HADEAN TO PALEOARCHEAN GEODYNAMICS DURING THE EMERGENCE OF LIFE DEFINED BY ROBUST PALEOMAGNETIC DATA

Deciphering the earliest paleomagnetic record is challenging because of ubiquitous metamorphism and deformation of Earth’s oldest rocks. A key to this endeavor is (i) finding natural magnetic recorders with a minimum of reactive iron that could form magnetite during sub-amphibolite grade metamorphism, as well as (ii) having primary magnetic grains dominantly in the single domain or single vortex state (versus multidomain state) that are able to retain magnetizations on billion-year timescales. Paleomagnetic tests, including those on a micro-scale that probe magnetizations that satisfy Maxwell-Boltzmann statistic limits for paleomagnetic recording, should support a primary remanence, and should include the identification of both (iii) primary and secondary (overprint) magnetizations. Paleomagnetic and paleointensity data from select relatively iron-poor single silicate crystals can meet these requirements. Here, we first use new data to examine magnetization from Paleoarchean rocks of the Pilbara and Eoarchean rocks of Greenland. We find that bulk rock analyses from the Pilbara reflect a widespread and prolonged remagnetization at ca. 2.7 Ga (failing criterion iii) and are unsuitable for testing plate motion. Similarly, we find that iron-rich remagnetized banded iron formation samples from Greenland have multiple generations of magnetite dominantly multidomain in size, failing criteria i, ii and iii, and are questionable recorders of paleolatitude and paleointensity. In contrast, paleomagnetic data from oriented single silicates (feldspar and quartz) meeting the criteria for robust recording from South Africa provide evidence for sluggish plate motion at 3.2 Ga. Moreover, paleointensity data from Eoarchean-Paleoarchean (ca. 3.9-3.4 Ga) zircons from both South Africa and Australia are consistent, and when properly analyzed provide evidence for a subdued geodynamo and latitude stasis consistent with stagnant lid tectonics. The combined data from South African and Australian sites suggest modern plate tectonics could have commenced between ca. 3.4 and 3.2 Ga . These analyses further highlight that life originated and was sustained on Earth without modern plate tectonics.