Calcium isotopic signatures of depleted mid-ocean ridge basalts from the northeastern Pacific

A number of high-temperature processes(e.g.,melt-rock reactions,metasomatism,partial melting)can produce signifi cant Ca isotopic fractionation and heterogeneity in the mantle,but the mechanism for such fractionation remains obscure.To investigate the eff ect of mantle partial melting on Ca isotopic fractionation,we reported high-precision Ca isotopic compositions of depleted mid-ocean ridge basalts(MORBs)from the East Pacifi c Rise and Ecuador Rift in the northeastern Pacifi c.Theδ44/40 Ca of these MORB samples exhibit a narrow variation from 0.84‰to 0.88‰with an average of 0.85‰±0.03‰,which are similar to those of reported MORBs(0.83‰±0.11‰)and back-arc basin basalts(BABBs,0.80‰±0.08‰)in literature,but are lower than the estimate value for the bulk silicate Earth(BSE,0.94‰±0.05‰).The lowδ44/40 Ca signatures of MORB samples in this study cannot be caused by fractional crystallization,since intermediate-mafi c diff erentiation has been demonstrated having only limited eff ects on Ca isotopic fractionation.Instead,the off set ofδ44/40 Ca between MORBs and the BSE is most likely produced by mantle partial melting.During this process,the light Ca isotopes are preferentially transferred to the melt,while the heavy ones tend to stay in the residue,which is consistent with the fact thatδ44/40 Ca of melt-depleted peridotites increases with partial melting in literature.The behavior of Ca isotopes during mantle partial melting is closely related to the inter-mineral(Cpx and Opx)Ca isotopic fractionation and melting mineral modes.Mantle partial melting is one of the common processes that can induce lowerδ44/40 Ca values in basalts and Ca isotopic heterogeneity in Earth’s mantle.

Magma Dynamics of Axial Melt Lens at Fast-Spreading Mid-Ocean Ridges

Multichannel seismic studies performed at fastspreading mid-ocean ridges revealed the presence of a thin(tens to hundreds of meters high), narrow(< 1-2 km wide) axial melt lens(AML) in the mid-crust, which is underlain by crystal/melt mush that is in turn laterally surrounded by a transition zone of mostly solidified material. In order to shed light on the complexity of magmatic and metamorphic processes ongoing within and at the roof of axial melt lenses, we have focused on the petrological and geochemical record provided by fossilized AMLs. Of particular significance is Hole 1256D in the equatorial Pacific drilled by the International Ocean Discovery Program(IODP), where for the first time, the transition between sheeted dikes and gabbros in intact fast-spreading crust was penetrated, providing a drill core with a more or less continuous record of the upper part of an AML(Teagle et al., 2006;Koepke et al., 2008). This can be regarded as rosetta stone to answer longstanding questions on the complex magmatic evolution within an AML, as well as on metamorphic and anatectic processes ongoing at the roof of a dynamic AML, rising upward in the midcrust as a consequence of a replenishment event. The plutonic rocks drilled from Hole 1256D consist of quartz-bearing gabbros, diorites and tonalites, which might represent the upper part of a fossilized AML. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate in varying ratios. Modeled trace elements support a model in which the tonalites originated from low-degree partial melting of the sheeted dikes overlying the AML, rather than extreme fractional crystallization(Erdmann et al., 2015;Zhang et al., 2017a). Therefore, the upper part of AML, largely composed of low density and high-viscosity felsic magmas, may serve as a barrier to eruptible MORB melts in the lower part of AML. Zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The high-Cl apatite core indicates assimilation of high-Cl brines at a magmatic stage, which is interpreted as immiscibility product from cycling seawater-derived fluids at a high temperature(Zhang et al., 2017b). The variation of F/Cl and Br/Cl ratios of bull rocks may reflect the mixing between MORB magmas and seawater-derived fluids, crystallization of apatite and amphibole, and/or extraction of magmatic fluids(Zhang et al., 2017c).

Model Forming of Mesoarchean Gridino Mafic Dyke Swarm during Subduction "Mid-Ocean Ridge – Continent"

The high-pressure metamorphosed Gridino dyke swarm comprises a major group of Mesoarchean 2.87-2.82 Ga mafic dykes intruded within the Mesoarchean continental crust of the Kola craton(the Belomorian tectonic province

TEM investigations of South Atlantic Ridge 13.2°S hydrothermal area

According to the exploration contract about polymetallic sulfides in the SWIR （Southwest Indian Ridge） signed by China with the International Seabed Authority, to delineate sulfide minerals and estimate resource quantity are urgent tasks. We independently developed our first coincident loop Transient Electromagnetic Method （TEM） device in 2010, and gained the TEM data for seafloor sulfide at South Atlantic Ridge 13.2°S in June 2011. In contrast with the widely applied CSEM （Marine controlled-source electromagnetic） method, whose goal is to explore hydrocarbons （oil/gas） of higher resistivity than seawater from 102 to 103 m below the sea floor, the TEM is for low resistivity minerals, and the target depth is from 0 to 100 m below the sea floor. Based on the development of complex sulfide geoelectrial models, this paper analyzed the TEM data obtained, proposing a new method for seafloor sulfide detection. We present the preliminary trial results, in the form of apparent resistivity sections for both half-space and full-space conditions. The results cor- respond well with the observations of the actual hydrothermal vent area, and the detection depth reached 50-100m below the bed, which verified the capability of the equipment.

Fractional crystallization processes of magma beneath the Carlsberg Ridge(57°–65°E)

Fractional crystallization of basaltic magma at variable depths influences strongly the geochemical compositions of mid-ocean ridge basalts(MORBs),especially at slow-spreading mid-ocean ridges.The Carlsberg Ridge is a typical slow-spreading ridge located in the northwestern Indian Ocean.In this study,we conducted petrological,geochemical and modelling studies of MORBs collected along the Carlsberg Ridge from 57°-65°E to understand the fractional crystallization processes of magma and the controls on variations in MORB geochemistry.Our results show that the mantle sources beneath the Carlsberg Ridge are heterogeneous even on the local scale of a segment;such heterogeneity may be ubiquitous beneath the Carlsberg Ridge.Mantle heterogeneity may be caused by the enriched components resulting in the"DUPAL"anomaly,whereas the effect of pyroxenite on mantle heterogeneity is negligible.The parental melts experienced crystallization of olivine,plagioclase and clinopyroxene prior to eruption,which played a significant role in the major and trace element variations in MORBs from the Carlsberg Ridge.The liquid lines of descent(LLDs),deduced from the forward modelling of three parental magma compositions using the Petrolog3 program at pressures between 1 atm and 10 kbar,demonstrate that clinopyroxene joined the olivine and plagioclase cotectic.The over-enrichment in highly incompatible elements relative to LLDs may be caused by the processes of replenishment-tapping-crystallization in magma chambers.The calculated crystallization pressures suggest that parental magmas beneath the Carlsberg Ridge experienced moderateto high-pressure crystallization and that crystallization beneath the slow-spreading Carlsberg Ridge may start at upper mantle depths.

Use of portable X-ray fluorescence in the analysis of surficial sediments in the exploration of hydrothermal vents on the Southwest Indian Ridge

Hydrothermal plumes released from the eruption of sea floor hydrothermal fluids contain large amounts of oreforming materials. They precipitate within certain distances from the hydrothermal vent. Six surficial sediment samples from the Southwest Indian Ridge（SWIR） were analyzed by a portable X-ray fluorescence（PXRF） analyzer on board to find a favorable method fast and efficient enough for sea floor sulfide sediment geochemical exploration. These sediments were sampled near, at a moderate distance from, or far away from hydrothermal vents. The results demonstrate that the PXRF is effective in determining the enrichment characteristics of the oreforming elements in the calcareous sediments from the mid-ocean ridge. Sediment samples（〉40 mesh） have high levels of elemental copper, zinc, iron, and manganese, and levels of these elements in sediments finer than 40 mesh are lower and relatively stable. This may be due to relatively high levels of basalt debris/glass in the coarse sediments, which are consistent with the results obtained by microscopic observation. The results also show clear zoning of elements copper, zinc, arsenic, iron, and manganese in the surficial sediments around the hydrothermal vent. Sediments near the vent show relatively high content of the ore-forming elements and either high ratios of copper to iron content and zinc to iron content or high ratios of copper to manganese content and zinc to manganese content. These findings show that the content of the ore-forming elements in the sediments around hydrothermal vents are mainly influenced by the distance of sediments to the vent, rather than grain size. In this way, the PXRF analysis of surface sediment geochemistry is found to satisfy the requirements of recognition geochemical anomaly in mid-ocean ridge sediments. Sediments with diameters finer than 40 mesh should be used as analytical samples in the geochemical exploration for hydrothermal vents on mid-oceanic ridges. The results concerning copper, zinc, arsenic, iron, and manganese and their ratio features can be used as indicators in sediment geochemical exploration of seafloor sulfides.

A Petrogenetic Model of Basalts from the Northern Central Indian Ridge:3-11°S

Mid-Ocean Ridge Basalts （MORB） from the Northern Central Indian Ridge （NCIR） were recovered between latitudes 3° and 11° S and are olivine tholeiite with higher abundances of K and Rb. They are of typical transitional MORB （T-MORB） variety and appear to have been generated from an enriched-mantle peridotite source. The primitive NCIR MORBs having Mg^# 〉 0.68 are the product of partial melting at an estimated pressure of - 1 GPa. It is inferred that the magma was subsequently modified at a pressure 〉 1 GPa by crystal fractionation and spinel was the first mineral to crystallize followed by separation of relatively Fe-rich olivine with subsequent decrease in pressure. During progressive fractionation at lower pressure （between 1-0.5 GPa）, the bulk composition of the magma became systematically depleted in MgO, and enriched in ∑FeO, TiO2, P2Os and Na20. There was, however, limited gradual depletion in Al2O3 and CaO and concomitant enrichment in K20. With the progressive fractionation these basalts became gradually enriched in V, Co, Y, Zr and to some extent in Sr, and depleted in Ni and Cro In addition, the T_JtEE of the magma also increased with fractionation, without any change in （La/Yb）n value.

Three-Dimensional Mantle Flow and Temperature Structure Beneath the Shatsky Rise Ridge-Ridge-Ridge Triple Junction

The Shatsky Rise ridge-ridge-ridge triple junction is an ancient triple junction in the Western Pacific Ocean whose initial geodynamic process is poorly understood and can only be inferred based on indirect geological and geophysical constraints.In this paper,we present three-dimensional numerical models that simulate the Shatsky Rise triple junction and calculate its coupled mantle flow and temperature structure.The mantle flow velocity field shows several distinctive features:1)stronger mantle upwelling closer to the ridge axis and triple junction;2)greater upwelling velocity at the faster-spreading ridges;and 3)the most significant increase in upwelling velocity for the slowest-spreading ridge toward the triple junction.The calculated mantle temperature field also reveals distinctive characteristics:1)sharp increases in the mantle temperature with depth and increases toward the spreading ridges and triple junction;2)the faster-spreading ridges are associated with higher temperatures at depth and identical distances from the triple junction;and 3)the slowest-spreading ridge shows the greatest increase in the along-ridge-axis temperature toward the triple junction.Compared to many present-day triple junctions with slower spreading rates,the along-ridge-axis velocity and thermal fields of the Shatsky Rise are more altered due to the presence of the triple junction.

Molybdenum isotope composition of the upper mantle and its origin:insight from mid-ocean ridge basalt

The molybdenum(Mo)isotope system is pivotal in reconstructing marine redox changes throughout Earth’s history and has emerged as a promising tracer for igneous and metamorphic processes.Understanding its composition and variation across major geochemical reservoirs is essential for its application in investigating high-temperature processes.However,there is debate regarding theδ^(98/95)Mo value of the Earth’s mantle,with estimates ranging from sub-chondritic to super-chondritic values.Recent analyses of global mid-ocean ridge basalt(MORB)glasses revealed significantδ^(98/95)Mo variations attributed to mantle heterogeneity,proposing a two-component mixing model to explain the observed variation.Complementary studies confirmed the sub-chondriticδ^(98/95)Mo of the depleted upper mantle,suggesting remixing of subduction-modified oceanic crust as a plausible mechanism.These findings underscore the role of Mo isotopes as effective tracers for understanding dynamic processes associated with mantle-crustal recycling.

Singularity of lithosphere mass density over the mid-ocean ridges and implication on floor depth and heat flow

The relation of heat flow and floor depth across the mid-ocean ridges versus lithosphere age can be described by linear functions of square root of age according to plate thermal conductive Half Space Models(HSM).However,one of the long-standing problems of these classical models is the discrepancies between predicted and observed heat flow and floor depth for very young and very old lithosphere.There have been several recent attempts to overcome this problem:one model incorporates temperature-and pressure-dependent parameters and the second model includes an additional low-conductivity crustal layer or magma rich mantle layer(MRM).Alternatively,in the current paper,the ordinary density of lithosphere in the plate conductive models is substituted with a reduction of lithosphere density towards axis that features the irregularity and nonlinearity of plates across the mid-ocean ridges.A new model is formulated incorporating the new form of density for predicting both peak heat flow and floor depth.Simple solutions of power-law forms derived from the model can significantly improve the predicting results of heat flow and floor depth over the mid-ocean ridges.Several datasets in the literature were reutilized for model validation and comparison.These datasets include both earlier datasets used for original model calibration and the more recently compiled high-quality datasets with both sedimentary and crustal loading corrections.The results indicate that both the heat flow and the slope(first orderderivative)of sea floor approach infinity(undifferentiability or singularities)around the mid-ocean ridges.These singularities are partially due to the boundary condition as it has been already known in the literature and partially to the reduction of density of lithosphere as discovered for the first time in the current research.

Microearthquake reveals the lithospheric structure at midocean ridges and oceanic transform faults

Mid-ocean ridge and oceanic transforms are among the most prominent features on the seafloor surface and are crucial for understanding seafloor spreading and plate tectonic dynamics,but the deep structure of the oceanic lithosphere remains poorly understood.The large number of microearthquakes occurring along ridges and transforms provide valuable information for gaining an indepth view of the underlying detailed seismic structures,contributing to understanding geodynamic processes within the oceanic lithosphere.Previous studies have indicated that the maximum depth of microseismicity is controlled by the 600-℃isotherm.However,this perspective is being challenged due to increasing observations of deep earthquakes that far exceed this suggested isotherm along mid-ocean ridges and oceanic transform faults.Several mechanisms have been proposed to explain these deep events,and we suggest that local geodynamic processes(e.g.,magma supply,mylonite shear zone,longlived faults,hydrothermal vents,etc.)likely play a more important role than previously thought.

基于AZ5214光致保护层的As2S3硫系脊型波导制备

实验研究发现AZ5214光刻胶在一定曝光剂量下显影后会留存一定厚度的底膜,该底膜可以在干法刻蚀过程中避免As-S薄膜与碱性显影液直接接触,减轻薄膜表面损伤,起到保护作用。基于此,采用该底膜作为保护层制备As2S3脊型波导,研究结果表明,在AZ5214光刻胶匀胶厚度为2.1μm、紫外曝光剂量为200 mJ/cm2、显影时间为45 s的条件下会留存约为220 nm厚的光致保护层,该条件下保护层均匀性较好,且在刻蚀阶段可以完全去除。实验表明利用此保护层制备的As2S3脊型波导具有良好的形貌特征,波导脊宽约为3μm、脊高约为800 nm的As2S3脊型波导的传输损耗约为0.74 dB/cm@1 550 nm。

Genesis of ^(230)Th excess in basalts from mid-ocean ridges and ocean islands:Constraints from the global U-series isotope database and major and rare earth element geochemistry

Based on 230Th-238U disequilibrium and major element data from mid-ocean ridge basalts(MORBs) and ocean island basalts(OIBs),this study calculates mantle melting parameters,and thereby investigates the origin of 230Th excess.(230Th/238U) in global MORBs shows a positive correlation with Fe8,Po,Na8,and Fmelt(Fe8 and Na8 are FeO and Na2O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%,Po=pressure of initial melting and Fmelt=degree of melt),while 230Th excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle melting.Furthermore,compared with the MORBs,higher(230Th/238U) in OIBs actually corresponds to a lower melting degree.This suggests that the 230Th excess in MORBs is controlled by mantle melting conditions,while the 230Th excess in OIBs is more likely related to the deep garnet control.The vast majority of calculated initial melting pressures of MORBs with excess 230Th are between 1.0 and 2.5 GPa,which is consistent with the conclusion from experiments in recent years that DU>DTh for Al-clinopyroxene at pressures of >1.0 GPa.The initial melting pressure of OIBs is 2.2-3.5 GPa(around the spinel-garnet transition zone),with their low excess 226Ra compared to MORBs also suggesting a deeper mantle source.Accordingly,excess 230Th in MORBs and OIBs may be formed respectively in the spinel and garnet stability field.In addition,there is no obvious correlation of K2O/TiO2 with(230Th/238U) and initial melting pressure(Po) of MORBs,so it is proposed that the melting depth producing excess 230Th does not tap the spinel-garnet transition zone.OIBs and MORBs in both(230Th/238U) vs.K2O/TiO2 and(230Th/238U) vs.Po plots fall in two distinct areas,indicating that the mineral phases which dominate their excess 230Th are different.Ce/Yb-Ce curves of fast and slow ridge MORBs are similar,while,in comparison,the Ce/Yb-Ce curve for OIBs shows more influence from garnet.The mechanisms generating excess 230Th in MORBs and OIBs are significantly different,with formation of excess 230Th in the garnet zone only being suitable for OIBs.

玻璃文物化学成分的相关性关系探究

本文的研究是基于2022年中国大学生数学建模竞赛的C题。针对该问题,按照高钾风化、高钾未风化、铅钡风化、铅钡未风化分为4组,采用Spearman相关性分析方法分析化学成分的相关性,作出热力图,并据此选取合适化学成分变量,建立岭回归模型来对各成分的相关规律做定量描述,分析并比较不同类别之间的化学成分关联关系的差异性,发现风化前,高钾玻璃化学成分的多元相关性弱于铅钡玻璃;且不论高钾/铅钡,风化后化学成分之间相关的多元性都有所降低。

武当山十堰地区二叠纪E-MORB型玄武岩识别及构造意义

大比例尺构造—岩相填图和测试分析研究显示南秦岭武当山并非统一前寒武纪基底组成,在十堰和两郧断裂之间的黄龙—方滩及丹江口银洞山等地区识别和厘定出一套中生代混杂岩带,开展混杂岩带内物质组成和成因研究对正确认识武当山造山作用和区域构造演化有重要意义。本文重点对武当山十堰黄龙—方滩地区混杂岩带内玄武岩块体岩石成因及时代进行研究,岩石地球化学显示玄武岩块体为亚碱性拉斑玄武岩系列,具有中等Ti、Zr、Hf、Nd含量、弱轻稀土富集重稀土亏损的E-MORB(Enriched Mid-ocean ridge basalt,富集型洋中脊玄武岩)特征,其岩浆为尖晶石二辉橄榄岩低压、高度部分熔融形成,源区可能受富集地幔或俯冲作用等影响导致富集高场强不相容元素;锆石U-Pb测年表明其为中二叠世火山作用产物;区域地质特征及岩石成因研究共同指示该套E-MORB形成于洋中脊或附近海山环境。综上,武当山十堰地区中—晚二叠世存在洋盆,该成果为理解十堰黄龙—方滩混杂岩带成因机制和正确认识南秦岭显生宙构造属性提供了基础资料。

太平洋洋中脊火山玻璃主量元素地球化学特征

洋中脊火山玻璃代表地幔源区熔体成分,携带了岩浆作用和洋壳形成的大量信息。通过研究太平洋洋中脊火山玻璃的主量元素地球化学特征,探讨了岩浆作用过程的元素分配特征,结果表明：洋中脊火山玻璃的主量元素成分呈现规律性演化,从基性向中酸性演化过程中,Na_2O的含量从2%逐渐升高至6%,K_2O从〈1%逐渐升高至5%;而Ca O含量从10%~13%逐渐降低至1%,Fe O从9%~13%逐渐降低至3%,Al_2O_3从18%逐渐降低至13%。洋中脊火山玻璃中主量元素分配特征揭示了其分配的不均一性,这可能是受到宏观上地幔不均一性的影响;在同一洋中脊中的火山玻璃成分也存在不均一性,可能是岩浆作用过程中局部动力学机制差异引起的元素分配不均一。

Ka频段宽带微带-波导转换

研究了在Ka频段上,宽带微带到波导的转换技术。转换通过波导-脊波导-绝缘子探针-微带线的方式实现。这样的实现方式具有插入损耗小、可密封、装配一致性好、可靠性高的优点。文中给出了转换的设计方法及脊波导参数的计算公式以计算转换的各项参数。设计完成后采用电磁场软件进行优化仿真,并根据仿真结果制作实物进行测试。最后给出了转换实物的测试数据。从测试数据可以看到,转换在整个Ka频段内具有良好的性能,插入损耗小于0.3 d B,驻波小于1.5。还可针对具体使用频段进行优化,以进一步提高性能。文中提出的转换完全能够满足实际工程应用,具有良好的应用前景。

An Intra-Oceanic Subduction System Influenced by Ridge Subduction in the Diyanmiao Subduction Accretionary Complex of the Xar Moron Area,Eastern Margin of the Central Asian Orogenic Belt

This study focuses on the geology,geochemistry,Sr-Nd isotopes and their tectonic settings of three types of basalts in Diyanmiao ophiolite in the Xar Moron area located on the eastern margin of the Central Asian Orogenic Belt.Type I basalts are oceanic tholeiites with a depleted light rare earth element(LREE)pattern,which are similar to the typical N-mid-oceanic ridge basalt(MORB)and suggests that they were formed at a mid-oceanic ridge.The initial 87Sr/86Sr ratios of Type I basalts range from 0.703966 to 0.705276 and theεNd(t)values are from 16.49 to 17.15,indicating that they were derived from a depleted mantle source.Type II basalts belong to the medium-potassium calc-akaline series and have the geochem-ical characteristics of Nb-enriched basalt(NEB)with high Nb content(14.5 ppm)and strong enrichment in LREEs,implying that they were created by the partial melting of mantle wedge peridotite that previously metasomatized by slab melts.Type III basalts are high-Al basalt(HAB)with high-Al contents(Al_(2)0_(3)=16.75 wt.%-18.00 wt.%),distinct Nb depletion and high Th/Yb ratios.Thus they were likely gen-erated in a normal island-arc setting.Therefore,the association of MORB,NEB,and HAB in the study area may be due to the subduction of a mid-oceanic ridge,and the Diyanmiao ophiolite is proposed to be formed in the forearc setting of a mid-oceanic ridge subduction system.

Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N:Evidence from High MgO MORB and Melt Inclusions

Mid-ocean ridge basalts （MORBs） from East Pacific Rise （EPR） 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO-Al2O3 and negative MgO-Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenoerysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium-bearing MORB sample E13-3B （MGO=9.52%） is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13-3B only by fractional crystallization at pressure 〉4±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber （at pressure -1 kbar）. The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization （PEC）. The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at 〉4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene＋plagioclase＋olivine crystallization at 〉4±1 kbar to mainly olivine＋plagioclase crystallization at 〈1 kbar, which contributes to the explanation of the “clinopyroxene paradox”.

Petrological and Os Isotopic Characteristics of Zedong Peridotites in the Eastern Yarlung–Zangbo Suture in Tibet

The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine （Fos9.7-91.2）, orthopyroxene （Enss-92）, clinopyroxene （En4-49Wo47-slFS2-4） and spinel [Mg^#=lOOxMg/（Mg＋Fe）]=49.1-70.7; Cr^#=（100xCr/（Cr＋Al）=18.8-76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%-40%, indicating that the Zedong mantle peridotites may experience a multi-stage process. The peridotites are characterized by depleted major element compositions and low REE content （0.08-0.62 ppm）. Their ＂spoon-shaped＂ primitivemantle normalized REE patterns with （La/Sm）N being 0.50-6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites （18.19-50.74 ppb） is similar with primary mantle with Pd/Ir being 0.54-0.60 and Pt/Pd being 1.09-1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with 187Os/18Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge （MOR） and were later modified in supra-subduction zone （SSZ） environment.