52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

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THE FE/S RATIO OF PYRRHOTITE GROUP SULFIDES IN CHONDRITES IS RELATED TO THE
DEGREE OF OXIDATION. D. L. Schrader1, J. Davidson1, T. J. McCoy2. T. J. Zega3, S. S. Russell4, K. J. Domanik3, and A. J. King4. 1Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA (devin.schrader@asu.edu), 2Dept. of Mineral Sciences, National Museum ofNatural History, Smithsonian Institution, Washington, DC USA, 3Lunar and Planetary Laboratory, University ofArizona, Tucson, Arizona 85721, USA, 4Department of Earth Sciences, Natural History Museum, Cromwell Road,London, SW7 5BD, UK.
Introduction: Fe-sulfides are ubiquitous in chondrites and are sensitive indicators of formation andalteration conditions in the protoplanetary disk andminor Solar System bodies [e.g., 1–12]. The compositions and textures of sulfides can be used to constrain
the oxygen and sulfur fugacities, and the histories ofaqueous alteration, thermal metamorphism, shockimpact processing, and cooling of the host rock [e.g.,3,5–7,11]. The most abundant sulfides in astromaterials are the pyrrhotite group (ideally Fe1–xS where0≤x≤0.125, but x can be ≤0.2; i.e., FeS [troilite] toFe0.8S; e.g., [13]), which can occur with pentlandite,(Fe,Ni)9S8, and pyrite, FeS2 (e.g., [1,5]). The compositions of pyrrhotite in carbonaceous chondrites varywith the degree of aqueous alteration experienced; theat.% Fe/S ratio of pyrrhotite decreases with increasing
degrees of aqueous alteration (i.e., Fe/S of pyrrhotite inCI < CM1 < CM2 [1,8,12]). However, troilite (FeS,Fe/S = 1) was observed in aqueously altered and heated CM and CY chondrites [2,4,8].
Here we discuss the Fe/S ratio of pyrrhotite inchondrite groups that cover a wide range of aqueous
and thermal histories to evaluate its usefulness as anindicator of parent-body processes and its application
to returned samples. Minimally altered samples andthose that have experienced varied types/degrees ofsecondary processing are included because: (1) asteroid Bennu is most like CM or CI chondrites [e.g., 14];(2) asteroid Ryugu is most like heated CI, CM [e.g.,15], or CY chondrites [16]; and (3) both asteroids likely include impact-delivered xenoliths [e.g., 17].
Samples and Analytical Procedure: We studied
the sulfide compositions of 58 different chondrites;
including CI (Alais [Fig. 1], Ivuna, and Orgueil), anungrouped C1 (MIL 090292), CY (B-7904, Y-82162,Y-86789 [Fig. 2], and Y-980115), CM1/2 (ALH 83100[Fig. 3] and Kolang) CM2 (Aguas Zarcas, Mighei, andQUE 97990), stage I (A-881458) and stage II (Y793321) heated CM2s (Nakamura, 2005), CM-like(Sutter’s Mill), CO3.00 (DOM 08006), CR1 (GRO95577), CR-an (Al Rais), CR2 (14 different meteorites), shock-heated CR2 (GRA 06100 and GRO03116), CV3OxA (Allende), CV3OxB (Bali), and CV3Red(Vigarano), CK4 (ALH 85002 and Karoonda), CK5(LAR 06868), CK6 (LEW 87009), H3.10 (NWA3358), L3.05 (QUE 97008), L(LL)3.05 (MET 00452),LL3 (Semarkona and Vicência), LL4 (Hamlet andSoko-Banja), LL5 (Chelyabinsk and Siena), LL6 (Appley Bridge and Saint-Séverin), R3 (Meteorite Hills[MET] 01149), R3.6 (LaPaz Icefield [LAP] 031275),
R5 (LAP 03639), and R6 (LAP 04840 and MIL 11207)
chondrites. Some Fe/S ratios were determined from
sulfide compositions that we previously published on
LL, R, CK, CM, CR2, and CO3 chondrites[5,7,9,10,11,18]. High-resolution images and chemicalcompositions were obtained with the JEOL-8530F
Hyperprobe electron microprobe analyzer (EPMA) at
Arizona State University, and the Cameca SX-100
EPMAs at the University of Arizona (UA) and at theNatural History Museum (NHM), London, following
procedures described in [11].

Figure 1. BSE image of pyrrhotite (po)-pentlandite(pn) intergrowth in Alais (CI).
Results: Pyrrhotite-group sulfides were identified
in all samples except the CK4 ALH 85002, CK5, andCK6 chondrites. Pyrite was identified in all CK chondrites. To avoid analyses that could overlap with submicron-sized grains of pentlandite, we define Ni-poor
pyrrhotite as containing less than 1 wt.% Ni [11]; theseanalyses were used to determine the at.% Fe/S ratios.
The pyrrhotite-group sulfide troilite (Fe/S ≈ 1) wasfound in the H3.10, L3.05, all LL chondrites, theCO3.00, all CV3s, the CYs, the CR1, all CR2s, someCM2s, the CM-like chondrite Sutter’s Mill, and the R3chondrite. Fe-depleted pyrrhotite (Fe1-xS; Fe/S from <1to 0.8) was found in the CIs (Fig. 1), CYs (Fig. 2), theC1-ung, the CR-an Al Rais, some sulfides within CR2

52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

chondrites, the CM1/2s (e.g., Fig. 3), the CM2s, theCM-like Sutter’s Mill, some sulfides in LL chondrites,
one sulfide in the H3.10, the R3 to R6 chondrites, anda single grain in the CK4 Karoonda. No troilite wasfound in the CK chondrites and only a single grain of
pyrrhotite was found in the CK4 Karoonda, which has
the lowest Fe/S ratio analyzed here.

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tionship is the case whether the sample is unaltered,aqueously altered, and/or thermally metamorphosed, asoxidation can occur in the protoplanetary disk or in theparent body during aqueous or thermal alteration.
Summary: While there are trends with the at.%
Fe/S ratio of pyrrhotite with thermal and aqueous alteration in some meteorite groups, there is a universal
trend with the Fe/S ratio with degree of oxidation. Thisfinding has important implications for the analysis ofsulfides in chondrites and returned samples.

Figure 2. BSE of pyrrhotite (po) laths in Y-86789(CY).
Discussion: The average Fe/S ratio of pyrrhotite
varies among meteorite groups and petrographic types
within meteorite groups. The trend in average Fe/S
ratio of all chondrites studied appears complicated, theorder from lowest to highest average Fe/S ratio separated by petrographic type is: CK4 < CI < R6 < CM1/2≤ CR-an ≤ C1-ung ≤ R5 ≤ R3.6 ≤ R3 ≤ CM2 ≤ CV3OxA≈ CR1 ≈ CR2 ≈ CY ≈ LL4 ≤ L3.05 ≤ LL5 ≤ LL3 ≈CV3OxB ≤ H3.10 ≈ LL6 ≤ CV3Red ≤ L(LL)3.05 ≈CO3.00. The average Fe/S ratio of pyrrhotite generally
trends with the degree of aqueous alteration in unheated carbonaceous chondrites. It is likely that internal
sample heterogeneities, thermal and aqueous alteration,and differences in sulfur and oxygen fugacities (e.g.,the CK4 and R chondrites) all contribute to the orderof Fe/S ratios. For example, as previously noted [2,8],
troilite is present in aqueously and thermally alteredcarbonaceous chondrites (e.g., the CYs studied here).
Therefore, using the at.% Fe/S ratio of pyrrhotite as auniversal proxy for the degree of aqueous or thermal
alteration is not warranted.
Comparing the average Fe/S ratio to the degree ofoxidation of each meteorite [e.g., 19], we find a relationship between the average Fe/S ratio and the bulk
meteorite oxidation state. The most reduced chondrites
contain troilite and the most oxidized (via thermal oraqueous alteration) chondrites contain Fe-depletedpyrrhotite. The presence of troilite or Fe-depleted pyrrhotite in a chondrite is an indicator of the relative oxygen fugacity of its formation or alteration. This rela-Figure 3. BSE image of pyrrhotite (po)-pentlandite(pn) intergrowth in ALH 83100 (CM1/2).
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Acknowledgements: We thank the NiPR, Smithsonian Institution, NASA, NSF, ASU, NHM, and IfP
for the samples used in this study, and NASA grant
NNX17AE53G (DLS PI, TJZ Co-I) for funding thisresearch.