Collaborative Research: Volatile Contents of Komatiites

知识价值。科马提岩为地球的热演化和成分演化提供了关键但有争议的约束条件。最重要的是科马提岩浆岩的含水量——在低含水量条件下，科马提岩浆岩的形成需要地幔电位温度比现代值高500℃，而湿熔融则需要更适度的升高。目前对科马地岩含水量的估计是多变的，争论激烈。此外，尽管这些元素有可能为地球化学循环和脱气模型提供主要约束，但对科马蒂岩岩浆及其地幔源中其他挥发性元素（S， F, Cl）的丰度知之甚少。我们建议合作研究科马提岩的挥发性成分。这将主要是通过测量来自津巴布韦Belingwe绿岩带保存完好的科马铁矿样品中硅酸盐和氧化物相中的熔体包裹体，但也将包括检查和初步测量来自其他低和中变质等级地体的科马铁矿。该研究将结合岩石学观察和微量分析技术：二次离子质谱法（SIMS）；激光烧蚀电感耦合等离子体质谱（LA-ICP-MS）和电子探针分析（EMPA）。以前报道的来自贝林圭样品中熔体包裹体的数据在估计科马地岩含水量方面并不确定，我们提出了一个仔细的分析方案来纠正这一点。我们还将测量其他挥发性物质（CO2， F， S, Cl）的丰度，以及主要和微量元素，S和H的形态和H同位素组成。这些额外的数据将有助于确定脱气、氢扩散、地壳同化、蚀变和其他有可能改变原始含水量的过程的影响，但也将为科马提岩岩浆的成分及其地幔来源提供新的见解。初步数据表明，我们手头的样品中的熔体包裹体含有大量可测量的H2O， CO2， S和Cl。总的来说，我们的研究将提供一些关于科马提岩岩浆和太古宙地幔以及科马提岩源区挥发性含量的第一个直接约束。更广泛的影响。这项工作将解决几个更广泛的影响。这项工作将支持两个项目负责人，促进俄勒冈州立大学和华盛顿卡内基研究所这两个研究机构之间的合作，并为这两个地方的地球化学分析设施的发展和维护作出贡献。这项工作还将提供有关地球化学和热演化的基本信息，并且是研究地幔挥发性结构和演化的长期项目的一部分。这项工作的一个特别重要的影响将是促进本科生的研究，并吸引有才华的本科生参与最先进的研究和使用俄勒冈州立大学和CIW的研究设施。俄勒冈州立大学的两名本科生将完成与该项目相关的高级论文研究，这将包括访问CIW和加拿大阿比提比绿岩带的科马提岩和伴生岩石的野外露头。两个学院都积极促进本科生的研究。PI-Kent正在协调俄勒冈州立大学岩石学小组的本科生研究，并将于2006年秋季为大三和大四学生共同教授一门本科生研究课程。俄勒冈州立大学的本科生研究目前正通过这些活动和最近雇用的本科生研究协调员而重新焕发活力。

Intellectual merit. Komatiites provide key, but controversial, constraints on the thermal and compositional evolution of the Earth. Of primary importance are the water contents of komatiite magmas - at low water contents formation of komatiites require mantle potential temperatures 500 C above modern values, whereas wet melting requires more modest increases. Present estimates of komatiite water contents are variable and hotly debated. Further, relatively little is known about the abundances of other volatile elements (S, F, Cl) in komatiite magmas and their mantle sources, although these have the potential to provide primary constraints for models of geochemical cycling and degassing. We propose a collaborative study of volatile contents in komatiites. This will primarily be through measurements of melt inclusions trapped within silicate and oxide phases in well-preserved komatiite samples from the Belingwe Greenstone Belt in Zimbabwe, but will also involve examination and preliminary measurements of komatiites from a selection of other low and medium metamorphic grade terranes. The study will use a combination of petrographic observations and microanalytical techniques: Secondary Ion Mass Spectrometry (SIMS); Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Electron Microprobe Analysis (EMPA). Previous data reported from melt inclusions in Belingwe samples are not definitive in terms of estimates of komatiite water contents and we propose a careful analytical scheme to rectify this. We will also measure abundances of other volatile species (CO2, F, S, Cl) as well as major and trace elements, S and H speciation and H isotope composition. This additional data will help define the effects of degassing, hydrogen diffusion, crustal assimilation, alteration and other processes that have the potential to alter primary water contents, but will also provide new insights into the compositions of komatiite magmas and their mantle sources. Preliminary data demonstrate that melt inclusions in samples we have in hand contain significant and measurable abundances of H2O, CO2, S and Cl. Overall our study will provide some of the first direct constraints on the volatile contents of komatiite magmas and Archean mantle and komatiites source regions. Broader impacts. This work will address several broader impacts. The work will support two PI's, foster collaboration between two research institutions, Oregon State University and the Carnegie Institute of Washington, and contribute to development and maintenance of geochemical analysis facilities in both places. The work will also provide fundamental information about the chemical and thermal evolution of the earth and is part of a long-term project to investigate the volatile structure and evolution of the mantle. A particularly important impact of this work will be to foster undergraduate research and to engage talented undergraduates in state-of-the-art research and use of research facilities at OSU and CIW. Two undergraduates from OSU will perform senior thesis research related to the project and this will involve visits to both CIW and to field outcrops of komatiite and associated rocks in the Abitibi Greenstone Belt in Canada. Both PI's are active in fostering undergraduate research. PI-Kent is coordinating undergraduate research for the OSU petrology group and will co-teach a course in undergraduate research for juniors and seniors in Fall 2006. Undergraduate research at Oregon State is currently being reinvigorated through these activities and via recent employment of an undergraduate research coordinator.