Carbon Enrichment in the Lithospheric Mantle:Evidence from the Melt Inclusions in Mantle Xenoliths from the Hainan Basalts

It is generally believed that the lithospheric mantle and the mantle transition zone are important carbon reservoirs.However,the location of carbon storage in Earth's interior and the reasons for carbon enrichment remain unclear.In this study,we report CO_(2)-rich olivine-hosted melt inclusions in the mantle xenoliths of late Cenozoic basalts from the Penglai area,Hainan Province,which may shed some light on the carbon enrichment process in the lithospheric mantle.We also present a detailed petrological and geochemical investigation of the late Cenozoic basalts and mantle xenoliths from northern Hainan Island.The collected samples of late Cenozoic Hainan Island basalts belong to both alkaline and subalkaline series,showing fractionated REE patterns with high(La/Yb)_(N)values of 3.52–11.77,which are typical for OIB.Based on Al-in-olivine thermometry,the temperatures estimated for the mantle xenoliths can be divided into two groups.One group has temperatures of less than 1050℃,and the other group has temperature ranging from 1050℃to 1282℃.Clinopyroxene(La/Yb)_(N)–Ti/Eu and clinopyroxene Ca/Al–Mg^(#)diagrams indicate that the mantle peridotite experienced metasomatism from both silicate and carbonate melts.Melt inclusions in the olivine of mantle xenoliths include(1)CO_(2)bubble–rich melt inclusions;(2)multiphase melt inclusions(glass+CO_(2)bubble+daughter minerals);(3)pure glass melt inclusions.Magnesite is a daughter mineral in the olivine-hosted melt inclusions,which could be interpreted as a secondary mineral formed by the interactions of CO_(2)-rich fluids with an olivine host,due to post-entrapment effects.The glasses in olivine-hosted melt inclusions have high SiO_(2)contents(60.21–77.72 wt%).Our results suggest that a considerable amount of CO_(2)-rich melt inclusions are captured in the lithospheric mantle during metasomatism.The lithospheric mantle can therefore act as is a‘carbon trap',with much CO_(2)being absorbed by the lithospheric mantle in this way.

Application of Hydrothermal Diamond Anvil Cell to Homogenization Experiments of Silicate Melt Inclusions

The homogenization of silicate melt inclusions （SMIs）,small droplets of silicate melt trapped in magmatic minerals,is an important component of petrogenetic and magmatic research.Conventional homogenization experiments on SMIs use microscope-mounted heating stages capable of producing high temperatures at 1 atm and cold-seal high-pressure vessels.Heating stages are generally used for SMIs with low internal pressures and allow in situ observations of the homogenization processes.In contrast,cold-seal high-pressure vessels are generally used to heat SMIs that have high internal pressures,although the homogenized SMIs can only be observed after quenching in this approach.Here we outline an alternative approach that uses a hydrothermal diamond anvil cell （HDAC） apparatus to homogenize SMIs.This is the only current method wherein phase changes in high-internal-pressure SMIs can be observed in situ during homogenization experiments,which represents an advantage over other conventional methods.Using an HDAC apparatus prevents high-internal-pressure SMIs from decrepitating during heating by elevating their external pressure,in addition to allowing in situ observations of SMIs.The type-V HDAC that is currently being used has a shorter distance between the sample chamber and the observation window than earlier types,potentially enabling continuous observation of the processes involved in heating and SMI homogenization through an objective lens with a long working distance.Homogenization experiments using HDAC require that a number of steps,including HDAC preparation,sample preparation,sample loading,preheating,and formal heating,be carefully followed.Homogenization experiments on SMIs within granite samples from the Jiajika pegmatite deposit （Sichuan,China） are best performed using an HDAC-based approach,because the elevated proper external pressure of these SMIs,combined with a short heating duration,helps to suppress material leakage and any reactions within the SMIs,in addition to allowing in situ observations during homogenization experiments.Furthermore,using the HDAC approach has other benefits：heating rates can be precisely controlled,wafer oxidization can be prevented,and samples can be subjected to in situ microbeam analysis.In summary,homogenization using HDAC provides more reliable results than those obtained using conventional heating equipment.Future developments will include improvements to the quenching method and temperature controls for the HDAC apparatus,thereby improving the utility of this approach for SMI homogenization experiments.

Magma Evolution Processes in the Southern Okinawa Trough:Insights from Melt Inclusions

The Okinawa Trough is an initial back-arc basin that is influenced by the subduction of the Philippine Sea Plate and develops on the continental crust.The Okinawa Trough is a natural laboratory for the study of basin evolution,magmatism,and crustmantle processes in the early stage of back-arc spreading.Melt inclusions are small droplets of magma that are captured during the mineral crystallization process and can record the geochemical composition changes during magma evolution.In this study,the geochemical compositions of melt inclusions in host plagioclases of two volcanic rock samples at Station nos.9-1 and 9-2 from the southern Okinawa Trough are systematically analyzed.Based on previous studies,the origin and evolution of magma and the introduction of subducting materials in the study area are discussed.Results show that melt inclusions are characterized by the relative enrichment of large-ion lithophile elements,depletion of high-field-strength elements,and slight enrichment of rare earth elements.Indeed,the subduction of the Philippine Sea Plate introduced sediment-derived melts and fluids into the magma source area of the southern Okinawa Trough.Subsequently,4%to 5%partial melting of the hydrated mantle produces basaltic magma.The melt inclusions of andesite and dacite investigated in this study were formed by fractional crystallization of basaltic magma.Finally,the crystallization of plagioclase,pyroxene,and magnetite occurred during the late stage of magma evolution.The temperature-pressure data show that the melt inclusions in plagioclase have two capture periods:one is at temperatures above 1250℃and the other is at temperatures between 1180℃and 1200℃.The capture pressure of the inclusions at temperatures between 1180℃and 1200℃is between 5.6 kPa and 6.1 kPa,corresponding to the depth of 15–17 km below the seafloor.The geochemical characteristics of major and trace elements in inclusions show that the samples from two stations(i.e.,9-1 and 9-2)have similar or identical magma source areas.However,the crystallization differentiation reflected by inclusions in sample 9-1 is more obvious than that in sample 9-2.The inclusions were captured during magma evolution and were not contaminated by crustal materials.

Magma Mixing as a Trigger for Sulphide Saturation in the UG2 Chromitite(Bushveld):Evidence from the Silicate and Sulphide Melt Inclusions in Chromite

It is of great importance to understand the origin of UG2 chromitite reefs and reasons why some chromitite reefs contain relatively high contents of platinum group elements（PGEs： Os, Ir, Ru, Rh,Pt, Pd） or highly siderophile elements（HSEs： Au, Re, PGE）. This paper documents sulphide-silicate assemblages enclosed in chromite grains from the UG2 chromitite. These are formed as a result of crystallisation of sulphide and silicate melts that are trapped during chromite crystallisation. The inclusions display negative crystal shapes ranging from several micrometres to 100 μm in size.Interstitial sulphide assemblages lack pyrrhotite and consist of chalcopyrite, pentlandite and some pyrite. The electron microprobe data of these sulphides show that the pentlandite grains present in some of the sulphide inclusions have a significantly higher iron（Fe） and lower nickel（Ni） content than the pentlandite in the rock matrix. Pyrite and chalcopyrite show no difference. The contrast in composition between inter-cumulus plagioclase（An_（68）） and plagioclase enclosed in chromite（An_（13））, as well as the presence of quartz, is consistent with the existence of a felsic melt at the time of chromite saturation.Detailed studies of HSE distribution in the sulphides and chromite were conducted by LA-ICP-MS（laser ablation-inductively coupled plasma-mass spectrometry）, which showed the following.（Ⅰ） Chromite contained no detectable HSE in solid solution.（Ⅱ） HSE distribution in sulphide assemblages interstitial to chromite was variable. In general, Pd, Rh, Ru and Ir occurred dominantly in pentlandite, whereas Os,Pt and Au were detected only in matrix sulphide grains and were clearly associated with Bi and Te.（Ⅲ）In the sulphide inclusions,（a） pyrrhotite did not contain any significant amount of HSE,（b） chalcopyrite contained only some Rh compared to the other sulphides,（c） pentlandite was the main host for Pd,（d）pyrite contained most of the Ru, Os, Ir and Re,（e） Pt and Rh were closely associated with Bi forming a continuous rim between pyrite and pentlandite and（f） no Au was detected. These results show that the use of ArF excimer laser to produce high-resolution trace element maps provides information that cannot be obtained by conventional（spot） LA-ICP-MS analysis or trace element maps that use relatively large beam diameters.

Lithogeochemistry of Volcanic Rocks and Hosted Melt Inclusions in Wudalianchi,Heilongjiang,China

Wudalianchi（五大连池） volcanoes are famous Quaternary ones with fairly good volcanic landscape in Northeast China.The volcanic rocks are phonotephrite,tephri-phonolite,trachyandesite,and basaltic trachyandesite with SiO2 contents of 47.3 wt.%-54.2 wt.%.The characteristic of high K2O content in Wudalianchi volcanic rocks suggests that they probably share a common potassic magma source.The magma evolutions of Wudalianchi volcanic rocks are similar,while the new eruption prod-ucts of Laoheishan（老黑山） and Huoshaoshan（火烧山） volcanoes are slightly more evolved than the old eruption materials,i.e.,the magma of the new eruptions in 1719-1721 from the above two volcanoes is the evolved magma in underground chamber.The main phenocrysts of Wudalianchi volcanic rocks are olivines,clinopyroxenes,and some characteristic K-rich leucites.The various-shaped melt inclu-sions are found in olivine phenocrysts from new eruption products of Laoheishan and Huoshaoshan volcanoes.By electron microprobe（EMP） analyzing,the contents of SiO2 and total alkali show a wide range,suggesting that the pre-eruptive magma was probably more complicated,or the melt inclusions were rather unhomogeneous.On the basis of EMP results,it is believed that the sulfur degassing rate from this new eruption was much higher than that of the millennium eruption of Tianchi（天池） volcano,while the chlorine degassing rate was a magnitude lower than that of Tianchi volcano.

Genesis of Rare Element Mineralized Granites in Southern China:Evidence from Melt and Fluid Inclusions

Rare element mineralized granites of me Yanshan period in Southern China are characterized by high contents of SiO2,Na2o,K2O,and F and are enriched in Nb,Ta,REE,W,Sn,Be and Li .Opinions differed over whether they are of metasomatk or magmatic origin .In this paper ,we present results of a study on melt and fluid inclusions in topaz-albite feldspar granite from Limo Nb-Ta-W-Sn granite and 414 Ta-Li albhe granite .Our data, which come from 15 mineralized granites, provide strong support magmatic origin .Mett inclusions are recognized in both topaz and quartz, and are associated with primary gas-rich incluaons.Crystallites of a Nb (Ta) -bearing mineral and cassiterite are also commonly present as inclusions in the topaz .Multiphase (mett+fluid) inclusions are observed in some quartz from granites and granite-related pegmatite .Mett inclusions start to mett at a temperature of approximately 540 ℃ in topaz and 650 ℃ in quartz .Final homogenization temperature is 700-900 ℃ in quartz ,700-800℃ in topaz and 440-550 ℃ for primary gas-rich inclusions coexisting with mett inclusions in topaz .The fluid inclusion data show that there was a continuous evobtion of the fluid from Nb-Ta granites and pegmatites to W-Sh hydrothermal vein. The coexistence of mett .vapour-rich fluid inclusions, and rare element crystaffites in topaz and quartz indicates that these minerals crystallized from a vapour-saturated mett enriched in Na.K,F and rare elements (Li,Be,Nb,Ta) .The evidence from fluid inclusion study shows that the possible genesis of the rare dement mineralization inLi-F granites of Yanshan period in Southern China results from magmatic differentiation .

Charnockite microstructures:From magmatic to metamorphic

Charnockites sensu lato （charnockite-enderbite series） are lower crustal felsic rocks typically characterised by the presence of anhydrous minerals including orthopyroxene and garnet. They either represent dry （H2O-poor） felsic magmas that are emplaced in the lower crust or granitic intrusions that have been dehydrated during a subsequent granulite facies metamorphic event. In the first case, post- magmatic high-temperature recrystallisation may result in widespread metamorphic granulite microstruc- tures, superimposed or replacing the magmatic microstructures. Despite recrystallisation, magmatic remnants may still be found, notably in the form of melt-related microstructures such as melt inclusions. For both magmatic charnockites and dehydrated granites, subsequent fluid-mineral interaction at inter- grain boundaries during retrogradation are documented by microstructures including K-feldspar micro- veins and myrmekites. They indicate that a large quantity of low-H2O activity salt-rich brines, were present （together with CO2 under immiscible conditions） in the lower crust.

Origin and evolution of alkali basaltic magma in the Okinawa Trough

The contents of major element composition of the phenocrysts and the matrix glass as well as the spinel inclusions and the melt inclusion in the phenocrysts of the basalt dredged from Station 133 of the Okinawa Trough are determined by electron microprobe. The results show that the basalt is a dor- galite consisting of phenocrysts of bytownite, chrysolite, clinopyroxene and magnetite as well as labradorite microcrystal, matrix glass and a few bits of broken vein quartz. Glassy melt inclusion and chromohercynite or chrompleonaste exist in bytownite and chrysolite. The formation of the spinels is re- lated to partial melting of mantle. The melt inclusions stand for a primary alkali dorgalitic magma, whose composition corresponds to olivine gabbro. The basaltic magma was generated from partial melting of spinel-lherzolite of the upper mantle and evolved in a process of 'alkali dorgalitic magma-trachy- basaltic magma-basdaltic trachytic magma-trachytic magma'. Assimilation and hybridization of crustal material may exist during magma upwelling in every evolutionary stage.

Study on Transformation of CaO-Al_2O_3-SiO_2-MgO System Inclusions in ZG06Cr13Ni4Mo Stainless Steel

This paper aims at generally studying on the inclusions of ZGG06Cr13Ni4Mo ultra-low carbon stainless steel used for large scale hydraulic turbine sets. On the basis of analysis the industrial test samples, thermodynamic software FactSage is used to study inclusions transformation process in CaO-SiO2-Al2O3-MgO system while stainless steel is been smelting. The results illustrate the inclusions melting points and plasticity variation law. The reasonable inclusions plasticity controlling area is found out in low melting point. The melting point is up to 1280℃ While MgO weight concentration is 10% in CaO-SiO2-Al2O3-MgO system, SiO2, Al2O3 and CaO in inclusions are in weight concentration range of 50-70%, 10-20% and 10-40%, respectively. For purpose of gaining low melting point inclusions, silicon alumina radio in deoxidant should be controlled between 2-5 for ultra-low carbon stainless steel refining in industry.