Application of 3D Raman Mapping on Complex Inclusions: A Case Study of the Styrian Basin Mantle Xenoliths (W-Carpathian Pannonian Region)

Abstract

Fluid and melt inclusions trapped in mantle xenoliths provide direct insights into the metasomatic agent in the lithospheric mantle, including its volatile content. We conducted 3D Raman mapping on fluid and melt inclusions in modally metasomatized mantle xenoliths from the Styrian Basin (W-Carpathian Pannonian Region) to explore how this method can be utilized to study the role of fluids and melts in the upper mantle. 3D Raman mapping revealed complex phase assemblages of coexisting fluid and solid phases in the inclusions. Fluid phases are CO₂ (49.2–98.4 mol%, 19.1–61.0 vol%) and H₂O (1.5–50.8 mol%, 8.6–35.3 vol%). This H₂O concentration range is considerably higher than in most mantle fluids (∼10–15 mol%). Solid phases are silicates, carbonates, sulfides, and sulfates present in varying volume% (vol%). 3D Raman mapping shows that liquid H₂O wets other phases in the mapped fluid inclusions and may be preferentially lost compared to the CO₂-rich phase during inclusion decrepitation. The accuracy of CO₂-H₂O mol ratios from Raman 3D mapping in fluid inclusions can be affected by variable Raman cross-sections of trapped phases. Therefore, thermodynamic modeling is recommended to validate measured CO₂-H₂O mol ratios. 3D Raman mapping may underestimate low Raman scatterers like silicate glass in fluid inclusions, but their volumes can be corrected based on FIB-SEM analyses. Thermodynamic modeling suggests that the fluid compositions in the Raman-mapped fluid inclusions may reflect non-equilibrium entrapment, whereas those in the melt inclusions reflect equilibrium entrapment in this mantle portion. The discovered C-O-H fluids provide new information on fluid-rock reactions in the lithospheric mantle.