¹Shohei Kaneko, ^1,2Masaaki Miyahara, ^1,3Eiji Ohtani, ⁴Tomoko Arai, ⁵Naohisa Hirao,⁶Kazuhisa Sato
¹Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
²Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan
³V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Science, 630090 Novosibirsk, Russia
⁴Planetary Exploration Research Center, Chiba Institute of Technology, Chiba 275-0016, Japan
⁵Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto Sayo, Hyogo 679-5198, Japan
⁶Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan * E-mail: miyahara@hiroshima-u.ac.jp

High-pressure polymorphs recovered in terrestrial craters are evidence of meteoroid impact events on the Earth’s surface. Despite countless impact craters on the Moon, high-pressure polymorphs have not been reported to date in returned Apollo samples. On the other hand, recent studies report that the high-pressure polymorphs of silica, coesite, and stishovite occur in shocked lunar meteorites. We investigated regolith breccia 15299, which was returned by the Apollo 15 mission, using the combined techniques of focused ion beam (FIB), synchrotron X-ray diffraction (XRD), and transmission electron microscopy (TEM). The regolith breccia 15299 studied here consists of a mafic impact melt breccia with millimeter-sized, coarse-grained, low-Ti basalt clasts. The mafic melt breccia consists of fragments of minerals (olivine, pyroxene, plagioclase, silica, and ilmenite) and glass. Several quartz, tridymite, and cristobalite grains of 10–100 μm across occur in the mafic impact melt breccia. Vesicular melt veins of less than ~200 μm wide cut across the mafic melt breccia matrix and mineral fragments. Some silica grains are entrained in the melt veins. One of the silica grains entrained in the melt veins consist of stishovite [a = 4.190(1), c = 2.674(1) Å, V = 46.95 Å3, space group P42/mnm] along with tridymite and silica glass. This is the first report of high-pressure polymorphs from returned lunar samples. TEM images show that the stishovite is needle-like in habit, and up to ~400 nm in size. Considering the lithologies and shock features of 15299, it is inferred that the stishovite possibly formed by the Imbrium impact or subsequent local impact event(s) in the Procellarum KREEP Terrane (PKT) of the nearside of the Moon.

Reference
Kaneko S, Miyahara M, Ohtani E, Arai T, Hirao N, Sato K (2015) Discovery of stishovite in Apollo 15299 sample.
American Mineralogist 100, 1308-1311
Link to Article [doi: 10.2138/am-2015-5290]

Copyright: The Mineralogical Society of America

¹Larry G. Evans et al. (>10)*
¹Computer Sciences Corporation, Lanham-Seabrook, MD 20706, USA

Orbital measurements obtained by the MESSENGER Gamma-Ray Spectrometer have been analyzed to determine the surface abundance of chlorine in Mercury’s northern hemisphere. The derived Cl/Si mass ratio is 0.0057 ± 0.001, which for an assumed Si abundance of 24.6 wt% corresponds to 0.14 ± 0.03 wt% Cl. The abundance of Cl is a factor of 2.9 ± 1.3 higher in the north polar region (>80° N) than at latitudes 0°−60° N, a latitudinal variation similar to that observed for Na. Our reported Cl abundances are consistent with measured bulk concentrations of neutron-absorbing elements on Mercury, particularly those observed at high northern latitudes. The Cl/K ratio on Mercury is chondritic, indicating a limited impact history akin to that of Mars, which accreted rapidly. Hypotheses for the origin of Mercury’s high metal-to-silicate ratio must be able to reproduce Mercury’s observed elemental abundances, including Cl. Chlorine is also an important magmatic volatile, and its elevated abundance in the northern polar region of Mercury indicates that it could have played a role in the production, ascent, and eruption of flood volcanic material in this region. We have identified several candidate primary mineralogical hosts for Cl on Mercury including the halide minerals lawrencite (FeCl2), sylvite (KCl), and halite (NaCl), as well as Cl-bearing alkali sulfides. Amphiboles, micas, apatite, and aqueously-deposited halides, in contrast, may be ruled out as mineralogical hosts of Cl on Mercury.

Reference
Evans LG et al. (2015) Chlorine on the Surface of Mercury: MESSENGER Gamma-Ray Measurements and Implications for the Planet’s Formation and Evolution. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.04.039]

Copyright Elsevier