Effects of melt percolation on platinum group elements and Re–Os systematics of peridotites from the Tan-Lu fault zone, eastern North China Craton
Effects of melt percolation on platinum group elements and Re–Os systematics of peridotites from the Tan-Lu fault zone, eastern North China Craton
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DOI：
10.1144/0016-76492010-113
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发表时间：
2011-08
影响因子：
2.7
通讯作者：
Yan Xiao;Hong‐fu Zhang
中科院分区：
文献类型：
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作者：
Yan Xiao;Hong‐fu Zhang
文摘：对中国克拉通东部潭庐断裂带内一套新生代北岩火山橄榄岩包体的铂族元素含量和Re-Os同位素组成进行了测定。以往的岩石学和地球化学数据表明，这些包体存在熔体-橄榄岩相互作用的相当大的变质作用。因此，这套包体提供了一个不同寻常的机会来研究熔体渗流对PGE丰度和Re-Os同位素的影响。我们的数据表明，大多数北岩橄榄岩具有扁平的球粒陨石标准化的PGE模式，而在二辉橄榄岩到富含CPx的高丰度二辉橄榄岩和魏橄榄岩中观察到的总PGE丰度有相当大的变化。总PGE含量为0.97~61.4ppb，PGE相对丰度为0.10~0.0001×CI球粒陨石。在这些岩石中，从二辉橄榄岩到富CPx的二辉橄榄岩和辉绿岩，总的PGE收支减少了90%以上，这可能是因为晶间硫化物被硅酸盐熔体完全清除了。而北岩橄榄岩的Re(0.0002-0.5118 ppb)和Os(0.194-10.4ppb)丰度较低，187Os/188Os(0.12167-0.14978)比值较高。部分榴辉岩的~(187)Os/~(188)Os比值远高于原始地幔。潭庐断裂带其他地区如山旺、女山的橄榄岩中也发现了较高的187Os/188Os比值。这可能是在熔体渗流过程中添加了放射性成因Os的结果。北岩橄榄岩中Os的极低丰度表明，Os在熔体渗流过程中可能表现为不相容元素，并可随硫化物的溶解而活化。潭庐断裂带内橄榄岩中187Os/188Os的超软骨性比值与远离该断裂带的橄榄岩中187Os/188Os的比值表明，潭庐断裂带作为熔体渗入通道，在熔体渗流导致的放射性成因Os的富集中起着重要的作用。这可能是在NCC东部观察到Os同位素不均匀的原因。这项研究还证实，在大规模熔体渗流过程中，Os同位素和PGE似乎是可移动的。
Abstract: Concentrations of platinum group elements (PGE) and Re–Os isotopic compositions were determined for a suite of peridotite xenoliths from Cenozoic Beiyan volcanoes within the Tan-Lu fault zone, eastern North China Craton (NCC). Previous petrographic and geochemical data indicate the existence of considerable modification by melt–peridotite interaction for these xenoliths. Thus, this suite of xenoliths provides an unusual opportunity to examine the effects of melt percolation on PGE abundances and Re–Os isotopes. Our data show that most of the Beiyan peridotites have flat chondrite-normalized PGE patterns whereas a considerable variation in total PGE abundances has been observed in lherzolites to highly fertile cpx-rich lherzolites and wehrlites. Their total PGE contents range between 0.97 and 61.4 ppb and their PGE relative abundances from 0.1 to 0.0001 × CI-chondrites, respectively. In these rocks, the total PGE budget was reduced by more than 90% from lherzolite to cpx-rich lherzolite and to wehrlite, probably because intergranular sulphides were completely removed by silicate melt. However, Beiyan peridotites have low and variable Re (0.0002–0.5118 ppb) and Os (0.194–10.4 ppb) abundances and high 187Os/188Os (0.12167–0.14978) ratios. The 187Os/188Os ratios in some wehrlites are much higher than the value for the primitive mantle. The high 187Os/188Os ratios have also been observed in peridotites found in other localities within the Tan-Lu fault zone such as Shanwang and Nüshan. This could be the result of the addition of radiogenic Os during the melt percolation. Extremely low Os abundances in Beiyan peridotites suggest that Os may behave as an incompatible element during melt percolation and could mobilize with the dissolution of sulphides. The suprachondritic 187Os/188Os ratios in peridotites within the Tan-Lu fault zone relative to those away from it indicate that the Tan-Lu fault zone played an important role as a melt infiltrating channel in the radiogenic Os enrichment induced by melt percolation. This could be the reason for the Os isotopic heterogeneity observed in the eastern NCC. This study also confirms that Os isotopes and PGE appear to be mobile during massive melt percolation.