Pentlandite: A Compositional Indicator of Oxygen Fugacity for Chondrites
D. L. Schrader1, T. J. Zega2, J. Davidson1, T. J. McCoy3, and K. J. Domanik2
Buseck Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, AZ 85287, USA(devin.schrader@asu.edu), 2Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA. 3Department ofMineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA.
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Introduction: Fe-sulfides are ubiquitous in chondrites and are sensitive indicators of formation and alteration conditionsin the protoplanetary disk and small Solar System bodies [e.g., 1–14]. We previously constrained a minimum oxygen fugacity(fO2) needed to form pentlandite, (Fe,Ni)9S8, [6,7] and identified trends with pentlandite compositions between distinct
meteorite groups [6,7]. We also identified a relationship between the at.% Fe/S ratio of pyrrhotite group sulfides and the fO2 offormation, either in the protoplanetary disk or the parent body during aqueous or thermal alteration [13]. Since pentlandite
formed with pyrrhotite (they are typically intergrown in these chondrite groups [1–14]; Figure 1), fO2 may have also influenced
pentlandite compositions. Here we report the chemical compositions of pentlandite in chondrites that experienced a wide rangeof formation and parent body alteration conditions (including fO2 [13]). This compositional analysis enables investigation ofpotential trend(s) with the pyrrhotite at.% Fe/S ratio, and by extension fO2.
Samples and Analytical Procedures: We determined themajor and minor element compositions of pentlandite in each of thefollowing 54 chondrites: CI (Alais), a C1-ungrouped (Miller Range[MIL] 090292), C2-ungrouped (Tarda and Tagish Lake), CY(Belgica [B]-7904), CM1/2 (Allan Hills [ALH] 83100 and Kolang[two lithologies]), unheated CM2s (Aguas Zarcas [multiplelithologies], Mighei, Queen Alexandra Range [QUE] 97990, TheilMountains [TIL] 91722, and D’Angelo Bluff [DNG] 06004), stage I
heated CM2 (Asuka [A]-881458), stage II heated CM2 (Yamato[Y]-793321), CM-like (Sutter’s Mill), CO3.00 (Dominion Range[DOM] 08006), CR1 (Grosvenor Mountains [GRO] 95577), CR-an(Al Rais), CR2 (Elephant Moraine [EET] 87770, EET 92048, EET96259, Gao-Guenie (b), Graves Nunatak [GRA] 95229, LaPaz Ice
Figure 1. Backscattered electron image of a pyrrhotite
Field[LAP] 02342, LAP 04720, MIL 090657, Northwest Africa(Po) – pentlandite (Pn) intergrowth in Tarda (C2-ung).
Mag = magnetite.
[NWA] 801, Pecora Escarpment [PCA] 91082, QUE 99177, Shişr033, and Y-793495), shock-heated CR2 (GRO 03116), CV3OxA(Allende), CV3OxB (Bali), and CV3Red (Vigarano), CK4 (ALH 85002 and Karoonda), CK5 (Larkman Nunatak [LAR] 06868),
CK6 (Lewis Cliff [LEW] 87009), L3.05 (EET 90161 and QUE 97008), LL3 (Semarkona and Vicência), LL4 (Hamlet andSoko-Banja), LL5 (Chelyabinsk and Siena), LL6 (Appley Bridge and Saint-Séverin), R3 (MET 01149), R3.6 (LAP 031275),
R5 (LAP 03639), and R6 (LAP 04840 and MIL 11207) chondrites. We acquired high-resolution backscatter electron images(e.g., Figure 1) and quantitative chemical compositions with the JEOL-8530F Hyperprobe electron microprobe analyzer(EPMA) at Arizona State University (ASU) and the Cameca SX-100 EPMA at the University of Arizona (UA) following [13].
Some pentlandite compositions were previously reported [6,7,10,11].
Results and Discussion: Our data show that there is a relationship between the at.% Fe/S ratio of pyrrhotite (data from[13]) and pentlandite compositions. Moreover, because the at.% Fe/S ratio of pyrrhotite is a proxy for fO2 [13], these results
indicate that there must also be a relationship between fO2 and pentlandite composition. This relationship holds whetherpentlandite formed: (1) during chondrule formation in the protoplanetary disk (i.e., during chondrule cooling, as previously
noted by [6,8,10,11] for CR2, CO3.00, and LL3.00 chondrites), (2) during thermal alteration on the parent asteroid underrelatively reducing (i.e., LL4–6 chondrites) or oxidizing (i.e., R4–6 and CK chondrites) conditions, or (3) during oxidizing
aqueous alteration (i.e., as in CI, CM1, C1-ung, and C2-ung chondrites). Therefore, in addition to the at.% Fe/S ratio of
pyrrhotite, the chemical compositions of pentlandite can also be used as a proxy for the fO2 of formation. This discovery has
implications for the interpretation of pentlandite compositions in meteorites and in asteroid returned samples from Ryugu andBennu.
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Acknowledgements: We thank the Smithsonian Institution, NASA/NSF, the ASU Buseck Center for Meteorite Studies(BCMS), and the National Institute of Polar Research (NiPR) for the loan of the meteorites used in this study, and NASA grant
NNX17AE53G (DLS PI, TJZ Co-I) and the BCMS for funding this research. US Antarctic meteorite samples are recovered by
the Antarctic Search for Meteorites (ANSMET) program which has been funded by NSF and NASA, and characterized andcurated by the Department of Mineral Sciences of the Smithsonian Institution and Astromaterials Curation Officeat NASA Johnson Space Center.