Geomagnetic Paleointensity As Recorded By the ~1.1 Ga Baraga-Marquette Dike Swarm (Michigan, USA

Abstract:

Paleointensity data from Precambrian rocks are of great importance for understanding the evolution of early geodynamo. However the Precambrian paleointensity database remains very limited. We present new rock magnetic and paleointensity results from 35 diabase dikes of the ~1.1 Ga Baraga-Marquette swarm (Michigan, USA) which belongs to the North American Mid-Continent Rift (MCR) system. All the studied dikes yielded a stable characteristic remanent magnetization component of normal or reversed polarity. Our rock magnetic experiments (including thermomagnetic, magnetic hysteresis, and first-order reversal curve analyses) indicate predominantly single-domain to pseudo-single-domain magnetite or low-Ti titanomagnetite as the principal magnetic carrier in these rocks. Minor amounts of hematite, maghemite, or pyrrhotite were also found in some dikes. Reversible thermomagnetic curves indicate no or negligible heating-induced alteration. Most of the dikes are analyzed using both LTD-Thellier and Pseudo-Thellier paleointensity methods. We will discuss the implications of our paleointensity results for understanding the state of geodynamo in the Mesoproterozoic, during the time of MCR formation.