Nanoscale Os isotopic quantification of Wadi Tayin dunite platinum group minerals by atom probe tomography

Serpentinites record processes that redistribute major and trace elements between mantle and crust. Platinum Group Elements (PGEs) are trace elements in serpentinites hosted in sulphides and alloys. Alloys are challenging to find, and most analytical techniques lack the spatial resolution to analyse them. This research adopts automatic mineral mapping technique to detect PGM grains in a sample from the Wadi Tayin (Oman) peridotite and uses atom probe tomography, a nanoscale quantitative analytical technique, to analyse the grain. This work applies and assesses the applicability of atom probe tomography to measure the ¹⁸⁷Os/¹⁸⁸Os isotopic ratio of natural alloys and uses the ratio to constrain the source of Os. A novel algorithm is used to automatically determine the number of counts of the ¹⁸⁷Os and ¹⁸⁸Os peaks, to calculate the isotopic ¹⁸⁷Os/¹⁸⁸Os ratio and the analytical uncertainty. The ¹⁸⁷Os/¹⁸⁸Os ratio is 0.126 ± 0.003, consistent with the isotopic composition reported by literature in the dunite of the Main Mantle Section of the Wadi Tayin ophiolite.

The analytical uncertainty is one order of magnitude higher than conventional bulk rock techniques, such as negative-thermal ionisation mass spectrometry (N-TIMS) and inductively coupled plasma mass spectrometry (ICP-MS). However, the precision is sufficient to conclude that the non-radiogenic ¹⁸⁷Os/¹⁸⁸Os ratio is compatible with a mantle origin for the alloy. Decreasing whole-rock Re with increasing LOI and the overprinting of magmatic pentlandite by magnetite demonstrate that progressive serpentinisation may have modified the Re budget. The results indicate that atom probe tomography can analyse ¹⁸⁷Os/¹⁸⁸Os ratio quantitatively in micron-sized natural alloys and provide insights into natural processes.