Previous studies have established a series of paleo-depositional redox proxies based on the form and content of iron in sedimentary rocks (e.g., degree of pyritization, FeHR/FeT, Fe/Al). These proxies were developed and applied on siliciclastic-rich marine sediments. Although marine carbonate rocks are generally considered to preserve the geochemical signals of ancient seawater, neither Fe nor Mn content in marine carbonate rocks (Fecarb, Mncarb) has been independently used as a proxy to quantify environmental cues in paleo-oceans. Both Fe and Mn are insoluble in oxic conditions (Fe2O3, Fe(OH)3, MnO2), while their reduced forms (Fe2+ and Mn2+) are soluble. Therefore, oxic seawater should have low concentrations of dissolved Fe2+ and Mn2+, and accordingly carbonate rocks precipitated from oxic seawater should have low Fecarb and Mncarb, and vice versa. To evaluate whether Fecarb and Mncarb can be used to quantify oxygen fugacity in seawater, we measured Fecarb and Mncarb of Upper Devonian marine carbonate rocks collected from nine sections in South China. Fecarb of intraplatform basin samples was significantly higher than that of shelf samples, while shelf and basin samples had comparable Mncarb. The modeling result indicates that the dramatic difference in Fecarb cannot be explained by variation in oxygen fugacity between the shelf and basin seawater. Instead, both Fecarb and Mncarb appear to be more sensitive to benthic flux from sediment porewater that is enriched in Fe2+ and Mn2+. Porewater Fe2+ and Mn2+ derive from bacterial iron and manganese reduction; flux was controlled by sedimentation rate and the depth of the Fe(Mn) reduction zone in sediments, the latter of which is determined by oxygen fugacity at the water–sediment interface. Thus, high Fecarb of the basin samples might be attributed to low sedimentation rate and/or low oxygen fugacity at the seafloor. However, invariant Mncarb of the shelf and basin samples might be the consequence of complete reduction of Mn in sediments. Our study indicates that marine carbonate rocks may not necessarily record seawater composition, particularly for benthic carbonate rocks. The influence of benthic flux might cause carbonate rocks’ geochemical signals to deviate significantly from seawater values. Our study suggests that interpretation of geochemical data from carbonate rocks, including carbonate carbon isotopes, should consider the process of carbonate formation.

Olivine trace element compositions in diamondiferous lamproites from India: Proxies for magma origins and the nature of the lithospheric mantle beneath the Bastar and Dharwar cratons

The ~1100 Ma CC2 and P13 lamproite dykes in the Wajrakarur Kimberlite Field (WKF), Eastern Dharwar Craton, and ~65 Ma Kodomali and Behradih lamproite diatremes in the Mainpur Kimberlite Field (MKF), Bastar Craton share a similar mineralogy, although the proportions of individual mineral phases vary significantly. The lamproites contain phenocrysts, macrocrysts and microcrysts of olivine set in a groundmass dominated by diopside and phlogopite with a subordinate amount of spinel, perovskite, apatite and serpentine along with rare barite. K-richterite occurs as inclusion in olivine phenocrysts in Kodomali, while it is a late groundmass phase in Behradih and CC2. Mineralogically, the studied intrusions are classified as olivine lamproites. Based on microtextures and compositions, three distinct populations of olivine are recognised. The first population comprises Mg-rich olivine macrocrysts (Fo89–93), which are interpreted to be xenocrysts derived from disaggregated mantle peridotites. The second population includes Fe-rich olivine macrocrysts (Fo82–89), which are suggested to be the product of metasomatism of mantle wall-rock by precursor lamproite melts. The third population comprises phenocrysts and overgrowth rims (Fo83–92), which are clearly of magmatic origin. The Mn and Al systematics of Mg-rich olivine xenocrysts indicate an origin from diverse mantle lithologies including garnet peridotite, garnet–spinel peridotite and spinel peridotite beneath the WKF, and mostly from garnet peridotite beneath the MKF. Modelling of temperatures calculated using the Al-in-olivine thermometer for olivine xenocrysts indicates a hotter palaeogeotherm of the SCLM beneath the WKF (between 41 and 43 mW/m2) at ~1100 Ma than beneath the MKF (between 38 and 41 mW/m2) at ~65 Ma. Further, a higher degree of metasomatism of the SCLM by precursor lamproite melts has occurred beneath the WKF compared to the MKF based on the extent of CaTi enrichment in Fe-rich olivine macrocrysts. For different lamproite intrusions within a given volcanic field, lower Fo olivine overgrowth rims are correlated with higher phlogopite plus oxide mineral abundances. A comparison of olivine overgrowth rims from the two fields shows that WKF olivines with lower Fo content than MKF olivines are associated with increased XMg in spinel and phlogopite and vice versa. Melt modelling indicates relatively Fe-rich parental melt for WKF intrusions compared to MKF intrusions. The Ni/Mg and Mn/Fe systematics of magmatic olivines indicate derivation of the lamproite melts from mantle source rocks with a higher proportion of phlogopite and/or lower proportion of orthopyroxene for the WKF on the Eastern Dharwar Craton compared to those for the MKF on the Bastar Craton. This study highlights how olivine cores provide important insights into the composition and thermal state of cratonic mantle lithosphere as sampled by lamproites, including clues to elusive precursor metasomatic events. Variable compositions of olivine rims testify to the complex interplay of parental magma composition and localised crystallisation conditions including oxygen fugacity variations, co-crystallisation of groundmass minerals, and assimilation of entrained material.

A compositional tipping point governing the mobilization and eruption style of rhyolitic magma.

The most viscous volcanic melts and the largest explosive eruptions on our planet consist of calcalkaline rhyolites. These eruptions have the potential to influence global climate. The eruptive products are commonly very crystal-poor and highly degassed, yet the magma is mostly stored as crystal mushes containing small amounts of interstitial melt with elevated water content. It is unclear how magma mushes are mobilized to create large batches of eruptible crystal-free magma. Further, rhyolitic eruptions can switch repeatedly between effusive and explosive eruption styles and this transition is difficult to attribute to the rheological effects of water content or crystallinity. Here we measure the viscosity of a series of melts spanning the compositional range of the Yellowstone volcanic system and find that in a narrow compositional zone, melt viscosity increases by up to two orders of magnitude. These viscosity variations are not predicted by current viscosity models and result from melt structure reorganization, as confirmed by Raman spectroscopy. We identify a critical compositional tipping point, independently documented in the global geochemical record of rhyolites, at which rhyolitic melts fluidize or stiffen and that clearly separates effusive from explosive deposits worldwide. This correlation between melt structure, viscosity and eruptive behaviour holds despite the variable water content and other parameters, such as temperature, that are inherent in natural eruptions. Thermodynamic modelling demonstrates how the observed subtle compositional changes that result in fluidization or stiffening of the melt can be induced by crystal growth from the melt or variation in oxygen fugacity. However, the rheological effects of water and crystal content alone cannot explain the correlation between composition and eruptive style. We conclude that the composition of calcalkaline rhyolites is decisive in determining the mobilization and eruption dynamics of Earth's largest volcanic systems, resulting in a better understanding of how the melt structure controls volcanic processes.