^1,2R. Liu,^1,3L. Hu,¹M. Humayun
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12803]
¹National High Magnetic Field Laboratory and Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida, USA
²Department of Geosciences, Texas Tech University, Lubbock, Texas, USA
³Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA
Published by arrangement with John Wiley & Sons

Rhenium is an important element with which to test hypotheses of isotope variation. Historically, it has been difficult to precisely correct the instrumental mass bias in thermal ionization mass spectrometry. We used W as an internal standard to correct mass bias on the MC-ICP-MS, and obtained the first precise δ187Re values (~±0.02‰, 2SE) for iron meteorites and chondritic metal. Relative to metal from H chondrites, IVB irons are systematically higher in δ187Re by ~0.14 ‰. δ187Re for other irons are similar to H chondritic metal, although some individual samples show significant isotope fractionation. Since 185Re has a high neutron capture cross section, the effect of galactic cosmic-ray (GCR) irradiation on δ187Re was examined using correlations with Pt isotopes. The pre-GCR irradiation δ187Re for IVB irons is lower, but the difference in δ187Re between IVB irons and other meteoritic metal remains. Nuclear volume-dependent fractionation for Re is about the right magnitude near the melting point of iron, but because of the refractory and compatible character of Re, a compelling explanation in terms of mass-dependent fractionation is elusive. The magnitude of a nucleosynthetic s-process deficit for Re estimated from Mo and Ru isotopes is essentially unresolvable. Since thermal processing reduced nucleosynthetic effects in Pd, it is conceivable that Re isotopic variations larger than those in Mo and Ru may be present in IVBs since Re is more refractory than Mo and Ru. Thus, the Re isotopic difference between IVBs and other irons or chondritic metal remains unexplained.

^1,2,3Kathleen E. Vander Kaaden, ^1,3Francis M. McCubbin, ⁴Larry R. Nittler, ⁵Patrick N. Peplowski, ⁴Shoshana Z. Weider, ⁴Elizabeth A. Frank,⁶Timothy J. McCoy
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.11.041]
¹Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA.
²Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA
³NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, TX 77058, USA.
⁴Department of Terrestrial Magnetism, Carnegie Institution of Washington, DC 20015, USA.
⁵The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
⁶Department of Mineral Sciences, National Museum of Natural History, 10th and Constitution Aves. NW, Smithsonian Institution, Washington, DC 20560, USA.
Copyright Elsevier

Orbital data from the MESSENGER mission to Mercury have facilitated a new view of the planet’s structure, chemical makeup, and diverse surface, and have confirmed Mercury’s status as a geochemical endmember among the terrestrial planets. In this work, the most recent results from MESSENGER’s X-Ray Spectrometer, Gamma-Ray Spectrometer, and Neutron Spectrometer have been used to identify nine distinct geochemical regions on Mercury. Using a variation on the classical CIPW normative mineralogy calculation, elemental composition data is used to constrain the potential mineralogy of Mercury’s surface; the calculated silicate mineralogy is dominated by plagioclase, pyroxene (both orthopyroxene and clinopyroxene), and olivine, with lesser amounts of quartz. Petrologically, the rocks on the surface of Mercury are highly diverse and vary from komatiitic to boninitic. The high abundance of alkalis on Mercury’s surface results in several of the nine regions being classified as alkali-rich komatiites and/or boninites. In addition, Mercury’s surface terranes span a wide range of SiO2 values that encompass crustal compositions that are more silica-rich than geochemical terranes on the Moon, Mars, and Vesta, but the range is similar to that of Earth. Although the composition of Mercury’s surface appears to be chemically evolved, the high SiO2 content is a primitive feature and a direct result of the planet’s low oxygen fugacity.