岩庄陨石的冲击熔融和元素再分配

Shock-induced melting and element redistribution of the Yanzhuang chondrite

岩庄陨石是受强烈冲击的H群普通球粒陨石,含有厘米级宽度的熔脉和熔块。冲击熔体中微量元素的分布,对了解在高温低压下从熔体分离结晶物相的元素再分配具有重要价值。本文以岩庄陨石中的全岩熔体、硅酸盐熔体和FeNi+FeS共结团粒中的FeNi金属三种物相为研究对象,利用激光剥蚀等离子体质谱技术,分别对其进行微量元素分析。结果表明:(1)该陨石冲击熔融后,Fe、Co和Ni等亲铁元素主要富集在全岩熔体内的FeNi金属相中,而Cr、V、Ti和Mn等则主要进入从硅酸盐熔体中晶出的微晶辉石晶格内,以置换M1位上的Al^(3+)。(2)Cu、Ga和Pb等亲铜元素也大部分都富集在FeNi金属相中,而挥发性较强的Zn,在陨石熔融时虽有少量丢失,但主要进入硅酸盐熔体相内,赋存在微晶橄榄石和辉石晶格之中以置换Fe^(2+)。(3)Sc、Zr、Nb和Ta等亲石元素,基本上都赋存在硅酸盐熔体相中,但Nb和Ta的分异程度低,而FeNi金属相的Nb/Ta比值比硅酸盐熔体要偏低一倍多,说明高温高压下陨石熔融时,Nb比Ta更为活泼。(4)硅酸盐熔体和全岩熔体的稀土元素分布均具有明显的奇偶效应,总体呈现轻稀土富集,重稀土亏损的特点,而FeNi金属则呈现轻稀土相对亏损的特点,且其稀土总量比硅酸盐熔体低近三倍,说明稀土元素特别是轻稀土元素主要赋存在硅酸盐熔体中。铂族元素在陨石局部熔融后转移到共结团粒的FeNi金属之中。

The Yanzhuang meteorite is a heavily shocked H-group chondrite. It contains centimeter-sized melt veins and melt pockets. Study on the trace element concentrations in shock-induced melt is of important significance for understanding element redistribution in different phases separately crystallized from the melt under high temperature and low pressure. By taking the whole-rock melt phase, silicate melt phase and FeNi metal phase in the metal-sulfide eutectic blobs as research objects, this paper conducted trace element measurement of each phase using the LA-ICPMS technique. The results show that （1） during the shock-induced melting of the Yanzhuang chondrite, the siderophile elements Fe, Ni and Co are mainly concentrated in the FeNi metal blobs embedded in the whole-rock melt phase, while the elements Cr, V, Ti and Mn are transferred into pyroxene crystallizing from silicate melt to substitute Al3＋; （2） the chalcophile elements Cu, Ga and Pb are also concentrated in the FeNi metal phase, while the volatile element Zn was partly lost upon melting, but it was mainly transferred into the micro-crystalline pyroxene and olivine crystallizing from the silicate melt to substitute Fe2＋; （3） the lithophile elements So, Zr, Nb, and Ta are basically enriched in the silicate melt phase, but the differentiation degrees of Nb and Ta are low, and the Nb/Ta ratio in the FeNi metal phase is lower than that in the silicate melt phase by more than 1 time, indicating that the activization ability of Nb is higher than Taupon high P-T melting; （4） the rare-earth elements （REE） patterns of whole-rock melt and silicate melt phases hold the characteristic features of Oddo-Harkins rule, and show the tendency of enrichment of light REE and that of depletion of heavy REE, but the FeNi metal phase displays the tendency of depletion of LREE, and its total REE content is lower than that in the silicate melt phase by about 3 times, This indicates that REE, especially the LREE, are enriched in the silicate melt phase; and （5） Pt-group elements are moved to the FeNi-metal phase after shock melting.