Did Impact-Generated Fields Magnetize the Rocks of Vredefort?

点击翻译按钮获取中文摘要

A key unresolved question in planetary science is whether the magnetization observed in meteorites and lunar rocks was generated internally in the metallic cores of the parent bodies or was instead produced by an external source. In particular, the discovery of large crustal magnetic fields associated with craters on the Moon has led to the speculation that large impacts could generate magnetic fields. Subsequent experimental studies have shown that strong magnetic fields are produced by electric currents in plasma clouds. If we wish to use extraterrestrial samples to learn about internally generated fields and the evolution of metallic cores on other bodies, it is essential that we determine whether impact-magnetization is a common process in the solar system.One of the best ways to test whether impacts can generate fields is to examine rocks from impact craters on Earth. Members of the team have recently hypothesized that magnetization in the rocks of the 2.02 billion year old Vredefort crater in South Africa also originated from impact-produced plasmas (Carporzen et al. 2005). They found that magnetization in these rocks is extremely intense and heterogeneously oriented on spatial scales ranging from tens of kilometers down to less than a few centimeters. They will address three important unanswered questions:Question 1. Were magnetic fields generated by the Vredefort impact and if so what were their intensities?Question 2. What was the characteristic length scales of any impact-generated magnetic fields?Question 3. What is the dominant magnetic mineral that carries impact-generated magnetization?The answers to these questions have critical implications for the interior structures, bulk compositions, and thermal histories of the terrestrial planets.The investigators plan to drill four 10-m deep cores into the basement rocks and sediments of the Vredefort impact structure in order to obtain unaltered samples that are likely to record the impact event. Because these samples should be free of lightning remagnetization, the PIs can test a persistent criticism that magnetization in Vredefort rocks is not from the impact but rather from recent lightning strikes. Analysis of the drill cores will be conducted in the MIT Paleomagnetism Laboratory using a 2G Superconducting Rock Magnetometer (spatial resolution of ~10 mm) and the SQUID Microscope (spatial resolution of 0.1 mm). The broader impact of this grant is that it would support the postdoctoral training of Laurent Carporzen at MIT as well as fieldwork in South Africa with MIT graduate students and South African geologists.