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

This session honors MSA Roebling Medalist M. Darby Dyar, celebrating her contributions to the mineralogy and spectroscopic properties of planetary bodies, especially the Earth, Moon, Mars, and Venus. Investigations probing mineral structures, iron valence, and planetary surface reactions are invited.

Monday, 20 October 2025
8:00 AM - 12:00 PM
HBGCC, 217A

“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

Paper number: 86-2
Program: Technical Sessions
Time: 8:25 AM – 8:40 AM

Authors

  • Fahui Xiong
    State Key Laboratory of Deep Earth and Mineral Exploration, Institute of Geology,, Beijing
  • Enrico Mugnaioli
    Department of Earth Sciences, University of Pisa, Pisa
  • Xiangzhen Xu
    State Key Laboratory of Deep Earth and Mineral Exploration, Institute of Geology, Beijing
  • Jingsui Yang
    School of Earth Sciences and Engineering, Nanjing University, Nanjing
  • Richard Wirth
    Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam
  • Peter Franke
    Karlsruhe Institute of Technology, IAM-AWP, Karlsruhe
  • Edward Grew Presenting Author
    School of Earth and Climate Sciences, University of Maine, Orono, Maine

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).

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412635

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