Molecular Fractionation of Ancient Organic Compounds in Deeply Buried Halite Crystals

The molecular fractionation of organic compounds through adsorption in minerals has wide implications, including tracing the origins of life, carbon sequestration, and climate change. Here we present the first in situ examination of molecular fractionation within individual crystals via optical-photothermal infrared (O-PTIR) spectroscopy. Our study focuses on a unique inclusion trail within deeply buried halite crystals, characterized by a distinctive orange-to-blue fluorescence gradient, providing primary evidence of molecular variation in ancient carbon-based fluids within the inclusion trail. The findings reveal a trend in the CH₂/C=O and CH₃/C=O ratios, conforming with a consistent decrease from the blue fluorescence region to the orange fluorescence region. The chemically influenced fluorescent behavior of these ancient liquid carbon-based compounds is attributed to the fractionation of fluids in the inclusions as a result of microfractures within the crystal acting as chromatography capillaries. These capillaries facilitated interactions between specific organic compounds, serving as adsorbates, and the halite mineral, representing the adsorbent. Our study provides insights into the fluid-solid physicochemical interactions within extreme environments and extends our understanding of molecular processes in such settings.