地幔矿物与水流体之间元素分配系数的研究及意义

流体是地球内部物质和能量迁移最为活跃的介质，它在造成地幔化学的富集和亏损，产生具有不同地球化学特征的幔源岩浆岩石，以及促进壳幔物质的再循环过程等诸多方面都起了重大作用，高温高压下实验模拟流体与地幔岩石和矿物之间痕量元素分配作用是揭示地幔流体的组成与性质，地幔中不同元素类型之间或内部的分异作用，地幔交代介质的类型与特征，岛孤玄武岩高场强元素亏损原因的一个重要的手段，并对近年来有关高温高压下流体与地幔矿物之间痕量元素分配作用的实验模拟研究进行了评述，分析了制约流体与地幔矿物之间痕量元素分配系数的因素，总结了这些研究的应用。     

磷在硅酸盐矿物与熔体之间分配系数的研究进展

硅酸盐矿物与熔体之间的磷分配系数对研究岩浆演化和结晶分异程度具有重要意义,也是了解地幔磷储库和建立地球各圈层间磷运移模式的基础。本文分析和总结了前人采用天然样品和合成实验样品研究不同硅酸盐矿物和熔体间的磷分配系数的成果,分析了不同物理化学参数对分配系数的影响,包括熔体组成(如Mg O、Al2O3含量)、矿物结构(分配系数与[Si O4]4-聚合度呈负相关关系)、温度、压力和氧逸度等,指出当前研究中有关更高压力条件(15 GPa)及有流体存在时分配系数的研究是不足的。     

元素分配系数和矿物、熔体成分(结构)的关系及其地球化学意义

元素的矿物—熔体间分配系数(简称分配系数,下同)是指元素在矿物和平衡共存熔体中重量浓度的比值,即K_i~(S/L)=C_i~S/C_i~L。它对于研究岩浆起源和演化有极其重要的意义,目前,巳被广泛用于与玄武岩、安山岩、花岗岩成因有关的深部岩石熔融分异和结晶分异以及碳酸岩、陨石、月岩成因等方面的研宄,并取得了很大的进展。研究表明,影响元素分配系数的因素很多,如温度、压力,氧逸度、元素浓度、挥发分以     

双沟蛇绿岩中地幔交代作用的质子微探针研究

采用空间分辨率高和高灵敏度的质子微探针和微束ＰＩＸＥ技术，测定地幔流体结晶后各组成矿物的微量元素组成及其在不同矿物内的分布状况。对云南双沟蛇绿岩中存在的地幔交代作用进行的研究结果表明，由地幔流体结晶而成的矿物（单斜辉石、尖晶石、石榴石、绿泥石），其微量元素组成与分布特征受地幔交代作用过程的控制。这对于进一步探讨双沟地幔交代作用过程中微量元素的地球化学行为很有意义     

超高压变质过程中的元素地球化学行为——CCSD主孔榴辉岩的矿物化学研究

中国大陆科学钻探主孔位于江苏东海县,苏鲁超高压变质带的南部.该钻孔的0～2050m深度获取了六种不同类型的榴辉岩和少量石榴石辉石岩岩心,它们是典型的基性超高压变质岩,为研究大陆深俯冲过程中的元素地球化学行为提供了非常好的样品.本文对各种超高压变质矿物的微量元素成分进行了系统的原位微区分析,结合全岩化学成分和矿物主量元素成分,深入地研究了超高压变质岩的微量元素赋存特征、分配规律、控制因素,及其对变质条件和流体.岩石相互作用的限定意义.结果表明,超高压榴辉岩中的LREE和Sr主要赋存在磷灰石、帘石和单斜辉石中,HREE赋存在石榴石中,Ba、Rb和Cs等LILE赋存在多硅白云母中,Ti、Nb和Ta等HFSE主要赋存在金红石、钛铁矿中,V、Sc、Co和Ni等元素大多赋存在石榴石和单斜辉石中.研究表明,全岩化学成分和矿物组成、及其含量的变化明显控制着超高压矿物的微量元素含量和分布形式.本研究也获得了如下重要的认识:超高压变质矿物之间的微量元素分配达到了化学平衡,并具有与地幔榴辉岩矿物之间类似的分配系数,表明榴辉岩的峰期变质温度很可能达到900℃～1000℃.部分高Ti和高Fe-Ti榴辉岩中的石榴石和绿辉石有明显的稀土元素成分环带,表明超高压变质岩经历了快速折返过程.金红石的Zr含量明显受到全岩成分和退变质作用影响,并不仅仅与形成温度有关,不是可靠的温度计.在超临界流体的作用下,榴辉岩中金红石的Nb、Ta发生了明显的分异,导致其Nb/Ta比值增大,由此推测俯冲到地幔深处的大量榴辉岩是地球内部高Nb/Ta比值的物质源区.在榴辉岩的不同程度退变质阶段,参与变质反应的流体具有不同的来源、成分和流体活动规模.     

铂族矿物中锇同位素比值的地质意义

铂族矿物中锇同位素比值的地质意义谢智（中国科技大学地球和空间科学系,合肥２３００２６）关键词锇同位素比值铂族矿物地幔地壳Ｏｓ元素是中等相容性元素,Ｒｅ是不相容性元素。在壳幔分异过程中,Ｒｅ／Ｏｓ分异很大,Ｏｓ倾向于存留在地幔中,而地壳中Ｏｓ的含量很低...     

角闪石-熔体间微量元素分配系数实验研究

微量元素在矿物与熔体间的分布与其物质组成、矿物结构以及温度、压力等有关。为了研究温度和压力对角闪石-熔体间微量元素分配系数的影响,我们对采自豫西伊川-汝阳的含水玄武岩进行了熔融结晶实验。在中国科学院地球化学研究所地球内部物质高温高压院重点实验室CS-3600t多顶砧压力机上完成了两个系列的实验:(1)温度系列,保持压力p=0.6 GPa和加热时间t=100 h不变,改变结晶温度(800~900℃)的实验;(2)压力系列,保持温度T=900℃和加热时间t=100 h不变,改变压力(0.6~2.6 GPa)的实验。结果显示,实验产物的固相主要为角闪石,另有少量磷灰石和尖晶石,得到的熔体为花岗闪长质成分。利用激光剥蚀电感耦合等离子质谱仪(LA-ICP-MS)测试实验产物中角闪石和熔体的微量元素,根据微量元素在平衡两相中分配的浓度比(即能斯特分配定律),我们计算了大离子亲石元素(LILE:Li,Be,Rb,Sr,Cs,Ba),稀土元素(REE:La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu,Y),高场强元素(HFSF:Nb,Ta,Zr,Hf,Pb4+)以及过渡金属元素(Sc,V,Cr,Co)在各体系内角闪石-熔体间微量元素的分配系数。从实验结果的整体特征上来看,角闪石的LILE和LREE的分配系数小于1(DRb=0.35,DLa=0.48),说明二者相对富集于熔体中;而HREE的分配系数大于1(DLu=1.37),说明其相对富集于角闪石中;HFSF的分配系数较小(DNb=0.81),表明其在角闪石中呈亏损状态;而过渡金属元素在熔体中表现为强烈的亏损,如DV=~30。在温度系列的实验产物中,随着温度从800℃升至900℃,各微量元素的分配系数没有明显的变化规律,但是LILE、REE和HFSF的分配系数都较压力系列的值要大。在压力系列的实验产物中,随着压力从0.6 GPa升至2.6 GPa,大部分微量元素的分配系数与压力呈负相关关系,特别是LILE和HFSF,其分配系数随压力的升高而明显减小。表明压力对微量元素在角闪石-熔体间分布的影响比温度对其分布的影响更大,而且压力越大,熔体中相对越富集LILE等微量元素。微量元素在晶体中的分配主要受离子半径和电荷的控制。对于相同电价的微量元素,其分配系数与矿物的晶格参数呈非线性关系。晶格应变弹性模型可以实现这一非线性拟合,其理论公式如下式所示:Di=D0×exp[-4πENA[r0/2(ri-r0)2+1/3(ri-r0)3]/RT]。其中:NA为阿伏加德罗常数;R为气体常数;T为温度(单位是k);E为杨氏模量;ri为元素i的离子半径;r0为矿物晶格的最佳离子半径;Di为元素i的分配系数;D0为无应变分配系数。假设+3价的REE全部进入角闪石晶格中的M4位,将各实验产物中REE的分配系数利用上述公式进行非线性拟合,可得到角闪石的D0、r0和E。拟合结果显示,在0.6 GPa下,温度从800℃升至900℃,角闪石的晶格参数没有明显的变化规律;而在900℃下,随着压力从0.6 GPa升至2.6 GPa,角闪石M4位的r0=~0.89-0.92,E的范围是139~534 GPa,而D0由1.17增大至1.53。这可能与压力利于REE富集于晶体中有关,特别是HREE选择性富集于角闪石晶体中。由于非线性拟合曲线与实验值有良好的一致性,表明理论计算结果和实验结果具有比较好的对应关系。     

地幔硫化物的物理化学性质及其地质意义

地幔硫化物是地球亲硫、亲铁元素的重要载体,与硅酸盐矿物有很大的物理性质上差别,它在高温高压条件下的物理化学性质是认识地幔物质组成及其不均一性的重要内容。此外,早期地球、火星、水星和月球幔部来源的硫化物对认识行星演化具有重要意义。地幔硫化物的存在条件、赋存方式以及元素分配系数受到硫化物和硅酸盐体系的化学平衡控制;硫化物在流体介质和固体介质中的迁移可以分别用斯托克斯方程和表面张力模型进行解释。基于目前的天然样品和实验研究结果,本文提出了地幔硫化物物理化学性质研究中尚存在的问题,并作展望：高温高压实验制约分配系数适用的元素含量远高于天然样品,导致亨利定律并不一定适用,要求开展元素含量更低的实验并进行测试;现有的线性或自然对数的经验方程不能准确描述硫化物-硅酸盐体系,机器学习可能揭示其错综复杂的热力学关系;硫化物熔体在深部地幔硅酸盐矿物表面的分布尚不清楚,对极端条件下的静水压力实验提出了需求。     

东太平洋海隆13°N附近分离结晶压力:模拟计算方法

在东太平洋海隆13°N附近在过去的30年里,尤其是在发现海底热液活动以来,对海隆内部玄武岩进行过大量详细的研究.研究表明洋中脊玄武岩地幔源区组成具有不一致性,并且即使在很小的区域范围玄武岩的同位素组成也表现具有差异性(Allègre和Condomines,1982;Niu,2002;Rudge等,2005;Anderson,2006).而一般地球化学家不倾向于利用微量元素限定地幔源区的微量元素的富集程度(O'Hara,2002),这是由于微量元素在岩浆过程中受到多种因素的影响,如:①地幔熔融程度,地幔熔融程度对不相容微量元素在熔体相中的富集程度影响很大,不同微量元素的相容性的差异性更增加了对地幔源区组成的不确定性;②参与部分熔融的地幔源区矿物组成的不确定性,在尖晶石一二辉橄榄岩源区微量元素的富集主要受控于单斜辉石的熔融程度;③地幔熔融的形式,分离部分熔融将比批式熔融对地幔不相容微量的亏损程度影响更大;④原始岩浆在迁移上升过程中受到岩浆分离结晶作用、岩浆混合及与围岩混染作用的影响.     

云南白马寨镍矿区煌斑岩地球化学Ⅰ.主要元素和微量元素

云南白马寨镍矿区煌斑岩呈岩墙和岩脉穿切矿区各时代地层、基性-超基性岩和矿体，为成矿期后产物；采自矿区不同中段和不同产状的煌斑岩具有相似矿物组合，岩石类型均为云煌岩；岩石化学特征表明岩石均为钾质-富钾质钙碱性煌斑岩；微量元素配分模式为相似的“骆峰型”，与MORB相比，富集LILE和HPSE；REE配分模式为相似的LREE富集型；主要元素和微量元素与镁指数(M值)之间存在一定的相关关系；在La—La／Sm图中样品既有水平分布趋势、也有倾斜分布趋势。总体上，该区煌斑岩为相同岩浆源区的产物，源区地幔部分熔融程度和岩浆结晶分异作用对岩石主要元素和微量元素地球化学性质均有一定的影响。比较老王寨金矿区煌斑岩形成地质背景、岩石类型、主要元素和微量元素地球化学特征，认为两地区煌斑岩地幔源区具有相同或相似、的性质。     

Silicate perovskite-melt partitioning of trace elements and geochemical signature of a deep perovskitic reservoir

We have determined the partitioning of a wide range of trace elements between silicate melts and CaSiO₃ and MgSiO₃ perovskites using both laser ablation-ICPMS and ion microprobe techniques. Our results show that, with the exception of Sc, Zr, and Hf, all trace elements we considered are incompatible in MgSiO₃ perovskite, from highly incompatible for U, Th, Ba, La, Sr and monovalent elements to slightly incompatible for heavy rare earth elements. MgSiO₃ perovskite-melt partition coefficients increase slightly with Al content in the perovskite. These observations contrast strongly with partitioning between CaSiO₃ perovskite and silicate melts. In the latter case, all rare earth elements are clearly compatible as are U and Th. Our data also suggest that, contrary to pressure and temperature, melt composition can significantly affect CaSiO₃ perovskite-melt partitioning; partition coefficients for rare earth elements and U and Th increase with decreasing CaO melt content. The presence of ∼0.4 wt% water in melt makes little difference, however. Partitioning of trace elements into the large site of both MgSiO₃ and CaSiO₃ perovskites follows the near-parabolic dependence on ionic radius predicted from the lattice strain model. The peaks of the parabolae are much higher for the CaSiO₃ phase, perhaps suggesting that the mechanisms of charge compensation for heterovalent substitution are different in the two cases. Our partitioning data have been used to assess the potential effect of perovskite fractionation into the lower mantle during early Earth history. Crystallisation of less than 8% of a mixture of CaSiO₃ and MgSiO₃ perovskites could have led to a ‘layer’ enriched in U and Th without disturbing the chondritic pattern of refractory lithophile elements in the primitive upper mantle. The resultant reservoir could have high Sm/Nd, U/Pb, Sr/Rb, Lu/Hf ratios similar to the HIMU component of ocean island basalts, but would not balance the observed depletion of the primitive upper mantle in Si and Nb.     

The partitioning of trace transition elements in crystal structures: a provocative review with applications to mantle geochemistry

Interrelationships between chemical compositions and crystal structures of minerals pioneered by Goldschmidt have been overlooked by modern geochemists. While analytical techniques and data for trace elements have become more sophisticated, progress has been slow in relating abundance data of individual elements to their relative enrichments in specific sites in mineral structures. The concept of diadochy has degenerated into an analytical relationship between major and trace elements, instead of its original crystallographic basis of replacement of one atom by another at a specific site in a crystal structure. Future interpretations of trace element data must consider the partitioning of atoms between different coordination sites in multisite mineral structures.The well-known partitioning of nickel and chromium into the earliest minerals during magmatic crystallization is extended to magma evolution in the mantle. The strong preference of Ni²⁺ and Cr³⁺ for octahedral sites in minerals leads to their enrichment in certain Iherzolites which are refractory residua during partial fusion of the mantle.     

Sc,Cr, Co and Ni partitioning between minerals from spinel peridotite xenoliths

The partitioning of divalent (Co, Ni) and trivalent (Sc, Cr) trace elements between olivine, ortho- and clinopyroxene and spinel from spinel peridotite xenoliths has been investigated. These peridotites cover a wide range in modal composition from dunite to primitive lherzolites and have equilibrated in the upper mantle between >900° C and <1,200° C.The distribution of Co and Ni shows only minor variation through the whole sequence. In contrast, Sc partitioning between ortho- and clinopyroxene and olivine and clinopyroxene as well as Cr partitioning between olivine and clinopyroxene or olivine and orthopyroxene display high but systematic variations which can be assigned to dependences upon equilibration temperatues. Empirical temperature calibrations are given for Sc-orthopyroxene/clinopyroxene, Sc-olivine/clinopyroxene and Cr-olivine/clinopyroxene which, in principle, may permit to estimate equilibration temperatures not only for lherzolites or harzburgites but for orthopyroxene-free peridotites, too.Sc and Ni partition coefficients between spinel and mantle silicate minerals are primarily dependent upon the major element composition of spinel (e.g. Cr and Al) although a temperature dependence can still be identified. Probably such compositional effects are not observed for trace element partitioning between pyroxenes and olivine or ortho- and clinopyroxene only for the reason that in normal spinel peridotites these minerals show much less variation in major element composition than their coexisting spinels.     

Experimental constraints on crystallization differentiation in a deep magma ocean

Major and trace element mineral/melt partition coefficients are presented for phases on the liquidus of fertile peridotite at 23-23.5 GPa and 2300 °C. Partitioning models, based on lattice-strain theory, are developed for cations in the ‘8-fold’ sites of majorite and Mg-perovskite. Composition-dependant partitioning models are made for cations in the 12-fold site of Ca-perovskite based on previously published data. D^min/melt is extremely variable for many elements in Ca-perovskite and highly correlated with certain melt compositional parameters (e.g. CaO and Al₂O₃ contents). The 8-fold sites in Mg-perovskite and majorite generally have ideal site radii between 0.8 and 0.9 Å for trivalent cations, such that among rare-earth-elements (REE) D^min/melt is maximum for Lu. Lighter REE become increasingly incompatible with increasing ionic radii. The 12-fold site in Ca-perovskite is larger and has an ideal trivalent site radius of ∼1.05 Å, such that the middle REE has the maximum D^min/melt. Trivalent cations are generally compatible to highly compatible in Ca-perovskite giving it considerable leverage in crystallization models. Geochemical models based on these phase relations and partitioning results are used to test for evidence in mantle peridotite of preserved signals of crystal differentiation in a deep, Hadean magma ocean.Model compositions for bulk silicate Earth and convecting mantle are constructed and evaluated. The model compositions for primitive convecting mantle yield superchondritic Mg/Si and Ca/Al ratios, although many refractory lithophile element ratios are near chondritic. Major element mass balance calculations effectively preclude a CI-chondritic bulk silicate Earth composition, and the super-chondritic Mg/Si ratio of the mantle is apparently a primary feature. Mass balance calculations indicate that 10-15% crystal fractionation of an assemblage dominated by Mg-perovskite, but with minor amounts of Ca-perovskite and ferropericlase, from a magma ocean with model peridotite-based bulk silicate Earth composition produces a residual magma that resembles closely the convecting mantle.Partition coefficient based crystal fractionation models are developed that track changes in refractory lithophile major and trace element ratios in the residual magma (e.g. convecting mantle). Monomineralic crystallization of majorite or Mg-perovskite is limited to less than 5% before certain ratios fractionate beyond convecting mantle values. Only trace amounts of Ca-perovskite can be tolerated in isolation due to its remarkable ability to fractionate lithophile elements. Indeed, Ca-perovskite is limited to only a few percent in a deep mantle crystal assemblage. Removal from a magma ocean of approximately 13% of a deep mantle assemblage comprised of Mg-perovskite, Ca-perovskite and ferropericlase in the proportions 93:3:4 produces a residual magma with a superchondritic Ca/Al ratio matching that of the model convecting mantle. This amount of crystal separation generates fractionations in other refractory lithophile elements ratios that generally mimic those observed in the convecting mantle. Further, the residual magma is expected to have subchondritic Sm/Nd and Lu/Hf ratios. Modeling shows that up to 15% crystal separation of the deep mantle assemblage from an early magma ocean could have yielded a convecting mantle reservoir with ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf isotopic compositions that remain internal to the array observed for modern oceanic volcanic rocks. If kept in isolation, the residual magma and deep crystal piles would grow model isotopic compositions that are akin to enriched mantle 1 (EM1) and HIMU reservoirs, respectively, in Nd-Hf isotopic space.     

The importance of adsorption in igneous partitioning of trace elements

For accurate mathematical modeling of trace-element partitioning during igneous fractionation, adsorption should be considered. Because of adsorption, the partitioning of elements between liquid and a surface layer of a crystal is often not the same as the partitioning between liquid and the solid crystal at true equilibrium. In some minerals e.g. high-calcium pyroxene, the effect of adsorption during crystal growth may be very important; this is suggested by the frequent occurrence of sector zoning in augite, and the wide range in measured partition coefficients for such elements as rare earths. The ions which are enriched by adsorption are usually those which are favored substituents according to Goldschmidt's rules. In other minerals, uptake of trace elements may be closer to equilibrium partitioning, rather than being determined by kinetic factors. For example, the relative partitioning of REE, U, Th and Pb into feldspars is qualitatively predicted by Pauling's rules for complex ionic crystals, rather than by Goldschmidt's rules.     

地幔流体作用——地幔捕虏体中流体包裹体的研究

被碱性玄武岩和金伯利岩带到地表的地幔捕虏体是认识地球深部信息的窗口 ,是人们能够直接观察到的一种上地幔样品 ,其矿物中流体包裹体的存在提供了上地幔流体活动的直接证据。流体 /地幔矿物之间元素的分配对约束地幔交代过程中流体相的作用和上地幔流体的组成 ,揭示俯冲带壳幔物质的再循环过程 ,解释岛弧玄武岩高场强元素亏损的原因有重要意义。文章对近年来有关地幔捕虏体中流体包裹体的研究进行了评述 ,并结合近年来流体 /地幔矿物之间元素分配的高温高压实验研究讨论了流体在地幔中的重要作用。     

金红石-榍石转变过程中元素地球化学行为——以雅鲁藏布江缝合带角闪岩为例

金红石边缘形成榍石冠状边结构在变质中-基性岩中普遍存在,是金红石与退变质流体携带的SiO_2与CaO作用的结果,反应形成的榍石微量元素特征受到金红石和流体的共同影响。雅鲁藏布江缝合带中角闪岩LZ06-04在抬升过程经历近等温降压退变质作用,石榴子石分解导致同一样品中含石榴子石部分与不含石榴子石部分的退变质流体成分的差异。两种流体分别与金红石反应,对应形成的榍石具有相似的Nb、Ta含量和Nb/Ta比值特征,但截然不同的REE特征。榍石的Nb、Ta来源于金红石,残余金红石与含水流体再平衡Nb、Ta的分配系数增大,且D_(Nb)~(Rt/Fluid)≥D_(Ta)~(Rt/Fluid);虽然Nb和Ta在含水流体中都表现为不活动元素,但相对于Nb,Ta在含水流体中活动性较高。榍石的Zr-Hf体系特征受到锆石、石榴子石等矿物的综合影响,并且Zr-Hf在含水流体中表现出比Nb-Ta更高的活动性。榍石的REE特征受流体中REE特征、榍石与流体配分系数以及共生矿物的影响。在岩浆或变质体系,榍石形成过程中,REE富集矿物(如石榴子石、锆石、褐帘石、独居石、磷灰石等)形成或分解将影响榍石的REE分布特征或形成REE环带结构。含水流体中金红石退变质形成榍石反应的进行受流体中TiO_2、CaO和SiO_2活度的影响。因此榍石常见于钙碱性岩浆岩、富Ca基性变质岩和矽卡岩中。流体中CaO活度的变化影响榍石的形成,进而影响Ti、Nb、Ta在流体中的运移能力。俯冲板片产生流体在交代上覆富Ca地幔楔物质过程中形成榍石残留同样可以造成部分熔融体具有亏损HFSE特征。     

俯冲带中水流体的化学性质研究进展

俯冲带中的水在壳幔演化和物质交换中起重要作用。本文就俯冲带中水的物理化学性质、溶液中的离子缔合、混溶、主量元素与微量元素组成等方面进行了阐述。重点剖析:1)超临界态下温度压力条件的改变导致水微观结构与性质的变化,扩散系数、粘度等性质随之改变,进而对水岩反应产生影响;2)富水流体中离子缔合影响着金属配合物数量,很大程度制约流体中矿物的溶解行为;3)低Cl流体中微量元素配分模式与岛弧玄武岩类似,意味着富水贫氯碱性硅酸盐成分的流体在地幔楔元素运移中起关键作用。并展望了俯冲带中流体化学性质研究的新手段。