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.

Project Report

The goal of this project was to study ultrahigh pressure minerals such as diamonds, metals, nitrides, carbides and silicides reported earlier from podiform chromitite of the Tibetan opiolite to understand nature of carbon and nitrogen reservoirs, and if the ultrahigh pressure minerals included in chromitite ores represent recycled material of the continental crust, or/and some microfragments of the deep mantle. Detailed petrographic studies with the aid of the high resolution electron beams and nano secondary ionization spectrometry for isotopic analyses were proposed to investigate the nature of the ultrahigh pressure minerals from paleosuboceanic mantle included in ordinary podiform chromitites. The novelty of this project is a discovery of a new mineral cubic Boron Nitride (cBN), named qingsongite. The cBN mineral and its name qingsongite have been approved by the Commission on New Minerals and Mineral Names (CNMMN) of the International Mineralogical Association (IMA). The mineral was named in honor of Qingsong Fang (1939-2010), Professor at the Institute of Geology , Chinese Academy of Geological Sciences, Beijing,who found the first diamond in the Luobasa chromitite of Tibet in late 1970s, and contributed to the discovery of four new spicies: yarlongite,zangbolite, qusongite and luobasite. The cBN-qingsongite was found in sourthern Tibet mountains of China within chromitites of the paleooceanic crust that was subducted to depth of 190 miles and recrystallized there at temperature ~2372 F and pressure ~118430 atm. About 180 million years ago the rocks were returned back to shallow levels of the Earth by plate tectonic processes leading the closure of huge Paleo-Thethys Ocean and collision of India with the Asian lihospheric plate. The Paleo-Thethys Ocean was an ancient Paleozoic ocean located between the paleo-continent Gondwana and the so-called Hunix terranes and existed from the Silurian (440 Ma) through the Jurassic periods. The cBN until this discovery was known only as an important technological material created first time in 1957 in laboratory. Because cBN has atomic structure similar to carbon bonds in diamond, it has a high density and its synthetic modifications promise to be as hard as diamond. The uniqueness of cBN-qingsongite is that it has a mixed parentage - Boron most likely crustal, and Nitrogen most likely mantle, nonetheless, cBN is a mantle mineral because it is formed under highly reducing condiions at high pressure and high temperature. Though Boron is "essentially crustal" element, our results show that it may be delivered to deep mantle, >190 miles, and due to its reaction with nitrogen, a mineral qingsongite was crystallized. It suggests that other boron-bearing high pressure minerals, such as boron carbides, for example, may be found in mantle rocks in a future. The results are important for improvement and re-consideration of some aspects of the plate tectonic theory, and open a new window in revisiting the existing concepts of the ophiolite formations in the world. In a wider sense, our results are addressed to clarification of the new role of Boron in the Earth's geochemical evolution. Our studies provide for the first time comprehensive knowledge of the ultrahigh pressure minerals from ophiolites and open a new avenue to search for similar ultrahigh pressure minerals in other ophiolites around the world, in order to learn if the minerals are the products of recycled crust, or products from deep subduction channels at convergent plate boundaries. At least now, it is clear that such ultrahigh pressure minerals can be preserved within refractory chromitite deposits of ophiolite. This part of the mantle history was "hidden" before, but now it has become very intriguing because we learn about some mineral assemblages that have never been found before either in the subcontinental mantle or in the suboceanic mantle. The crust-mantle and deep mantle recycling unrevealed before through podiform chromitite will be a further target for thoughtful search of preserved deep mantle , or ultrahigh pressure metamorphic rock fragments within other ophiolitic formations as well as can be a target for future oceanic mantle deep drilling projects.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Jennifer Wade
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University of California Riverside
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