Ion Probe Studies of Mantle-Melting Run Products

本申请申请资金支持对现有的10kbar地幔融化流产物进行离子探测研究，该研究是P.I.在2001年秋季休假期间在伍兹霍尔海洋研究所进行的。这些运行的产品代表了由P.I.和两位最近的博士毕业生5年多来在nsf资助下的工作。研究了10种橄榄岩起始材料，每种材料都是由手工挑选和酸洗矿物分离物以不同的模式重组而成的。其中5种矿物来自基尔伯恩洞肥沃的伊热橄榄岩捕虏体，而另外5种矿物来自澳大利亚东南部Noorat山的中间伊热橄榄岩样品。总共有54种含玻璃和橄榄石的产品可供选择，36种含cpx， 41种含opx， 47种含尖晶石。计算的熔体分数范围为1-48 wt %；22种含熔点小于5%。所有的实验都采用了一种金刚石骨料技术的变体（例如，Hirose和Kushiro, 1993； Baker和Stolper， 1994），其中一层70-100微米直径的玻璃碳球取代了粉末金刚石，以提供一个熔体池，其中一些熔体可以从残余晶体中分离出来，从而防止在实验结束时其淬火改性。所有起始材料的主要元素矿物和玻璃数据随着温度的升高以系统和合理的方式变化，结合2-10天的运行持续时间，支持实验中接近平衡的方法。这项工作的主要目标是测量稀土、钛、钒、铬、锶、钇和锶（be、B、Rb、Nb和Ba在辉石中可能过低）在熔体与cpx和opx之间的分布系数，并分析这些名义上无水运行产物的含水量。据我所知，这将是第一个与地幔熔融有关的数据，这些数据来自于橄榄岩的实验，而不是玄武岩的大块成分。早期在玄武岩体系中的工作已经揭示了辉石/熔体Ds的复杂成分依赖性，表明在建模研究中使用单值Ds是不合适的。我们的主要元素数据极大地扩展了实验所涵盖的组合空间，并将允许扩展和更好地定义这些依赖关系，从而改进预测模型。我们还发现了17个含cpx的产品，熔体含量低于5%，其中一些熔体的Na2O和SiO2含量较高，Baker等人（1995）首次注意到这是非常接近固体的地幔熔体的特征。考虑到大洋中脊环境中部分熔融的重要性，确定这些“外来”熔体和地幔辉石之间关键微量元素的分配行为将是至关重要的

JohnstonEAR-0105500This proposal requests funds to support an ion probe study of existing 10 kbar mantle-melting run products which the P.I. proposes to pursue while at Woods Hole Oceanographic Institution during his sabbatical in Fall, 2001. These run products represent 5+ years of NSF-funded work by the P.I. and two recent Ph.D. graduates. Ten peridotitic starting materials, each constructed by recombining hand-picked and acid-washed mineral separates in differing modes, were investigated. Five of these utilized minerals from a fertile lherzolite xenolith from Kilbourne Hole while the other five utilized minerals from a compositionally intermediate lherzolite sample from Mt. Noorat, S.E. Australia. In all, 54 glass- and olivine-bearing run products are available, 36 with cpx, 41 with opx, and 47 with spinel. Calculated melt fractions range from 1-48 wt. %; 22 contain less than 5 wt% melt.All experiments employed a variation of the diamond aggregate technique (e.g., Hirose and Kushiro, 1993; Baker and Stolper, 1994) in which a layer of 70-100 micron diameter vitreous carbon spheres was substituted in lieu of powdered diamond, to provide a melt sink in which some melt could be isolated from the residual crystals, thereby preventing its quench modification upon termination of the experiments. Major element mineral and glass data for all starting materials vary in a systematic and sensible manner with increasing temperature and this, combined with run durations of 2-10 days, supports a close approach to equilibrium in the experiments.The primary goals of the proposed work will be to measure the distribution coefficients between melt and cpx and opx for REE, Ti, V, Cr, Sr, Y, and Sr (Be, B, Rb, Nb, and Ba will also be attempted but will probably be too low in the pyroxenes), and to analyze the water contents of these nominally anhydrous run products. To my knowledge, these will be the first such data germane to mantle melting derived from experiments on peridotitic, as opposed to basaltic, bulk compositions. Earlier work in basaltic systems has revealed complex compositional dependencies of pyroxene/melt Ds demonstrating that the use of single-valued Ds in modeling studies is inappropriate. Our major element data greatly expand the composition space covered by experiment and will allow extension and better definition of these dependencies, thus improving predictive models. We also have 17 cpx-bearing run products with less than 5% melt and some of these melts have the elevated Na2O and SiO2 contents that Baker et. al. (1995) first noted characterize very near-solidus mantle melts. Given the likely importance of fractional melting in the mid-ocean ridge environment, determination of the partitioning behavior of key trace elements between these "exotic" melts and mantle pyroxenes will be critical