Formation of phosphorus-rich olivine in Dar al Gani 978 carbonaceous chondrite through fluid-assisted metamorphism

Yang Li, Ai-Cheng Zhang, Jia-Ni Chen, Li-Xin Gu, Ru-Cheng Wang

American Mineralogist
DOI: 10.2138/am-2017-5881
First Published on January 03, 2017

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“Phosphorus-rich olivine (P2O5 > 1 wt%) is a mineral that has been reported only in a few terrestrial and extraterrestrial occurrences. Previous investigations suggest that P-rich olivine mainly forms through rapid crystallization from high-temperature P-rich melts. Here, we report a new occurrence of P-rich olivine in an ungrouped carbonaceous chondrite Dar al Gani (DaG) 978. The P-rich olivine in DaG 978 occurs as lath-shaped grains surrounding low-Ca pyroxene and olivine grains. The lath-shaped olivine shows a large variation in P2O5 (0–5.5 wt%). The P-rich olivine grains occur in a chondrule fragment and is closely associated with chlorapatite, merrillite, FeNi metal, and troilite. Tiny Cr-rich hercynite is present as inclusions within the P-rich olivine. The lath-shaped texture and the association with Cr-rich hercynite indicates that the P-rich olivine in DaG 978 formed by replacing low-Ca pyroxene precursor by a P-rich fluid during a thermal event, rather than by crystallization from a high-temperature melt. The large variation of P2O5 within olivine grains on micrometer-scale indicates a disequilibrium formation process of the P-rich olivine. The occurrence of P-rich olivine in DaG 978 reveals a new formation mechanism of P-rich olivine.”