Accurate electron probe microanalysis of key petrogenetic minor and trace elements in Cr-spinel

Abstract

The trace element composition of Cr-spinel is paramount for interpreting the petrogenesis of a large group of mafic to ultramafic rocks. Although laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has proven to be very useful for the determination of trace element abundances of Cr-spinel, the characterization of Cr-spinel grains that are inhomogeneous over micrometer length scales requires the use of techniques that provide a better spatial resolution than LA-ICP-MS. In this work, we develop a protocol for the determination of trace and minor elements in Cr-spinel by electron probe microanalysis (EPMA) using the software provided by the manufacturer. The optimized analytical conditions (25 kV accelerating voltage, 900 nA beam current, 60–480 s peak counting times, aggregate spectrometer data) allowed us to achieve detection limits (3σ) in the range 4–26 ppm with relative analytical precisions (2σ) in the range 1–3 % for all analyzed elements (Ti, V, Mn, Co, Ni, and Zn), except for Sc and Ga, for which the precision was much lower (36–46 %). The developed methodology was applied to the analysis of Cr-spinel grains from the Mayarí-Baracoa Ophiolitic Belt (eastern Cuba), in correspondence with previous LA-ICP-MS analyses, which were used to assess the reliability of the EPMA results. The root mean square percentage deviation (RMS) between the EPMA and LA-ICP-MS data was in the range 4.7–22.5 % for Ti, V, Mn, Co, Ni, and Zn, while for Ga the RMS value was 32.5 %. For Sc, the RMS value was much higher (171 %), despite the low detection limit achieved (4 ppm) for this element. Our results indicate that the trace element composition of primary Cr-spinel obtained by EPMA can be readily used, except for Sc, to constrain petrogenetic information on chromitites in a reliable way.