¹Christopher S. Noyes,²Guy. J. Consolmagno,²Robert J. Macke,^3,4Daniel T. Britt,¹Cyril P. Opeil
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13895]
¹Department of Physics, Boston College, 140 Commonwealth Avenue, Chestnut Hill, Massachusetts, 02467 USA
²Vatican Observatory, Vatican City, V-00120 Vatican City State
³Department of Physics, University of Central Florida, 4111 Libra Dr, Orlando, Florida, 32816 USA
⁴Center of Lunar and Asteroid Surface Science, 12354 Research Pkwy, Suite 214, Orlando, Florida, 32826 USA
Published by arrangement with John Wiley & Sons

We have measured the thermal conductivity and specific heat capacity of subsamples from four iron meteorites with nickel concentrations between 5% and 8% (Agoudal, Canyon Diablo, Muonionalusta, and Sikhote-Alin) at temperatures between 5 and 300 K. From these, we have calculated their thermal diffusivity and thermal inertia values across this temperature range. For comparison, we also measured subsamples from two L chondrites (NWA 11038 and NWA 11344) at the same time, using the same methods. The thermal diffusivity results of the irons show a relatively constant value for T > 100 K with a characteristic low-temperature maxima at ∼5 K for the iron meteorites; by contrast, the diffusivities of the L chondrites fell by a factor of two over this range and reached low-temperature maxima at ∼20 K. Thermal inertia values show a crossover behavior, with a strong increase in thermal inertia as temperatures drop below 55 K and a less dramatic change at higher temperatures. Our new diffusivity and inertia values cover a wider range of temperatures than previous literature data for iron meteorites. They also provide a useful ground truth in understanding remotely sensed thermal inertias of potentially metal-rich asteroids, including 16 Psyche, target of the NASA Psyche mission.

¹Megan E.Guenther,¹Stephanie M.Brown Krein,¹Timothy L.Grove
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.07.023]
¹Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences 54-1212, 77 Massachusetts Ave, Cambridge, MA 02139, United States
Copyright Elsevier

We present the results of high pressure, high temperature multiple saturation experiments at variable oxygen fugacity ( conditions (IW+1.5 and IW-2.1) on three lunar high titanium ultramafic glasses: the Apollo 17 Orange glass (A17O, 9.1 wt. % TiO2), the Apollo 15 Red glass (A15R, 13.8 wt. % TiO2), and the Apollo 14 Black glass (A14B, 16.4 wt. % TiO2). We performed experiments in graphite ( = IW+1.5) and iron ( = IW-2.1) capsules. The experimentally determined multiple saturation points (MSPs) in graphite capsules are 2.5 GPa and ∼1530℃ (A17O), 1.3 GPa and ∼1350℃ (A15R), and 1.55 GPa and ∼1425℃ (A14B). In iron, we found MSPs of 3.3 GPa and ∼1565℃ (A17O), 2.8 GPa and ∼1490℃ (A15R), and 4.0 GPa and ∼1540℃ (A14B). These results, when combined with previous experiments on the lunar ultramafic glasses, indicate that the increase in the pressure of multiple saturation is linearly proportional to the TiO2 content of the melt , R2 = 0.93, RMSE = 0.2 GPa). The high depths of melting correlated with the lowest conditions are hard to reconcile with buoyancy constraints on these iron and titanium rich magmas. In addition, measurements of on the orange glass as well as the presence of iron blebs in the glasses suggest that the glasses were reduced during eruption. To reconcile buoyancy constraints with estimates, we present a model in which the high titanium magmas experienced higher conditions at their source, but underwent subsequent reduction at shallow depths (4-52 km) just prior to their eruption. In this model, we can then further bracket the depth of melting to be from the minimum multiple saturation pressure in graphite to the deepest depth at which the magmas are buoyant: assuming the Hess and Parmentier (1995) post overturn cumulate mantle, the depths of melting range from ∼550-770 km for the A17O glass, ∼260-490 km for the A15R glass, and ∼320-350 km for the A14B glass.

^1,2A.P.Jordan,³M.L.Shusterman,⁴C.J.Tai Udovicic
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115184]
¹Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA
²Solar System Exploration Research Virtual Institute, NASA Ames Research Center, Moffett Field, CA, USA
³School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
⁴Northern Arizona University, Flagstaff, AZ, USA
Copyright Elsevier

Micrometeoroid impacts, solar wind bombardment, and dielectric breakdown driven by solar energetic particles all potentially alter the optical properties of the lunar regolith by creating submicroscopic metallic iron (smFe0), which includes both nanophase (<33nm) and microphase (>33nm) iron. We create a simple model that describes the time-dependent accumulation of optically active smFe0 in the lunar highlands. Our model synthesizes recent analyses of how space weathering processes form smFe0-bearing agglutinates and rims on soil grains and how impact gardening controls the exposure time of these grains. It successfully reproduces the results of a prior analysis of the formation of smFe0 in the highlands, particularly in regard to nanophase iron, showing that there is consistency among diverse analyses of Apollo samples and of orbital observations. We find that the results of our model are not consistent with the solar wind directly forming smFe0 (although the solar wind may play a role in optical maturation via hydrogen implantation). Our model results are consistent with smFe0 in the lunar highlands being created mainly by micrometeoroid impacts, with a possible contribution from dielectric breakdown weathering.

¹Philipp Gleißner,¹Julie Salme,¹Harry Becker
Earth and Planetary Science Letters 593, 117680 Link to Article [https://doi.org/10.1016/j.epsl.2022.117680]
¹Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstr. 74-100, 12249 Berlin, Germany
Copyright Elsevier

Elevated water contents in various lunar materials have invigorated the discussion on the volatile content of the lunar interior and on the extent to which the volatile element inventory of lunar magmatic rocks is controlled by volatility and degassing. Abundances of moderately volatile and siderophile elements can reveal insights into lunar processes such as core formation, late accretion and volatile depletion. However, previous assessments relied on incomplete data sets and data of variable quality. Here we report mass fractions of the siderophile volatile elements Cu, Se, Ag, S, Te, Cd, In, and Tl in lunar magmatic rocks, analyzed by state-of-the-art isotope dilution-inductively coupled plasma mass spectrometry. The new data enable us to disentangle distribution processes during the formation of different magmatic rock suites and to constrain mantle source compositions. Mass fractions of Cu, S, and Se in mare basalts and magnesian suite norites clearly correlate with indicators of fractional crystallization. Similar mass fractions and fractional crystallization trends in mafic volcanic and plutonic rocks indicate that the latter elements are less prone to degassing during magma ascent and effusion than proposed previously. The latter processes predominate only for specific elements (e.g., Tl, Cd) and complementary enrichments of these elements also occur in some brecciated highland rocks. A detailed comparison of elements with different affinities to metal or sulfide and gas phase reveals systematic differences between lunar magmatic rock suites. The latter observation suggests a predominant control of the variations of S, Se, Cu, and Ag by mantle source composition instead of late-stage magmatic degassing. New estimates of mantle source compositions of two low-Ti mare basalt suites support the notion of a lunar mantle that is strongly depleted in siderophile volatile elements compared to the terrestrial mantle.