Unlike the familiar Phanerozoic history of life, evolution during the earlier and much longer Precambrian segment of geological time centred on prokaryotic microbes. Because such microorganisms are minute, are preserved incompletely in geological materials, and have simple morphologies that can be mimicked by nonbiological mineral microstructures, discriminating between true microbial fossils and microscopic pseudofossil 'lookalikes' can be difficult. Thus, valid identification of fossil microbes, which is essential to understanding the prokaryote-dominated, Precambrian 85% of life's history, can require more than traditional palaeontology that is focused on morphology. By combining optically discernible morphology with analyses of chemical composition, laser--Raman spectroscopic imagery of individual microscopic fossils provides a means by which to address this need. Here we apply this technique to exceptionally ancient fossil microbe-like objects, including the oldest such specimens reported from the geological record, and show that the results obtained substantiate the biological origin of the earliest cellular fossils known.

The field of Mössbauer spectroscopy (MS) has recently enjoyed renewed visibility in the diverse geoscience communities as a result of the inclusion of Mössbauer spectrometers on the Mars Exploration Rovers. Furthermore, new improvements in technology have made possible studies involving very small samples (1-5 mg or less) and samples with very low Fe contents (such as feldspars), in addition to samples measured in situ in thin sections. Because of these advances, use of Mössbauer spectroscopy in Earth science applications is expected to continue to grow, providing information on site occupancies; valence states; magnetic properties; and size distributions of (largely) Fe-bearing geological materials, including minerals, glasses, and rocks. Thus, it is timely to review here the underlying physics behind the technique, with a focus on the study of geological samples. With this background, recent advances in the field, including (a) changes in instrumentation that have allowed analysis of very small samples and of surface properties, (b) new models for fitting and interpreting spectra, and (c) new calculations of recoil-free fraction, are discussed. These results have made possible increasingly sophisticated studies of minerals, which are summarized here and organized by major mineral groups. They are also facilitating processing and interpretation of data from Mars. MS: Mössbauer spectroscopy XPS: X-ray photoelectron spectroscopy EELS: electron-energy loss spectroscopy XANES: X-ray absorption near-edge spectroscopy Mössbauer effect: emission or absorption of a gamma photon without energy loss (or gain) in a transition between the ground state and an excited state of certain nuclei bound in a solid viii Contents

Laser-Raman imagery is a non-intrusive, non-destructive analytical technique, recently introduced to Precambrian paleobiology, that can be used to demonstrate a one-to-one spatial correlation between the optically discernible morphology and kerogenous composition of permineralized fossil microorganisms. Made possible by the submicron-scale resolution of the technique and its high sensitivity to the Raman signal of carbonaceous matter, such analyses can be used to determine the chemical-structural characteristics of organic-walled microfossils and associated sapropelic carbonaceous matter in acid-resistant residues and petrographic thin sections. Here we use this technique to analyze kerogenous microscopic fossils and associated carbonaceous sapropel permineralized in 22 unmetamorphosed or little-metamorphosed fine-grained chert units ranging from approximately 400 to approximately 2,100 Ma old. The lineshapes of the Raman spectra acquired vary systematically with five indices of organic geochemical maturation: (1) the mineral-based metamorphic grade of the fossil-bearing units; (2) the fidelity of preservation of the fossils studied; (3) the color of the organic matter analyzed; and both the (4) H/C and (5) N/C ratios measured in particulate kerogens isolated from bulk samples of the fossil-bearing cherts. Deconvolution of relevant spectra shows that those of relatively well-preserved permineralized kerogens analyzed in situ exhibit a distinctive set of Raman bands that are identifiable also in hydrated organic-walled microfossils and particulate carbonaceous matter freed from the cherts by acid maceration. These distinctive Raman bands, however, become indeterminate upon dehydration of such specimens. To compare quantitatively the variations observed among the spectra measured, we introduce the Raman Index of Preservation, an approximate measure of the geochemical maturity of the kerogens studied that is consistent both with the five indices of organic geochemical alteration and with spectra acquired from fossils experimentally heated under controlled laboratory conditions. The results reported provide new insight into the chemical-structural characteristics of ancient carbonaceous matter, the physicochemical changes that accompany organic geochemical maturation, and a new criterion to be added to the suite of evidence by which to evaluate the origin of minute fossil-like objects of possible but uncertain biogenicity.

Features attributed to ferric iron in remotely sensed spectral data of Mars and the magnetic nature of Martian soil at the Viking landing sites are consistent with the occurrence of hematite (o•-Fe203) as both superparamagnetic (nanocrystalline) hematite (sp-Hm) and larger-diameter hematite (bulk-Hm) particles. These hematite particles most likely occur in pigmentary form, that is, as particles dispersed throughout the volume of a relatively spectrally neutral (silicate?) material. Likely physical forms of this pigmented volume include rocks, dust and soil particles, and coatings (weathering rinds) thereon. Accommodation of Martian data by hematite is a result of differences in optical and magnetic properties of sp-Hm and bulk-Hm particles. Optical, magnetic. and Mossbauer properties of sp-Hm particles dispersed within particles of high-area silica gel are reported in this study and compared to the corresponding properties of bulk-Hm powders. Samples were prepared by calcining (---550øC) powders of high-area silica gel that had been impregnated with ferric nitrate solutions. The samples are classified according to type of Mossbauer spectrum observed at 293 K. (1) Type S + D samples, which by Mossbauer granulometry contain hematite particles both larger and smaller than 10(2) nm, are characterized by a hematite sextet plus superparamagnetic doublet. (Uncertainties are given in parentheses and refer to the final digit(s).) (2) Type D samples, which contain hematite particles smaller than 10(2) nm, are characterized by only a superparamagnetic doublet and so contain only sp-Hm. The presence of larger particles in type S + D samples is consistent with X ray diffraction data; the diffraction patterns of type S + D samples are characterized by a few, broad hematite lines, and type D samples have no lines because the particles are too small to coherently scatter X rays. Measurements of internal field strengths (Hint) at 22 K for both type S + D and type D samples show that Hin t is not constant but decreases with decreasing particle diameter from 54.0 T for bulk-Hm to 46.6 T for 5.4-nm sp-Hm. This dependence implies that phase identifications based solely on comparisons to bulk values of Hin t are equivocal when superparamagnetic particles are present. Sp-Hm (< 10-nm diameter) is much more magnetic than bulk-Hm; the saturation magnetization at 293 K for type D samples is 7(2) A m2/kg as compared to 0-0.5 A m2/kg for bulk-Hm. Optical properties of type S + D samples are similar to those of bulk-Hm; in particular, a well-defined band minimum is present near 860 nm. Optical properties of type D samples, with only sp-Hm at 293 K, are significantly different in that a step-shaped feature instead of a well-defined band is centered near 860 nm. The transition from well-defined band to step-shaped feature occurs at a hematite particle diameter of -• 10 nm. The position of the UV-visible absorption edge and the absorption strength at 860 nm depend on the number density of sp-Hm particles, the Fe20 3 concentration, and the physio...

Widespread detections of phyllosilicates in Noachian terrains on Mars imply a history of near-surface fluid-rock interaction. Ferrous trioctahedral smectites are thermodynamically predicted products of basalt weathering on early Mars, but to date only Fe 3+ -bearing dioctahedral smectites have been identified from orbital observations. In general, the physicochemical properties of ferrous smectites are poorly studied because they are susceptible to air oxidation. In this study, eight Fe 2+ -bearing smectites were synthesized from Fe 2+ -Mg-Al silicate gels at 200°C under anoxic conditions. Samples were characterized by inductively coupled plasma optical emission spectrometry, powder X-ray diffraction, Fe K-edge X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy, and visible/near-infrared (VNIR) reflectance spectroscopy. The range of redox states was Fe respectively. The spectra for ferrous saponites are distinct from those for dioctahedral ferric smectites, permitting their differentiation from orbital observations. X-ray diffraction patterns for synthetic high-Mg ferrosaponite and high-Mg ferrian saponite are both consistent with the Sheepbed saponite detected by the chemistry and mineralogy (CheMin) instrument at Gale Crater, Mars, suggesting that anoxic basalt alteration was a viable pathway for clay mineral formation on early Mars.