¹Ananya Mallik,²Sabrina Schwinger,¹Arkadeep Roy,¹Pranabendu Moitra
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13921]
¹Department of Geosciences, University of Arizona, Tucson, Arizona, 85721 USA
²Institute of Planetary Research, German Aerospace Center, Berlin, 12489 Germany
Published by arrangement with John Wiley & Sons

The Moon is much wetter than previously thought. The estimated bulk H2O concentrations based on the analyses of H2O in lunar materials show a wide range from 5 to 1650 ppm. To better constrain bulk H2O in the lunar magma ocean (LMO), we model LMO crystallization and vary DH (concentration of H2O in LMO mineral/concentration of H2O in melt), interstitial melt fraction, and initial LMO depth. We take the highest and lowest values of DH reported in the literature for the LMO minerals. We assess the bulk H2O content required in the initial magma ocean to satisfy two observational constraints: (1) H2O measured in plagioclase grains from ferroan anorthosites and (2) crustal mass from GRAIL. We find that the initial bulk LMO H2O that best explains the H2O content in crustal plagioclase is strongly dependent on DH rather than interstitial melt fractions or initial LMO depths, with a drier magma ocean (10 ppm H2O) being favored with higher DH and a wetter magma ocean (100–1000 ppm H2O) with lower DH. This underscores the importance of constraining DH specific to lunar conditions in future studies. We also demonstrate that crustal mass is not an effective hygrometer.

¹Daniel L.Burgin,¹James M.Scott,¹Petrus J.le Roux,²Geoffrey Howarth,¹Marshall C.Palmer,¹Thomas A.Czertowicz,¹Marianne Negrini,^1,3Malcolm R.Reid,^1,3Claudine H.Stirling
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.11.011]
¹Department of Geology, University of Otago, Dunedin 9054, New Zealand
²University of Cape Town, Rondebosch, South Africa
³Centre for Trace Element Analysis, University of Otago, Dunedin 9054, New Zealand
Copyright Elsevier

The 87Sr/86Sr isotopic properties of martian meteorites, measured by isolation and purification of Rb and Sr fractions, have long been used to partially characterise Mars’ mantle. However, this method is time-consuming, destructive, and subject to incorporation of terrestrial contamination due to precipitation of phases in fractures and/or grain boundaries prior to meteorite collection. In this study, we test the effectiveness of in situ acquisition of 87Sr/86Sr by laser ablation in maskelynite – a typically low Rb/Sr (< 0.1) feldspar-composition glass formed during high-P shock metamorphism – as a method of rapid characterisation of Mars’ mantle Sr isotope ratios. Element concentration maps and the results of unwashed and gently surface-leached bulk rock analyses indicate that terrestrial Sr has infiltrated the studied meteorites and affected bulk rock trace element budgets. However, maskelynite trace element concentrations are largely unaffected and their martian igneous Rb/Sr is preserved. In situ 87Sr/86Sr analyses of maskelynite, checked against a newly developed Sr feldspar reference material (KAN, presented here and available on demand), accurately and precisely distinguish different shergottite mantle reservoirs. The measurements reproduce published data, have uncertainties on the 4th to 5th decimal place, and yield statistically indistinguishable results in analytical sessions separated by long periods of time. The data from 11 enriched shergottites reveal there to be subtle Sr isotope variation within the enriched shergottite mantle reservoir, or that there was crustal assimilation of radiogenic components into the shergottites, or that the shergottite liquids were extracted over a time period during which the mantle reservoir 87Sr/86Sr evolved. The limited published age range of the enriched shergottites, coupled with the absence of a correlation between in situ 87Sr/86Sr and REE, Sr or Pb concentration in maskelynite, suggests that the isotope range is best explained by small variations in the enriched mantle reservoir. Given the presence of maskelynite (or plagioclase) in many meteorites, the in situ method will be useful when there are only very small volumes of material available and/or where terrestrial contamination is suspected.