¹Ivan S. Bazhan, ^1,2Konstantin D. Litasov, ^1,3Eiji Ohtani, ³Shin Ozawa3
American Mineralogist 102, 1279-1286 Link to Article [https://doi.org/10.2138/am-2017-5892]
¹V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, 630090, Russia
²Novosibirsk State University, Novosibirsk, 630090, Russia
³Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Copyright: The Mineralogical Society of America

High-pressure minerals—majorite-pyrope garnet and jadeite—were found in the Pervomaisky L6 ordinary chondrite. Majorite-pyrope (79 mol% majorite) was observed within the fine-grained silicate matrix of a shock-melt vein (SMV), coexisting with olivine and high-Ca pyroxene. This is the first report of a garnet–olivine–high-Ca pyroxene assemblage that crystallized from the melt in the SMV matrix of meteorite. P-T conditions of the formation of the SMV matrix with olivine fragments are 13.5–15.0 GPa and 1750–2150 °C, the lowest parameters among all known majorite-bearing (H,L)-chondrites. The estimated conditions include the olivine/(olivine + ringwoodite) phase boundary and there is a possibility that observed olivine is the result of wadsleyite/ringwoodite back-transformation during a cooling and decompression stage. In the framework of this hypothesis, we discuss the problem of survival of the high-pressure phases at the post-shock stage in the meteorites and propose two possible P-T paths: (1) the high-pressure mineral is transformed to a low-pressure one during adiabatic decompression above the critical temperature of direct transformation; and (2) quenching below the critical temperature of direct transformation within the stability field of the high-pressure phase and further decompression. The aggregates with plagioclase composition (Ab81.1An14.9Or4.1) occur in host-rock fragments near (or inside) of the SMV, and have a radial, concentric “spherulite-like” microstructure previously described in the Novosibirsk meteorite, and that is very similar to the texture of tissintite in the Tissint martian meteorite. It is likely that jadeite is related to crystallization of the SMV and could have formed from albitic feldspar (plagioclase) melt at 13.5–15.0 GPa and ~2000 °C.

¹Hiroyuki Sato, ¹Mark S. Robinson, ²Samuel J. Lawrence, ³Brett W. Denevi, ⁴Bruce Hapke, ⁵Bradley L. Jolliff, ⁶Harald Hiesinger
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.06.013]
¹School of Earth and Space Exploration, Arizona State University, 1100 S. Cady Mall, INTDS A, Tempe, AZ 85287-3603, USA
²Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058, USA
³Applied Physics Laboratory, Johns Hopkins University, 11100 John Hopkins Rd, Laurel, MD 20723-6005, USA
⁴Department of Geology and Planetary Science, University of Pittsburgh, 4107 O’Hara Street, Pittsburgh, PA 15260, USA
⁵Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University, One Brookings Drive, St Louis, Missouri 63130, USA
⁶Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

The visible (VIS; 400-700 nm) and near-infrared (NIR; 700-2800 nm) reflectance of the lunar regolith is dominantly controlled by variations in the abundance of plagioclase, iron-bearing silicate minerals, opaque minerals (e.g., ilmenite), and maturation products (e.g., agglutinate glass, radiation-produced rims on soil grains, and Fe-metal). The same materials control reflectance into the near-UV (250-400 nm) with varying degrees of importance. A key difference is that while ilmenite is spectrally neutral in the VIS and NIR, it exhibits a diagnostic upturn in reflectance in the near-UV, at wavelengths shorter than about 450 nm. The Lunar Reconnaissance Orbiter Wide Angle Camera (WAC) filters were specifically designed to take advantage of this spectral feature to enable more accurate mapping of ilmenite within mare soils than previously possible. Using the reflectance measured at 321 and 415 nm during 62 months of repeated near-global WAC observations, first we found a linear correlation between the TiO2 contents of the lunar soil samples and the 321/415 nm ratio of each sample return site. We then used the coefficients from the linear regression and the near-global WAC multispectral mosaic to derive a new TiO2 map. The average TiO2 content is 3.9 wt% for the 17 major maria. The highest TiO2 values were found in Mare Tranquillitatis (∼ 12.6 wt%) and Oceanus Procellarum (∼ 11.6 wt%). Regions contaminated by highland ejecta, lunar swirls, and the low TiO2 maria (e.g., Mare Frigoris, the northeastern units of Mare Imbrium) exhibit very low TiO2 values (<2 wt%). We find that the Clementine visible to near-infrared based TiO2 maps (Lucey et al., 2000) have systematically higher values relative to the WAC estimates. The Lunar Prospector Gamma-Ray Spectrometer (GRS) TiO2 map is consistent with the WAC TiO2 map, although there are local offsets possibly due to the different depth sensitivities and large pixel scale of the GRS relative to the WAC. We find a wide variation of TiO2 abundances (from 0 to 10 wt%) for early mare volcanism (>2.6 Ga), whereas only medium- to high-TiO2 values (average = 6.8 wt%, minimum = 4.5 wt%) are found for younger mare units (<2.6 Ga).