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

“这项奖励是根据2009年美国复苏和再投资法案（公法111-5）资助的。”知识价值：金伯利岩是最深层的陆生岩浆，它将含有矿物的钻石带到地表，这些矿物被认为起源于过渡带或下地幔。关于金伯利岩岩浆活动的成因和来源，以及原金伯利岩熔体与岩石圈相互作用的性质仍然存在激烈的争论。原金伯利岩熔体是在厚厚的大陆岩石圈底部或底部附近产生的熔体，可能是对局部或区域地幔上涌的反应，在与岩石圈橄榄岩的反应和同化后，喷发成金伯利岩。我们建议通过测量南部非洲五个地点的金伯利岩巨晶的Pb-Sr-Nd-Hf同位素和微量元素组成来更好地表征原金伯利岩岩浆的来源和早期演化。来自金伯利岩的巨晶被认为是上升的原金伯利岩熔体与深部大陆岩石圈地幔之间熔融-岩石反应的产物。因此，它们提供了关于金伯利岩的深层来源和与之反应和同化的岩石圈地幔围岩性质的信息。最能约束这种相互作用的同位素数据是罕见的，并且还没有Sr， Nd， Pb和Hf同位素比值表征的巨型晶体。我们将根据构造背景、金伯利岩类型和代表太古宙（Kaapvaal和津巴布韦克拉通）和元古宙岩石圈的五个金伯利岩地点的巨晶成分范围进行分析。分析将集中在斜辉石上，因为它在各种巨晶形成条件下结晶，并且含有大量的Sr， Nd， Pb和Hf。我们还打算测量亲石微量元素，以帮助选择样品进行同位素分析。所有微量元素和同位素测量以及亲子元素浓度测量（通过同位素稀释）将使用亚利桑那州立大学的SIMS和多收集器ICPMS设施进行。通过这些分析，我们将编制迄今为止最大的南非巨晶岩同位素数据集，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，解决巨晶岩-金伯利岩关系的悬而未决的问题，并限制原金伯利岩熔体的可能来源。更广泛的影响;金伯利岩作为经济钻石矿床的主要来源，在经济上是非常重要的，特别是对一些发展中国家。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，可以影响这类矿床的寻找。在科学上，金伯利岩的重要性在于其捕虏体提供了关于上地幔组成的独特直接信息，为影响全球气候的地幔挥发物的行为提供了线索，并为它们与全球地幔动力学的大规模特征（例如，非洲“超级羽流”）的潜在联系提供了线索。这一提案所产生的结果将提高我们对这些联系的理解，因为它们将被整合到一个广泛的国际合作研究项目（由法国ANR资助的“TopoAfrica”）中，该项目旨在研究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化之间的联系。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关系的国际研究合作（由美以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，建立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）和远程学习的一部分，介绍火山活动和地幔，以及在亚利桑那州立大学火星教育计划的主持下，为科学教育工作者提供为期一天的实地体验。这次实地考察将带领24名教育工作者前往亚利桑那州东南部圣卡洛斯的世界级地幔捕虏体地区进行探索。

"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.