Study of oxygen fugacity during magma evolution and ore genesis in the Hongge mafic–ultramafic intrusion, the Panxi region, SW China

Economic concentrations of Fe–Ti oxides occurring as massive layers in the middle and upper parts of the Hongge intrusion are different from other layered intrusions(Panzhihua and Baima) in the Emeishan large igneous province, SW China. This paper reports on the new mineral compositions of magnetite and ilmenite for selected cumulate rocks and clinopyroxene and plagioclase for basalts. We use these data to estimate the oxidation state of parental magmas and during ore formation to constrain the factors leading to the abundant accumulation of Fe–Ti oxides involved with the Hongge layered intrusion. The results show that the oxygen fugacities of parental magma are in the range of FMQ-1.56 to FMQ+0.14, and the oxygen fugacities during the ore formation of the Fe–Ti oxides located in the lower olivine clinopyroxenite zone(LOZ) and the middle clinopyroxenite zone(MCZ) of the Hongge intrusion are in the range of FMQ-1.29 to FMQ-0.2 and FMQ-0.49 to FMQ+0.82, respectively.The MELTS model demonstrates that, as the oxygen fugacity increases from the FMQ-1 to FMQ+1, the proportion of crystallization magnetite increases from 11 % to 16 % and the crystallization temperature of the Fe–Ti oxides advances from 1134 to 1164 °C. The moderate oxygen fugacities for the Hongge MCZ indicate that the oxygen fugacity was not the only factor affecting the crystallization of Fe–Ti oxides. We speculated that theinitial anhydrous magma that arrived at the Hongge shallow magma chamber became hydrous by attracting the H_2O of the strata. In combination with increasing oxygen fugacities from the LOZ(FMQ-1.29 to FMQ-0.2) to the MCZ(FMQ-0.49 to FMQ+0.82), these two factors probably account for the large-scale Fe–Ti oxide ore layers in the MCZ of the Hongge intrusion.

Oxygen Fugacity and Water Activity during Thermal Peak and Retrogression of Granulites around the Sarvapuram Area, Karimnagar Granulite Terrane, Andhra Pradesh, India

The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks.

The effect of oxygen fugacity on ionic conductivity in olivine

The oxygen fugacity(f_(O2)) may affect the ionic conductivity of olivine under upper mantle conditions because Mg vacancies can be produced in the crystal structure by the oxidization of iron from Fe^(2+) to Fe3+. Here we investigated olivine ionic conductivity at 4 GPa, as a function of temperature, crystallographic orientation, and oxygen fugacity, corresponding to the topmost asthenospheric conditions. The results demonstrate that the ionic conductivity is insensitive to f_(O2) under relatively reduced conditions(f_(O2) below Re-ReO_(2) buffer), whereas it has a clear f_(O2)-dependence under relatively oxidized conditions(f_(O2) around the magnetite-hematite buffer). The ionic conduction in olivine may contribute significantly to the conductivity anomaly in the topmost asthenosphere especially at relatively oxidized conditions.

In-situ control of oxygen fugacity for laboratory measurements of electrical conductivity of minerals and rocks in a multi-anvil press

In this paper, a new of oxygen fugaeity controltechnique that can be widely applied to in-situ measurement of the grain interior electrical conductivities of minerals and rocks is presented for high temperature and high pressure. Inside the sample assembly, a metal and corresponding metal oxide form a solid oxygen buffer. The principle of this technique is to randomly monitor and adjust oxygen fugacity in the large-volume multi-anvil press by changing the types of solid oxygen buffer, metal shielding case and electrodes. At a pressure of up to 5.0 GPa and a temperature of up to 1423 K, the electrical conductivities of the dry peridotite are tested under the conditions of different oxygen fugacities. By virtue of this new technique, more and more reasonable and accurate laboratory electrical property data will be successfully obtained under controlled thermodynamic conditions.

A new technology for controlling in-situ oxygen fugacity in diamond anvil cells and measuring electrical conductivity of anhydrous olivine at high pressures and temperatures

We present a novel technique for controlling oxygen fugacity,which is broadly used to in-situ measure the electrical conductivities in minerals and rocks during diamond anvil cell experiments.The electrical conductivities of olivine are determined under controlled oxygen fugacity conditions(Mo–MoO2)at pressures up to 4.0 GPa and temperatures up to 873 K.The advantages of this new technique enable the measuring of the activation enthalpy,activation energy,and activation bulk volume in the Arrhenius relationship.This provides an improved understanding of the mechanism of conduction in olivine.Electrical conduction in olivine is best explained by small polaron movement,given the oxygen fugacity-dependent variations in conductivity.

Geochemistry and mineral chemistry of the armoor granitoids,eastern dharwar craton: implications for the redox conditions and tectono-magmatic environment

The mineralogical and geochemical characteristics of the K-rich granites from the Armoor granitic rocks in the northeastern portion of the Eastern Dharwar Craton(EDC) are presented.In order to understand its physicochemical conditions,the petrogenesis of the granitoid was explained from biotite chemistry and geochemical systematics.Studies of mineral chemistry expose that compositionally,K-feldspar and plagioclase in Armoor granite rocks range from An0,Ab_(3-5.9),Or_(94-96.9) and An_(5-29,-Ab71.9-94.9),Or_(0-1.5),respectively.The mineral chemistry of biotite crystals exhibits composition that varies from primary to re-equilibrated primary biotites.Although biotites from the Armoor granites generally exhibit an I-type trend,with calc-alkaline parental magma in a subduction setting.Biotite chemistry of granites displays magnetite(oxidized)series nature,which has oxygen fugacity(fO_(2))=-15.1 to-16.7(log_(10) bar),under high oxidizing conditions.Temperature and pressure estimates for the crystallization of Armoor granites based on biotite composition are T=612-716 ℃ and 1.0-0.4 kbar,respectively.Geochemically,these rocks are metaluminous to slightly peraluminous and magnesian,with calc-alkaline potassiumrich granite.On the chondrite normalized REE diagram,the granites have positive europium anomalies;rich Sr/Y,(Dy/Yb)_(N) ratios and reduced Mg#,Rb/Sr,Rb,Sr indicate that the melting of earlier rocks,crystal accumulation and residual garnet source formed at high pressures.The examined granites show that they are produced from the melting of crustal sources.Thus,the extensive analyses of the described Armoor granite suggest that they were produced by crust sources and developed under oxidizing conditions in subduction setting.

Reduced magma generation and its implications for the Pulang giant porphyry Cu-polymetallic deposit in Northwest Yunnan,China

The Pulang giant porphyry Cu-Mo polymetallic deposit is located in the Zhongdian area in the center of the Sanjiang Tethys tectonic domain,which was formed by the westward subduction of the Garze-Litang oceanic slab beneath the Zhongza massif.Chalcopyrite-pyrrhotite-pyritemolybdenite occurs as disseminations,veins,veinlets,and stockworks distributed in the K-silicate alteration zone in the monzonite porphyry,which is superimposed by propylitization.The chemical compositions of biotite and amphibole analyzed by electron probe microanalysis(EPMA)indicate that the ore-forming magma and exsolved fluids experienced a continuous decrease in the oxygen fugacity(fO_(2)).Primary amphibolite and biotite(type I)crystallized at relatively high temperatures(744-827°C)and low fO_(2)(log fO_(2)=−12.26 to−11.91)during the magmatic stage.Hydrothermal fluids exsolved from the magma have a relatively lower temperature(621-711°C)and fO_(2)(log fO_(2)=−14.36 to−13.32)than the original magma.In addition,the presence of a high abundance of pyrrhotite and an insufficiency of primary magnetite and sulfate in the ore(i.e.,anhydrite and gypsum)indicate that the deposit may be a reduced porphyry deposit.Magma and fluid fO_(2)results,combined with previous research on magmatic fO_(2)at the Pulang deposit,indicate that the magma associated with the reduced Pulang ore assemblages was initially generated as a highly oxidized magma that was subsequently reduced by sedimentary rocks of the Tumugou Formation.

Geology and mineralization of the Dongping supergiant alkalic-hosted Au-Te deposit(>100 t Au)in Northern Hebei Province,China:A review

The Dongping deposit is the largest alkalic-hosted gold deposit in China containing>100 t of Au.This paper presents a new understanding for Dongping ore system,based on the previous studies.The mineralization originally occurred at 400-380 Ma,simultaneous with emplacement of the Shuiquangou alkaline complex,and was overprinted by the hydrothermal activity in the Yanshanian.Isotope compositions of ores indicate metals of the deposit are mainly provided by the Shuiquangou complex.Ore-forming fluids are characterized by increasing oxygen fugacity and decreasing sulfur fugacity,while tellurium fugacity increased in the Stage II-2 and decreased in Stage II-3.These systematic changes are closely related to the processes of mineral precipitation and fluid evolution.Sulfide precipitation from Stage Ⅰ to Stage Ⅱ was triggered by fluid boiling,which leads to the precipitation of Pb-Bi-Te,due to decrement of sulfur fugacity.Condensation of gas phase containing high concentration of H_2Te leads to precipitation of Te-Au-Ag minerals and native tellurium.Based on these hypotheses,this paper present a polyphase metallogenic model as follow.During the Devonian,fluids were released from alkaline magmas,which carried ore-forming materials form the surrounding rocks and precipitate the early ores.During the Jurassic-Cretaceous,fluorine-rich fluids exsolved from highly factionated Shangshuiquan granite,which extracted and concentrated Au from the Shuiquangou complex and the Sanggan Group metamorphic rocks,and finally formed the Dongping gold deposit.

Experimental Study on the Electrical Conductivity of Orthopyroxene at High Temperature and High Pressure under Different Oxygen Fugacities

At presure 1.0-4.0 GPa and temperature 1073-1423 K and under oxygen partial pressure conditions, a YJ-3000t multi-anvil solid high-pressure apparatus and Sarltron-1260 Impedance/Gain-Phase analyzer were employed to conduct an in-situ measurement of the electrical conductivity of orthopyroxene. The buffering reagents consist of Ni＋NiO, Fe＋Fe3O4, Fe＋FeO and Mo＋MoO2 in order to control the environmental oxygen fugacity. Experimental results made clear that： （1） within the measuring frequency range from 10-1 to 106 Hz, the complex impedance （R） is of intensive dependence on the frequency; （2） The electrical conductivity （a） tends to increase along to the rise of temperature （T）, and Log a vs. 1/ T fit the Arrenhius linear relations; （3） Under the control of oxygen buffer Fe＋Fe3O4, with the rise of pressure, the activation enthalpy tends to increase whereas the electrical conductivity tends to decrease. The activation energy and activation volume of the main current carders of orthopyroxene have been obtained, which are （1.715±0.035） eV and （0.03±0.01） cm^3/mol, respectively; （4） Under given pressure and temperature, the electrical conductivity tends to increase with increasing oxygen fugacity, while under given pressure the activation enthalpy tends to decrease with increasing oxygen fugacity; and （5） The sample＇s small polarons mechanism has provided a reasonable explanations to the conduction behavior at high temperature and high pressure.

Unveil the Redox Evolution of Ore-forming Fluids using Sulfur Isotope:A Case Study of the Zhengguang Intermediate Sulfidation Epithermal Au-Zn Deposit,NE China

Oxygen fugacity(fO_(2))is a key intensity variable during the entire magmatic-hydrothermal mineralization courses.The redox state and its variations between different stages of the ore-forming fluids of intermediate sulfidation epithermal deposits are rarely deciphered due to the lack of appropriate approaches to determine fO_(2)of the fluids.Here,we reported theδ^(34)S of the sulfides from three different stages(stageⅠ,Ⅱ,Ⅲ)of Zhengguang,an Early Ordovician Au-rich intermediate sulfidation(IS)epithermal deposit,to decipher the redox evolution of the ore-forming fluids.The increasingδ^(34)S values from stageⅠpyrite(pyl,average-2.6‰)through py2(average-1.9‰)to py3(average-0.2‰)indicates a decrease of the oxygen fugacity of the ore-forming fluids.A compilation ofδ^(34)S values of sulfides from two subtypes of IS deposits(Au-rich and Ag-rich)from NE China shows that theδ^(34)S values of sulfides from Au-rich IS deposits are systematically lighter than those of Ag-rich IS Ag-Pb-Zn deposit,indicating the ore-forming fluids of the former are more oxidized than the latter.We highlight that sulfur isotopic composition of hypogene sulfides is an efficacious proxy to fingerprint the oxygen fugacity fluctuations of epithermal deposits and could potentially be used to distinguish the subtypes of IS deposits.

Formation and Evolution of Chromitites in the Hongshishan Complex,Beishan Orogenic Collage,NW China:Constraints from Mineralogical Compositions,Re-Os Isotopes and Platinum-Group Element Geochemistry

The Hongshishan chromitite deposits are situated to the north of the Beishan orogenic collage,in the southern part of the Central Asian Orogenic Belt.This study describes the mineral chemistry,Re-Os isotopes and platinum-group elements geochemistry of the Hongshishan chromitites for the purpose of constraining the origin,evolution and composition of their parental melts.The restricted ranges of Al_(2)O_(3),Cr_(2)O_(3)and Cr#-Mg#variation of chromite-cores and chromites fall within the field of the mid-ocean ridge and ophiolitic podiform chromite settings.The(^(187)Os/^(188)Os)i ratios of the chromitites are in the range of 0.12449–0.12745(average 0.12637)and theγOs are from-1.92 to-0.06(average-0.83).In the Re-Os isotope diagrams,all the samples fall in the field of chromitites and show a residual peridotitic trend.The range of Os isotopic compositions andγOs values indicate that they overlap the depleted MORB mantle(DMM)as well as being close to global Os isotopic data andγOs of ophiolite chromitites.The characteristics of the PGE contents can be roughly subdivided into two groups:podiform chromitites and Ural-Alaskan type complexes.For the ferritchromite cores,the calculated Al_(2)O_(3)concentrations of the parental melt are higher(average 16.65 wt%)in high-Cr than high-Al chromitite(average 16.17 wt%)and for the chromite,the calculated Al_(2)O_(3)concentrations are even higher(average 16.48 wt%)in the high-Cr than the high-Al examples(average 15.38 wt%).In the(TiO_(2))melt vs.TiO_(2)diagrams,most high-Al melts fall in the MORB,while the high-Cr melts fall in the ARC field.The calculated Fe O/Mg O ratios for the parental melt show the closest resemblance to a MORB magma composition.The inferred parental melt composition for studied chromitites falls in the field of mid-ocean ridge basalt(MORB)magmas and far away from boninite.The calculated degrees of partial melting producing the chromitites are 16%-22%(average 19%),which is around the range of those of the MORB magmas.The chromitites are suggested to have been formed in a MORB setting.The chromites and ferritchromite cores are mostly scattered along the MORB and SSZ harzburgite–dunite fields.Ferritchromite rims and ferritchromites with high YFes formed as a result of alteration during serpentinization..

The formation of porphyry copper deposits

Copper is a moderately incompatible chalcophile element.Its behavior is strongly controlled by sulfides.The speciation of sulfur is controlled by oxygen fugacity.Therefore,porphyry Cu deposits are usually oxidized(with oxygen fugacities > AFMQ +2)(Mungall 2002;Sun et al.2015).The problem is that while most of the magmas at convergent margins are highly oxidized,porphyry Cu deposits are very rare,suggesting that high oxygen fugacity alone is not sufficient.Partial melting of mantle peridotite even at very high oxygen fugacities forms arc magmas with initial Cu contents too low to form porphyry Cu deposits directly(Lee et al.2012;Wilkinson 2013).Here we show that partial melting of subducted young oceanic slabs at high oxygen fugacity(>AFMQ +2) may form magmas with initial Cu contents up to >500 ppm,favorable for porphyry mineralization.Pre-enrichment of Cu through sulfide saturation and accumulation is not necessarily beneficial to porphyry Cu mineralization.In contrast,remelting of porphyritic hydrothermal sulfide associated with iron oxides may have major contributions to porphyry deposits.Thick overriding continental crust reduces the "leakage" of hydrothermal fluids,thereby promoting porphyry mineralization.Nevertheless,it is also more difficult for ore forming fluids to penetrate the thick continental crust to reach the depths of 2—4 km where porphyry deposits form.

Contrasting oxidation states of low-Ti and high-Ti magmas control Ni-Cu sulfide and Fe-Ti oxide mineralization in Emeishan Large Igneous Province

Magmatic Ni-Cu-(PGE) sulfide and Fe-Ti oxide deposits in plume-related large igneous provinces(LIPs)are commonly related to low-Ti and high-Ti series magmas, respectively, but the major factors that control such a relationship of metallogenic types and magma compositions are unclear. Magma fOcontrols sulfur status and relative timing of Fe-Ti oxide saturation in mafic magmas, which may help clarify this issue. Taking the Emeishan LIP as a case, we calculated the magma fOof the high-Ti and low-Ti picrites based on the olivine-spinel oxygen barometer, and the partitioning of V in olivine. The obtained fOof the high-Ti series magma(FMQ + 1.1 to FMQ + 2.6) is higher than that of the low-Ti series magma(FMQ-0.5to FMQ + 0.5). The magma fOof the high-Ti and low-Ti picrites containing Fo > 90 olivine reveals that the mantle source of the high-Ti series is likely more oxidized than that of the low-Ti series. The results using the ’lambda REE’ approach show that the high-Ti series may have been derived from relatively oxidized mantle with garnet pyroxenite component. The S contents at sulfide saturation(SCSS) of the two series magmas were calculated based on liquid compositions obtained from the alpha Melts modeling, and the results show that the low-Ti series magma could easily attain the sulfide saturation as it has low fOwith S being dominantly as S. In contrast, the oxidized high-Ti series magma is difficult to attain the sulfide saturation, but could crystallize Fe-Ti oxides at magma MgO content of ~7.0 wt.%. Thus, contrasting magma fOof low-Ti and high-Ti series in plume-related LIPs may play an important role in producing two different styles of metallogeny.

Mantle Transects in Africa According to Data of Mantle Xenocrysts and Diamond Inclusions

We designed the mantle transects using the PTXFO2 diagrams(Ashchepkov et al.,2010,2013,2017)constructed(Figs.1 a–c)for mantle columns beneath kimberlite and sections of the lithospheric mantle(SCLM)under the Kaapvaal and the Congo cratons.The set of the pipes is in(Zinchenko et al.,2020,2021).

Mica Types as Indication of Magma Nature,Central Anatolia,Turkey

This study focuses on the nature of giant micas occurring at the contact between theÖzvatan(foid-bearing)syenites and the metamorphic basement in Central Anatolia.The studied micas are dark greenish-black in color and crystallized within vein shape like bodies as a narrow lens.The origin and processes responsible for the formation of these independent crystals of the giant micas were investigated by mineralogical,petrographical and geochemical analyses with the use of Confocal Raman Spectroscopy(CRS),Fourier Transform Infrared(FTIR)Spectroscopy,X-Ray Diffraction(XRD),Polarized Energy Dispersive X-Ray Fluorescence Spectrometer(PED-XRF)and Electron Probe Micro Analysis(EPMA).According to XRD,CRS,FTIR and EPMA data,the giant micas are phlogopite.EPMA results reveal that studied mica minerals represent the products of re-equilibrated primary mica characterized by high MgO and FeO and low Al2O3 and TiO2 contents.The trace element concentrations of the giant micas display similar patterns with the upper crust.The giant micas are crystallized within small cubicles from an alkaline magma and their composition is possibly modified by a mixing event between the crust-and mantle-derived magmas and contaminated at varying extent by the basement metamorphic rocks.

Genesis and magma fertility of gold associated high-K granites:LA-ICP-MS zircon trace element and REEs constraint from Bakoshi–Gadanya granites in NW Nigeria

High-K granites dominate the rock units in the Bakoshi and Gadanya areas located in the northwestern Nigerian subshield,part of the Trans-Saharan Belt,West Africa.In this contribution,the LA-ICP-MS zircon trace element revealed the fertility of magma responsible for the high-K granites that hosts the Bakoshi–Gadanya gold mineralization.Two likely metallogenic granites types are 1)Gadanya alkali granite,with high Ce^(4+)/Ce^(3+)(mean 1485)and limited range of Eu anomalies may likely be associated with the gold mineralization,and 2)Bakoshi porphyritic granite,Jaulere biotite granite,Shanono coarsegrained granite,and Yettiti granite,all have low Ce^(4+)/Ce^(3+) ratios(mean\100,except second Bakoshi granite D2-1)with wider ranges of Eu/Eu^(*) values,thus are considered reduced granites.These reduced granites have oxygen fugacity values and Eu anomalies comparable to reduced granites associated with tin belts in Myanmar and Zaaiplaats granites in Bushveld Complex,South Africa.Ti-inZircon thermometric study revealed two thermal regimes during the crystallization of the Bakoshi–Gadanya granites:the high temperature(746–724℃):Shanono coarsegrained granite,Bakoshi granite D2-1,and Jaulere biotite granite;and relatively low temperature(705–653℃):Bakoshi porphyritic granite D1-1,Yettiti medium-grained granite,and Gadanya alkali granite.Zircon trace elements including U,Yb,Y,Nb,and Sc ratios constraint the magma source of Bakoshi–Gadanya granites to an enriched mantle metasomatized during the subduction process before its melting.Except for Gadanya alkali granite,fractionation of titanite and apatite dominate the magma evolution with limited amphibole fractionation.Melt that crystallized Gadanya alkali granite is rather saturated in zircon without accessory titanite or apatite.

From Mantle Transition Zone with Microstructures: A New Paradigm in Ophiolite Studies

The SE Ladakh(India) area displays one of the best-preserved ophiolite sections in this planet, in places up to 10 km thick, along the southern bank of the Indus River. Recently, in situ ultra-high pressure(UHP) microstructural evidences from mantle transition zone(MTZ ~ 410–660 km) with diamond and reduced fluids were discovered from two peridotite bodies in the basal mantle part of this Indus ophiolite(Das et al., 2015;2017). Ultrahigh-pressure phases were also found by early workers from podiform chromitites of another equivalent Neo–Tethyan ophiolite in southern Tibet(e.g., Yang et al., 2007;Yamamoto et al., 2009). However, the MTZ phases in the Indus ophiolite are found in silicate peridotites not metallic chromitites and the peridotitic UHP phases show systematic and contiguous phase transitions from the MTZ to shallower depth, unlike the discrete ultrahighpressure inclusions, all in Tibetan chromitites. The gradual change in oxygen fugacity(fo2) and fluid composition from(C-H + H2) to(CO2 + H2O) in the upwelling peridotitic mantle causing melting to produce MORB. At shallow depths(< 100 km) the free water stabilizes into hydrous phases, such as amphiboles and serpentines, capable of storing water and prevent melting(Fig. 1). The results from Indus ophiolite provide unique insights into deep sub-oceanic mantle processes, and link deep mantle upwelling and MORB genesis(Fig. 1). The tectonic setting of Neo-Tethyan ophiolites has been a difficult problem since the birth of plate tectonics concept. This problem for the origin of ophiolites in mid-ocean ridge versus supra subduction-zone settings clearly confused the Geoscience community. However, Indian Ocean –type isotopic characteristics are present in Neo-Tethyan ophiolites(Zhang et al., 2005). Recently, continental materials(quartz, k-feldspar etc.) bearing old zircons(up to 2700 Ma) are also recovered from UHP chromitite of Tibetan ophiolite(Yamamoto et al., 2013). Eventually, the presence of older continental material can produce non-MORB like basalts in Neo-Tethyan ophiolites in mid-oceanic-ridge following the ―historical contingency‖ model(Moores et al., 2000).

撞击过程对月表氧逸度的改造:来自嫦娥五号月壤中撞击玻璃珠的启示

Lunar materials are overall more reducing compared with their terrestrial counterparts,but the mechanism remains to be elucidated.In this study,we present a possible explanation for the changes in redox state of the lunar regolith caused by impact events,based on our investigations of the impact glass beads from Chang’e-5 mission.These glass beads contain iron metal grains and show concentration gradients of FeO and K_(2)O(with or without Na_(2)O)from their rims to centers.The compositional profiles exhibit errorfunction-like shapes,which indicates a diffusion-limited mechanism.Our numerical modeling results suggest that the iron metal grains on the surface of the glass beads were generated through the reduction of FeO by elemental K and(or)Na produced during the impact events.Meanwhile,the iron metal grains inside the bead may have formed due to oxygen diffusion driven by redox potential gradients.Furthermore,our study suggests that impact processes intensify the local reducing conditions,as evidenced by the presence of calcium sulfide particles within troilite grains that coexist with iron metal grains on the surface of the glass beads.This study provides insights into the oxygen diffusion kinetics during the formation of iron metal spherules and sheds light on the changes in redox conditions of lunar materials caused by impact events.

The Dabie-Sulu orogenic peridotites:Progress and key issues

Orogenic peridotites in the Dabie-Sulu orogenic belt are commonly subdivided into‘crustal’type and‘mantle’type.They exhibit distinct mineral textures,metamorphic evolution,and whole-rock and mineral compositions.Most‘mantle’type peridotites originated from the subcontinental lithospheric mantle(SCLM)of the North China Craton and thus provide direct evidence of crust-mantle interactions in the continental subduction channel.In garnet peridotites,both garnet and Cr-spinel can be equilibrated at peak pressure conditions.Their stabilities are mainly controlled by the refertilized degree of whole-rock;therefore,spinel composition cannot be used to discriminate the partial melting degree of orogenic peridotites.Refractory mantle-derived dunites contain the textures of low Mg and high Ca olivine veins that crosscut orthopyroxene porphyroblasts,which is considered as evidence for silica-undersaturated melt-rock reactions.Such reactions occurring before subduction may potentially affect Re-Os isotopic compositions.Rutile,Ti-clinohumite and zircon in mantle-derived peridotites or pyroxenites provide direct mineralogical evidence for the transport of high field strength elements(HFSEs)from the subducted crust into the mantle wedge.Based on detailed in situ element and isotope analyses,we can constrain the source of metasomatic agents,the metasomatic time and the process of mass transfer.The mantle wedge above continental subduction zones has a wide range of oxygen fugacity values(FMQ=?5.50–1.75),showing a roughly negative correlation with the subducted depths.However,the calculated results of oxygen fugacity are significantly affected by mineral assemblages,P-T conditions and dehydrogenation-oxidation of nominally anhydrous mantle olivine during exhumation.Although significant progress has been made in the study of orogenic peridotites in the Dabie-Sulu orogenic belt,many critical questions remain.With new approaches and advanced technologic applications,additional knowledge of the phase relation in the peridotite-pyroxenite complex system,the mantle geodynamic process before continental subduction,the effects of crustal metasomatism on chemical composition,the oxygen fugacity,and the physical properties of the mantle wedge is anticipated.

Mineral Chemistry and Crystallization Conditions of the Late Cretaceous Mamba Pluton from the Eastern Gangdese, Southern Tibetan Plateau

The Late Cretaceous Mamba granodiorite belongs to a part of the Mesozoic Gangdese continental magmatic belt. No quantitative mineralogical study has been made hitherto, and hence the depth at which it formed is poorly constrained. Here we present mineralogical data for the Mamba pluton, including host rocks and their mafic microgranular enclaves（MMEs）, to provide insights into their overall crystallization conditions and information about magma mixing. All amphiboles in the Mamba pluton are calcic, with ~B（Ca＋Na）〉1.5, and Si=6.81-7.42 apfu for the host rocks and Si=6.77-7.35 apfu for the MMEs. The paramount cation substitutions in amphibole include edenite type and tschermakite type. Biotites both in the host rocks and the MMEs collectively have high Mg O（13.19 wt.%-13.03 wt.%） contents, but define a narrow range of Al apfu（atoms per formula unit） variations（2.44-2.57）. The oxygen fugacity estimates are based on the biotite compositions cluster around the NNO buffer. The calculated pressure ranges from 1.2 to 2.1 kbar according to the aluminum-in-hornblende barometer. The computed pressure varies from 0.9 to 1.3 kbar based on the aluminum-in-biotite barometer which corresponds to an average depth of ca. 3.9 km. Besides, the estimates of crystallization pressures vary from 0.8 to 1.4 kbar based on the amphibole barometer proposed by Ridolfi et al.（2010）, which can be equivalent to the depths ranging from 3.1 to 5.2 km. The MMEs have plagioclase oscillatory zonings and quartz aggregates, probably indicating the presence of magma mixing. Besides, core-to-rim element variations（Rb, Sr, Ba, and P） for the K-feldspar megacrysts serve as robust evidence to support magma mixing and crystal fractionation. This indicates the significance of the magma mixing that contributes to the formation of K-feldspar megacryst zonings in the Mamba pluton.