Multi-scale analyses of magnetic carriers in the Jack Hills metaconglomerates, Western Australia: Further evidence for a primary magnetization

Abstract:

Metaconglomerates from the Jack Hills, Western Australia, host zircons up to approximately 4.4 billion years old; these zircons are targets for single crystal paleointensity analyses (Tarduno et al. 2006, 2014, 2015). Cobble-sized quartz clasts from the metaconglomerate contain a high unblocking temperature component of magnetization that is resolvable only with the most sensitive DC SQUID magnetometers optimized for low intensity measurements. The high unblocking component passes a conglomerate test, overprinted by a complex series of low unblocking temperature magnetizations (Tarduno and Cottrell, 2013). Here we investigate the physical, chemical and magnetic properties of magnetic minerals within the cobbles to better understand their origin and the nature of their remanence records. We first use reflected light microscopy, scanning electron microscopy, and energy dispersive spectroscopy to study magnetic grain morphology and basic chemical composition. Our data detail a suite of magnetic minerals that match the unblocking temperature interpretations of Tarduno and Cottrell, 2013. These data also show as false the claim of Weiss et al., 2015 that high unblocking temperature phases such as magnetite are exclusively secondary products of laboratory heating. Magnetic hysteresis measurements on the cobbles are a challenge because the material is dominantly quartz. Within the context of careful calibration measurements with high sensitivity probes, we next discuss magnetic hysteresis and First Order Reversal Curve (FORC) analyses on samples ranging in size from submillimeter to several millimeters and compare these within a cobble, and between cobbles. Finally, given the background of our microscopy, spectroscopy, and magnetic hysteresis results, we evaluate hypotheses for the origin of the low unblocking temperature components of magnetization. Overall, our new data adds further support for the preservation of primary magnetizations in some Jack Hills sedimentary rocks.