PGE (PLATINUM GROUP ELEMENTS) COSMOCHEMISTRY ON IRON

本文叙述了用电感耦合等离子体质谱(ICPMS)进行的同位素测量，并介绍了旨在进一步发展同位素地球化学的新的实验结果。利用激光烧蚀-ICPMS原位测量铂族元素(PGE)和Re，可以在宿主(铁闪石)和富镍出溶相(网纹岩和闪锌矿)之间建立分馏模式。对IAB的两个铁陨石(Canyon Diablo和Odessa)的测量表明，对于富Ni相，a)Ru和Rh没有明显的分馏，b)Pd富，c)Ir，Os和Pt强烈亏损。我们认为，对于铁闪石、铁闪石和闪锌矿来说，控制机制是离子大小，离子大小的顺序是Ir-Os-Pt-(Re)-Rh-Ru-Pd。这一级数在相对丰度分布与离子大小的关系图上绘制为一条平滑的曲线。铁陨石的PGE和Re数据与发布的…数据的比较来自CI球粒陨石的更多EDD数据表明，元素之间的相对分馏很少或没有。例外的是Re，相对于球粒陨石，Re明显富含铁镁石。这证实了早先关于块状IAB铁和CI值之间Re/Os分馏的观察结果。Canyon Diablo IAB陨石中切割石墨包裹体的金属脉被解释为铁陨石的熔融部分。PGE和Re的丰度比在铁晶石相和金属矿脉之间类似于铁晶石相和富镍相之间。具有较大离子半径的元素(Rh、Ru和Pd)集中在金属矿脉中，这一事实增加了它们代表熔体分数的观点。我们对PGE和Re的原位测量表明，元素分馏既发生在熔体分离过程中，也发生在固态扩散(出溶)过程中。这些数据对Re/Os地质年代学具有重要意义，并表明测量铁陨石中各相的187Os/186Os比值可能提供一个潜在的矿物等时线。较少

This paper describes in isotopic measurements that have been made with an inductively coupled plasma mass spectrometry (ICP-MS) and presentsresults of new experiments aimed at further progress of the isotope geochemistry. In situ measurement of the platinum group elements (PGE) and Re by laser ablation-ICPMS allows fractionation patterns to be established between host (kamacite) and Ni-rich exsolved phases (taenite and plessite). Measurements on two IAB iron meteorites (Canyon Diablo and Odessa) show that for the Ni-rich phases ; a) there is no significant fractionation for Ru and Rh, b) Pd is enriched and c) Ir, Os and Pt are strongly depleted. We suggest that in the case of kamacite, taenite and plessite, the controlling mechanism is ionic size with the order of ions (small to large) being Ir-Os-Pt- (Re) -Rh-Ru-Pd. This progression plots as a smooth curve on a diagram of relative abundance distribution versus ionic size. A comparison of PGE and Re data on iron meteorites with publish … More ed data from CI chondrites indicate that there is little or no relative fractionation between the elements. The exception is Re that is clearly enriched in kamacite relative to chondrites. This confirms earlier observations on the fractionation of Re/Os between bulk IAB irons and CI values. Metallic veins cutting graphite inclusions within the Canyon Diablo IAB meteorite are interpreted as a melt fraction from the iron meteorite. Abundance ratios for PGE and Re between kamacite and the metallic vein are similar to abundance ratios between kamacite and Ni-rich phases. The fact that those elements with large ionic radii (Rh, Ru and Pd) are concentrated in the metallic veins adds weight to the view that they represent a melt fraction.Our in-situ measurements of the PGE and Re demonstrate that elemental fractionation takes place both during melt segregation and solid state diffusion (exsolution). The data have important implications for the Re/Os geochronometer and indicate that measurements of 187Os/186Os ratios on individual phases within iron meteorites may provide a potential mineral isochron. Less