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

摘要

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 5 Ma (95% confidence). Vector analyses results of the step-wise alternating field demagnetization (NRM up to 100 mT) and thermal demagnetization (from NRM up to 650 degrees C) yield three components of magnetization. Curie point determinations indicate three characteristic temperatures, one at 150-200 degrees C, a second one at 450 degrees C and a third one at 580 degrees 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 degrees C and up to at least 580 degrees 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 degrees 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 similar to 6.4 +/- 0.68 (N = 11) micro-Teslas with a Virtual Dipole Moment (VDM) of 1.51 +/- 0.81 x 10(22) Am-2, from a medium-to high -temperature component ranging from 300 to 590 degrees 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. (C) 2016 Elsevier B.V. All rights reserved.