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“To See a World in a Grain of Sand:” An Enigmatic 0.8mm Fragment from the Luobusa Ophiolite in Tibet, China Contains 10 Minerals with Crustal and Mantle Affinities

Session: Advancing Mineralogy and Spectroscopy Across the Solar System in Honor of MSA Roebling Medalist M. Darby Dyar

Presenting Author:

Edward S. Grew

Authors:

Xiong, Fahui¹, Mugnaioli, Enrico², Xu, Xiangzhen³, Yang, Jingsui⁴, Wirth, Richard⁵, Franke, Peter⁶, Grew, Edward Sturgis⁷

(1) 1Center for Advanced Research on the Mantle (CARMA), State Key Laboratory of Deep Earth and Mineral Exploration, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China, State Key Laboratory of Deep Earth and Mineral Exploration, Institute of Geology,, Beijing, China, (2) Department of Earth Sciences, University of Pisa, Pisa, Italy, (3) State Key Laboratory of Deep Earth and Mineral Exploration, Institute of Geology, Beijing, China, (4) School of Earth Sciences and Engineering, Nanjing University, Nanjing, China, (5) Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany, (6) Karlsruhe Institute of Technology, IAM-AWP, Karlsruhe, Germany, (7) School of Earth and Climate Sciences, University of Maine, Orono, Maine, USA,

Abstract:

Poet William Blake’s famous line (ca. 1807) about how we can find meaning in the smallest things in Nature (“a grain of sand”) is well illustrated by sample M11483 extracted from chromitite in the Luobusa ophiolite in Tibet, China. Since its first description by Yang et al. (2004), 10 mineral species have been confirmed in this fragment, which is only 0.8 mm in the largest dimension. It has an alloy core with an inner rim of α-Ti and an outer rim of kyanite + coesite pseudomorphs of stishovite + amorphous Al-Ti silicate. The alloy core has a “patchwork-quilt” appearance composed of the new minerals maurogemmiite (ideally Ti₁₀Fe₃O₃) and paulrobinsonite (ideally Ti₈Fe₄O₂) interspersed with vermicular intergrowths of osbornite (TiN) and unnamed TiFe. We suggest that the rim of α-Ti surrounding the alloy core crystallized from a Ti-rich liquid, which then reacted with solid α-Ti to precipitate maurogemmiite, which in turn reacted with liquid to precipitate paulrobinsonite, and finally TiFe and osbornite precipitated in a eutectic. However, this scenario does not explain the relationship between the inner rim of α-Ti and the outer rim of silicate. We suggest that an explanation can be found in a 10 µm-wide wenjiite (ideally Ti₁₀Si₆) border around α-Ti, which formed by metasomatic exchange of Ti and Si between the α-Ti and silicate rims due to the incompatibility of SiO₂ with α-Ti. Wenjiite is calculated to be stable only at log aO2 < ‒21.9 for P = 0.1 MPa and T = 1300 °C, that is, under strongly reducing conditions (cf. ‒10.7 for Fe/wüstite under these conditions). The silicate outer rim has a crustal affinity (high Si, Al content; B present), whereas the α-Ti inner rim and alloy core appear to have a mantle affinity (mantle isotope signature of N in TiN). We propose that crustal and mantle material were brought in contact relatively late in their respective histories, which limited the extent of exchange of Si and Ti, but not of N, which has a mantle signature in the silicate rim. This interpretation more precisely defines the “mixed parentage” that Dobrzhinetskaya et al. (2014) attributed to qingsongite (BN), a constituent of the silicate rim. The coesite pseudomorphs imply the “mixed parentage” developed at P ≥ 10 GPa (Yang et al., 2007).