Rochechouart impact crater melt breccias record no geomagnetic field reversal

Impact melt breccias from the Rochechouart (France) meteorite crater possess paleomagnetic directions with both normal and reverse polarities, raising the question whether shock from the collision initiated a geomagnetic field reversal. Stepwise thermal demagnetization together with a suite of rock magnetic experiments, optical microscopy, Raman spectroscopy and electron microprobe analyses identify adjacent, multiphase, titanohematite in the samples containing normal and mixed polarities typical of lithologies bearing self-reversal behavior stemming from magnetic exchange interaction. Melt breccias possessing mostly titanium-free hematite as the magnetic remanence carrier have solely reverse-polarity directions, leading to the conclusion that the Rochechouart meteorite impact did not spawn a geomagnetic event. Samples displaying paleomagnetic directions with normal or mixed polarity yield unreliable paleointensity data. 30 samples with reverse polarity that pass stringent data selection criteria from Thellier-Thellier paleointensity experiments with alteration, tail and additivity checks define an average field value of 12.8 +/- 3.7 mu T. This translates into a virtual dipole moment of 2.7 +/- 0.8 x 10(22) A m(2), which is relatively low, but within uncertainty of other Mesozoic paleointensity data. (C) 2013 Elsevier B.V. All rights reserved.