Application of the 190Pt-186Os isotope decay system to dating platinum-group minerals

Abstract

Constraining the formation age of platinum-group minerals (PGM) in placer deposits has traditionally been very challenging. The 190Pt-186Os decay system, measured by laser ablation multi-collector ICPMS (LA-MC-ICPMS), has been demonstrated as a useful geochronometric tool for detrital PGM grains. However, it is difficult to appraise the method accuracy using detrital PGM. In this thesis two examples are presented that verify the accuracy and precision of the technique in geologically well constrained situations.
The age of the Bushveld Complex has been measured using various radioisotope chronometers which yield ages ranging from 1921 – 2165 Ma, although an age of 2054 Ma is widely accepted. We present an accurate isochron age of 2012 ± 47 Ma (2σ, n = 50, MSWD = 1.19) for in situ Bushveld PGM, demonstrating the validity of multi-grain Pt-Os ages. Precision on the Bushveld LA-MC-ICPMS Pt-Os age is comparable to the Re-Os and Rb-Sr ages but data acquisition is easier and less time consuming. Scatter on the isochron may result from analytical artefacts or from post-crystallisation disturbance of the Pt-Os system within Pt-rich PGM grains via exsolution or deformation followed by incomplete sampling of the resulting within-grain heterogeneities.
Placer PGM derived from the Meratus ophiolite, southeast Borneo yield a precise Pt-Os isochron age of 197.8 ± 8.1 Ma (2σ, n = 260, MSWD = 0.90). This is consistent with published age constraints and provides the first direct date for the igneous rocks of the Meratus ophiolite, highlighting the utility of the Pt-Os geochronometer as a tool for dating ophiolites, layered intrusions and placer platinum mineralisation despite potential derivation of PGM from various isolated chromitites.
Analysis of large PGM populations has allowed improvement on published constraints for Pt/Os inter-element fractionation occurring during ablation analysis of PGE sulphides and alloys. This is estimated to amount to between 2 – 2.5 %.