¹A. J. King,^1,2E. Mason,^1,3H. C. Bates,¹P. F. Schofield,^3,4K. L. Donaldson Hanna,³N. E. Bowles,¹S. S. Russell
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13734]
¹Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD UK
²Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ UK
³Planetary Spectroscopy Facility, Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU UK
⁴Department of Physics, University of Central Florida, Orlando, Florida, 32816–2385 USA
Published by arrangement with John Wiley & Sons

The CM carbonaceous chondrites are an important resource in our efforts to understand the role of volatiles in the formation of planetary systems. We report the bulk mineralogy, water abundance, and infrared (IR) reflectance spectra of the CM chondrites LaPaz Icefield (LAP) 04514, LAP 04796, LAP 04565, and LAP 02333. They contain abundant Fe- and Mg-rich serpentines (˜70–80 vol%), and based on their phyllosilicate fractions, we classify LAP 04514, LAP 04796, and LAP 04565 as petrologic subtype 1.6 and LAP 02333 as 1.4. This is consistent with estimated water abundances of 9.9 (±1.1) wt% for LAP 04796, 10.4 (±0.1) wt% for LAP 04565, and 11.5 (±0.5) wt% for LAP 02333. However, LAP 04514 contains less water (8.8 ± 0.3 wt%), has a shallower 3 µm band depth, and lacks tochilinite having experienced posthydration temperatures of ˜300–400 °C. We conclude that LAP 04514, LAP 04796, and LAP 04565 are among the least altered CM chondrites, which retain primitive features from the initial building blocks of the CM parent body. Finally, we use the IR spectral features of LAP 04514, LAP 04796, and LAP 04565 to identify C-complex asteroid surfaces that record mild levels of hydration.

¹Lindsay P. Keller,^2,1Eve L. Berger,^1,3Shouliang Zhang,⁴Roy Christoffersen
Meteoritics & Planteray Science (in Press) Link to Article [https://doi.org/10.1111/maps.13732]
¹NASA Johnson Space Center, Mail Code XI3, Houston, Texas, 77058 USA
²Texas State University − Jacobs JETS − NASA Johnson Space Center, Houston, Texas, 77058 USA
³Samsung Austin Semiconductor, Analysis Engineering, 12100 Samsung Blvd, Austin, Texas, 78754 USA
⁴Jacobs, NASA Johnson Space Center, Mail Code X13, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

Transmission electron microscope (TEM) imaging techniques combined with focused ion beam sample preparation were used to calibrate the solar energetic particle track production rate in lunar samples. Track density measurements by TEM as a function of depth were obtained from lunar rock 64455 that has a well-constrained exposure age of 2 Myr giving a track production rate of 4.4 ± 0.4 × 104 tracks cm−2 yr−1 for a 2π exposure at 1 AU. The typical space weathering effects in mature lunar soils (both vapor-deposited rims and solar wind-damaged rims) accumulate in ˜106 yr based on the new calibration applied to track densities in individual grains. Solar wind-damaged rim widths in anorthite and olivine follow a power law relationship with track density and achieve steady-state widths in a few Myr. Vapor-deposited rim widths show no correlation with exposure age suggesting that their formation is episodic with the full width of vapor-deposited rims accumulating in a single or a few rare impact events. Solar wind-damaged rim development was modeled using the stopping range of ions in matter code. Modeling shows that the solar wind-damaged rims develop rapidly and approach steady-state values in 105–106 yr. Anorthite and olivine record similar track densities for similar exposure ages, but their structural response to solar wind irradiation differs significantly. Solar wind-damaged rims on olivine are not amorphous in contrast to modeling and high flux laboratory experiments and a model is proposed to account for their different response to solar wind irradiation.

¹Masashi Yoshida,¹Masaaki Miyahara,^1,2,3Hiroki Suga,^4,5Akira Yamaguchi,⁶Naotaka Tomioka,⁷Takeshi Sakai,^7,8Hiroaki Ohfuji,⁸Fumiya Maeda,^7,8,9Itaru Ohira,⁸Eiji Ohtani,¹⁰Seiji Kamada,¹¹Takuji Ohigashi,¹¹Yuichi Inagaki,¹²Yu Kodama,³Naohisa Hirao
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13735]
¹Graduate School of Advanced Science and Engineering, Hiroshima University, Hiroshima, Higashi, 739-8526 Japan
²Department of Earth and Planetary, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo, Bunkyo-ku, 113-0033 Japan
³Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto Sayo, Hyogo, 679-5198 Japan
⁴National Institute of Polar Research, Tokyo, 190-8518 Japan
⁵Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, 190-8518 Japan
⁶Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kochi, Nankoku, 783-8502 Japan
⁷Geodynamics Research Center, Ehime University, Matsuyama, 790-8577 Japan
⁸Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai, 980-8578 Japan
⁹Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Tokyo, Toshima-ku, 171-8588 Japan
¹⁰Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, 980-8578 Japan
¹¹UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki, Aichi, 444-8585 Japan
¹²Marine Works Japan, Kochi, Nankoku, 783-8502 Japan
Published by arrangement with John Wiley & Sons

The (plagioclase) lherzolitic shergottite Northwest Africa (NWA) 7397 consists of poikilitic and non-poikilitic lithologies. Coarse-grained low-Ca pyroxene oikocrysts enclose olivine and chromite grains in the poikilitic lithology. The major constituents of the non-poikilitic lithology are olivine, Ca-pyroxene, and plagioclase. Minor amounts of chromite, ilmenite, alkali feldspar, Ca-phosphate, and iron-sulfide are included in the non-poikilitic lithology. Most plagioclase grains in the non-poikilitic lithology have become maskelynite. A melt pocket occurs in the non-poikilitic lithology. Plagioclase in contact with the melt pocket has dissociated into zagamiite + stishovite. Apatite and merrillite entrained in the melt pocket have transformed into tuite. Olivine in contact with the melt pocket has dissociated into bridgmanite (almost vitrified) + ferroan-periclase. Alteration products, iron oxides and hydroxides, also occur in the dissociated olivine although it is not clear when the aqueous alteration occurred. The dissociation reactions of olivine and plagioclase into the high-pressure polymorphs (bridgmanite, ferroan-periclase, zagamiite, and stishovite) are found from lherzolitic shergottites for the first time. The estimated peak shock-pressure and -temperature conditions recorded in melt pockets of NWA 7397 are ˜23 GPa and 2,000 °C at least, respectively, based on the high-pressure mineral assemblages.