Absolute Paleointensity Estimates from Precambrian India and the Long-Term Thermal Evolution of the Earth

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Summary Despite significant progress in paleomagnetic research over the last century, the origin, evolution, and long-term behavior of the geomagnetic field remains poorly understood. One significant open question is when and how the inner core nucleated. Since geomagnetic field behavior is intrinsically linked to the thermal evolution of the core, scientists have turned to the global paleointensity record to search for proxies for inner core nucleation. From this record, two signals have been identified as possible indicators of inner core nucleation: (1) A spike in magnetic field strength between 1.5–1.0 Ga, and (2) an initially strong, but gradually decreasing field strength that resulted in a weak dynamo in the Ediacaran. Although both these hypotheses are vastly different, they do have one common challenge hindering rigorous testing: A paucity of paleointensity data. This is especially true for the Precambrian time period for which well-preserved outcrops are scarce and weathering/alteration is nearly inescapable. Despite making up almost 90% of Earth's history, data from this super eon comprises <10% of the global paleointensity database. This lack of data for most of Earth's history represents a considerable gap in our knowledge and greatly impedes our ability to understand the origin and evolution of our planet and its magnetic field. In an effort to fill in this gap, we performed paleointensity experiments on Precambrian-aged mafic dikes from India (Malani Igneous Suite and Bastar, Dharwar, and Bundelkhand cratons) with ages ranging from ∼740 Ma to ∼2.36 Ga. To monitor thermal alteration and minimize the effects of non-ideal grain sizes, the Thellier method following the IZZI protocol was used. Successful results were obtained for samples from the Bundelkhand (∼740 Ma) and Bastar (∼1.89 Ga) cratons. The Bastar results fall in a ∼40 Myr gap in the database and corroborate field trends predicted by the Monte Carlo axial dipole moment model (MCADAM), which suggests that intensity values were moderately low (2–4 × 1022 Am2) in the middle Paleoproterozoic. The Bundelkhand result suggests that the field may have been rapidly decaying in the late Tonian to early Cryogenian.