Marian Horstmann^a, Munir Humayun^b and Addi Bischoff^a

^aInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany.
^bNational High Magnetic Field Laboratory & Department of Earth, Ocean and Atmospheric Science, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA.

Enstatite (E) chondrites are a group of texturally highly variable meteorites formed under strongly reducing conditions giving rise to unique mineral and chemical characteristics (e.g., high abundances of various sulfides and Si-bearing metal). In particular the abundant metal comprises a range of textures in E chondrites of different petrologic type, but available in situ siderophile trace element data on metal are limited. Nine samples of E chondrites from the recent Almahata Sitta fall [one EH3, two EL3/4, two EL6, two EL impact melt rocks (IMR), two EH IMR] were investigated in this study in addition to St. Mark’s (EH5) and Grein 002 (EL4/5), with a focus on the nature of their metal constituents. Special attention was given to metal-silicate intergrowths (MSSI) that occur in many primitive E chondrites, which have been interpreted as post-accretionary asteroidal impact melts or primitive nebular condensates. This study shows that siderophile trace element systematics in E chondrite metal are independent of petrologic type of the host rock and distinct from condensation signatures. Three basic types of siderophile trace element signatures can be distinguished, indicating crystallization from a melt, thermal equilibration upon metamorphism/complete melting, and exsolution of schreibersite-perryite-sulfide. Textural and mineral-chemical constraints from EL3/4s are used to evaluate previously proposed formation processes of MSSI (impact melting vs. nebular condensation) and elucidate which other formation scenarios are feasible. It is shown that post-accretionary (in situ) impact melting or metallic melt injection forming MSSI on the thin section scale, and nebular condensation, are unlikely formation processes. This leads to the conclusion that MSSIs are pre-accretionary melt objects that were formed during melting processes prior to the accretion of the primitive E chondrites. The same can be concluded for metal nodules in the EH3 chondrite examined. The pre-accretionary origin of MSSIs in E chondrites is consistent with a growing body of evidence for early differentiation followed by impact disruption of early formed planetesimals in all major chondrite types.

Reference
Horstmann M, Humayun M and Bischoff A (in press) Clues to the origin of metal in Almahata Sitta EL and EH chondrites and implications for primitive E chondrite thermal histories. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.041]
Copyright Elsevier

Link to Article

Wolf Uwe Reimold^1,2, Ludovic Ferrière³, Alex Deutsch⁴ and Christian Koeberl^3,5

¹Museum für Naturkunde Berlin, Berlin, Germany
²Humboldt-Universität zu Berlin, Berlin, Germany
³Natural History Museum, Vienna, Austria
⁴Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Muenster, Germany
⁵Department of Lithospheric Research, University of Vienna, Vienna, Austria

This is a letter to the editor with no abstract.

Reference
Reimold WU, Ferrière L, Deutsch A and Koeberl C (in press) Impact controversies: Impact recognition criteria and related issues. Meteoritics & Planetary Science
[doi:10.1111/maps.12284]
Published by arrangement with John Wiley & Sons

Link to Article

Jafar Arkani-Hamed^a,b and Daniel Boutin^b

^aDepartment of Physics, University of Toronto, Toronto, Ontario, Canada
^bDepartment of Earth and Planetary Sciences, McGill University, Montreal, Quebec, Canada

We investigate the possibility that a strong core dynamo of the Moon has magnetized the lunar crust. The magnetic data from two missions, Lunar Prospector and Kaguya, are used and the magnetic fields of two different features are examined: The isolated small magnetic source bodies with almost no topographic signatures, and the impact craters with diameters larger than 100 km. Five data sets are examined separately for each of the isolated magnetic anomalies: the r, θ, and φ components of the Lunar Prospector data, the rcomponent of a 150-degree spherical harmonic model of the lunar magnetic field, and the r component of the Kaguya data. The r component of the Lunar Prospector data is also used to derive the magnetic field over the impact craters. We conclude that most of the ancient lunar far side crust is heterogeneously magnetized with coherency wavelength about a few hundred km. The paleomagnetic north poles determined from modeling the magnetic field of both features show some clustering whereas the source bodies are widely distributed, suggesting that the magnetizing field may have been a core dynamo field. Paleintensity data suggest that the core field intensity was at least 1 mT at the core mantle boundary. There is also evidence for core field reversals, because further clustering occurs when the south poles of some features are considered.

Reference
Arkani-Hamed J and Boutin D (in press) Analysis of Isolated Magnetic Anomalies and Magnetic Signatures of Impact Craters: Evidence for a Core Dynamo in the Early History of the Moon. Icarus
[doi:10.1016/j.icarus.2014.04.046]
Copyright Elsevier

Link to Article