Characterization of mesostasis regions in lunar basalts: Understanding late-stage melt evolution and its influence on apatite formation

1,2,3Nicola J. Potts,1,4,5Romain Tartèse,1,6Mahesh Anand,2Wim van Westrenen,1,7Alexandra A. Griffiths,1Thomas J. Barrett,1Ian A. Franchi
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12681]
1Planetary and Space Sciences, The Open University, Milton Keynes, UK
2Faculty of Earth and Life Sciences, VU University Amsterdam, 1081 HV Amsterdam, the Netherlands
3School of GeoSciences, University of Edinburgh, Edinburgh, UK
4Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, 5Muséum National d’Histoire Naturelle, Sorbonne Universités, CNRS, UPMC & IRD, Paris, France
6Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
7School of Earth, Atmospheric and Environmental Sciences, University of Manchester, UK
Published by arrangement with John Wiley & Sons

Recent studies geared toward understanding the volatile abundances of the lunar interior have focused on the volatile-bearing accessory mineral apatite. Translating measurements of volatile abundances in lunar apatite into the volatile inventory of the silicate melts from which they crystallized, and ultimately of the mantle source regions of lunar magmas, however, has proved more difficult than initially thought. In this contribution, we report a detailed characterization of mesostasis regions in four Apollo mare basalts (10044, 12064, 15058, and 70035) in order to ascertain the compositions of the melts from which apatite crystallized. The texture, modal mineralogy, and reconstructed bulk composition of these mesostasis regions vary greatly within and between samples. There is no clear relationship between bulk-rock basaltic composition and that of bulk-mesostasis regions, indicating that bulk-rock composition may have little influence on mesostasis compositions. The development of individual melt pockets, combined with the occurrence of silicate liquid immiscibility, exerts greater control on the composition and texture of mesostasis regions. In general, the reconstructed late-stage lunar melts have roughly andesitic to dacitic compositions with low alkali contents, displaying much higher SiO2 abundances than the bulk compositions of their host magmatic rocks. Relevant partition coefficients for apatite-melt volatile partitioning under lunar conditions should, therefore, be derived from experiments conducted using intermediate compositions instead of compositions representing mare basalts.

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