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

Stromatolite structures in Early Archean carbonate deposits form an important clue for the existence of life in the earliest part of Earth's history. Since Mars is thought to have had similar environmental conditions early in its history, the question arises as to whether such stromatolite structures also evolved there. Here, we explore the capability of Raman spectroscopy to make semiquantitative estimates of solid solutions in the Ca-Mg-Fe(+Mn) carbonate system, and we assess its use as a rover-based technique for stromatolite characterization during future Mars missions. Raman microspectroscopy analysis was performed on a set of carbonate standards (calcite, ankerite, dolomite, siderite, and magnesite) of known composition. We show that Raman band shifts of siderite-magnesite and ankerite-dolomite solid solutions display a well-defined positive correlation (r(2) > 0.9) with the Mg# = 100 x Mg/(Mg + Fe + Mn + Ca) of the carbonate analyzed. Raman shifts calibrated as a function of Mg# were used in turn to evaluate the chemical composition of carbonates. Raman analysis of a suite of carbonates (siderite, sidero-magnesite, ankerite, and dolomite) of hydrothermal and sedimentary origin from the ca. 3.2 Ga old Barite Syncline, Barberton greenstone belt, South Africa, and from the ca. 3.5 Ga old Dresser Formation, Pilbara Craton, Western Australia, show good compositional agreement with electron microprobe analyses. These results indicate that Raman spectroscopy can provide direct information on the composition and structure of carbonates on planetary surfaces.

UV and VIS Raman spectra of natural lonsdaleites: towards a recognised standard.

A UV laser has now been used to measure the Raman spectrum of lonsdaleite. This mineral species is a little-known hexagonal form of carbon having no known P-T field of stability. Lonsdaleite is known to coexist with diamond and/or graphite in certain impact structures and meteorites. Its presence in microinclusions in some ultrahigh-pressure eclogites is under discussion as there is a considerable wavenumber overlap of the sp(3) Raman band of lonsdaleite in the 1200-1400 cm(-1) region with certain bands of haematite, graphite and diamond, and also with "disordered-diamond" having a downshifted wavenumber. Various incoherent previously published values of the Raman bands are briefly reviewed and an attempt is made to establish a reference spectrum. Four samples of lonsdaleite from the Zapadnaya and Popigai impact structures (Ukraine) were measured with three different laser sources (488, 514.5 and 325 nm) with two Raman spectrometers. UV-Raman was less fluorescent. All the new data were coherent in establishing an sp(3) band centred at 1324+/-4 cm(-1) with a FWHM about five times wider than that of diamond and an intensity about 500 times weaker. The presence of a second band giving a weak shoulder around 1225 cm(-1) is discussed with respect to the alternative of one continuous asymmetrical band.