Diamonds and Other Ultra-high Pressure Minerals From Paleo-suboceanic Mantle - possible Mantle Recycling

钻石和其他来自古海底地幔的超高压矿物——可能是地幔循环。根据地球化学特征，蛇绿岩（包括脚状铬铁矿）可分为两类，一类形成于发散型板块边界，另一类形成于辐合型板块边界（即超俯冲带）的弧后扩张。脚状铬铁矿对了解海底地幔的熔融过程具有重要意义，是一种有价值的矿石。脚状铬铁矿的成因可以归结为：(1)类似于堆积岩中带状铬铁矿的超镁铁质岩浆结晶；(2)上地幔部分熔融残余物质；(3)莫霍边界附近熔融/岩石相互作用的幔源部分熔体结晶产物；(4)物质经地幔柱从核-幔边界经地幔柱的底流体运输（连同伴生的泥质岩和辉锌矿）；(5)地幔深部的形成（cr尖晶石最初是高压多晶岩形成的）。然而，近年来在西藏罗布沙地块的铬铁矿中发现了原位超高压（UHP）矿物包裹体（金刚石、Fe-Ti合金、铁长石后伪晶钴矿、TiN、cBN、TiO2 II以及具有ca -铁素体结构的铬铁矿残余高压多晶），对脚状铬铁矿的岩浆成因(3)提出了质疑。这些包裹体都是在高压下形成的，对应于250？380公里。一项为期两年的项目拟研究罗布萨铬铁矿中不寻常的超高压矿物，以了解它们与宿主铬铁矿的关系及其形成的深度和年龄。这项工作将包括微观结构分析、微量元素和同位素分析，以及UHP矿物和宿主铬铁矿中流体/熔体和固体/流体纳米包裹体的表征。该项目将侧重于两个工作假设：(1)脚状铬铁矿中的超高压矿物是在低压下产生的，但已经俯冲到深处并回收到它们目前的位置；(2)至少部分铬铁矿形成于古俯冲带的一个可能较深的沟槽内，后来在莫霍带附近被硼岩质熔体改造。详细的岩石学、高分辨率扫描和分析透射电子显微镜、聚焦离子束（FIB）和纳米二次电离光谱（nano SIMS）分析C、B、N、Os、Re同位素，研究了UHP矿物的性质。这一结果将为研究幔源岩中以前未观察到的异型超高压矿物组合打开一扇新的窗口。更广泛的影响。拟议的项目通过与劳伦斯利弗莫尔国家实验室的长期合作，促进加州大学院系利用先进的科学技术。该项目将资助一名研究生和两名本科生。学生将学习如何使用FIB， nanoSIMS， SEM和TEM，这些技术共同代表了21世纪科学和技术的前沿。该项目鼓励一名女科学家担任PI。国家实验室（LLNL）设施与学术研究和教育的整合将加强科学家与社会之间的联系，并促进对地球的了解。S内部及其内部过程的动力学。它将鼓励学生的创新，在未来，这将影响美国在科学和技术方面的竞争力。

Diamonds and other ultra-high pressure minerals from paleo-suboceanic mantle - possible mantle recyclingIntellectual merit. According to geochemical characteristics ophiolites (including podiform chromitites) are classified into two groups formed at either divergent plate boundaries or via back-arc spreading at convergent plate boundaries (i.e., supra-subduction zone settings). Podiform chromitites play a significant role in understanding the processes of melting in the suboceanic mantle, and constitute valuable ores. Origin of podiform chromitites has been attributed to: (1) crystallization from ultramafic magmas similar to banded chromitite in cumulate rocks; (2) residual material from partial melting in the upper mantle; (3) products of crystallization of mantle-derived partial melts as a result of melt/rock interaction near the Moho boundary; (4) diapiric transport (along with associated dunite and harzburgite) of materials from the core-mantle boundary via mantle plumes, and (5) formation in the deep mantle (where Cr-spinel originally formed as a high-pressure polymorph). However, recently the magmatic origin (3) of podiform chromitite is questioned by many scientists after the discovery of in-situ ultrahigh-pressure (UHP) mineral inclusions (diamond, Fe-Ti alloy, coesite pseudomorphous after stishovite, TiN, cBN, TiO2 II, and relict high-pressure polymorph of chromite with Ca-ferrite structure) in the chromitite of the Luobusa massif, Tibet. These inclusions all formed at high-pressure, corresponding to depths of 250?380 km. A two-year project is proposed to investigate unusual UHP minerals from the Luobusa chromitites to understand their relationships with host chromites and the depth and age of their formation. This work will include microstructural analysis, trace-element and isotope analyses, and characterization of fluid/melt and solid/fluid nanoinclusions in both UHP minerals and host chromite. The project will focus on two working hypotheses: (1) the UHP minerals from podiform chromitite were created at low pressure but have been subducted deeply and recycled to their current locations; (2) part, at least, of the chromitites were formed within a presumably deep channel of a paleo-supra-subduction zone and later modified by boninitic melt near the Moho. Detailed petrographic, high-resolution scanning and analytical transmission electron microscopy, focused ion beam (FIB), and nano secondary-ionization spectrometry (nano SIMS) for C,B,N,Os,Re isotopic analyses are proposed to investigate the nature of the UHP minerals. The results will open a new window to study xenomorphic UHP mineral assemblages that have not previously been observed in mantle-derived rocks. Broader impacts. The proposed project promotes utilization of advanced scientific technologies available to University of California faculties through long-term collaboration with Lawrence Livermore National Laboratory. The project will support a graduate student and two undergraduate students. The students will be taught how to work with FIB, nanoSIMS, SEM, and TEM, which together represent a frontier of 21st-century science and technologies. The project promotes a female scientist acting as PI. Integration of national laboratory (LLNL) facilities into academic research and education will strengthen ties between scientists and society, and promote understanding of the Earth?s interior and the dynamics of its internal processes. It will encourage student innovation that, in the future, will impact the competitiveness of the United States in science and technology.