Fe-Ni sulfides in Tagish Lake: Implications for nebular and parent body conditions of formation

1Miranda C. Holt,1Christopher D. K. Herd
Meteoritics & Planetary Science (in Press) Link tp Article [https://doi.org/10.1111/maps.13819]
1Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 Canada
Published by arrangement with John Wiley & Sons

This study examined nine pristine samples representing seven lithologies of the ungrouped C2 carbonaceous chondrite Tagish Lake from the University of Alberta Meteorite Collection using scanning electron microscope and electron probe microanalyzer analyses to characterize the sulfide mineralogy, textures, and compositions present. Four distinct sulfide morphologies were identified including pyrrhotite containing exsolved pentlandite, unexsolved pyrrhotite, and unexsolved pentlandite, and a unique “bull’s-eye” sulfide morphology. The at% Fe/Ni of the pyrrhotite grains within these samples decreases with increasing degree of alteration and roughly places them in the alteration order of TL11v chip1 < TL4 < TL11v chip2 < TL5b ≤ TL10a < TL 11h < TL1 < TL11i. The at% Fe/Ni of low Ni (<1 wt% Ni) pyrrhotite indicates that the overall degree of alteration of Tagish Lake lies between that of CM1/2 and CI chondrites. Comparison of the composition of the sulfides to established Fe-Ni-S phase diagrams at different temperatures indicates two separate generations of sulfide formation. They are (1) high-temperature formation of exsolved pyrrhotite–pentlandite and much of the unexsolved pentlandite at ~500–600 °C, likely by cooling of a monosulfide solid solution melt during chondrule formation; and (2) low-temperature formation of unexsolved pyrrhotite, some unexsolved pentlandite, pyrrhotite containing flame-like pentlandite bodies, and bull’s-eye sulfides at ~25–100 °C, likely formed during aqueous alteration events on the Tagish Lake parent body.

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