Origin and implications of two Verwey transitions in the basement rocks of the Vredefort meteorite crater, South Africa

摘要

Two populations of magnetite exist in the shocked basement rocks of the Vredefort meteorite impact crater: one associated with original crustal genesis and metamorphism 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 mu m in size and formed within the interstices of planar deformation features or within the reaction rims of biotite, both of which were created during impact. Our study shows that each of these populations possesses specific Verwey transition temperatures: one around 124 K associated with pre-impact magnetite and the other around 102 K associated with impact-related magnetite. The high temperature Verwey transition is attributed to stoichiometric magnetite while the low temperature Verwey transition to non-stoichiometric magnetite. Pre-impact rocks containing both Verwey transitions are ubiquitous throughout the crater. Pseudotachylites formed during impact have a single Verwey transition spanning temperatures from 94 to 111 K. Heating the basement rocks above similar to 550-600 degrees C for 3 min or above similar to 500 degrees C for 1 h irreversibly modifies the 124 K Verwey transition by shifting it to lower temperatures. Based on these findings, it is possible that no wholesale heating of the crater occurred above 550-600 degrees C for 3 min or above 500 degrees C for 1 h 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 exist in the crust during and after impact. (c) 2006 Elsevier B.V. All rights reserved.