Degassing pathways of Cl-, F-, H-, and S-bearing magmas near the lunar surface: Implications for the composition and Cl isotopic values of lunar apatite

Abstract:

Experimental degassing of H-, F-, Cl-, C-, and S-bearing species from volatile-bearing magma of lunar composition at low P and fO₂ close to quartz-iron-fayalite indicates that the composition of the fluid/vapor phase that is lost changes over time. A highly H-rich vapor phase is exsolved within the first 10 min. of degassing leaving behind a melt that is effectively dehydrated. Some Cl, F, and S is also lost during this time, presumably as HCl, HF, and H₂S gaseous species; however much of the original inventory of Cl, F, and S components are retained in the melt. After 10 min., the exsolved vapor is dry and dominated by S- and halogen-bearing phases, presumably consisting of metal halides and sulfides, which evolves over time towards F enrichment. This vapor evolution provides important constraints on the geochemistry of volatile-bearing lunar phases that form subsequent to or during degassing. The rapidity of H loss suggests that little if any OH-bearing apatite will crystallize from surface or near surface (≈7m) melts and that degassing of lunar magmas will cause the compositions of apatites to evolve first towards the F-Cl apatite binary and eventually towards end member fluorapatite during crystallization. During the stage of loss of primarily H component from the melt, Cl would have been lost primarily as HCl, which is reported not to fractionate Cl isotopes at magmatic temperatures. After the loss of H-bearing species, continued loss of Cl would result in the degassing of metal chlorides, as a mechanism to fractionate Cl isotopes. After the onset of metal chloride degassing, the δ³⁷Cl of the melt would increase to +6 (82% Cl loss), +8 (85% Cl loss), and +20‰ (95% Cl loss) at 1, 4, and 6 hours, respectively, approximated in a computed trajectory of δ³⁷Cl values in basalt during degassing of FeCl₂. This strong enrichment of ³⁷Cl in the melt after metal chloride volatilization is fully consistent with values measured for the non-leachates of a variety of lunar samples and would be reflected in apatites crystallized from a degassing melt. We suggest that a range in δ³⁷Cl from 0 to > 20‰ is expected in lunar apatite, with heavy enrichment being the norm. Importantly, such enrichments can occur in spite of high initial H-contents, and therefore, demonstrate that elevated values of δ³⁷Cl cannot be used as supporting evidence for an anhydrous Moon.