Sulfide compositions of young Chang’e-5 basalts and implications for sulfur isotopes in lunar basalt sources

Abstract

Sulfides are accessory phases in lunar rocks butare important forunderstanding lunar interior processes as well as impacts on the lunar surface.Whether or not the lunar mantle had achieved sulfide saturation during magma ocean evolution and displays homogeneous sulfur isotopes remains under debate. TheChang’e-5 (CE-5) mission returned young (2.0 Ga) basalts from a mare terrain in thenorthern Oceanus Procellarum.Here we study chemical and sulfur isotopic compositions (δ34SV-CDT) of sulfides from CE-5 basaltic fragments and combine them with δ34S of other young (3.1–3.0 Ga) lunar low-Ti basalt (NWA 10597 and NWA 4734) and gabbro meteorites (NWA 6950) to compare them with Apollo low-Ti and high-Ti mare basalts. The sulfides in basaltic fragments of CE-5 are troilites (FeS) with lowabundances ofNi, Co, and Cu(e.g., Ni < 0.04 wt% and Ni/Co < 0.3). Textures and chemical compositions indicate that most troilites arelate-stage crystallization products fromthe highly evolvedCE-5 basalts. Several troilites occur in the matrices of impactite clasts and are intergrown with Fe–Ni metal(12–36 wt% Ni, Ni/Co of 12–39). These troilites are distinct from the major population of troilites withnoticeably higher Ni abundances (mostly > 0.2 wt% with Ni/Co of 1–3) andreconcile with the addition of meteoritic materials into the impact melts.The δ34SV-CDTof large troilite grains (>10 μm) from the CE-5 basaltic fragments and lunar meteorites were obtained by high-precision, high-spatial-resolution femtosecond laser ablation MC-ICP-MS which achieved external uncertainty (0.65 ‰, 2SD at 8-μm laser spots) like nano-SIMS. Sulfur degassing during surficial effusive lava flow likely led to a slightdecrease inδ34S (by ∼ 1 ‰) for some basaltic fragments; however, such effects were limited to the scale of bulk rock samples, consistent with previous results. The mean δ34Sof troilites in CE-5 basaltic fragments (0.35 ± 0.25 ‰, 2SE, n = 45) is similar to those of ancient (3.8–3.1 Ga old) Apollo low-Ti and high-Ti mare basalts and the young gabbro cumulate NWA 6950 (0.56 ± 0.21 ‰, 2SE, n = 10). The paired NWA 10597 and NWA 4734 show consistent δ34S, lower than most values by ∼ 0.5 ‰. Current data thus indicate that most mantle sources of lunar basalts would be homogeneous for δ34S(0.6 ± 0.3 ‰) and minor regions may be different. The overall homogenous δ34S from different mantle sources with variably low sulfur contents supports sulfide-undersaturated accumulation of the lunar magma ocean, which was inherited from strong volatile loss and evaporative fractionation during the formation of the Moon.