Trace element abundances in megacrysts and their host basalts: Constraints on partition coefficients and megacryst genesis
Trace element abundances in megacrysts and their host basalts: Constraints on partition coefficients and megacryst genesis
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DOI：
10.1016/0016-7037(84)90056-5
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发表时间：
1984-06
影响因子：
5
通讯作者：
A. Irving;F. Frey
中科院分区：
文献类型：
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作者：
A. Irving;F. Frey
为了获得基性岩高压成岩过程中矿物/熔体微量元素分配信息，测定了斜辉石、正辉石、角闪洞、云母、斜长石、磷灰石、锆石等巨晶及其寄主玄武岩中的稀土等微量元素丰度。一般来说，通过实验分配研究和斑晶/基质测量建立的矿物/熔体分配系数范围与巨晶/宿主丰度比范围重叠。我们的Hf， Sc， Ta和Th分区的数据代表了一些唯一可用的估计。相平衡、主元素分配和同位素比值表明，大部分辉石和角闪洞巨晶可能在高压下（大部分为10-25 kb）与寄主岩浆处于平衡状态。云母、斜长石、磷灰石和锆石巨晶不太可能与寄主玄武岩平衡形成；相反，我们得出结论，它们是从更演化的岩浆中沉淀出来的，并与它们现在的宿主岩浆混合在一起。因此，巨晶/寄主的微量元素丰度比不应被解释为分配系数，而只能作为理解高压成岩过程中微量元素分配的指导。基于此，我们得出结论，巨晶/主体微量元素丰度数据表明，高压分馏过程中玄武岩体系中的矿物/熔体分配系数与低压分馏时的分配系数并没有显著差异。
In an effort to obtain information about mineral/melt trace element partitioning during the high pressure petrogenesis of basic rocks, we determined rare earth and other trace element abundances in megacrysts of clinopyroxene, orthopyroxene, amphibole, mica, anorthoclase, apatite and zircon, and in their host basalts. In general, the ranges of mineral/melt partition coefficients established from experimental partitioning studies and phenocryst/matrix measurements overlap with the ranges of megacryst/host abundance ratios. Our data for Hf, Sc, Ta and Th partitioning represent some of the only estimates available. Consideration of phase equilibria, major element partitioning and isotopic ratios indicate that most of the pyroxene and amphibole megacrysts may have been in equilibrium with their host magmas at high pressures (mostly 10–25 kb). In contrast, it is unlikely that mica, anorthoclase, apatite and zircon megacrysts formed in equilibrium with their host basalts; instead, we conclude that they were precipitated from more evolved magmas and have been mixed into their present host magmas. Consequently, the trace element abundance ratios for megacryst/host should not be interpreted as partition coefficients, but only as guides for understanding trace element partitioning during high pressure petrogenesis. With this caveat, we conclude that the megacryst/ host trace element abundance data indicate that mineral/melt partition coefficients in basaltic systems during high pressure fractionation are not drastically different from partition coefficients valid for low pressure fractionation.