Archean basement in the Lonar impact breccia, Deccan Volcanic Province: Implications for depth of excavation and mixing of target rocks in simple craters

Details Archean basement in the Lonar impact breccia, Deccan Volcanic Province: Implications for depth of excavation and mixing of target rocks in simple craters Archean basement in the Lonar impact breccia, Deccan Volcanic Province: Implications for depth of excavation and mixing of target rocks in simple craters

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr51b0972c&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #MR51B-0972

Other

1040 Radiogenic Isotope Geochemistry, 1065 Major And Trace Element Geochemistry, 6022 Impact Phenomena (5420, 8136)

Scientific paper

The Lonar impact crater in the Deccan Traps of the Indian peninsula provides unique opportunities to study physical and chemical processes of impact cratering on basaltic targets, because terrestrial impact craters on basalts are extremely rare. Such studies may have implications for similar cratering processes in the Moon, Mars and other basaltic targets in the solar system. Considering some of these objectives, we analyzed trace elements and Nd, Sr, Pb- isotopes of impact breccia and target basalts from the Lonar crater. Chondrite-normalized rare earth element (REE) patterns in the target basalts and impact breccia show similar light REE-enriched patterns, although in detail, the breccia are more fractionated in La/Sm compared to the target basalts. The breccia are also significantly enriched in Rb, Ba and Pb, and to a lesser extent in Th and U, compared to the target rocks and show more radiogenic Sr, less radiogenic Nd and higher Rb/Sr and lower Sm/Nd ratios, indicating that an additional component, other than the target basalt was involved in the formation of the breccia. The Deccan traps are underlain by Archean to mid-Proterozoic cratonic rocks. The overall geochemical signatures of the impact breccia are identical for both in situ and non-in situ samples. Specifically their negative υNd values, radiogenic Sr isotopic composition as well as high 207Pb/204Pb at low 206Pb/204Pb indicate that a major component of the Lonar impact breccia was derived from melting of the Archean basement. However, no fragments of the Archean basement rocks have been found in the Lonar impact breccia. Archean geochemical signatures in the impact breccia cannot be explained by the incorporation of paleosols that are weathering products of the target basalts, or inter-trappean sediments that are most commonly cherts and limestones of Mesozoic age. Similarly, the possible role of eolian sediments in causing the Archean Pb-isotopic signature, identical to those of the Deccan basement, in the breccia rocks can be excluded. We suggest that spherules in the breccia, which formed from impact melting of the target rocks, are possibly the carrier phase of the Archean geochemical and isotopic signatures in these breccia rocks. Theoretical models predict that for simple craters, the depth of the crater is almost 0.33 times the diameter of the transient crater (1.2 km for Lonar), which in turn is almost 0.64 times the final crater diameter (1.8 km for Lonar). Based on these relations, the depth of the transient crater at Lonar should be ~400m which is similar to the estimated thickness of the basaltic target rocks of the Deccan Traps at Lonar. It is however important to know the depth of melting and excavation during simple cratering. Based on theoretical modeling, it has been recently suggested that only the top one-third of the transient crater (135m in case of the Lonar crater) might actually be excavated. This estimated depth of excavation (135m) is much lower than the estimated thickness (400m) of the host basalts at Lonar. Thus the theoretical data are in contrast with our interpretation based on geochemical evidence that the Archean basement signatures observed in the Lonar breccia spherules must be derived from depths greater than 135m. It is possible that the thickness of the Deccan basalts at Lonar crater is over-estimated. We suggest that the dynamics of crater formation, especially when compact target rocks of distinctly different compositions are involved, such as basalts underlain by granites, are more complex that those estimated by available theoretical models.

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