岩浆演化对岛弧岩浆岩的铀系不平衡的影响  

Effect of Magma Differentiation on U-series Disequilibria in Arc Lavas

作　　者：黄方[1] 张鞠琳[1] 

机构地区：[1]中国科学院壳幔物质与环境重点实验室,中国科学技术大学地球和空间科学学院,合肥230026

出　　处：《矿物岩石地球化学通报》2013年第2期204-211,共8页Bulletin of Mineralogy, Petrology and Geochemistry

基　　金：国家自然科学基金资助项目(41173031)

摘　　要：本文模拟了岛弧岩浆岩的演化过程对铀系不平衡的影响。结果显示地幔楔熔融产生的初始岩浆同化混染老的地壳物质可改变岩浆的铀系不平衡,同时产生(226 Ra/230 Th)和10Be/Be或(238 U/230 Th)之间的正相关关系,这个结果和南美安第斯山智利的年轻岩浆岩的铀系不平衡数据非常一致。(226 Ra/230 Th)和Sr/Th(或者Ba/Th和10Be/Be)之间的关系常被用来支持岛弧岩浆岩的铀系不平衡的流体加入模型。但考虑到岛弧岩浆岩普遍经历演化,我们的模型表明该相关性反映了幔源岩浆在陆壳内部的岩浆演化过程,而不是俯冲流体加入地幔楔的结果。地幔部分熔融结合地壳内部的岩浆演化更好地解释铀系不平衡及其和其它地球化学指标的相关性,具有不同半衰期的不平衡体系和10 Be/Be体系制约的岛弧岩浆作用的时间尺度其实是一致的。This study establishes a mathematic model to simulate the effect of magma differentiation processes on U- series disequilibria in arc lavas. Our results show that the assimilating old crustal material by initial magma pro- duced by the melting of the mantle wedge can decrease the U-series disequilibria and produce the positive correla- tions between （226 Ra/230 Th） and 10 Be/Be or （228 U/230 Th）. This result agrees with the U-series disequilibria data of the lavas from the Andean in the south America. The correlations between （226 Ra/230 Th） with ＂fluid indices＂ （e. g. , Sr/Th, Ba/Th, and 20 Be/Be） have been used as strong evidences for U-series disequilibria reflecting recent flu- id addition from the slab to the wedge. But considering the geological fact of magma differentiation, our model shows these correlations are more likely to reflect the differentiation of mantle-derived magma within the continental crust instead of the fluid addition model. Therefore, the conclusions regarding fluid addition model and fast ascent rate of magmas from mantle wedge should be reconsidered. Our model further suggests that different parent and daughter disequilibria as well as observation of 10 Be in arc lavas are produced by the same geological processes.

关 键 词：铀系不平衡 岩浆演化 流体加入 数值模拟 

分 类 号：P588.1[天文地球—岩石学]