HR: 0800h
AN: P51A-0909    [Abstracts]
TI: Bulk Chemical Fractionation Between Basalt, Impact-Melts and Spherules of Lonar Impact Crater, India
AU: * Misra, S
EM: saumitramisra@yahoo.co.in
AF: Dept. of Geology and Geophysics, Indian Institute of Technology, Kharagpur, 721302 India
AU: Prasad, M
EM: shyam@darya.nio.org
AF: Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa, 403004 India
AU: Saha, D
EM:
AF: Dept. of Geology and Geophysics, Indian Institute of Technology, Kharagpur, 721302 India
AU: Sengupta, D
EM:
AF: Dept. of Geology and Geophysics, Indian Institute of Technology, Kharagpur, 721302 India
AU: Khedekar, V D
EM:
AF: Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa, 403004 India
AU: Dube, A
EM:
AF: Dept. of Geology and Geophysics, Indian Institute of Technology, Kharagpur, 721302 India
AU: Newsom, H E
EM: newsom@unm.edu
AF: Institute of Meteoritics, Univ. of New Mexico, Albuquerque, NM 87131 United States
AB: The ~50,000 year old Lonar Crater, India, is one of the two known continental impact craters that were excavated on basalt. In-situ impact-melts from within ejecta from the SE crater rim are associated with splash form impact-spherules of two types: mm-size non-magnetic spherules, also found in other parts of the ejecta, and slightly smaller magnetic spherules with smoother surfaces. Both impact-melts and the two types of spherules have homogeneous compositions, though minor systematic variations exist in impact melts. Compared to unaltered target basalt, average impact-melt characteristically shows ~21-22% depletion in Na₂O and ~44-61% enrichment in K₂O, while impact-spherules show more depletion in Na₂O (~41-42%) and less enrichment in K₂O (~21-29%). Additionally average magnetic spherules are enriched in MgO (~22%) and depleted in CaO (~9%) and Fe₂O_{3}^T (~7%), while non-magnetic spherules are depleted in MgO (~11%) and enriched in CaO (~13%) and Fe2O3^T (~2%) over target-basalt. #Mg for impact-melts and non-magnetic spherules have restricted and overlapping values between ~0.35 and 0.43, but for magnetic spherules it varies between 0.38 and 0.56. The small variation in mobile CaO (~3.4 wt%), Na2O (~2.4 wt%) and K2O (~0.8 wt%) of target-rock and impactites suggest limited weathering of the samples. Lonar impactites were formed by plagioclase- dominated partial melting of target-basalt, which caused enrichment of K2O in impactites and Rb, Cs, Ba, La, Ce, Nd and Th in impact-melts and non-magnetic spherules, while another important target- rock constituent, augite, was variably mixed with plagioclase-melt only in the highest stage of shock metamorphism. Depletion of Na2O in impactites occurred due to limited separation of xenocrystic augite (Na2O ~0.23 and 0.26 wt%) from these molten impactites. Mixing models suggest proportions of plagioclase melt and augite in average impact-melts and non-magnetic spherules is close to ~1:0.4, in magnetic spherules it varies between 1:0.4 and 1:2.3. Proportions of ulvöspinel, a minor component of target-basalt, varied between 1:0.3 and 1:0.2. Volatile phases active during solidification of impactites were mainly H2O and Br. H2O was derived from target-basalt during impact, but depleted by a factor of ~9 times in average impact-melt. Inter-trappean ground water or pre-impact altered basalt flows was possible sources for Br. Enrichment of Br in average impact-melts and non-magnetic spherules (~3-4 times) over target-basalt suggests entrapment of this volatile inside impactites during their formation. The magnetic nature of high #Mg spherules is not clearly understood, may be due to a lower proportion of magnetite within these spherules. Studies of Lonar Crater ejecta have clear application to the nature of impact deposits on Mars being studied by MER rovers and in Columbia Hills.}
DE: 1039 Alteration and weathering processes (3617)
DE: 1065 Major and trace element geochemistry
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 6225 Mars
DE: 8136 Impact phenomena (5420, 6022)
SC: Planetary Sciences [P]
MN: Fall Meeting 2005