Petrographic and compositional indicators of formation and alteration conditions from LL chondrite sulfides

Abstract

Sulfide minerals occur in many types of extraterrestrial samples and are sensitive indicators of the conditions under which they formed or were subsequently altered. Here we report that chemical and petrographic analyses of Fe,Ni sulfides can be used to determine the metamorphic type of the host LL chondrite, and constrain their alteration conditions. Our data show that the major- and minor-element compositions of the pyrrhotite-group sulfides (dominantly troilite) and pentlandite vary with degree of thermal metamorphism experienced by their host chondrite. We find that Fe,Ni sulfides in LL3 chondrites formed during chondrule cooling prior to accretion, whereas those in LL4 to LL6 chondrites formed during cooling after thermal metamorphism in the parent body, in agreement with previous work. High degrees of shock (i.e., ≥S5) caused distinct textural, structural, and compositional changes that can be used to identify highly shocked samples. Distinct pyrrhotite-pentlandite textures and minerals present in Appley Bridge (LL6) suggest that they cooled more slowly and therefore occurred at greater depth(s) in the host parent body than those of the other metamorphosed LL chondrites studied here. Sulfides in all LL chondrites studied formed under similar sulfur fugacities, and the metamorphosed LL chondrites formed under similar oxygen fugacities. The data reported here can be applied to the study of other LL chondrites and to sulfides in samples of asteroid Itokawa returned by the Hayabusa mission in order to learn more about the formation and alteration history of the LL chondrite parent body.

Figure 1.

BSE images of sulfides containing pyrrhotite (po) – pentlandite (pn) intergrowths in (a) Semarkona (LL3.00, S2), (b) Vicência (LL3.2, S1), (c) Hamlet (LL4, S3), (d) Soko-Banja (LL4, S3), (e) Chelyabinsk (LL5, S5), (f) Siena (LL5, S3/4), (g) Appley Bridge (LL6, S3), and (h) Saint-Séverin (LL6, S2). The pyrrhotite (po) in Chelyabinsk (e) displays multiple 120° triple junctions. Where: Ch = chondrule; OA = opaque assemblage; MOA = matrix opaque assemblage; Mx = matrix; met = Fe,Ni metal; chr = chromite.

Figure 2.

BSE images of uncommon sulfide textures and associated minerals in (a) Chelyabinsk (LL5, S5) and (b) Appley Bridge (LL6, S3). The grain of pyrrhotite (po) in Chelyabinsk (a) contains numerous 120° triple junctions. (b) Appley Bridge contains regions of ‘mottled’ pyrrhotite-pentlandite (pn) intergrowths, which border larger regions of po and pn, as well as metallic Cu, Ni-rich metal (met), and a low-Ni high-Co metal. Where: OA = opaque assemblage.

Figure 3.

(a) Ni vs. Co (wt.%) and (b) Ni vs. Cu (wt.%) compositions of sulfides. Analyses along the x- and y-axes are below the element’s detection limit (Table EA-1). The vertical line is the compositional boundary between pyrrhotite and pentlandite.

Figure 4.

Fe-Ni-S ternary phase diagrams that most closely match the sulfides data from each LL chondrite studied here: (a) Semarkona (LL3.00, S2); (b) Vicência (LL3.2, S1), (c) Hamlet (LL4, S3); (d) Soko-Banja (LL4, S3); (e) Chelyabinsk (LL5, S5); (f) Siena (LL5, S3/4); (g) Appley Bridge (LL6, S3); and (h) Saint-Séverin (LL6, S2). All LL chondrite sulfides studied here are consistent with equilibrating at ≤230°C; since pentlandite in each sample is near the pentlandite stability field and troilite is along the tie line between troilite and the pentlandite field. Since pentlandite in each sample is near but more Ni-poor than the stability field, the equilibration temperature is ≤230°C. Where pn = pentlandite and mss = monosulfide solid solution. Phase diagrams adapted from Raghavan (2004); original data for 230 °C diagram from Misra and Fleet (1973).