Sulfur isotope signatures of eucrites and diogenites

Nanping Wu, James Farquhar, James W. Dottin III, Nivea Magalhães

Geochimica et Cosmochimica Acta,
Volume 233,15 July 2018,Pages 1-13


“High precision sulfur isotope data on eight eucrites and two diogenites are reported to explore the provenance of sulfur and processes that affected the sulfur isotope composition and sulfur abundance within the HED (Howardite, Eucrite, Diogenite) parent body (Asteroid 4 Vesta) or parent bodies for eucrite-like meteorites. Sulfur isotope values for a subset of six of the eucrites (EET 90020, PCA 82502, BTN 00300, MIL 11290, QUE 97014, and Sioux County) and two diogenites (Johnstown and Shalka) converge on a singular sulfur isotope composition, yielding a mean δ34S of 0.26 ± 0.17‰ (2 SD), Δ33S of 0.010 ± 0.007‰ (2 SD), and Δ36S of −0.22 ± 0.21‰ (2 SD). The variances on these mean values can be entirely accounted for by analytical uncertainties. The homogenous positive Δ33S for these samples suggests that sulfur in Asteroid 4 Vesta has a Δ33S of 0.010 ± 0.002‰ (2 SE) which is consistent with the occurrence of a process that allowed for promoted-isotopic-exchange within the parent body to homogenize Δ33S. This process was previously recognized in the homogenous Δ17O composition that has been linked to large-scale silicate melting and mixing. The positive δ34S of this group of samples is attributed to fractionations associated with sulfur loss, and the homogeneity of sulfur isotopes suggests that this loss predates the mixing event. The sulfur isotope data alone do not differentiate whether this loss occurred prior to or post core formation, or even prior to accretion.

Two eucrites (GRA 98098 and CMS 04049) yield isotopic compositions that are distinct from this mean for δ34S, and in the case of CMS 04049, for Δ33S as well. GRA 98098 yields δ34S of 0.81 ± 0.18‰ (2 σest, estimated external precision), Δ33S of 0.010 ± 0.008‰ (2 σest) and Δ36S of 0.01 ± 0.34‰ (2 σest). CMS 04049 yields δ34S of −0.22 ± 0.18‰ (2 σest), Δ33S of 0.024 ± 0.008‰ (2 σest), and Δ36S of −0.17 ± 0.34‰ (2 σest). The more positive δ34S of GRA98098 suggests fractionation of sulfur (34S/32S) by a combination of parent body processes that may be linked to evaporation. The more positive Δ33S and the negative δ34S of CMS 04049 suggests that this meteorite may be from another ‘Vesta-like’ parent body.”