A whole rock absolute paleointensity determination of dacites from the Duffer Formation (ca. 3.467 Ga) of the Pilbara Craton, Australia: An impossible task?

Abstract

We have conducted a whole-rock type magnetic and absolute paleointensity determination of the red dacite of the Duffer Formation from the Pilbara Craton, Australia. The age of the dated rock unit is 3467±5Ma (95% confidence). Vector analyses results of the step-wise alternating field demagnetization (NRM up to 100mT) and thermal demagnetization (from NRM up to 650°C) yield three components of magnetization. Curie point determinations indicate three characteristic temperatures, one at 150–200°C, a second one at ∼450°C and a third one at ∼580°C. Magnetic grain-size experiments were performed on small specimens with a variable field translation balance (VFTB). The coercivity of remanence (Hcr) suggests that the NRM is carried by low-coercivity grains that are associated with a magnetite fraction as is shown by the high-temperature component with blocking temperatures above 450°C and up to at least 580°C. The ratios of the hysteresis parameters plotted as a modified Day diagram show that most grain sizes are scattered within the Single Domain (SD) and the Superparamagnetic and Single Domain SP-SD domain ranges. In addition to the rock magnetic experiments we have performed absolute paleointensity experiments on the samples using the modified Thellier-Coe double heating method to determine the paleointensities. Partial-TRM (p-TRM) checks were performed systematically to document magnetomineralogical changes during heating. The temperature was incremented by steps of 50°C between room temperature and 590°C. The paleointensity determinations were obtained from the slope of Arai diagrams. Our paleointensity results indicate that the paleofield obtained was ∼6.4±0.68 (N=11) micro-Teslas with a Virtual Dipole Moment (VDM) of 1.51±0.81×1022Am2, from a medium-to high-temperature component ranging from 300 to 590°C that has been interpreted to be the oldest magnetization yet recorded in paleomagnetic studies of the Duffer Formation. The absolute paleointensity is relatively low and we interpret this low-paleofield bias a result of a thermochemical remanent magnetization (TCRM) process that indicates a possible underestimate of the paleofield by a factor of four for the red dacite of the Duffer Fm.