ANALYTICAL TECHNIQUES

Transcription

1 ANALYTICAL TECHNIQUES Raman Spectra Raman spectra were obtained by use of a T64000 (JY Horiba) triple-stage laser- Raman system having both macro-raman and confocal micro-raman capability which permits acquisition both of individual point spectra and of Raman images that map the two-dimensional spatial distributions of molecular-structural components. A Coherent Innova argon ion laser was focused through the 50 objective of an Olympus BX41 microscope to a ~1 µm spot; the laser provided excitation at nm with a power at the sample of ~1-8 mw, which is well below the threshold that could lead to radiation damage to the specimens studied here (Schopf et al., 2002). Due to its confocal capability, a vertical resolution of 2-3 µm and a horizontal resolution of ~1.5 µm are achieved by this system. Backscattered Raman radiation was collected over a range from 320 to 3100 cm -1 by use of a single spectral window centered at 1800 cm -1 that afforded simultaneous acquisition of both of the major bands (at ~1374 and ~1587 cm -1 ) and the second-order band (at ~2700 cm -1 ) of graphite, as well as of the major band of apatite (at ~965 cm -1 ) and of quartz (at 467 cm -1 ). Spectra were acquired at a resolution of <3 cm -1. Two- and Three-dimensional Raman Images Raman spectra were collected over a rectangular (25 x 35 µm) area enclosing the apatite grain and its surrounding quartz matrix at 1.5-µm-spaced vertical increments by use of a programmed motorized high precision microscope stage (SCAN 75x50, Märzhäuser GmbH & Co. KG) that moved the specimen in the x and y directions under a laser beam fixed in position and sequentially increased the depth of the image focal plane. To obtain an acceptable signal-to-noise ratio, each spectral element was analyzed for 2 seconds, resulting in a total data collection time of ~20 minutes for each twodimensional image. The several hundred spexels comprising such images were then processed (using LabSpec v4.14) to construct maps of the intensities in the spectral windows corresponding to the "G" band (~1587 cm -1 ) of graphite and the major bands of apatite and of quartz. The two-dimensional molecular-structural maps thus produced document the distribution of the minerals that comprise the specimen at a spatial resolution ~1.5-2 μm (given by the convolution of step size and laser beam focus). To obtain a "through focus" series from which three-dimensional images could be

2 constructed, 65 sequential two-dimensional maps were acquired and processed by use of the VolView v2.0 three-dimensional rendering program. Because the VolView software can process only one dataset at a time, we first constructed separate three-dimensional images of the quartz, apatite, and graphite present in the specimen and then merged these images by use of the Adobe Photoshop v7.0 image processing program. Construction of three-dimensional images such as those presented here requires processing of data having comparable overall quality and similar spatial resolution throughout an entire sampled volume (Schopf and Kudryavtsev, 2005). Two principal difficulties inhibit achievement of this requirement: (1) spherical aberration results in a decrease of vertical resolution with increasing depth; and (2) the intensity of the detectable backscattered Raman signal becomes increasingly faint with depth. Under our experimental conditions, these constraints become limiting at depths of ~65 70 µm (Schopf et al., 2002; Schopf and Kudryavtsev, 2005). To correct for diminishing Raman intensity with depth, data were normalized to the intensity of the quartz band in each of the sequentially acquired two-dimensional images. Ion Microscope Images Ion microscope images and carbon isotopic measurements of the graphitic inclusion contained in apatite grain #1 were obtained by use of a CAMECA ims1270 ion microprobe utilizing a mass-filtered 20 kev Cs + primary ion beam and analyzing negative secondary ions. Prior to isotopic analysis, the thin section was cut (by use of a diamond-impregnated wafering saw) such that the apatite grain was located approximately at the center of a roughly centimeter-sized section. This section had been hand-polished to remove its uppermost ~46 μm in order to expose the apatite and bring its inclusion to within ~0.5 μm of the thin section surface. Following cutting and polishing, the section was cleaned, positioned in the center of a 2.5-cm-diameter aluminum disk and sputter-coated with a thin gold film. The spatial distributions of 12 C, 28 Si, and 31 P in an area centered on apatite grain #1 were mapped by capturing the realtime ion images on the microchannel plate of the ims1270 with a CCD-camera and a standard PC-based image capture board. A contrast diaphragm was inserted into the secondary ion beam path in order to restrict image crossover and to obtain a spatial resolution of ~1 μm. The spatial resolution of the resultant ion images is sufficient to

3 verify that isotopic analyses were acquired from the same inclusion that had been imaged previously by optical and Raman methods and that these measurements did not include a significant contribution from any other carbon source. Carbon Isotopic Measurements Carbon isotopic ratios were measured by using the molecular ions 12 C 2 and 12 C 13 C since these species typically have higher intensity and exhibit less instrumental mass fractionation than atomic ions sputtered from carbonaceous material. Data were collected at high mass resolving power (~7000) by magnetic field-peak switching, a configuration in which all molecular ion mass interferences are quantitatively resolved (Fig. DR1). A normal incidence electron gun was used to achieve charge compensation during the analyses and the energy bandpass of the spectrometer was set at 30 ev. Appropriate corrections were subsequently made for electron multiplier dead time and for signal drift during the analysis. Both such corrections were relatively small under the analytical conditions used because the sputter rate was kept low (so as to not degrade the grain too quickly) by defocusing the Cs + beam slightly and using a low current (~150 pa) C 2 12 C 2 H Intensity (counts/sec) C 12 C Mass (amu) Figure. DR1. High mass resolution scan of m/q =24 and m/q =25 regions sputtered from E. Coli standard. Plotted is the logarithm of measured ion intensity (c/s) as a function of mass (i.e., magnetic field setting). The molecular ion interference from C 2 H is well resolved from the 12 C 13 C peak at a mass resolution (m/δm) = ~7000.

4 Instrumental mass fractionation (IMF) was calibrated by use of an E. coli bacterial standard (prepared by C. House, PennState University) which has an established isotopic composition of δ 13 C = (relative to PDB, as are all carbon isotopic data reported here). Previous analyses at UCLA have demonstrated that this biologic standard is homogenous to within ~1 on a micron scale. Ten analyses of randomly selected small (<10-μm-sized) clumps of this E. coli standard yielded an IMF = 2.1 /amu with a standard deviation = 1.5 (1σ standard error = 0.54 ), results consistent with counting statistics and the "in-run" precision on any one clump of such bacteria (reduced χ 2 = 0.94). Prior investigations have documented that SIMS matrix effects, a possible dependence of instrumental fractionation on the analyzed isotopic composition of a sample analyzed, are typically less than a few permil between graphite and biogenic carbonaceous matter (House et al., 2000; McKeegan et al., 1985; Orphan et al., 2002; Sangely et al., 2005). We thus consider that systematic errors in the isotopic compositions reported here, due to instrument effects and/or sample composition, are <5. Carbon isotope data are reported in Table DR1. Shown are the mean count rates for each analyzed spot, the measured ( 13 C 12 C)/ 12 C 2 ratios corrected for a detector deadtime of 25ns and IMF, and the δ 13 C values (relative to PDB) with one standard error of the mean. The larger uncertainty in the second analysis of the graphitic inclusion in Akilia apatite#1 was due to a declining signal, and no further measurements were attempted so as to not completely destroy the specimen. Table DR1. Carbon isotope measurements. Sample C 2 ( 13 C 12 C) / δ 13 C PDB 1 σ (10 5 cts/sec) C 2 ( ) ( ) E. Coli E. Coli E. Coli E. Coli E. Coli E. Coli E. Coli Ak. apatite# Ak. apatite# E. Coli E. Coli E. Coli

5 Additional Images of Graphitic Inclusions in Apatite Examples of two additional apatite grains in thin section G92-197B that contain graphitic inclusions are shown in Figure DR2. The opaque inclusions are identified as graphite by Raman spectra shown in Figure DR3. Figure DR2. Optical photomicrographs of quartz-embedded apatite grains #2 and #3 that contain opaque inclusions shown by Raman spectroscopy (Fig. DR3) to be composed of disordered graphite, in the same thin section (G92-197B) of a >3800 Ma quartz-pyroxene supracrustal rock from Akilia, southwest Greenland that contains apatite grain #1; magnification of all parts is shown by the scale in (A). A and B: Photomicrographs of apatite grain #2 at sequentially deeper focal planes showing its opaque graphitic inclusion (arrows). After polishing of the thin section to expose the inclusion in apatite grain #1 for analyses by SIMS, the upper surface of apatite grain #2, ~40 28 μm in size, was situated ~42 μm below the surface of the thin section and the median plane of the ~12 8 μm-sized inclusion was ~21 μm beneath the upper surface of the grain. C and D: Photomicrographs of apatite grain #3 at sequentially deeper focal planes showing the

6 presence of two opaque graphitic inclusions (arrows). After polishing, the upper surface of this ~40 22 μm-sized apatite grain was situated ~6 μm below the surface of the thin section; the median plane of the upper of the two inclusions, an elongate ~3 10 μmsized graphitic body (C, arrow), was ~12 μm beneath the upper surface of the grain; and the median plane of the lower, approximately equant, ~6 8 μm-sized graphitic inclusion (D, arrow) was ~18 μm beneath the upper surface of the grain. Figure DR3. Raman spectra that establish the graphitic composition of the apatite-hosted inclusions illustrated in Fig. DR2. A: The Raman spectrum of the inclusion in apatite grain #2 (denoted by the arrows in Fig. DR2, A and B). B: The spectrum of the upper of the two inclusions in apatite grain #3 (Fig. DR2, C, arrow). C: The spectrum of the lower inclusion in apatite grain #3 (Fig. DR2, B, arrow). The Raman spectra of these three inclusions are similar to one another and closely resemble that of the inclusion in apatite grain #1 (Fig. 2), all exhibiting the G, D', and D" bands characteristic of graphite (at 1587, 1374, and 1621 cm -1, respectively) as well as a broad second-order band at ~2730

7 cm -1 (not shown). The presence of the D' and D" bands in the spectrum of the inclusion in apatite grain #2 (A, the uppermost of the three spectra), like that of the same bands in the spectrum of the graphitic inclusion in apatite grain #1 (Fig. 2), shows that this carbonaceous matter has a moderately disordered crystal structure. The somewhat more narrow and less intense D' and D" bands in the spectra of the inclusions in apatite grain #3 (B and C, the lower two of the three spectra) and the bifurcation of the G band (evident especially in C, the spectrum of the lower inclusion in apatite grain #3), indicate that the carbonaceous material of these inclusions, though not crystalline graphite, is more graphitized than that in apatite grains #1 and #2. Three-dimensional Raman Images of Apatite #1 and surrounding Quartz Figure DR4. Three dimensional Raman image of Apatite #1; see also video presentation in the Data Repository.

8 REFERENCES House, C.H., Schopf, J.W., McKeegan, K.D., Coath, C.D., Harrison, T.M., and Stetter, K.O., 2000, Carbon isotopic composition of individual Precambrian microfossils: Geology, v. 28, p McKeegan, K.D., Walker, R.M., and Zinner, E., 1985, Ion microprobe isotopic measurements of individual interplanetary dust particles: Geochimica et Cosmochimica Acta, v. 49, p Orphan, V.J., House, C.H., Hinrichs, K.U., McKeegan, K.D., and DeLong, E.F., 2002, Multiple archaeal groups mediate methane oxidation in anoxic cold seep sediments: Proceedings of the National Academy of Sciences of the United States of America, v. 99, p Sangely, L., Chaussidon, M., Michels, R., and Huault, V., 2005, Microanalysis of carbon isotope composition in organic matter by secondary ion mass spectrometry: Chemical Geology, v. 223, p Schopf, J.W., and Kudryavtsev, A.B., 2005, Three-dimensional Raman imagery of Precambrian microscopic organisms: Geobiology, v. 3, p Schopf, J.W., Kudryavtsev, A.B., Agresti, D.G., Wdowiak, T.J., Czaja, A.D., 2002, Laser-Raman imagery of Earth's earliest fossils: Nature, v. 416, p

9 3-D D Raman Image, Apatite Grain #1 2-D D Optical Images Viewed from Top Quartz Apatite Graphite 10 μm Graphite 10 μm Quartz 3-D Apatite Raman Image Viewed from Side K. D. McKeegan, A. B. Kudryavtsev, and J. W. Schopf. Raman and ion microscopic imagery of graphitic inclusions in apatite from >3830 Ma Akilia supracrustals, West Greenland 1

10 3-D D Raman Image, Apatite Grain #1 2-D D Optical Images Viewed from Top Quartz Apatite Graphite 3-D Raman Image Viewed from Side Quartz Apatite Graphite 10 μm 10 μm 2