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

University Of California-Riverside, Riverside CA

Investigators

Abstract

Diamonds and other ultra-high pressure minerals from paleo-suboceanic mantle - possible mantle recycling Intellectual 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.