Calibration of carbonate composition using micro-Raman analysis: application to planetary surface exploration

Area of Science:

• Astrobiology
• Geochemistry
• Planetary Science

Background:

• Stromatolites in Early Archean carbonate deposits are key indicators of early life on Earth.
• Early Mars may have had similar environmental conditions conducive to stromatolite formation.
• Characterizing Martian carbonates is vital for astrobiological investigations.

Purpose of the Study:

• To assess Raman spectroscopy's capability for semiquantitative analysis of carbonate solid solutions (Ca-Mg-Fe(+Mn)).
• To evaluate Raman spectroscopy as a rover-based technique for stromatolite characterization on Mars.
• To establish a calibration for determining carbonate composition using Raman band shifts.

Main Methods:

• Raman microspectroscopy was performed on known carbonate standards (calcite, ankerite, dolomite, siderite, magnesite).
• Calibration curves were developed correlating Raman band shifts with Mg# (Mg/(Mg + Fe + Mn + Ca)) in solid solutions.
• Analysis of ancient terrestrial stromatolitic carbonates (South Africa, Western Australia) was conducted and compared with electron microprobe analyses.

Main Results:

• A strong positive correlation (r(2) > 0.9) was observed between Raman band shifts and Mg# in siderite-magnesite and ankerite-dolomite solid solutions.
• Calibrated Raman spectroscopy accurately determined the chemical composition of analyzed carbonate samples.
• Results showed good agreement between Raman spectroscopy and electron microprobe analyses for terrestrial samples.

Conclusions:

• Raman spectroscopy is a viable technique for semiquantitative analysis of carbonate solid solutions.
• This method can provide direct information on carbonate composition and structure on planetary surfaces, including Mars.
• Raman spectroscopy is a promising tool for future Mars missions searching for signs of ancient life.