Detrital orthopyroxene as a tracer of geodynamic setting:: A Raman and SEM-EDS provenance study

Raman spectroscopy is a powerful and efficient tool that is being used more and more frequently in high-resolution provenance analysis. This study is dedicated to Ca-poor orthorhombic pyroxene, which for its low durability during diagenesis can be considered as a diagnostic first-cycle phase. We focused on 15 samples of modern sands representing a full range of rock units exposed at different structural levels of continental, arc, or oceanic lithosphere and generated in diverse geodynamic settings. By coupling optical observations, SEM-EDS mineral-chemistry and Raman spectroscopy analyses on the same grains we show how orthopyroxene from mantle, deep crustal, and volcanic rocks ranging in composition from andesite to dacite and felsic differentiates can be robustly distinguished. The Mg# [Mg/(Mg + Fe)] content of orthopyroxene grains can be accurately estimated by recording the characteristic position of their six main characteristic peaks (ν1 to ν6, vibrational modes) even during routine point-counting under the Raman spectroscope. In particular, the position of the strong and narrow ν3 peak (~655 cm−1) also allows estimation of the Ca content when compared to the position of other peaks (especially ν1, ~330 cm−1). High-Mg orthopyroxene is exclusively derived from mantle rocks, whereas metagabbros of the lower continental crust and gabbroic rocks of arc crust are distinguished by their lower Ca. The lowest Mg# was recorded in detrital orthopyroxene derived from the silicic Amiata Volcano in central Italy, whereas trachytes in rift-related settings did not show unique characteristics apart from their slightly higher Ca content.