¹Patrick N. Peplowski et al. (>10)*
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
Patrick N. Peplowski
*Find the extensive, full author and affiliation list on the publishers Website

The first map of variations in the abundances of thermal-neutron-absorbing elements across Mercury’s surface has been derived from measurements made with the anti-coincidence shield on MESSENGER’s Gamma-Ray Spectrometer (GRS). The results, which are limited to Mercury’s northern hemisphere, permit the identification of four major geochemical terranes at the 1000-km horizontal scale. The chemical properties of these regions are characterized from knowledge of neutron production physics coupled with elemental abundance measurements acquired by MESSENGER’s X-Ray Spectrometer (XRS) and GRS. The results indicate that the smooth plains interior to the Caloris basin have an elemental composition that is distinct from other volcanic plains units, suggesting that the parental magmas were partial melts from a chemically distinct portion of Mercury’s mantle. Mercury’s high-magnesium region, first recognized from XRS measurements, also contains high concentrations of unidentified neutron-absorbing elements. At latitudes north of ∼65° N, there is a region of high neutron absorption that corresponds closely to areas known to be enhanced in the moderately volatile lithophile elements Na, K, and Cl, and which has distinctly low Mg/Si ratios. The boundaries of this terrane differ from those of the northern volcanic plains, which constitute the largest geological unit in this region.
Reference
Peplowski PN et al. (2015) Geochemical terranes of Mercury’s northern hemisphere as revealed by MESSENGER neutron measurements. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.02.002]

Copyright Elsevier

^1,2Jingao Liu, ¹Miriam Sharp, ¹Richard D. Ash, ³David A. Kring, ¹Richard J. Walker
¹Department of Geology, University of Maryland, College Park MD 20742 USA
²Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton AB T6G 2E3 Canada
³Center for Lunar Science and Exploration, Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, Texas 77058, USA

Concentrations of highly siderophile elements (HSE) and 187Os/188Os isotopic compositions for eleven impact related rocks from the Apollo 15 and 16 landing sites are reported and combined with existing geochronological data to investigate the chemical nature and temporal changes in the large impactors implicated in the formation of the lunar basins. Data for the samples all define linear trends on plots of HSE versus Ir concentrations, whose slopes likely reflect the relative HSE compositions of the dominant impactors that formed the rocks.
The inferred Imbrium basin impactor that generated Apollo 15 impact melt rocks 15445 and 15455 was characterized by modestly suprachondritic 187Os/188Os, Ru/Ir, Pt/Ir and Pd/Ir ratios. Diverse impactor components are revealed in the Apollo 16 impact melt rocks. The 187Os/188Os and HSE/Ir ratios of the impactor components in melt rocks 60635, 63595 and 68416, with reported ages < 3.84 Ga, are within the range of chondritic meteorites, but slightly higher than ratios characterizing previously studied granulitic impactites with reported ages > 4.0 Ga. By contrast, the impactor components in melt rocks 60235, 62295 and 67095, with reported ages of ∼3.9 Ga, are characterized by suprachondritic 187Os/188Os and HSE/Ir ratios similar to the Apollo 15 impact melt rocks, and may also sample the Imbrium impactor. Three lithic clasts from regolith breccias 60016 and 65095, also with ∼3.9 Ga ages, contain multiple impactor components, of which the dominant composition is considerably more suprachondritic than those implicated for Imbrium and Serenitatis (Apollo 17) impactors. The dominant composition recorded in these rocks was most likely inherited from a pre-Imbrium impactor. Consideration of composition versus age relations among lunar impact melt rocks reveals no discernable trend.
Virtually all lunar impact melt rocks sampled by the Apollo missions, as well as meteorites, are characterized by 187Os/188Os and HSE/Ir ratios that, when collectively plotted, define linear trends ranging from chondritic to fractionated compositions. The impact melt rocks with HSE signatures within the range of chondritic meteorites are interpreted to have been derived from impactors that had HSE compositions similar to known chondrite groups. By contrast, the impact melt rocks with non-chondritic relative HSE concentrations could not have been made by mixing of known chondritic impactors. These signatures may instead reflect contributions from early solar system bodies with bulk chemical compositions that have not yet been sampled by primitive meteorites present in our collections. Alternately, they may reflect the preferential incorporation of evolved metal separated from a fractionated planetesimal core.
Pre-4.0 Ga ages for at least some impactor components with both chondritic and fractionated HSE raise the possibility that the bulk of the HSE were added to the lunar crust prior to the later-stage basin-forming impacts, such as Imbrium and Serenitatis, as proposed by Fischer-Gödde and Becker (2012). For this scenario, the later-stage basin-forming impacts were more important with respect to mixing prior impactor components into melt rocks, rather than contributing much to the HSE budgets of the rocks themselves.

Reference
Liu J, Sharp M, Ash RD, Kring DA, Walker RJ (2015) Diverse impactors in Apollo 15 and 16 impact melt rocks: evidence from osmium isotopes and highly siderophile elements. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.004]

Copyright Elsevier

^1,2Emmanuel Jacquet, ³Olivier Alard, ^2,4Matthieu Gounelle
¹Canadian Institute for Theoretical Astrophysics, 60 St George Street, Toronto, ON, M5S 3H8, Canada
²Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, CNRS & Muséum National d’Histoire Naturelle, UMR 7590, 57 rue Cuvier, 75005 Paris, France
³Géosciences Montpellier, UMR 5243, Université de Montpellier II, Place E. Bataillon, 34095 Montpellier cedex 5, France
⁴Institut Universitaire de France, Maison des Universités, 103 boulevard Saint-Michel, 75005 Paris, France

We report trace element concentrations of silicate phases in chondrules from LL3 ordinary chondrites Bishunpur and Semarkona. Results are similar to previously reported data for carbonaceous chondrites, with rare earth element (REE) concentrations increasing in the sequence olivine < pyroxene < mesostasis, and heavy REE (HREE) being enriched by 1-2 orders of magnitude (CI-normalized) relative to light REE (LREE) in ferromagnesian silicates, although no single olivine with very large LREE/HREE fractionation has been found. On average, olivine in type II chondrules is poorer in refractory lithophile incompatible elements (such as REE) than its type I counterpart by a factor of ∼2. This suggests that olivine in type I and II chondrules formed by batch and fractional crystallization, respectively, implying that type II chondrules formed under faster cooling rates (> ∼ 10 K/h) than type I chondrules. Appreciable Na concentrations (3-221 ppm) are measured in olivine from both chondrule types; type II chondrules seem to have behaved as closed systems, which may require chondrule formation in the vicinity of protoplanets or planetesimals. At any rate, higher solid concentrations in type II chondrule forming regions may explain the higher oxygen fugacities they record compared to type I chondrules. Type I and type II chondrules formed in different environments and the correlation between high solid concentrations and/or oxygen fugacities with rapid cooling rates is a key constraint that chondrule formation models must account for.

Reference
Jacquet E, Alard O, Gounelle M (2015) Trace element geochemistry of ordinary chondrite chondrules: the type I/type II chondrule dichotomy. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.005]

Copyright Elsevier

¹Christa Göpel, ¹Jean-Louis Birck, ²Albert Galy, ³Jean-Alix Barrat, ⁴Brigitte Zanda
¹Institut de Physique du Globe de Paris, Sorbonne, Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, F-75005 Paris, France
²Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK
³Université Européenne de Bretagne and CNRS UMR 6538, U.B.O-I.U.E.M., F-29280 Plouzané Cedex, France
⁴Laboratoire Minéralogie et Cosmochimie du Muséum, MNHN and CNRS UMR 7202, F-75005 Paris, France

Cr isotopic compositions have been measured on carbonaceous chondrites (CC): Tafassasset, Paris, Niger I, NWA 5958, NWA 8157 and Jbilet Winselwan. In bulk samples, the 54Cr/52Cr ratios (expressed as ε54Cr) range from 0.93 to 1.58 ε units. These values are in agreement with values characteristic for distinct petrologic types. Despite this 54Cr heterogeneity, the variability in the 53Cr/52Cr ratios (expressed as ε53Cr) of 0.2ε units and the Mn/Cr ratios is consistent with the previous finding of an isochron in the Mn-Cr evolution diagram.
The Mn/Cr ratio in CC corresponds to variable abundances of high-T condensate formed and separated at the beginning of the solar system, thus the canonical 53Mn/55Mn ratio can be defined. Based on a consistent chronology for U-Pb and Mn-Cr between the earliest objects formed in the solar nebula and the D’Orbigny angrite we define a canonical 53Mn/55Mn ratio and ε53Cri of 6.8 x 10-6 and – 0.177, respectively.
The internal Mn/Cr systematics in Tafassasset and Paris were studied by two approaches: leaching technique and mineral separation. Despite variable ε54Cr values (up to > 30 ε) linear co-variations were found between ε53Cr and Mn/Cr ratio. The mineral separates as well as the leachates of Tafassasset fall on a common isochron indicating that 1) cooling of the Tafassasset’s parent body occurred at 4563.5 +/-0.25 Ma, and that 2) 54Cr is decoupled from the other isotopes even though temperatures > 900 °C have been reached during metamorphism. In the case of Paris, the leachates form an alignment with a 53Mn/55Mn ratio higher than the canonical value. This alignment is not an isochron but rather a mixing line. Based on leachates from various CM and CI, we propose the occurrence of three distinct Cr reservoirs in meteoritic material: PURE54, HIGH53 and LOW53 characterized by a ε53Cr and ε54Cr of 0 and 25,000, -2.17 and 8, and 0.5 and -151, respectively. PURE54 has already been described and is carried by highly refractory nano-spinel; HIGH53 is Mn-rich and most probably carried by sulfides in the matrix, whereas LOW53 is characterized by low Mn/Cr ratios and it is sensitive to metamorphism. This component could correspond to mineral phases such as refractory oxides and carbide. Variable mixing proportions of HIGH53 and LOW53 would explain the larger-than-expected uncertainty (MSWD of 5.5) on the CC bulk regression line. A Monte Carlo simulation allows us to evaluate the impact of the dispersion of the initial Cr isotopic ratios (as a function of variable HIGH53). The co-variation of the Mn/Cr ratio and the ε53Cr defined by the mineral separates from Paris corresponds to an age of 4566.44 +0.66/-0.75 Ma, while their ε54Cr still differ by at least 0.42 ε. This age is likely to date the segregation of forsteritic olivines (most probably from type I chondrules) from fayalitic olivines (from type II chondrules) and, given the sampling procedure by handpicking of hundreds of grains, corresponds to the average age of chondrule formation.

Reference
Göpel G, Birck J-L, Galy A, Barrat J-A, Zanda B (2015) Mn-Cr systematics in primitive meteorites: insights from mineral separation and partial Dissolution. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.008]

Copyright Elsevier