Marion Auxerre, François Faure, and Delphine Lequin
Meteoritics & Plaentary Science (in Press)
Link to Article [https://doi.org/10.1111/maps.13830]
CNRS, CRPG, UMR 7358, 15 rue Notre Dame des Pauvres F-54501 Vandoeuvre-lès-Nancy France
Published by arrangement with John Wiley & Sons

Chondrules, the major constituent of chondrites, are millimeter-sized igneous objects resulting from the crystallization of silicate liquids produced by the partial or complete melting of chondritic precursors, whose exact nature remains disputed. Various chondrule textures are observed as a function of the extent of the initial melting event. Here, we report dynamic crystallization experiments performed with a broad range of cooling rates (2–750 °C h⁻¹) from superliquidus or subliquidus initial conditions to demonstrate the control of nucleation on the final chondrule texture. Classical crypto-porphyritic, micro-porphyritic, and porphyritic olivine textures were reproduced in subliquidus experiments in which heterogeneous nucleation dominates. In contrast, we were unable to reproduce barred olivine textures, regardless of the cooling rates investigated from superliquidus conditions; instead, macro-porphyritic textures were systematically obtained at low cooling rates (<10 °C h⁻¹). The small number and large size of crystals in the macro-porphyritic texture are consistent with the initial step of superheating and the presence of long embayments that indicate an initial episode of rapid growth due to delayed nucleation. Crystals then acquired polyhedral shapes during a subsequent episode of slow growth. When the growth rate is too slow to decrease the degree of supersaturation in the liquid, a new episode of rapid growth produces a new generation of melt inclusions.

David J. Lawrence¹, Patrick N. Peplowski¹, Jack T. Wilson¹, and Richard C. Elphic²
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2022JE007197]
¹Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland
²NASA Ames Spaceflight Center, Moffett Field, California
Published by arrangement with John Wiley & Sons

A global map of bulk hydrogen abundances on the Moon is presented. This map was generated using data from the Lunar Prospector Neutron Spectrometer. This map required corrections for variations due to rare-earth elements, and was calibrated to Apollo sample hydrogen abundances. Since neutron-derived measurements sample hydrogen content to a depth of tens of cm, these results provide complementary insights to those provided by studies using spectral reflectance data, which sample depths of order μm. Comparison of these abundances to Apollo sample values suggest that the samples reflect actual hydrogen content on the lunar surface, not dominantly from non-lunar contamination. The average lunar hydrogen abundance is 47 ppm with a systematic uncertainty of ∼10 ppm. This is consistent with bulk hydrogen from solar wind emplacement. A bulk hydrogen enhancement (50–68 ppm) has been identified at the Moon’s largest pyroclastic deposit (Aristarchus Plateau), which corroborates prior observations that hydrogen and/or water plays a role in lunar magmatic events. Global data show a correlation between hydrogen and evolved materials rich in incompatible trace elements (i.e., KREEP type rocks), with a hydrogen excess of 14–36 ppm in these materials. Based on this hydrogen enhancement, we estimate a lower-limit water abundance within urKREEP materials (i.e., the final ∼2% of the lunar magma ocean) of 320–820 ppm H₂O. This observation implies that water played a role in the original magma-ocean formation and solidification with a lower-limit water content in the original lunar magma ocean of 7–16 ppm or higher.