Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
Proc Natl Acad Sci U S A. 2013 Jun 11;110(24):9663-8. doi: 10.1073/pnas.1302040110. Epub 2013 May 28.
Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure-temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge.
板片衍生流体在俯冲带的热质传递中起着重要作用。通过相平衡和流体流动模拟研究了俯冲板块中矿物的脱水和脱碳酸反应。然而，对流体化学和压力-温度条件的直接观察却很少。本报告描述了从 1991 年皮纳图博火山灰中采集的尖晶石-蓝方石捕虏体中含 CO2 的盐水包裹体。包裹体中填充有盐水溶液，其中含有 5.1 ± 1.0%（wt）NaCl 当量方镁石晶体、含 CO2 的蒸气泡，以及壁上的滑石和/或纤蛇纹石层。这些捕虏体中含有透闪石角闪石，在最上部地幔中温度低于 830°C 时稳定存在。皮纳图博火山位于吕宋弧的火山前缘，与温暖的大洋板块俯冲有关。目前的观察表明，俯冲板片衍生的含 CO2 盐水流体使前弧地幔和最上部地幔水合。脱水和脱碳酸作用发生，海水样盐水流体从俯冲板块迁移到地幔楔。盐水流体的存在很重要，因为它们比纯 H2O 溶解更多的金属，并影响地幔楔的化学演化。
