Misra S., Newsom H.E., Prasad M.S., Geissman J.W., Dube A., Sengupta D.
Institute of Meteoritics, Department of Earth and Planetary Sciences, 1 University of New Mexico, Albuquerque, NM 87131, United States; National Institute of Oceanography, Dona Paula, Goa-403004, India; Department of Earth and Planetary Sciences, MSC03 2040, 1 University of New Mexico, Albuquerque, NM 87131, United States; P 147/3, Janak Road, Kolkata-700 029, India; Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721302, India; School of Geological Sciences, University of Kwazulu-Natal, Durban-4000, South Africa
Misra, S., Institute of Meteoritics, Department of Earth and Planetary Sciences, 1 University of New Mexico, Albuquerque, NM 87131, United States, Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721302, India, School of Geological Sciences, University of Kwazulu-Natal, Durban-4000, South Africa; Newsom, H.E., Institute of Meteoritics, Department of Earth and Planetary Sciences, 1 University of New Mexico, Albuquerque, NM 87131, United States; Prasad, M.S., National Institute of Oceanography, Dona Paula, Goa-403004, India; Geissman, J.W., Department of Earth and Planetary Sciences, MSC03 2040, 1 University of New Mexico, Albuquerque, NM 87131, United States; Dube, A., P 147/3, Janak Road, Kolkata-700 029, India; Sengupta, D., Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721302, India
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 ∼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 ∼2 Am2/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 Fe2O 3 (by ∼1.5 wt%), MgO (∼1 wt%), Mn (∼200 ppm), Cr (∼200 ppm), Co (∼50 ppm), Ni (∼1000 ppm) and Zn (∼70 ppm), and low Na2O (∼1 wt%) and P2O5 (∼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. We interpret 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 Na2O and P2O5 contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume.
