Magmatic Na-rich phlogopite in a suite of gabbroic crustal xenoliths from Volcán San Pedro, Chilean Andes: Evidence for a solvus relation between phlogopite and aspidolite
Magmatic Na-rich phlogopite in a suite of gabbroic crustal xenoliths from Volcán San Pedro, Chilean Andes: Evidence for a solvus relation between phlogopite and aspidolite
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DOI：
10.2138/am-2001-0104
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发表时间：
2001
影响因子：
3.1
通讯作者：
F. Costa;M. Dungan;B. Singer
中科院分区：
文献类型：
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作者：
F. Costa;M. Dungan;B. Singer
摘要富钠金云母（1- 5wt%Na_2 O）是安第斯弧火山圣佩德罗（智利南纬36°）两组结构和矿物学特征截然不同的辉长岩捕虏体中的晚结晶矿物。金云母的特征是高100·Mg/（Mg + Fe）（高达83）和高Cr2 O3含量（高达0.4wt%），并且它们总是被发现围绕可再吸收的橄榄石、辉石、铬尖晶石，并且在某些情况下，斜长石。我们解释这些云母作为开放系统过程的结果，涉及富水的演变熔体渗透[高Na/（Na + K）]和耐火矿物的反应。如果不考虑其他因素，金云母中最高的100·Na/（Na + K）（~70）和Na 2 O浓度（~ 5wt%）似乎需要与不切实际的高Na/（Na + K）的液体反应。这一点，以及金云母由交替的富钠和贫钠解理平行带组成的观察结果，可以最好地解释为在螺云母（Na）和金云母（K）端元之间存在固溶线。高比例（高达15体积%）的富钠金云母在两个不同的辉长岩捕虏体组表明，它可能是一个更常见和丰富的矿物比以前已经认识到，它可以被用来作为一个指标的开放系统的过程。
Abstract Magmatic Na-rich phlogopite (1-5 wt% Na2O) is present as a late-crystallizing mineral in two groups of texturally and mineralogically distinct gabbroic xenoliths at Volcán San Pedro (36°S, Chile), an Andean arc volcano. Phlogopites are characterized by high 100·Mg/(Mg + Fe) (up to 83) and high Cr2O3 contents (up to 0.4 wt%), and they are always found surrounding variably resorbed olivine, pyroxenes, Cr-spinel, and in some cases, plagioclase. We interpret these micas as the result of open-system processes involving infiltration of water-rich evolved melts [with high Na/(Na + K)] and reaction with refractory minerals. The highest 100·Na/(Na + K) (~70) and Na2O concentrations (~5 wt%) in phlogopite appear to require reaction with liquids of unrealistically high Na/(Na + K) if no other factor is considered. This, together with the observation that phlogopites consist of alternating Na-rich and Na-poor cleavage-parallel bands, can be best interpreted by the presence of a solvus between the aspidolite (Na) and phlogopite (K) end-members. The high proportions (up to 15 vol%) of Na-rich phlogopite in two different groups of gabbroic xenoliths suggest that it might be a more common and abundant mineral than has been previously recognized, and that it may be used as an indicator of open-system processes.