The transition from blueschist to greenschist facies modeled by the reaction glaucophane + 2 diopside + 2 quartz = tremolite + 2 albite
The transition from blueschist to greenschist facies modeled by the reaction glaucophane + 2 diopside + 2 quartz = tremolite + 2 albite
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DOI：
10.1007/s00410-011-0621-8
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发表时间：
2011-03
影响因子：
3.5
通讯作者：
D. Jenkins
中科院分区：
文献类型：
--
作者：
D. Jenkins
蓝闪石+2透辉石+2石英=透闪石+2钠长石是蓝片岩相向绿片岩相转变的模式。在1.0-2.1 GPa和500-800°C的范围内，使用化学系统Na 2 O-CaO-MgO-Al 2 O3-SiO2-H2O中的合成相对该反应进行了实验研究。在500 °C和550°C下，这种反应可能发生逆转，而在600°C下，低压组合体的生长是可能的;然而，在600°C或更高的温度和1.6 GPa以上的压力下，绿辉石成核（以透辉石和钠长石为代价）变得相当强烈，阻止了这种反应的明显逆转。在角闪石的组成变化，确定由电子探针分析和相关性之间的单位细胞的尺寸和组成。蓝闪石，特别是透闪石，显示出明显的成分再平衡迹象，并在约754°C时合并为单一的蓝闪石成分。用不对称多组分热力学混合模型参数WTR，GL = 68 kJ，α TR = 1.0，α GL = 0.75，或简单的两点不对称热力学混合模型参数WNaCa = 13.4 kJ，WCaNa = 19.3 kJ，对蓝闪石和透闪石之间的溶解度间隙进行了模拟。结合这些热力学模型和其他相的现有热力学数据，计算出了上述反应的位置，包括纯透辉石和钠长石，在0.94-1.93 GPa和400-754°C范围内，与实验观察到的反转和角闪石组成非常一致。翡翠固溶于透辉石的作用是使dP/dT斜率从0.0028降低到0.0021 GPa/°C，并使754°C时的压力降低0.28 GPa。该反应边界的dP/dT斜率接近13°C/km的线性地热，并且与用于模拟蓝片岩-绿片岩相变的其他固-固反应的斜率一致。
The reaction glaucophane + 2 diopside + 2 quartz = tremolite + 2 albite is proposed to model the transition from the blueschist to greenschist facies. This reaction was investigated experimentally over the range of 1.0–2.1 GPa and 500–800°C using synthetic phases in the chemical system Na2O–CaO–MgO–Al2O3–SiO2–H2O. Reversals of this reaction were possible at 500 and 550°C and growth of the low-pressure assemblage at 600°C; however, at temperatures of 600°C and higher and at pressures above 1.6 GPa omphacite nucleation (at the expense of diopside and albite) became quite strong and prevented attaining clear reversals of this reaction. Compositional changes in the amphiboles were determined by both electron microprobe analyses and correlations between unit-cell dimensions and composition. Glaucophane and particularly tremolite showed clear signs of compositional re-equilibration and merged to a single amphibole of winchite composition by about 754°C. These data were used to model the miscibility gap between glaucophane and tremolite using either the asymmetric multicomponent formulism parameters ofWTR,GLof 68 kJ with αTRof 1.0 and αGLof 0.75 or a simple two-site asymmetric thermodynamic mixing expression with Margules parametersWNaCaof 13.4 kJ andWCaNaof 19.3 kJ. Combination of these thermodynamic models of the miscibility gap with extant thermodynamic data for the other phases yields a calculated location of the above reaction, involving pure diopside and albite, that is in good agreement with the observed experimental reversals and amphibole compositions over the range of 0.94–1.93 GPa and 400–754°C. The calculated effect of jadeite solid solution into diopside is to reduce the dP/dT slope from 0.0028 to 0.0021 GPa/°C and decrease the pressure by 0.28 GPa at 754°C. The dP/dT slope of this reaction boundary lies close to a linear geotherm of 13°C/km and is consistent with the slopes of other solid–solid reactions that have been used to model the blueschist-to-greenschist facies transition.