Recovering Mesoproterozoic geomagnetic field intensity using anorthosite xenoliths hosted in Midcontinent Rift diabase

Abstract:

Recovering paleointensity records during the Precambrian is essential for understanding the long-term evolution of Earth’s core dynamics. Recent paleointensity results from Midcontinent Rift volcanics reveal a geomagnetic field strength at 1.1 Ga similar to today. Those results could be interpreted to be consistent with the hypothesis of an onset of inner core nucleation before this time in the Mesoproterozoic. However, these estimates contrast with other lower ones from the Meso- and Neoproterozoic that have been used to argue for waning power sources to the dynamo at the time and interpreted to reflect later inner core growth. The Midcontinent Rift contains abundant well-preserved intrusive rocks coeval with the volcanics. Together, these lithologies can provide many paleointensity estimates of geomagnetic field strength over a period of >20 Myr, improving constraints on the time-averaged field. In this study, we target anorthosite xenoliths that were emplaced within the Beaver River diabase ca. 1091 Ma. These units provide an opportunity to develop paleointensity estimates from two distinct lithologies that cooled together in the same ancient field. Given that silicate minerals such as plagioclase have the potential to protect magnetite from alteration, these anorthosites could be excellent material for recovering geomagnetic field intensity. We conducted IZZI Thellier paleointensity experiments on both diabase and anorthosite samples. Most diabase specimens exhibit sagging or zig-zagging Arai plots. In contrast, many anorthosite specimens produced straight Arai plots and most sites yielded consistent estimates amongst specimens. The cooling-rate-corrected anorthosite paleointensity estimates are similar to the strength of the modern geomagnetic field. The diabase specimens which passed selection criteria yielded similar results. Combined with existing data, these results demonstrate that Earth’s geomagnetic field strength was similar to modern field strength for a 20 million year interval of the late Mesoproterozoic.