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首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >Hydrous phase equilibria of a Mexican high-silica andesite: A candidate for a mantle origin?
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Hydrous phase equilibria of a Mexican high-silica andesite: A candidate for a mantle origin?

机译：墨西哥高硅安山岩的水相平衡：地幔成因的候选者？

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摘要

A group of Pleistocene-recent lavas, with 62 to 64% SiO_2 and 5.6 to 3.5% MgO, have erupted along a linear fault system near the volcanic front of central Mexico. Not only is the composition of this group of lavas distinctive in comparison to andesites from other parts of the Mexican Volcanic Belt, but their typical pyroxene phenocryst assemblages may include hornblende and rarely plagioclase. Consequently, the experimental (water-saturated) P-T conditions, up to 3 kbar, necessary to reproduce the various phenocryst assemblages have been explored for the most Mg-rich andesite. The phenocryst assemblages in this group of lavas can be reproduced over a range of water concentrations (～6 to ～3 wt%) at temperatures consistent with those derived from phenocryst mineral geothermometers (910 to 1055 ℃). The phase fields of the phenocryst assemblages are ～200 ℃ below the water-saturated liquids (> 1100 ℃), which is defined by orthopyroxene, followed by olivine, ～30 ℃ below. However, the orthopyroxene and olivine assemblage has never been found in the Valle de Bravo lavas. Three observations are consistent with a mantle source for this andesitic lava that contains up to 176 ppm Ni and 257 ppm Cr. Both experimental liquidus olivine (Fo_(90)) and orthopyroxene (En_(88)) have compositions appropriate to a lherzolite source; the composition of a water-saturated partial melt of lherzolite at 1000 ℃ and 10 kbar is comparable to these andesites. Last, a contemporaneous andesite lava that erupted along the same fault system contains nodules of hornblende-spinel-lherzolite, representative of the Mexican subarc mantle.
机译：墨西哥中部火山前缘附近的线性断层系统喷发了一批更新世的熔岩，其中SiO_2含量为62％至64％，MgO含量为5.6％至3.5％。与来自墨西哥火山带其他部分的安山岩相比，这组熔岩的组成不仅具有独特性，而且它们典型的辉石相隐辉石组合可能包括角闪石和很少的斜长石。因此，对于镁含量最高的安山岩，已经探索了实验（水饱和）的P-T条件（高达3 kbar），该条件是再现各种非晶体组合所必需的。这组熔岩中的phenocryst组合物可以在一定范围内的水浓度（〜6〜〜3 wt％）和与phenocryst矿物地热仪（910〜1055℃）一致的温度范围内繁殖。隐晶相组合的相场在水饱和液体（> 1100℃）以下约200℃，由邻二甲苯定义，其后为橄榄石，在约30℃以下。但是，在Valle de Bravo熔岩中尚未发现邻苯二酚和橄榄石组合。三个观察结果与该安山岩熔岩的地幔来源一致，该熔岩含高达176 ppm的镍和257 ppm的铬。实验液相线橄榄石（Fo_（90））和邻苯二甲酚（En_（88））均具有适合锂铁矿来源的成分;在1000℃和10 kbar的压力下锂沸石的水饱和部分熔体的组成与这些安山岩相当。最后，沿着同一断层系统喷发的同时期安山岩熔岩中含有代表墨西哥亚弧地幔的角闪石-尖晶石-锂铁矿结节。

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