Lunar and Planetary Science XXIX 1403.pdf

IRON-NICKEL SULFIDES IN CARBONACEOUS CHONDRITE MATRIX AND CHONDRULE RIMS Tanya Di Valentin1 and Michael Zolensky2, 1Department of Geology, University of Ottawa, Ottawa, Canada and Lunar and Planetary Institute, Houston, TX 77058 USA; 2Earth Science and Solar System Exploration Division, NASA Johnson Space Center, Houston TX 77058 USA

INTRODUCTION micron, since we wished to investigate only fine- Iron-nickel sulfides are found in most or all solar grained material. The samples analyzed include a system environments, and are probably the only reduced lithology from the brecciated Vigarano CV3 minerals found in all extraterrestrial materials on chondrite, Mokoia - an oxidized CV3, and the CM2 hand. Despite this ubiquity, they have not received chondrites Mighei and Nogoya. Sulfides were the attention they deserve. The most common Fe-Ni analyzed using a JEOL 200FX TEM equipped with a sulfides in chondrites are troilite (FeS), pyrrhotite Link EDX detector. The analyses were calibrated (Fe1-xS) and pentlandite (Fe,Ni)9S8. Troilite is using natural mineral standards, and analytical believed to have resulted from sulfidation of metal results are accurate to within 1% relative. (Fe-Ni) grains in an H2S-containing environment. RESULTS Pyrrhotite is produced when friable troilite grains, The sulfides analyzed were randomly selected which are exfoliated from the metal nucleus, are from different areas of microtomed sections. Their submitted to continued sulfidation [1]. Some morphologies ranged from rounded, oval, spherical, asteroids are known to have experienced aqueous rectangular to anhedral, and their sizes extended alteration, forming products including new from 1 down to approximately 0.1 micron. generations of sulfides (pyrrhotite and pentlandite). The Fe, S and Ni atom percentages from the Pentlandite in particular is known to form during various analyses were plotted in triangular atomic such alteration [1]. However, recent experimental diagrams (Figure 1). The data for the two CM2 work by Lauretta has indicated that pentlandite may chondrites reveals that sulfides from the chondrule also have been formed during the initial sulfidation rims and the meteorite matrix have similar process [2], due to the faster diffusion rate of nickel compositions for each meteorite. Many grains have into the forming sulfide, as compared to iron. compositions intermediate between pyrrhotite and Finally, there is considerable evidence [1,3&4] for a pentlandite. In contrast, results for CV3 matrix and family of phases intermediate between pyrrhotite and rims are different. Vigarano matrix contains only pentlandite, following the trend of the high troilite, while chondrule rims contain troilite and the temperature monosulfide solid solution [5], intermediate phase. Mokoia matrix contains troilite something not encountered in terrestrial rocks. Each and pentlandite, whereas the rims contain sulfide has its own particular stability conditions, pentlandite and the intermediate phase. In general, which have been determined for most phases. What the Mokoia sulfides are more Ni-rich than those remains unclear is how these laboratory-determined from Vigarano, consistent with its more oxidized stability conditions apply to actual environments in nature. These matrix results are consistent with the early solar system. The long-term objective of those obtained by McSween [6]. this research is to characterize sulfides in chondritic Observation of the triangular plots of all the meteorites in order to better establish the conditions Mighei and Nogoya analyses, as well as the Mokoia under which they formed, and the subsequent and Vigarano rim analyses reveals the dominating processes they experienced. Ultimately, it will be presence of a sulfide with an Fe-Ni content similar to possible to infer whether the sulfides in the pentlandite, but with an increased S content. It is chondrites were formed in the solar nebula or on possible that a solid solution not observed in asteroids, and if formed on the asteroids, deduce how terrestrial sulfides exists within this area of the much alteration has occurred there. Here we explore diagram, as mentioned above. Apparently, the Ni the relationships between the finest grain size content of the Fe-Ni sulfide is related to the degree of portions of carbonaceous chondrites, these being nonstoichiometry of the monosulfide solid solution matrix and chondrule rims; fine-grained materials [7]. Therefore the presence of these unusual are the most sensitive to their environment. intermediate sulfides should be telling us something EXPERIMENTAL PROCEDURE important about the Fe-Ni ratio of the mother gas or Sulfides from four chondrites were analyzed fluid. during the course of this study. We specifically Because pentlandite is a known alteration excluded from analysis any sulfides larger than 1 product, the presence of Fe-Ni sulfides could indicate Lunar and Planetary Science XXIX 1403.pdf

Fe-Ni Sulfides in Carbonaceous Chondrite Matrix and Chondrule Rims: T. Di Valentin and M. E. Zolensky

an increased degree of alteration in the Nogoya The CV3 sulfides are a different story. Matrix meteorite, and lesser alteration in Mighei and and chondrule rim sulfides have distinct Mokoia. The results for CMs concur with the compositions for both Vigarano (relatively unaltered) alteration model established by Browning et al. [8]. and Mokoia (relatively altered, though still less so Assuming this view, the predominant presence of than CM2s). These results indicate that CV3 Ni-poor sulfides in reduced lithology of the Vigarano chondrule rims are probably not directly related to CV3 chondrite, seems to indicate that this meteorite matrix, although both have experienced similar is the least altered of the four. These findings agree environments. CV3 chondrule rim material contains with many studies [9], which find that the effects of the intermediate sulfides, matrix does not. However, aqueous alteration on CV chondrites are not as it is impossible to know what this means before we pervasive as in the CM meteorites. Recent better understand the exact nature of this experimental work by Lauretta [2] to the effect that intermediate sulfide material. pentlandite can be produced by gas-solid reactions REFERENCES deserves consideration here as well. However, in a [1] Zolensky and Thomas (1995) GCA 59, 4707- analytical study of chondritic interplanetary dust 4712; [2] Lauretta et al. (1996) Proc. NIPR Symp. particles (IDPs), pyrrhotite, pentlandite and other Antarctic Mets. No 9, 97-110; [3] Scott and Taylor Ni-rich sulfides were only located in aqueously (1985) Proc. 15th Lunar Planet. Sci. Conf., C699- altered IDPs; in anhydrous IDPs only troilite was C709; [4] Alexander et al. (1989) GCA 53, 3045- found [1]. 3057; [5] Vaughn and Craig (1978) Mineral CONCLUSIONS Chemistry of Metal Sulfides, pp. 305-308; [6] The chondrule rim sulfides in CM2 chondrites McSween (1977) GCA 41, 1777-1790; [7] Ahmed El generally have nickel-rich compositions resembling Goresy, personal communication, 1997; [8] the matrix sulfides. It is hypothesized that these Browning et al. (1996) GCA 60, 2621-263; [9] materials were formed in similar conditions, Zolensky and McSween (1988) In Meteorites and probably on the asteroid. Due to the occurrence of the Early Solar System, p.114-143. alteration processes, inferred from the Ni-rich composition of the sulfides, we suggest that the majority of primary sulfides in CMs that were originally present were modified during hydrous reactions. Some primary troilite grains do remain in Mighei matrix and chondrule rims.

S Figure 1: Sulfide Analyses (atomic %)

Vigarano Matrix S Vigarano Rim

PYRRHOTITE Mighei Matrix S Mighei Rim TROILITE 50 S Nogoya Matrix Nogoya50 Rim Mokoia Matrix Mokoia Rim

50

Fe 50 Ni 50

Fe 50 Ni PENTLANDITE

Fe 50 Ni

Fe 50 Ni