Leading research of supercritical metamorphic fluids

高温高压区域变质岩被认为是最接近化学平衡的岩石。然而，在本项目的研究中，我们不仅在斯里兰卡和东南极洲的高温（麻粒岩相）区域变质岩中，而且在中国的高压（榴辉岩相）区域变质岩中，成功地发现了理论上意想不到的特征。它们是先前存在的Al_2SiO_5矿物（高温条件下为硅线石，高压条件下为蓝晶石）在石英（SiO_2）附近形成刚玉（Al_2O_3），中间有厚度小于0.1 mm的长石薄膜。超临界流体是热输运和物质输运的最有效介质，因此，在石英存在的情况下，会阻止在Al_2SiO_5矿物后形成刚玉的反应，因为与地壳和上地幔中的Al_2SiO_5矿物相比，刚玉+石英组合是不稳定的。然而，如果由于富h2o的超临界流体沿构造渗透而使岩石发生部分熔融，则会有机会在Al_2SiO_5矿物之后形成刚玉，因为SiO_2和Al_2O_3都不会在石英周围形成的硅酸盐熔体中迅速扩散。因此，岩石被分为饱和和欠饱和两部分。我们对高温粗粒泥质岩在富h2o超临界流体作用下进行了部分熔融实验，以证实在残余石英颗粒周围熔体中形成陡峭成分梯度的推断。因此，富h2o2超临界流体在区域变质作用中的作用在固体下和固体上条件下有明显的不同。此外，利用SiO_2在熔体中的扩散模型估计了高温条件的持续时间。

High-temperature and/or high-pressure regional metamorphic rocks have been regarded as the rocks most close to chemical equilibrium. During the research of this program, however, we have successfully discovered theoretically unexpected features not only in high-temperature (granulite-facies) regional metamorphic rocks from Sri Lanka and East Antarctica but also in high-pressure (elcogite-facies) regional metamorphic rocks from China. They are the formation of corundum (Al_2O_3) after pre-existing Al_2SiO_5 minerals (sillimanite in the high-temperature case and kyanite in the high-pressure case, respectively) in the vicinity of quartz (SiO_2) with intervening thin films of feldspar (less than c. 0.1 mm in thickness). The supercritical fluids are the most effective media of transportation of heat and material and, therefore, would prevent such reactions to form corundum after Al_2SiO_5 minerals in the presence of quartz, because the corundum+quartz assemblage is unstable compared with Al_2SiO_5s minerals in the crust and upper mantle. If, however, partial melting takes place in the rocks by the infiltration of H_2O-rich supercritical fluids along fructures, a chance to form corundum after Al_2SiO_5 minerals would happen, because both SiO_2 and Al_2O_3 do not diffuse rapidly in silicate melts formed around quartz. Thus rocks are separated into two portions, SiO_2-saturated and SiO_2-undersaturated.We performed experiments of partial melting of high-temperature coarse-grained pelitic rocks in the presence of H_2O-rich supercritical fluids to confirm the inferred formation of steep compositional gradients in the melts surrounding relict quartz grains. Thus, the role of H_2O-rich supercritical fluids during regional metamorphism has been revealed to be distinctly different under the subsolidus and "above-solidus" conditions.In addition, the duration of high-temperature conditions is estimated by the diffusion models of SiO_2 in melts.