Pushing Back the Record of Tectonic Motion & Geodynamo Reversals via Magnetic Imaging, Roger Fu
Channel: Magnetics Information Consortium (MagIC)
Date: 2023-03-16

"Pushing Back the Record of Tectonic Motion and Geodynamo Reversals using Magnetic Imaging" by Roger Fu

Talk presented at the 2023 Magnetics Information Consortium (MagIC) Workshop: Magnetism and Earth History: Field Evolution, Environmental Change and Paleogeography, Feb 28th-Mar 2nd, 2023. Convened at the Scripps Institute of Oceanography, UCSD in La Jolla, California and sponsored by the National Science Foundation (NSF).

Workshop website: https://earthref.org/events/MAGIC/2023/

Talk Abstract:

Roger R. Fu, Alec R. Brenner, Sarah C. Steele, Anna Mittelholz
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.

A magnetic core dynamo and mobile lid plate tectonics are two planetary-scale geophysicalprocesses that exert fundamental controls on the surface environment. A dynamo can promoteor prevent atmospheric loss to interplanetary space while plate tectonics shape surface chemistryand temperature by governing the input and erosion fluxes of biologically important lightelements such as C and P. Due to these effects, describing the onset and cessation of theseprocesses is key to understanding planetary habitability.We will present the results of studies on the martian meteorite ALH 84001 and 3.18-3.25 Ga terrestrial rocks from the Pilbara Craton. First, we find that 100 μm scale ferromagnetic assemblages in ALH 84001 are magnetized in two sets of directions approximately 140 ̊ apart (Fig. 1; Steele et al., in revision). Rock magnetic characterization of these chromite-sulfide assemblages indicates that they are capable of recording microtesla-level magnetic fields. Meanwhile, mesoscale impact modeling shows that shock events between 10 and 30 GPa can remagnetize 100 μm-scale sub-volumes of ALH 84001 material while only weakly heating neighboring regions.

Combining these observations, we conclude that the two magnetization directions record ancient martian magnetic fields during two distinct impact events. Given the history of ALH 84001, these events may have occurred at 4.1, 3.9, or 1.2 Ga, thereby requiring a strong surface magnetic field onMarsatorbefore3.9Ga. Further, our statistical analysis shows a reversing magnetic core dynamo is more likely to explain the large angular separation between the two directions. The persistence of a martian core dynamo until at least 3.9 Ga implies the presence of a global magnetic field during at least the first several hundred My of valley network formation on Mars. This magnetic field likely played an active role in controlling atmospheric composition during this hydrologically active portion of martian history, although its exact effect on atmospheric loss remains to be quantified.

Second, our paleomagnetic analyses of metavolcanic rocks from the 3.18 Ga Honeyeater basalts and 3.25 Ga Kunagunarrina Formation show that the Pilbara Craton underwent translation and rotation at rates greater than 0.55 ̊ per My during the 3.34-3.18 Ga interval (Brenner et al. 2020, 2022). The Euler pole positions for these motions are difficult to reconcile with true polar wander while the high minimum velocity is likely inconsistent with single-plate, net lithosphere rotation. We therefore conclude that mobile-lid plate tectonics, with its implications for global temperature regulation and light element cycling, were likely established on Earth by 3.25 Ga, which is earlier than inferred from some recent geochemical studies. Finally, Kunagunarrina Formation rocks record a geomagnetic reversal at 3.25 Ga, implying a relatively dipolar, core- generated geodynamo with implications for Earth's atmospheric loss history and the validity of Archean paleomagnetism.

Micrometer-scale magnetic field imaging played a critical role in both sets of studies described above. The high spatial resolution of the quantum diamond microscope (QDM) enabled net magnetic moment recovery from individual chromite-sulfide assemblages in ALH 84001 for the first time. Meanwhile, identification of the ferromagnetic sources with specific alteration mineral assemblages in the Honeyeater basalt and Kunagunarrina Formation metavolcanics shows with high confidence that the magnetization is a thermochemical remanent magnetization (TCRM) acquired during seafloor alteration. This observation then allowed direct dating of the magnetization using the associated secondary phases. These results therefore demonstrate the potential of the QDM for expanding the set of magnetic materials amenable to paleomagnetic analysis and for strengthening the interpretation of traditional samples by revealing the mineralogical basis of magnetization.