Probing the Sources and Early Evolution of Kimberlite: A Sr-Nd-Pb-Hf Isotopic Study of Megacrysts From Southern Africa

探索金伯利岩的来源和早期演化：南部非洲巨晶的 Sr-Nd-Pb-Hf 同位素研究

• 批准号: 0911599
• 负责人: Philip Janney
• 金额: $ 11.44万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-08-01 至 2012-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911599&HistoricalAwards=false
• 关键词: Probing; Sources; Early; Evolution; Kimberlite

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual Merit: Kimberlites are the most deeply derived terrestrial magmas and have brought diamonds to the surface that contain minerals believed to originate in the transition zone or lower mantle. The causes and sources of kimberlite magmatism remain hotly debated, as does the nature of interaction of protokimberlite melts with the lithosphere through which it erupts. Protokimberlite melts are those generated beneath or near the base of thick continental lithosphere, presumably in response to local or regional mantle upwelling, which, upon reaction with and assimilation of lithospheric peridotite, erupt as kimberlite. We propose to better characterize the sources and early evolution of protokimberlite magmas through the measurement of Pb-Sr-Nd-Hf isotopic and trace element compositions of kimberlite-hosted megacrysts from five localities in southern Africa. Megacrysts from kimberlite are thought to represent products of melt-rock reaction between ascending protokimberlite melts and the deep continental lithospheric mantle. As such, they provide information on both the deep source(s) of kimberlites and the nature of lithospheric mantle wallrock with which they react and assimilate. Isotope data that best constrain this interaction are rare and no megacrysts have yet been characterized for Sr, Nd, Pb and Hf isotope ratios. We will analyze megacrysts from five kimberlite localities, chosen on the basis of tectonic setting, kimberlite type, and megacryst compositional range representing localities on both Archean (Kaapvaal and Zimbabwe cratons) and Proterozoic lithosphere. Analysis will focus on clinopyroxene because it crystallizes across a wide range of megacryst formation conditions and contains significant quantities of Sr, Nd, Pb and Hf. We also intend to measure lithophile trace elements to aid in selecting samples for isotopic analysis. All trace element and isotope measurements and parent-daughter element concentration measurements (via isotope dilution) will be performed using SIMS and multicollector ICPMS facilities at Arizona State University. Through these analyses, we will compile the largest isotopic data set yet available for southern African megacrysts, which will be used to test quantitative models of protokimberlite melt evolution and melt-lithosphere interaction, resolve outstanding questions of megacryst-kimberlite relationships, and constrain the possible sources of protokimberlite melts.Broader Impacts; Kimberlites are very important economically, especially to some developing countries, as the primary source of economic diamond deposits. Models of kimberlite-lithosphere interaction, and the interpretations of lithospheric structure derived from them can impact the search for such deposits. Scientifically, kimberlites are important for the unique direct information that their xenoliths provide on the composition of the upper mantle, for the clues to behavior of mantle volatiles that can impact global climate, and for their potential connection to large scale features in global mantle dynamics (e.g., the African "superplume"). The results generated in this proposal will improve our understanding of these connections, because they will be integrated in a broad collaborative international research program ("TopoAfrica" funded by ANR France) on the connection between deep mantle dynamics and the evolution of surface topography, sedimentary basins, climate and biodiversity on the African continent. It will also be integrated with our ongoing international research collaboration on connections between diamonds and early kimberlite evolution (funded by US-Israel BSF). At ASU the work will contribute to interaction between research laboratories, building capacity for interactive research and education. The results of this work will be disseminated to K-12 science educators as part of on-site (at ASU) and distance learning presentations on volcanism and the Earth's mantle, as well as a one-day field experience for science educators, run under the auspices of ASU's Mars Education program. The field trip will take 24 educators on a guided exploration of the world-class mantle xenolith locality at San Carlos, in southeastern Arizona.

“该奖项是根据2009年美国复苏和再投资法案（公法111 - 5）资助的。"智力优点：金伯利岩是最深的陆地岩浆，并将钻石带到表面，其中含有被认为起源于过渡带或下地幔的矿物。金伯利岩岩浆活动的原因和来源仍然存在激烈的争论，就像原始金伯利岩熔体与岩石圈相互作用的性质一样，它通过岩石圈喷发。原金伯利岩熔体是在厚大陆岩石圈底部或附近产生的熔体，可能是对局部或区域地幔上涌的响应，在与岩石圈橄榄岩反应和同化后，喷发为金伯利岩。我们建议更好地表征的来源和早期演化的原始金伯利岩岩浆通过测量的Pb-Sr-Nd-Hf同位素和微量元素组成金伯利岩托管巨晶从南部非洲的五个地方。金伯利岩巨晶被认为是上升的原金伯利岩熔体和大陆岩石圈地幔之间的熔体-岩石反应的产物。因此，它们提供了关于金伯利岩的深部来源和与它们反应和同化的岩石圈地幔围岩性质的信息。同位素数据，最好的限制这种相互作用是罕见的，还没有巨晶的特点是Sr，Nd，Pb和Hf同位素比值。我们将分析巨晶从五个金伯利岩的地方，选择的构造背景，金伯利岩类型的基础上，巨晶成分范围代表太古代（Kaapvaal和津巴布韦）和元古代岩石圈的地方。分析将集中在单斜辉石，因为它在大范围的巨晶形成条件下结晶，并含有大量的Sr，Nd，Pb和Hf。我们还打算测量亲石微量元素，以帮助选择同位素分析样品。所有微量元素和同位素测量以及母-子元素浓度测量（通过同位素稀释）将使用亚利桑那州立大学的SIMS和多集电极ICPMS设施进行。通过这些分析，我们将汇编迄今为止关于南部非洲巨晶的最大的同位素数据集，这些数据集将用于测试原金伯利岩熔体演化和熔体-岩石圈相互作用的定量模型，解决巨晶-金伯利岩关系的未决问题，并限制原金伯利岩熔体的可能来源。金伯利岩在经济上非常重要，特别是对一些发展中国家来说，是经济钻石矿床的主要来源。金伯利岩-岩石圈相互作用的模型，以及由此得出的岩石圈结构的解释可能会影响对此类矿床的寻找。从科学上讲，金伯利岩是重要的，因为它们的捕虏体提供了关于上地幔成分的独特的直接信息，对于可以影响全球气候的地幔挥发物行为的线索，以及它们与全球地幔动力学中大尺度特征的潜在联系（例如，非洲"超级羽流"）。该提案产生的结果将提高我们对这些联系的理解，因为它们将被纳入一个广泛的国际合作研究计划（由法国ANR资助的"TopoAfrica"），该计划涉及深部地幔动力学与非洲大陆表面地形、沉积盆地、气候和生物多样性演变之间的联系。它还将与我们正在进行的关于钻石与早期金伯利岩演化之间联系的国际研究合作（由美国-以色列BSF资助）相结合。在亚利桑那州立大学，这项工作将有助于研究实验室之间的互动，建设互动研究和教育的能力。这项工作的结果将传播给K-12科学教育工作者作为现场（在亚利桑那州立大学）和远程学习介绍火山作用和地球地幔的一部分，以及科学教育工作者为期一天的实地经验，在亚利桑那州立大学的火星教育计划的主持下运行。这次实地考察将带领24名教育工作者在亚利桑那州东南部的圣卡洛斯对世界级地幔捕虏体进行指导性探索。