Effects of H2O on the Disequilibrium Breakdown of Muscovite+Quartz
Effects of H2O on the Disequilibrium Breakdown of Muscovite+Quartz
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H2O对白云母石英不平衡分解的影响
DOI：
10.1093/petrology/31.4.925
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发表时间：
1990
影响因子：
3.9
通讯作者：
D. Rubie
中科院分区：
文献类型：
--
作者：
A. Brearley;D. Rubie
我们使用低孔隙率岩石样品，在实验不平衡条件下详细研究了 H2O 对白云母+石英分解过程中产生的结构和机制的影响。在 H2O 条件下（添加 1 wt.% H2O），白云母在 757°C 下于 5 个月内完全反应成过铝莫来石 + 黑云母的亚稳态组合体，该反应延迟了稳定平衡组合体的形成。相比之下，H2O不饱和实验中的白云母通过产生钾长石+硅线石+黑云母的稳定脱水反应和同一样品中的亚稳态熔融反应而分解。亚稳态熔体的成分由许多动力学因素控制，这些因素随着时间和反应进程而变化。由于石英相对于白云母的快速溶解，加上白云母中的钠石组分的熔化与黑云母的结晶不一致，早期熔体具有高硅质和钠质。莫来石的延迟成核导致熔体中铝过饱和，从而抑制熔融反应速率。一旦莫来石成核，铝熔体过饱和度就会降低，白云母溶解速率就会增加。白云母完全反应后，随着硅从邻近石英扩散到假晶中，熔体化学性质继续发生变化。即使在 757°C 温度下 5 个月后，熔体中仍然存在 Si 和 Al 的大成分梯度。在 H2O 实验中，亚稳态熔化最初发生，由微量晶界流体或白云母脱水释放的流体催化。然而，一旦熔化开始，任何流体都会强烈分馏到熔化相中，减少μH2O。在这些条件下，进一步熔化被抑制，稳定的脱水反应将继续。对低压下捕虏岩和接触变质岩中不平衡条件下反应的天然白云母实例的研究表明，亚稳态熔融是某些地质条件下的重要过程。它可能广泛存在于接触变质岩中，但在此类岩石的长期冷却历史中，表明亚稳态熔融反应的结构被掩盖。
We have examined in detail the effect of H2O on the textures and mechanisms produced during the breakdown of muscovite+quartz under experimental disequilibrium conditions using low-porosity rock samples. Under H2O conditions (1 wt.% H2O added) muscovite reacts completely in <5 months at 757°C to a metastable assemblage of peraluminous meltmullite+biotite, a reaction which delays the formation of the stable equilibrium assemblage. In contrast, muscovite in the H2O undersaturated experiments breaks down by both the stable dehydration reaction producing K feldspar+sillimanite+biotite and by metastable melting reactions in the same sample. The com position of the metastable melt is controlled by a number of kinetic factors which change as a function of time and reaction progress. Early melts are highly siliceous and sodic due to the rapid dissolution of quartz relative to muscovite, coupled with the incongruent melting of the paragonite component of muscovite and crystallization of biotite. The delayed nucleation of mullite results in Al supersaturation of the melt, which inhibits the rate of the melting reaction. Once mullite nucleates the degree of Al melt supersaturation is decreased and the rate of muscovite dissolution increases. After complete reaction of muscovite the melt chemistry continues to change as Si diffuses into the pseudomorphs from adjacent quartz. Even after 5 months at 757°C large compositional gradients in Si and Al still persist within the melt. In the H2O experiments metastable melting occurs initially, catalysed by traces of grain boundary fluid or by fluid released from the dehydration of muscovite. However, once melting starts any fluid is strongly fractionated into the melt phase, reducing μH2O Under these conditions further melting is inhibited and the stable dehydration reaction will continue. Examination of examples of natural muscovite reacted under disequilibrium conditions in xenolithic and contact metamorphic rocks at low pressures suggests that metastable melting is an important process under certain geological conditions. It is possible that it is widespread in contact metamorphic rocks, but the textures indicative of metastable melting reactions are obscured during the extended cooling histories of such rocks.