Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr54a..03c&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #MR54A-03
Mathematics
Logic
1519 Magnetic Mineralogy And Petrology, 1527 Paleomagnetism Applied To Geologic Processes, 1540 Rock And Mineral Magnetism, 3662 Meteorite Mineralogy And Petrology (1028, 6240), 5420 Impact Phenomena, Cratering (6022, 8136)
Scientific paper
Two populations of magnetite exist in the shocked basement rocks of the Vredefort meteorite impact crater: one associated with original crustal genesis and metamophism around 3.0 Ga, and the other related to the impact itself at 2.02 Ga. Pre-impact magnetite is mostly micron to millimeter in size, lying within the multidomain to pseudo-single domain range. The second population of magnetite is less than 10 microns in size and formed within the interstices of planar deformation features that were created during impact. Our study shows that each magnetite population in the Vredefort basement rocks possess specific Verwey transition (T_v) temperatures: one around 124 K associated with pre-impact magnetite and the other around 102 K associated with impact-related magnetite. The Verwey transition (T_v) in magnetite occurs due to a crystallographic phase transition at low temperatures. The T_v of Fe-pure, stoichiometric magnetite is 124 K, whereas non-stoichiometry or cation (Ti, Al, Zn, etc.) substitution decrease the T_v temperature. Pressure appears to decrease the T_v temperature by about -3 K/GPa although recent experiments have noted that a coordination crossover occurs under pressure, resulting in an increase of magnetite's isotropic point by more than 10 K/GPa. We attribute the high temperature T_v in the Vredefort rocks to stoichiometric magnetite while the low temperature T_v to non-stoichiometric magnetite. Pre-impact rocks containing both T_v are ubiquitous throughout the crater. Pseudotachylites formed during impact have a single T_v spanning temperatures from 94 to 111 K. Heating the basement rocks above ~ 550-600 °C for three minutes or above ~ 500 °C for one hour irreversibly modifies the 124 K T_v by shifting it to lower temperatures. These results have important implications for modeling the thermal consequences of cratering. Our findings raise the possibility that no wholesale heating of the crater occurred above 550-600 °C for three minutes or above 500 °C for one hour during or since the time of impact, although some places of more localized heating are identified. An unresolved problem remains to reconcile these data with temperatures thought to persist in the crust during and after impact.
Carporzen Laurent
Gilder Stuart A.
Hart Rodger J.
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