Evidence of amorphous carbon enclosed in hydrothermal gypsum deposits using combined LIBS and Raman spectroscopy. Geochemical implications in planetary exploration

This study presents a multi-analytical spectroscopic methodology designed to characterize solid micro-inclusions trapped within complex evaporitic matrices, focusing on selenite gypsum crystals (CaSO 4 ·2H 2 O) from the Mina Rica mine (Pulpí, Spain). Gypsum is regarded a high-priority a Martian analog, as recent findings by the Perseverance rover have confirmed the presence of calcium sulfates in various hydration states on the Martian surface. Furthermore, gypsum is a key target for geochemistry due to its high potential for material entrapment and long-term preservation (Wang et al., 2024 [1] ). The primary objective is to evaluate the synergistic capability of complementary techniques to identify and spatially resolve distinct co-existing phases within mineral interlayers. For the first time, carbonaceous material has been detected within these specific crystals using Laser-Induced Breakdown Spectroscopy (LIBS). High-resolution depth profiling and spatial rastering demonstrated the potential of LIBS to track carbon signals despite a highly heterogeneous distribution. Complementary micro-Raman spectroscopy resolved the structural nature of this carbon, identifying it as an amorphous, disordered phase preferentially localized along the mineral cleavage planes. Furthermore, Raman analysis successfully identified co-existing iron carbonate (siderite) and iron oxyhydroxide micro-granules. While the spatial overlap of these phases frequently induces spectral interferences, the integration of elemental LIBS data and structural Raman deconvolution enables the effective differentiation between carbonate-bound carbon and independent amorphous carbonaceous residues. The latter enclosed in gypsum cristals may stem from the remobilization of bituminous compounds derived from metamorphic bedrock in phreatic conditions during crystallization. The results validate this dual-spectroscopic approach as a robust, low-destructive framework for indexing organic and inorganic fluid-transported inclusions in sub-surface sulfate deposits.