Significance of "stretched" mineral inclusions for reconstructing P-T exhumation history
Significance of "stretched" mineral inclusions for reconstructing P-T exhumation history
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DOI：
10.1007/s00410-015-1149-0
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发表时间：
2015-06-01
影响因子：
3.5
通讯作者：
Law, Richard D.
中科院分区：
文献类型：
--
作者：
Ashley, Kyle T.;Darling, Robert S.;Law, Richard D.
事实证明，对具有化学和物理抵抗力的基质中的矿物包裹体进行分析对于重建高品位变质岩的 P-T 折返历史具有重要价值。纽约州戈尔山石榴石中出现方英石包裹体是出人意料的，因为所达到的峰值变质条件已经从这种通常在高温火山环境中形成的低密度二氧化硅多晶型物的稳定场中去除（> 600 摄氏度太冷）。先前对该地区样品的研究将由方英石、钠长石和钛铁矿组成的多矿物包裹体解释为代表石榴石反应过程中产生的熔体结晶液滴，随后包裹体中的水分流失，解释了包裹体压力的降低，从而推动了石英向方英石的转变。然而，最近在附近的胡珀矿发现的方英石单矿物包裹体不能用这个过程来解释。对于这些包裹体，我们认为在折返到地球表面期间对压力和温度变化的体积响应导致二氧化硅相内产生很大的拉伸应力，这足以导致向低密度（低压）形式的转变。其他常见包裹体宿主系统的弹性模型表明，这个石英到方石英的例子可能不是一个独特的案例。在蓝晶石稳定场内的 P-T 条件下，铝硅酸盐多晶型蓝晶石作为斜长石内含物被捕获后，如果折返到地球表面，则还具有保留拉应力的能力，折返过程中产生的应力足以产生向红柱石的转变。这些结果突出了折返过程中可能出现的弹性环境，并为观察到的包裹体提供了潜在的解释，这些包裹体的稳定性场已从折返过程中遵循的 P-T 路径中很好地消除了。
Analysis of mineral inclusions in chemically and physically resistant hosts has proven to be valuable for reconstructing the P-T exhumation history of high-grade metamorphic rocks. The occurrence of cristobalite-bearing inclusions in garnets from Gore Mountain, New York, is unexpected because the peak metamorphic conditions reached are well removed (>600 degrees C too cold) from the stability field of this low-density silica polymorph that typically forms in high temperature volcanic environments. A previous study of samples from this area interpreted polymineralic inclusions consisting of cristobalite, albite and ilmenite as representing crystallized droplets of melt generated during a garnet-in reaction, followed by water loss from the inclusion to explain the reduction in inclusion pressure that drove the transformation of quartz to cristobalite. However, the recent discovery of monomineralic inclusions of cristobalite from the nearby Hooper Mine cannot be explained by this process. For these inclusions, we propose that the volume response to pressure and temperature changes during exhumation to Earth's surface resulted in large tensile stresses within the silica phase that would be sufficient to cause transformation to the low-density (low-pressure) form. Elastic modeling of other common inclusion-host systems suggests that this quartz-to-cristobalite example may not be a unique case. The aluminosilicate polymorph kyanite also has the capacity to retain tensile stresses if exhumed to Earth's surface after being trapped as an inclusion in plagioclase at P-T conditions within the kyanite stability field, with the stresses developed during exhumation sufficient to produce a transformation to andalusite. These results highlight the elastic environment that may arise during exhumation and provide a potential explanation of observed inclusions whose stability fields are well removed from P-T paths followed during exhumation.