Sulfur-in-apatite: An indicator of the volatile evolution during lunar magmatism

The volatile content of lunar magmas remains controversial despite nearly five decades of interrogating samples from the NASA Apollo missions. Recently, the mineral apatite in lunar mare basalts has been investigated owing to its potential to constrain the volatile (F, Cl, H, S) budget of magmas [1-3]. The F-Cl-H signatures of lunar apatite were interpreted to record fractional crystallization, with nucleation and growth of apatite from a late-stage, interstitial, nearly anhydrous (<10 μg/g H2O), rhyolitic melt that evolved from a sulfide-undersaturated mare basalt [1]. The enigmatic S signature reported for those apatite grains was not interpreted due to the absence of published thermodynamic (partitioning) data for S. Here, we report new experimentally determined apatite/melt partition coefficients for S (DSap/m) at conditions applicable to lunar systems. The DSap/m values and thermodynamically modeled S content (XS) of lunar residual melt were used to constrain plausible S contents of lunar apatite produced by crystal fractionation (Sap = XS * DSap/m). Our results demonstrate that apatite crystallizing under lunar-like conditions from rhyolitic melt cannot obtain the reported 430 μg/g of S [2] by fractional crystallization. The results indicate that 5-35x higher S contents than feasible in sulfide-undersaturated, hydrous and dry rhyolitic melt, respectively, would be required to support crystal fractionation models [1]. Even elevated water concentrations in a sulfide-saturated rhyolitic melt cannot explain the S contents of lunar apatite rims. We propose two plausible scenarios: (A) The necessary concentration of S in rhyolitic melts may be achieved at >5 orders of magnitude higher fO2 (>ΔFMQ+1.2) than reported for lunar magmas, where S6+ is the prevalent oxidation state of S in rhyolitic melt, related to the significant degassing and preferential loss of H2 that drives oxidation of the residual melt [4]. (B) The volatile (F-Cl-H-S) signatures of lunar apatites may reflect cryptic metasomatic reactions between apatite and a S-Cl-rich, F-poor volatile phase released by underlying magma reservoirs. [1] Boyce et at., 2014, Science 344:400-402. [2] Greenwood et al., 2011, Nat. Geosci 4:79-82. [3] Webster et al., 2009, Geochim. Cosmochim. Acta 73, 559-581. [4] McCanta et al., 2017, Icarus 285, 95-102.