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Geochemical identification of impactor for Lonar crater, India

DRS at CSIR-National Institute of Oceanography

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Title Geochemical identification of impactor for Lonar crater, India
 
Creator Mishra, S.
Newsom, H.E.
ShyamPrasad, M.
Geissman, J.W.
Dube, A.
Sengupta, D.
 
Subject oceanite
basalts
concretions
craters
 
Description The only well-known terrestrial analogue of impact craters in basaltic crusts of the rocky planets is the Lonar crater, India. For the first time, evidence of the impactor that formed the crater has been identified within the impact spherules, which are approx. 0.3 to 1 mm in size and of different aerodynamic shapes including spheres, teardrops, cylinders, dumbbells and spindles. They were found in ejecta on the rim of the crater. The spherules have high magnetic susceptibility (from 0.31 to 0.02 SI-mass) and natural remanent magnetization (NRM) intensity. Both NRM and saturation isothermal remanent magnetization (SIRM) intensity are approx. 2 Am sup(2)/kg. Demagnetization response by the NRM suggests a complicated history of remanence acquisition. The spherules show schlieren structure described by chains of tiny dendritic and octahedral-shaped magnetite crystals indicating their quenching from liquid droplets. Microprobe analyses show that, relative to the target basalt compositions, the spherules have relatively high average Fe sub(2)O sub(3) (by approx. 1.5 wt%), MgO (approx. 1 wt%), Mn (approx. 200 ppm), Cr (approx. 200 ppm), Co (approx. 50 ppm), Ni (approx. 1000 ppm) and Zn (approx. 70 ppm), and low Na sub(2) O (approx. 1 wt%) and P sub(2) O sub(5) (approx. 0.2 wt%). Very high Ni contents, up to 14 times the average content of Lonar basalt, require the presence of a meteoritic component in these spherules. It is interpreted that the high Ni, Cr, and Co abundances in these spherules to indicate that the impactor of the Lonar crater was a chondrite, which is present in abundances of 12 to 20 percent by weight in these impact spherules. Relatively high Zn yet low Na sub(2)O and P sub(2) O sub(5) contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume
 
Date 2009-11-16T11:08:17Z
2009-11-16T11:08:17Z
2009
 
Type Journal Article
 
Identifier Meteoritics & Planetary Science, vol.44(7); 1001-1018
http://drs.nio.org/drs/handle/2264/3438
 
Language en
 
Rights Copyright The Meteoritical Society [2009]. All efforts have been made to respect the copyright to the best of our
knowledge. Inadvertent omissions, if brought to our notice, stand for correction and withdrawal of document from this repository.
 
Publisher The Meteoritical Society