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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Xenocryst assimilation and formation of peritectic crystals during magma contamination: An experimental study
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Xenocryst assimilation and formation of peritectic crystals during magma contamination: An experimental study

机译：岩浆污染过程中异晶的同化和包晶的形成：一项实验研究

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摘要

Contamination of magmas by country rocks may contribute xenoliths and xenocrysts to the magma, but also melt and peritectic crystals that form through incongruent melting or dissolution of the original contaminants. Identifying contaminant-derived peritectic crystals and former melt components in igneous rocks is particularly challenging, but also particularly important, because their assimilation significantly affects melt composition and magma temperature. To facilitate the identification of peritectic crystals in igneous rocks, the aim of this study was to experimentally control partial assimilation of xenocrysts and examine the formation, textures, and composition of resulting peritectic crystals. Our experiments mimic contaminant melting and contamination of a partially crystallized basaltic andesite by melanorite- and monzodiorite-derived xenocrysts and micro-xenoliths. Micro-xenoliths and xenocrysts partially survive assimilation, and yet peritectic crystals form ～1/3 of all solid contaminants. Anhydrous xenocrysts either develop laterally continuous, subhedral to euhedral, and inclusion-poor overgrowths, or progressively decompose. Hydrous and partially altered xenocrysts decompose to peritectic crystals. The peritectic crystals form clusters of subhedral to euhedral, randomly-oriented olivine, clinopyroxene, olivine-plagioclase, and olivine-plagioclase-clinopyroxene that texturally resemble primary magmatic crystals. We propose that natural peritectic crystals with short residence times form clusters of one or more minerals with textures as those of our experiments, and that peritectic crystals with longer residence times likely anneal to subhedral or euhedral single crystals or coarse-grained mineral clusters. They hold crucial evidence for largely assimilated country-rock components and estimates of open-system magma evolution, but the longer their magma residence time the more easily they are overlooked.
机译：乡村岩石对岩浆的污染可能为岩浆贡献了异岩和异晶，但也由于原始污染物的不完全融化或溶解而形成的熔体和包晶晶体。鉴定火成岩中污染物衍生的包晶晶体和前熔体成分特别具有挑战性，但也特别重要，因为它们的同化作用会显着影响熔体成分和岩浆温度。为了便于识别火成岩中的包晶晶体，本研究的目的是通过实验控制异晶的部分同化作用，并检查所形成的包晶晶体的形成，织构和组成。我们的实验模拟了污染物的融化以及部分结晶的玄武质安山岩受到黑铁矿和单闪闪石派生的异晶和微异石的污染。微小异种岩和异种晶在同化作用下部分幸存，而包晶形成了所有固体污染物的约1/3。无水异种晶要么横向连续，从半面到全面，包涵体过度生长，要么逐渐分解。含水的和部分改变的异晶分解为包晶晶体。包晶晶体形成了从表面到从表面到随机的橄榄石，斜向辉石，斜长石，斜长石，斜长石和斜长石的簇，它们在结构上类似于初级岩浆晶体。我们建议，具有短停留时间的天然包晶晶体形成一种或多种具有我们实验纹理的矿物的簇，而具有更长停留时间的包晶晶体可能退火成亚面或全面单晶或粗粒矿物簇。他们拥有重要的证据，可以大致了解同化的国家岩石成分和对开放系统岩浆演化的估计，但岩浆停留时间越长，越容易被忽视。

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