Determination of water partition coefficients between upper mantle minerals and melts as a function of pressure, temperature, and CO2 content using a rapid quench cell in multi-anvil experiments

在多砧实验中使用快速淬火室确定上地幔矿物和熔体之间的水分配系数作为压力、温度和 CO2 含量的函数

• 批准号: 402695659
• 负责人: Professor Dr. Tomoo Katsura
• 金额: --
• 依托单位: Bayerisches Forschungsinstitut für Experimentelle Geochemie und Geophysik (BGI)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/402695659?language=en
• 关键词: Determination; water; partition; coefficients; between

It is commonly accepted that the Earth’s mantle contains trace amounts of water varying from 10^(1) to 10^(3) wt. ppm in different regions. However, the question about how water is distributed and cycled in the earth interior remains. Although the behaviors of water in the shallow part of upper mantle are already well constrained by natural sampling, those in the deep asthenosphere are still under debate due to the lack of natural samples. Knowledge about water partitioning between minerals and basaltic melts under asthenosphere conditions is the key for understanding the water behavior in the mantle. In spite of that a series of experimental studies about water partition coefficients between the major upper mantle minerals (olivine and pyroxene) and melts have been reported, all of those data are based on experiments performed at pressures below 4 GPa, corresponding to lithosphere or topmost asthenosphere conditions. The water partitioning in the deep asthenosphere (up to 13 GPa) is unknown since it should be pressure-dependent inferred from the significant pressure dependence of water solubility in olivine. The reason for the limited pressure condition is that basaltic melts cannot be quenched to glass in a typical multi-anvil or piston-cylinder experiment at higher than 4 GPa, and therefore impossible to directly measure the water contents in melts, sequentially, unable to determine the water partition coefficients between minerals and melts.In order to investigate the water partitioning between upper mantle minerals and basaltic melts at high pressures corresponding to those in the deep asthenosphere, in this study, we will design a multi-anvil cell assembly with much faster quenching rate (here called “rapid quench cell”) than typical multi-anvil experiments to quench the basaltic melts to glass at high pressures. By directly measuring the water contents in the glass and coexisted forsterite and enstatite, their water partition coefficients will be obtained as a function of pressure from 1 to 13 GPa, temperature from 1300 to 1900 K. The preliminary experiments demonstrated that melt coexisting with olivine can be quenched from 1870 K at a pressure of 8 GPa, and implied a possibility that the previous study might misinterpreted the pressure and/or temperature dependence of the partition coefficients. Additionally, since CO2 content in melts may significantly affect the water partitioning, the CO2-content dependence of water partition coefficients will also be investigated.

人们普遍认为，在不同的地区，地幔中含有微量的水，从10^(1)到10^(3)百万分之多不等。然而，关于水如何在地球内部分布和循环的问题仍然存在。虽然上地幔浅部的水的行为已经被自然采样很好地约束了，但由于缺乏自然样本，深层软流层的水的行为仍然存在争议。了解软流圈条件下矿物和玄武岩熔体之间的水分配是理解地幔中水行为的关键。尽管已经报道了一系列关于上地幔主要矿物（橄榄石和辉石）与熔体之间的水分配系数的实验研究，但所有这些数据都是基于在低于4 GPa的压力下进行的实验，对应于岩石圈或顶部软流圈条件。深层软流层（高达13 GPa）的水分配是未知的，因为它应该是由橄榄石中水溶性的显著压力依赖性推断出来的压力依赖性。限制压力条件的原因是，在典型的多砧或活塞缸实验中，玄武岩熔体在高于4 GPa的温度下无法淬火成玻璃，因此无法直接测量熔体中的含水量，从而无法确定矿物与熔体之间的水分分配系数。为了研究上地幔矿物和玄武岩熔体在高压下对应于深部软流圈的水分配，在本研究中，我们将设计一个比典型的多砧实验具有更快淬灭速率的多砧池组件（这里称为“快速淬灭池”），以在高压下将玄武岩熔体淬成玻璃。通过直接测量玻璃和共存的长辉石和长辉石中的水分含量，可以得到它们的水分分配系数是压力为1 ~ 13 GPa，温度为1300 ~ 1900 K的函数。初步实验表明，与橄榄石共存的熔体可以在8gpa的压力下从1870 K开始淬火，这意味着先前的研究可能误解了分配系数对压力和/或温度的依赖关系。此外，由于熔体中的CO2含量会显著影响水分配，因此还将研究CO2含量对水分配系数的依赖关系。