Methane in Fluid Inclusions in Ophiolitic Chromitites Revealed by Raman Spectroscopy: Preliminary Results

Zaccarini, Federica and Baloghné Kiss, Gabriella and Garuti, Giorgio and Mauro, Daniela and Economou-Eliopoulos, Maria and Hegedűs, Máté and Biagioni, Cristian (2025) Methane in Fluid Inclusions in Ophiolitic Chromitites Revealed by Raman Spectroscopy: Preliminary Results. MINERALS, 15 (4). ISSN 2075-163X

Preview

Text Zaccarini_etal2025_minerals-15-00335.pdf - Published Version Available under License Creative Commons Attribution. Download (9MB) | Preview

Abstract

This contribution provides a petrographic and Raman investigation of fluid inclusions found in chromitites collected in the ophiolites of Santa Elena (Costa Rica), Bracco (Italy), Otrhys and Vourinos (Greece), and Troodos (Cyprus). Most of the analyzed chromites are classified as high-Cr, with the exception of those from Bracco and some of the Othrys complexes that are high-Al. Although the investigation of fluid inclusions in chromitites is very challenging due to the poor transparency of the host chromite, the studied samples contain numerous fluid inclusions. The fluid inclusions look to be more abundant in the high-Cr chromitites, related to a subduction zone environment, compared to the high-Al chromitites generated in a mid-ocean ridge. This is in agreement with the petrogenetic model for the formation of podiform chromitites that implies the presence of a metasomatic event caused by hydrous fluids that reacted pervasively with variable depleted mantle tectonites, especially in the subduction zone setting. The fluid inclusions, between 1 and 15 µm in size, show negative crystal or irregular angular shapes. They occur when enclosed in chromite crystals that have not been affected by low-temperature processes. The fluid inclusions consist of liquid (L), vapour(V~30–50 area%) and L + V (V~40–60 area% rarely 10–80 area%). The fluid inclusions may contain only vapour and a vapour and a solid phase, too. The Raman spectra reveal the presence of CH4 in certain fluid inclusions. Considering the high number of fluid inclusions that potentially contain CH4, we suggest that the fluid inclusions in the chromite crystals and their leaching can be a possible source in order to explain the high amount of CH4 detected in some podiform chromitites, previously attributed to the Sabatier reaction. The mode of the occurrences of the studied CH4 bearing fluid inclusions, i.e., entrapped in unaltered chromite crystals formed at a magmatic temperature, suggest their abiotic origin from mantle-derived fluids, rather than those related to the low-temperature serpentinization processes. The investigation of fluid inclusions, although it is difficult and challenging or even impossible when the chromite is too opaque, can be applicable to other chromitites worldwide to verify the presence of H2O, CH4 or other gases. This information will greatly improve our understanding of the nature of the fluid phases during the formation of podiform chromitites.

Actions (login required)

Edit Item