Northwest Africa 6486: Record of large impact events and fluid alteration on the L chondrite asteroid

1C. A. Lorenz,1E. V. Korochantseva,1M. A. Ivanova,2J. Hopp,3I. A. Franchi,4M. Humayun,1M. O. Anosova,1S. N. Teplyakova,2M. Trieloff
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13774]
1Vernadsky Institute RAS, Kosygin St. 19, Moscow, 119991 Russia
2Institut für Geowissenschaften, Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 234-236, Heidelberg, 69120 Germany
3Planetary & Space Sciences, School of Physical Sciences, Open University, Milton Keynes, MK7 6AA UK
4National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, Florida, 32310 USA
Published by arrangement with John Wiley & Sons

We report the results of petrological, geochemical, and geochronological investigations of the unusual K-rich L chondrite melt rock Northwest Africa 6486 (NWA 6486). The rock has slightly fractionated siderophile elements and a mostly unfractionated L chondrite pattern of lithophile elements with the exceptions of enrichments in K and Rb and chondritic Sr abundance similar to the K-rich inclusions found in the ordinary chondrites and indicating a fractionation of alkaline elements through the vapor. We suggest that NWA 6486 and related K-rich chondritic inclusions were formed in situ on the OC parent bodies and that K and Rb enrichment of these rock most probably is a result of the selective impact evaporation of volatile alkali elements followed by the reaction of a vapor with shock melt. NWA 6486 recorded a breakup event of the L chondrite parent asteroid at 470 Ma during which it was formed. Unusual veins, depleted in K, Na, Ca, and Al relative to the host rock were found in NWA 6486. We suggest that NWA 6486 was affected by aqueous fluids that produced alteration zones depleted in a feldspar component on the walls of opened fractures. The melt veins could be formed during a subsequent impact event by in situ melting of the fracture walls or due to decomposition of an injected supercritical aqueous silicate fluid. The aqueous alteration and the second impact event had no detectable effect on Ar and oxygen isotopic systems. Cosmic ray exposure ages indicate that NWA 6486 was ejected from its parent asteroid ~3–4 Ma ago.

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