H and Cl isotope systematics of apatite in brecciated lunar meteorites Northwest Africa 4472, Northwest Africa 773, Sayh al Uhaymir 169, and Kalahari 009

Abstract

AbstractWe have investigated the H and Cl systematics in apatite from four brecciated lunar meteorites. In Northwest Africa (NWA) 4472, most of the apatites contain ∼2000–6000 ppm H2O with δD between −200 and 0‰, except for one grain isolated in the matrix, which contains ∼6000 ppm H2O with δD of ∼500–900‰. This low‐δD apatite contains ∼2500–7500 ppm Cl associated with δ37Cl of ∼15–20‰, while the high‐δD grain contains ∼2500 ppm Cl with δ37Cl of ∼7–15‰. In NWA 773, apatites in a first group contain ∼700–2500 ppm H2O with δD values averaging around ∼0 ± 100‰, while apatites in a second group contain ∼5500–16500 ppm H2O with δD ∼250 ± 50‰. In Sayh al Uhaymir (SaU) 169 and Kalahari (Kal) 009, apatites are similar in terms of their H2O contents (∼600–3000 ppm) and δD values (−100 to 200‰). In SaU 169, apatites contain ∼6000–10,000 ppm Cl, characterized by δ37Cl of ∼5–12‰. Overall, most of the analyzed apatite grains have δD within the range reported for carbonaceous chondrites, similar to apatite analyzed in ancient (>3.9 Ga) lunar magmatic. One grain in NWA 4472 has H and Cl isotope compositions similar to apatite from mare basalts. With an age of 4.35 Ga, this grain could be a representative of the oldest known lunar volcanic activity. Finally, since numerous evolved clasts in NWA 773 formed through silicate liquid immiscibility, the apatite grains with extremely high H2O contents, reaching pure hydroxylapatite composition, could provide insights into the effects of such process on the evolution of volatiles in lunar magmas.