Micro-conglomerate tests and the Hadean to Paleoarchean geodynamo as recorded in zircons of the Jack Hills

Abstract:

The Jack Hills (JH) belt of Western Australia has tremendous potential for increasing our understanding of the ancient geomagnetic field through application of the single silicate crystal paleointensity (SCP) method (e.g. Tarduno et al., 2006). We have recently reported SCP data from JH zircons (Tarduno et al., 2013, 2015) that suggest the presence of a core dynamo > 750 million years earlier than prior estimates (to as old as 4.2 Ga). Here we elaborate on these measurements with an emphasis on our microconglomerate test. The JH belt has been metamorphosed to at least 475 °C, but the interiors of some cobble-sized clasts preserve regions with a magnetite-dominated magnetization that passes a conglomerate test after demagnetization to temperatures of 550 °C (Tarduno and Cottrell, 2013). High unblocking temperature components of magnetization are also suggested in the data of Weiss et al. (2015), but these data are extremely noisy. This leads us to believe that the Weiss et al. (2015) data lack the resolution to examine Paleoarchean to Hadean magnetizations. Locally, components of the JH sediments have the potential to preserve high unblocking temperature magnetizations that can be characterized with the SCP method. As a further step to assess the ancient recording fidelity of the JH meta-conglomerates, we have conducted a micro-conglomerate test on oriented small (~500 to 800 μm) samples, each centered on a single large (200-300 μm) zircon. We use an ultra-high resolution 3-component DC SQUID magnetometer that affords an order of magnitude greater sensitivity than other high-resolution SQUID rock magnetometers. Thermal demagnetization using a CO₂ laser shows unblocking between 565 and 580 °C consistent with a magnetite carrier. The characteristic magnetization directions are well defined but together cannot be distinguished from those drawn from a random distribution. Importantly, this positive micro-conglomerate test also addresses the debate over the primary nature of inclusions within JH zircons. We will also discuss our ongoing efforts to further define the geodynamo history as recorded by JH zircons.