Isochemical breakdown of garnet in orogenic garnet peridotite and its implication to reaction kinetics
Isochemical breakdown of garnet in orogenic garnet peridotite and its implication to reaction kinetics
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DOI：
10.1007/s00710-012-0260-4
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发表时间：
2013-01
影响因子：
1.8
通讯作者：
M. Obata;K. Ozawa;K. Naemura;A. Miyake
中科院分区：
文献类型：
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作者：
M. Obata;K. Ozawa;K. Naemura;A. Miyake
在摩尔多瓦带捷克部分的石榴石橄榄岩中，发现了一种具有与前体石榴石几乎相同的整体化学成分的等化学钾霞石（斜方辉石+尖晶石+斜长石），它位于普通钾霞石（斜方辉石+单斜辉石+尖晶石±角闪石）基质中。结果表明，石榴子石的钾英石化分三个阶段进行：（1）石榴石-橄榄石反应，伴随着跨越反应区的远距离物质转移，和（2）石榴石的等化学分解，基本上在化学封闭系统中，和最后，（3）开放系统水合反应，产生薄水合区（角闪石+尖晶石+斜长石），位于等化学钾长石和残余石榴石之间。残余石榴石的存在表明，这一分解反应的第二阶段没有进行到一个完整的可能是由在反应前沿的水合区的形成阻碍。电子背散射衍射结果表明，在第一类钾镁矾中，斜方辉石和尖晶石不表现出拓扑关系，而在等化学钾镁矾中，斜方辉石和尖晶石表现出局部拓扑关系。本文根据对两种钾镁矾过渡带的详细观察，讨论了第一类钾镁矾向第二类钾镁矾的过渡。更广泛的出现的等化学钾镁矾预计将出现在造山橄榄岩以及从火山岩带来的捕虏体。
An isochemical kelyphite (orthopyroxene+spinel+plagioclase) that has nearly the same bulk chemical composition as the precursor garnet was found within a matrix of ordinary kelyphites (orthopyroxene+clinopyroxene+spinel±amphibole) in garnet peridotites from the Czech part of the Moldanubian Zone. It was shown that the kelyphitization of garnet took place in three stages: (1) the garnet-olivine reaction, accompanied by a long-range material transfer across the reaction zone, and (2) the isochemical breakdown of garnet, essentially in a chemically-closed system, and finally, (3) an open-system hydration reaction producing a thin hydrous zone (amphibole+spinel+plagioclase), which is located between the isochemical kelyphite and relict garnet. The presence of relict garnet suggests that this breakdown reaction of the second stage did not proceed to a completion probably being hindered by the formation of the hydrous zone at the reaction front. It was found by electron back-scattered diffraction method that orthopyroxene and spinel do not show any topotaxic relationship in the first type of kelyphite; whereas they show locally topotaxic relationship in the isochemical kelyphite. The transition from the first type to the second type of kelyphite is discussed on the basis of the detailed observations in the transition zone between the two kelyphites. More widespread occurrence of isochemical kelyphite is expected to occur in orogenic peridotites as well as from xenoliths brought by volcanics.