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Leading research of supercritical metamorphic fluids

超临界变质流体的领先研究

基本信息

批准号：
14340158
负责人：
HIROI Yoshikuni
金额：
$ 9.6万
依托单位：
Chiba University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2004
项目状态：
已结题

项目摘要

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.

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，在这个项目的研究过程中，我们不仅成功地在斯里兰卡和东南极的高温(麻粒岩相)区域变质岩中发现了理论上意想不到的特征，而且在中国的高压(榴辉岩相)区域变质岩中也成功地发现了意想不到的特征。它们是在石英(SiO_2)附近预先存在Al_2SiO_5矿物(高温情况下为硅线石，高压情况下为蓝晶石)后形成的刚玉(Al_2O_3)，中间夹有厚度小于0.1 mm的长石薄膜。超临界流体是最有效的热量和物质传输介质，因此，由于刚玉+石英组合与地壳和上地幔中的Al_2SiO_5S矿物相比不稳定，超临界流体将阻止这种反应在存在石英的Al_2SiO_5矿物之后形成刚玉。然而，如果富H_2O的超临界流体沿晶体渗透到岩石中发生部分熔融，则有可能在Al_2SiO_5矿物之后形成刚玉，因为SiO_2和Al_2O_3都不会在形成于石英周围的硅酸盐熔体中迅速扩散。因此，岩石被分成两部分，即SiO_2饱和和SiO_2欠饱和。在富H_2O的超临界流体存在下，我们进行了高温粗粒泥质岩的部分熔融实验，以证实在残留石英颗粒周围的熔体中形成了陡峭的成分梯度。因此，富H_2O的超临界流体在区域变质过程中的作用在亚固相线和“固相线上”条件下明显不同。此外，根据SiO_2在熔体中的扩散模型估算了高温条件下的持续时间。