Deuterium-Hydrogen (D/H) Fractionation at High Pressure and High Temperature

本研究旨在通过高压高温实验，了解压力、温度和成分对氢-氢同位素分馏的影响。高压下的同位素分馏在理论上是有趣的，它既影响从俯冲板块释放的流体的组成，也影响携带在高压相中的残余水的组成。在有利的条件下，这些含水相可能在下地幔深层被隔离。这种“循环”水的同位素特征将部分取决于地幔条件下发生的分馏过程。尽管目前上地幔的D/H比总体上是相对恒定的，但地球的原始D/H比是未知的。了解与地球脱气有关的高压D/H分馏系数将有助于限制原始D/H比值和来自地幔的海水比例。与俯冲有关的玄武岩的D/H比值相对较高(例如Poreda，1985；Dobson和O‘Neil，1987)，表明它们的源区与亏损的上地幔具有不同的D/H比值。虽然这部分归因于俯冲水的D/H比，但演化水的D/H比可能受到分馏过程的影响，因为水是从深部的含水矿物中释放出来的。在地幔的大部分，名义上的无水矿物被认为是水的主要储集层(例如Bell和Rossmann，1992；Kohlstedt等人，1996)。有人认为，上地幔名义上的无水矿物中水的D/H比是通过与地幔楔体中的含水流体平衡而固定的(Bell和Ihinger，2000)。因此，含水相(在板片中)、流体(离开板片)、含水岩浆(在地幔楔体中)和名义上无水矿物(地幔残留物)之间的D/H分馏可以解释整个上地幔的D/H系统。然而，在压力大于几千巴的情况下，D/H分馏系数还没有被测量到，也不能可靠地预测；因此，我们建议在地幔条件下进行实验测量。在这个为期一年的试验项目期间，我们将在压力(3-6 Gpa)和温度(500-700摄氏度)升高的情况下，初步研究压力和温度对水镁石与水之间D/H分馏的影响。这项工作将确定压力对D/H分馏的影响程度。在这个简单的氧化镁-水体系中工作，拥有这样一种富含水的矿物，也将使我们能够发展和改进我们的实验和分析技术。这一系统研究的结果将是我们理解高压下同位素分馏的基础。此外，我们还将开始研究人造玄武岩熔体和H2O气相之间的同位素分馏系数，以了解地幔脱气过程中的D/H分馏。还将考察溶解H2O含量(羟基与分子H2O之比)对分馏系数的影响。

ZhangEAR-0106718This proposal is aimed at understanding the influence of pressure, temperature and composition on deuterium-hydrogen (D/H) isotopic fractionation through high-pressure, high temperature experiments. Isotope fractionation at high pressure is theoretically interesting and affects both the composition of fluids released from subducted slabs, and of the residual water that is entrained in high-pressure phases. Under favorable conditions, these water-containing phases may be sequestered at great depth in the lower mantle. The isotopic signature of such "recycled" water will depend in part upon fractionation processes occurring at mantle conditions.Although the D/H ratio in the present upper mantle is known to be on the whole relatively constant, the primordial D/H ratio for the earth is unknown. Knowledge of high-pressure D/H fractionation factors relevant to degassing of the earth will help constrain the primitive D/H ratio and the proportion of ocean water that originated from the mantle .The D/H ratios of subduction related basalts are relatively high (e.g. Poreda, 1985; Dobson and O'Neil, 1987), indicating that their source has a different D/H ratio to the depleted upper mantle. Although this is attributed in part to the D/H ratio of subducted water, the D/H ratio of the evolved water may be affected by fractionation processes as water is released from hydrous minerals at depth. In the bulk of the mantle, nominally anhydrous minerals are thought to constitute the major reservoir for water (e.g. Bell and Rossmann, 1992; Kohlstedt et al., 1996). It has been suggested that the D/H ratio of water in nominally anhydrous minerals of the upper mantle is fixed by equilibration with hydrous fluids in the mantle wedge (Bell and Ihinger, 2000). Thus, fractionation of D/H between hydrous phases (in the slab), fluid (leaving the slab), hydrous magmas (in the mantle wedge) and nominally anhydrous minerals (the mantle residue) could explain the D/H systematics of the entire upper mantle. D/H fractionation factors at pressures greater than a few kilobars have not yet been measured, however, and cannot be reliably predicted; thus we propose to measure fractionation experimentally under mantle conditions. In the period of this one-year pilot project, we will carry out a preliminary investigation of the effects of pressure and temperature on D/H fractionation between brucite and water at elevated pressure (3-6 GPa) and temperature (500-700 degrees C). This work will determine the magnitude of the pressure effect on D/H fractionation. Working in this simple MgO-H2O system with such a water rich mineral will also allow us to develop and improve our experimental and analytical technique. The results of this systematic study will be fundamental to our understanding of isotopic fractionation at high pressure. In addition, we will begin to investigate the isotopic fractionation factor between a synthetic basalt melt and H2O vapor phase to understand the D/H fractionation during mantle degassing. The effect of dissolved H2O content (hydroxyl to molecular H2O ratio) on the fractionation factor will also be examined.