A Raman Study of Carbonates and Organic Contents in Five CM Chondrites

Introduction: Carbonates comprise the second most abundant class of carbon-bearing phases in carbonaceous chondrites after organic matter (~2 wt.%), followed by other C-bearing phases such as diamond, silicon carbide, and graphite [1]. Therefore, understanding the abundances of carbonates and the associated organic matter provide critical insight into the genesis of major carbonaceous components in chondritic materials. Carbonates in CM chondrites mostly occur as calcite (of varying composition) and dolomite [2-4]. Properly performed, Raman spectroscopy provides a non-destructive technique for characterizing meteorite mineralogy and organic chemistry. It is sensitive to many carbonaceous phases, allows the differentiation of organic from inorganic materials, and the interpretation of their spatial distribution. Here, with the use of Raman spectroscopy, we determine the structure of the insoluble organic matter (IOM) in the matrix and carbonate phases in five CM chondrites: Jbilet Winselwan, Murchison, Nogoya, Santa Cruz, and Wisconsin Range (WIS) 91600, and interpret the relative timing of carbonate precipitation and the extent of the associated alteration events. Samples and Analytical Techniques: We studied five CM chondrites Jbilet Winselwan (hereafter, Jbilet) (CM2.3, a find from the Western Sahara, 2013), Murchison (CM2.5, a fall from Australia, 1969), Nogoya (CM2.2, a fall from Argentina, 1879), Santa Cruz (CM2, a fall from Mexico, 1939), and WIS 91600 (CM-an, a find from Antarctica, 1991). WIS 91600 was provided by the Meteorite Working Group (MWG) and the JSC Meteorite Curator. The remaining samples were provided by meteorite collectors. We prepared polished thin sections of the meteorite samples with isopropanol, and then identified carbonates in each sample with an optical microscope. Standard carbonates aragonite (CaCO3), calcite (CaCO3), dolomite (CaMg(CO3)2) and siderite (FeCO3) were provided by M. Zolensky and R. Morris (NASA JSC). The samples were analyzed using a Jobin-Yvon Horiba LabRam HR (800 mm) Raman microprobe at the Department of Geosciences, Virginia Tech. The excitation wavelength was 514.53 nm (green) provided by ModuLaser Stellar Pro-L 514 nm, 100 Mw solid-state laser operated at 50 mW at the source). The Raman probe spatial resolution at the analyzed spot was 1.14 μm at the 40x magnification. Three accumulated spectra were collected from each analytical spot to identify and discard spurious signals, such as those from cosmic rays. Wavelength calibration against a silicon wafer sample was checked daily prior to sample analyses. Custom software written in the Python programming language [5] was used to fit the Raman data using Lorentzian profiles and applying a linear baseline correction in order to determine the peak center positions (ω) and full width half-maximum (FWHM, Γ) of each Raman band.. Results and Discussion: We compared the ~1100 cm band positions of the CM carbonates to the dolomite and calcite standards (Table 1), and concluded that most selected carbonates identified in this study are calcite, except for a single dolomite grain in Nogoya (Grain 5). This does not preclude the presence of other carbonate phases in the CMs. The v1 positions of the CM calcites are 2−3 cm higher than pure calcites, which suggests that they contain significant impurity cations. The cations in the Cacarbonates are mainly Fe, and occasionally Mn and Mg in CMs [2, 3], and Sr in CIs [6].