Application of laser ablation-ICP-mass spectrometry for 2-dimensional mapping of element distributions in a Late Archean impact spherule layer

摘要

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has been successfully applied to visualize the 2-dimensional distribution of various major (Mg, Mn, Fe) and trace (Cr, Co, Ni, Ir, Pt) elements in the Late Archaean Paraburdoo impact spherule layer (PB SL). As this spherule layer is highly enriched in meteoritic material, this study particularly focuses on the distribution of siderophile elements. Multi-elemental maps with a lateral resolution of 15 μm, covering surface areas of approximately 1.5 mm × 1.5 mm and multiple impact spherules, were acquired by LA-ICP-MS using two different ablation -standard (circular) and teardrop - cells. It was observed that with the teardrop cell, the elemental maps follow the mineralogy as displayed by the optical microscopy image more closely, with limited memory and washout effects in comparison to the standard cell. In order to acquire representative quantitative data, 100 μrn diameter spot LA-ICP-MS analysis in a vertical transect through the SL was performed additionally. Quantification of element concentrations in the PB SL was carried out using external calibration procedures. The validity of this approach was evaluated by comparison of the averages of the determined transect spot concentrations to bulk rock geochemical data and it was found that the mean concentrations fall within the uncertainty of the bulk geochemical analysis, but the results show biases of 5.9% for Pt, 16% for Ir, 11 % for Cr and 38% for Ni. In addition, the bulk rock Pt/Ir elemental ratio obtained upon transect LA-ICP-MS analysis was reported and it was observed that the slope of the regression line (1.06) and the mean value (1.23) for all data points measured with LA-ICP-MS equal the bulk rock Pt/Ir value of 1.08-1.43 measured by fire assay/solution ICP-MS for all PB SL outcrops at the Governor site.