Two distinct Verwey transitions in the shocked rocks from the Vredefort meteorite impact crater

Details Two distinct Verwey transitions in the shocked rocks from the Vredefort meteorite impact crater Two distinct Verwey transitions in the shocked rocks from the Vredefort meteorite impact crater

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmgp13a0042c&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #GP13A-0042

Mathematics

Logic

1518 Magnetic Fabrics And Anisotropy, 1519 Magnetic Mineralogy And Petrology, 1527 Paleomagnetism Applied To Geologic Processes, 1540 Rock And Mineral Magnetism

Scientific paper

The Vredefort meteorite impact crater, among the oldest (2.02 Ga) and largest (original crater diameter ~300 km) impact structures on Earth, has ~1000 km2 of well-exposed Archean crystalline basement that contains original Archean multidomain magnetite as well as new single domain magnetite that crystallized during the impact event. To better understand the magnetic mineralogy of the Vredefort rocks reported in Carporzen et al. (Nature 435, 198 - 201, 2005), we measured the magnetizations of 47 specimens during warming between 10 and 300 K after zero field cooling (ZFC) using an MPMS. We identified two distinct Verwey transitions (Tv) in the shocked rocks, one at 102.8±3.6 K and another at 124±2.9 K (IRM Quarterly 14(1), 3, 2004). We then crushed the same samples, and then separated the magnetite by sieving and hand picking it into representative size fractions of >100μm and <10μm. While it is possible the small size fraction would be contaminated with some of the pulverized material of the original multidomain magnetite, it is very unlikely the large-size fraction would be contaminated. We performed MPMS experiments on both size fractions from several different samples. For the small fraction, we still observe the two Tv but for the large grains size, we identified only the high temperature Tv. We conclude the multidomain magnetite contributes to the " high " temperature Tv and the single domain magnetite contributes to the " low " temperature Tv. After heating whole rock samples to 700°C and again repeating the MPMS experiments, the low temperature behavior is modified, creating a single Tv whose temperature is intermediate between the two. This leads to the preliminary interpretation that the Vredefort rocks were never heated above 700°C during or after the impact event.

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