Crystallization and Fractionation Trends in the System Andesite-H2O-CO2-O2 at Pressures to 10 Kb
Crystallization and Fractionation Trends in the System Andesite-H2O-CO2-O2 at Pressures to 10 Kb
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安山岩-H2O-CO2-O2 系统在 10 Kb 压力下的结晶和分馏趋势
DOI：
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发表时间：
1973
期刊：
影响因子：
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通讯作者：
C. Burnham
中科院分区：
文献类型：
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作者：
D. Eggler;C. Burnham
Hood山安山岩具有平均造山安山岩的组成，其相关系已被确定为1 atm下的O2逸度和10 kb压力下的H2 O逸度的函数，在石英-铁橄榄石-磁铁矿（QFM）缓冲层的O2逸度下。所有的运行中包含的H2 O或H2 O-CO 2流体相，熔体在运行与H2 O-CO 2流体相H2 O不饱和。根据NaAlSi 3 O 8 -H2O热力学数据，在H2O-CO2系统理想混合的假设下，计算了熔体中H2O含量和H2O逸度。一个大气压的运行表明，硅酸盐的熔融关系受fo 2的影响很小，但钛铁矿和磁铁矿的温度提高了较高的fo 2。钛铁矿比磁铁矿在更高的温度下沉淀。在这些运行中，并在所有运行在高压下与H2 O和H2 O-CO 2流体相，氧化物是不稳定的温度下的硅酸盐液相线。氧化物可能是稳定的硅酸盐液相线，如果fo 2上升两个或更多的日志单位以上的Ni-NiO（NNO）缓冲。然而，计算表明，在天然岩浆中，那些可能改变fo 2的过程--晶体-液体平衡或H2交换，或H2和H2O与围岩的交换--不能使fo 2升高那么大。由于玄武岩熔体向安山岩的分异必须涉及富铁氧化物相的消减，这种分馏模型显得不合理。对于胡德山安山岩组成，斜长石在H2O饱和条件下至5 kb为液相，在H2O不饱和条件下当熔体中H2O含量小于4.7wt%时在10 kb为液相。当H2O含量较高时，斜方辉石或角闪石在压力大于8 Kb的H2O饱和状态下呈液相线.在所有情况下，单斜辉石结晶温度低于斜方辉石。熔融曲线中的H 2 O下饱和区域可以轮廓要么作为熔体中的H 2 O的百分比或作为P e H 2 O ;在任何一种情况下，各种硅酸盐熔融曲线的拓扑结构是不同的情况下的H 2 O饱和熔融。因此，在H2O饱和条件下测定的熔融关系不能成功地用于预测H2O欠饱和区域的熔融关系。角闪石熔融关系进行了研究等压在5 kb的温度和流体相组成的函数。角闪石的最高稳定温度为940 ± 15°C，适用于100 - 44摩尔% H2 O的流体;对于CO2含量超过56%（或相当于熔体中H2 O含量低于4.4重量%）的流体，熔融温度较低。如果不存在CO2，熔体为H2O不饱和熔体，也可以看到同样的关系.这些相当低的熔融温度，相对于其他硅酸盐相，排除安山岩生成玄武岩分馏涉及角闪石在压力小于10千巴。
Phase relations of a Mount Hood andesite, which has the composition of an average orogenic andesite, have been determined as a function of O 2 fugacity at 1 atm and of H 2 O fugacity to pressures of 10 kb, at O 2 fugacities of the quartz-fayalite-magnetite (QFM) buffer. All runs contained either a H 2 O or H 2 O–CO 2 fluid phase; melts in runs with a H 2 O–CO 2 fluid phase were H 2 O undersaturated. The H 2 O contents of the melts and H 2 O fugacities were calculated from NaAlSi 3 O 8 –H 2 O thermo-dynamic data on the assumption of ideal mixing in the system H 2 O–CO 2 . One-atmosphere runs show that melting relations of silicates are little affected by f o 2 but that both ilmenite- and magnetite-out temperatures are raised by higher f o 2 . Ilmenite precipitates at higher temperature than magnetite. In these runs and in all runs at high pressure with H 2 O and H 2 O–CO 2 fluid phases, oxides were not stable at temperatures of the silicate liquidus. Oxides might be stable on the silicate liquidus if f o 2 rose two or more log units above the Ni–NiO (NNO) buffer. However, calculations indicate that in natural magmas, those processes which might change f o 2 —crystal-liquid equilibria or exchange of H 2 , or H 2 and H 2 O with the wall rocks—cannot raise f o 2 by that magnitude. Because differentiation of basalt melts to andesite must involve iron-rich oxide phase subtraction, such fractionation models appear unreasonable. For the Mount Hood andesite composition, plagioclase is the liquidus phase under H 2 O–saturated conditions to 5 kb and under H 2 O–undersaturated conditions at 10 kb when the H 2 O content of the melt is less than 4.7 wt percent. For higher H 2 O contents, either orthopyroxene or, at H 2 O saturation at pressure greater than 8 kb, amphibole assumes the liquidus. In all cases, clinopyroxene crystallizes at lower temperature than orthopyroxene. Melting curves in the H 2 O–under-saturated region may be contoured either as percent H 2 O in melt or as P e H 2 O ; in either case, the topology of the various silicate melting curves is different from the case of H 2 O–saturated melting. Therefore, melting relations determined at H 2 O–saturated conditions cannot be used successfully to predict melting relations in the H 2 O–undersaturated region. Amphibole melting relations were studied isobarically at 5 kb as a function of temperature and fluid-phase composition. Amphibole has a maximum stability temperature of 940 ± 15°C for fluid compositions of 100 to 44 mole percent H 2 O; for fluids containing more CO 2 than 56 percent (or, equivalently, less than 4.4 wt percent H 2 O in melt), the melting temperature is lower. The same relations would be seen if CO 2 were not present and the melt were H 2 O undersaturated. These rather low melting temperatures, relative to other silicate phases, preclude andesite generation by basalt fractionation involving amphibole at pressures less than 10 kb.