Discovery of moissanite in a peralkaline syenite from the Azores Islands

Received 6 August 2018 Accepted 30 October 2018 Available online 03 November 2018 Keywords: Moissanite Alkaline magmas Oceanic crust Mantle Azores oxygen fugacity carbon cycle. 1. Introduction Since its discovery in 1893, moissanite (SiC) was recovered in an in- creasing number of terrestrial tectonic settings: as inclusions in diamonds (Klein-Bendavid et al., 2007), in kimberlites (Leung et al., 1990; Marshintsev et al., 1967; Mathez et al., 1995; Shiryaev et al., 2011), in ophiolites (Trumbull et al., 2009), in granulites and metamor- phic rocks (Janák et al., 2015; Machev et al., 2018; Perraki and Faryad, 2014), and in volcanic breccias (Di Pierro et al., 2003; Dobrzhinetskaya et al., 2018). However, despite the increased findings from continental crust sources, moissanite has never been reported from present-day oceanic geodynamic settings. Doubts are often raised on the natural origin of SiC grains because they are commonly recovered in heavy fractions after separation, and only rarely in situ (Di Pierro et al., 2003; Dobrzhinetskaya et al., 2018; Machev et al., 2018). Nevertheless, both the morphology and the occur- rence/nature of inclusions inside moissanite usually support a natural origin. The presence of moissanite in such different rocks opened an intense debate to explain its origin (Di Pierro et al., 2003; Dobrzhinetskaya et al., 2018; Griffin et al., 2016; Machev et al., 2018; Shiryaev et al., 2011), ⁎ Correspondingauthor. E-mail address: sabrina.nazzareni@unipg.it (S. Nazzareni). https://doi.org/10.1016/j.lithos.2018.10.036 0024-4937/© 2018 Published by Elsevier B.V. abstract Our discovery of moissanite grains in a peralkaline syenite from the Água de Pau Volcano (São Miguel, Azores Islands, Portugal) represents the first report of this mineral in present day oceanic geodynamic settings. Raman spectroscopy and single-crystal X-ray diffraction show the presence of both the 6H and 4H polytypes with the predominance of the first one. The distribution of trace elements is homogeneous, except for Al and V. Azorean moissanite often hosts rounded inclusions of metallic Si and other not yet identified metallic alloys. A process involving a flushing of CH4-H2 ultra-reducing fluids in the alkaline melts might be considered as a possible mech- anism leading to the formation of natural SiC, thus calling for strongly reducing conditions that were locally met in the crust-mantle beneath the São Miguel Island.