¹Brennecka, G. A., ¹Budde, G.,¹Kleine, T.
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany

A handful of events, such as the condensation of refractory inclusions and the formation of chondrules, represent important stages in the formation and evolution of the early solar system and thus are critical to understanding its development. Compared to the refractory inclusions, chondrules appear to have a protracted period of formation that spans millions of years. As such, understanding chondrule formation requires a catalog of reliable ages, free from as many assumptions as possible. The Pb-Pb chronometer has this potential; however, because common individual chondrules have extremely low uranium contents, obtaining U-corrected Pb-Pb ages of individual chondrules is unrealistic in the vast majority of cases at this time. Thus, in order to obtain the most accurate 238U/235U ratio possible for chondrules, we separated and pooled thousands of individual chondrules from the Allende meteorite. In this work, we demonstrate that no discernible differences exist in the 238U/235U compositions between chondrule groups when separated by size and magnetic susceptibility, suggesting that no systematic U-isotope variation exists between groups of chondrules. Consequently, chondrules are likely to have a common 238U/235U ratio for any given meteorite. A weighted average of the six groups of chondrule separates from Allende results in a 238U/235U ratio of 137.786 ± 0.004 (±0.016 including propagated uncertainty on the U standard [Richter et al. 2010]). Although it is still possible that individual chondrules have significant U isotope variation within a given meteorite, this value represents our best estimate of the 238U/235U ratio for Allende chondrules and should be used for absolute dating of these objects, unless such chondrules can be measured individually.

Reference
Brennecka GA, Budde G, Kleine T (2015) Uranium isotopic composition and absolute ages of Allende chondrules. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12567]
Published by arrangement with John Wiley & Sons

¹Birlan, M et al. (>10)*
¹Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE), Observatoire de Paris, CNRS UMR8028, 77 avenue Denfert-Rochereau, 75014 Paris Cedex, France
*Find the extensive, full author and affiliation list on the publishers website

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Birlan M et al. (2015) Characterization of (357439) 2004 BL86 on its close approach to Earth in 2015. Astronomy & Astrophysics 581, A3
Link to Article [http://dx.doi.org/10.1051/0004-6361/201526460]

¹Harry Y. McSween Jr.
¹Department of Earth and Planetary Sciences and Planetary Geoscience Institute, University of Tennessee, Knoxville, Tennessee 37996-1410, U.S.A.

Petrologic investigations of martian rocks have been accomplished by mineralogical, geochemical, and textural analyses from Mars rovers (with geologic context provided by orbiters), and by laboratory analyses of martian meteorites. Igneous rocks are primarily lavas and volcaniclastic rocks of basaltic composition, and ultramafic cumulates; alkaline rocks are common in ancient terranes and tholeiitic rocks occur in younger terranes, suggesting global magmatic evolution. Relatively uncommon feldspathic rocks represent the ultimate fractionation products, and granitic rocks are unknown. Sedimentary rocks are of both clastic (mudstone, sandstone, conglomerate, all containing significant igneous detritus) and chemical (evaporitic sulfate and less common carbonate) origin. High-silica sediments formed by hydrothermal activity. Sediments on Mars formed from different protoliths and were weathered under different environmental conditions from terrestrial sediments. Metamorphic rocks have only been inferred from orbital remote-sensing measurements. Metabasalt and serpentinite have mineral assemblages consistent with those predicted from low-pressure phase equilibria and likely formed in geothermal systems. Shock effects are common in martian meteorites, and impact breccias are probably widespread in the planet’s crustal rocks. The martian rock cycle during early periods was similar in many respects to that of Earth. However, without plate tectonics Mars did not experience the thermal metamorphism and flux melting associated with subduction, nor deposition in subsided basins and rapid erosion resulting from tectonic uplift. The rock cycle during more recent time has been truncated by desiccation of the planet’s surface and a lower geothermal gradient in its interior. The petrology of Mars is intriguingly different from Earth, but the tried-and-true methods of petrography and geochemistry are clearly translatable to another world.

Reference
McSween Jr. HY (2015) Petrology on Mars. American Mineralogist 100, 2380-2395
Link to Article [doi: 10.2138/am-2015-5257]

Copyright: The Mineralogical Society of America