Analyzing Calcites With In-Situ LA-ICP-MS to Obtain an Age for the McArras Brook Formation

Abstract

The McArras Brook Formation is located on the Northwest coast of Nova Scotia, Canada, consists of interlayered red-bed sedimentary rocks, and amygdaloidal basalt. This formation plays a part in Dalhousie University’s intermediate field school course each Summer, however other than this it has not been often studied. The formation has been placed stratigraphically in the early Devonian period, but no absolute (numerical) age is known, nor are there absolute ages known for surrounding units. The amygdaloidal basalts in the unit contain calcite, forming the amygdules, which has been dating using U-Pb techniques previously for both eruption/volcanism and tectonic events for basalt. The basalts in this unit have also been characterized previously in a Dalhousie project, and this project aims to continue work done on this unit. This project uses Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) to analyze over thirty isotopes in-situ on calcite samples taken from the McArras Brook basalt. Data acquired from analysis is used to image isotopic distribution on sampled locations, to create temperature maps of sampled locations, and ultimately to plot U and Pb ratios to obtain an absolute age for the unit. Keywords: Calcite dating; Carbonate dating; LA-ICP-MS; U-Pb geochronology; McArras Brook Formation