Deciphering the evolution of ecological interactions among the metabolic types during the early diversification of life on Earth is crucial for our understanding of the ancient biosphere. The stromatolites from the genus Conophyton cylindricus represent a datum for the Proterozoic (Meso to Neoproterozoic) on Earth. Their typical conical shape has been considered a result of a competition between microorganisms for space, light and nutrients. Well-preserved records of this genus from the "Paleontological Site of Cabeludo ", Vazante Group, Sao Francisco Craton (Southern Brazil) present in situ fossilized biofilms, containing preserved carbonaceous matter. Petrographic and geochemical analyses revealed an alternation between mineral laminae (light grey laminae) and fossilized biofilms (dark grey laminae). The dark grey laminae comprise three different biofilms recording a stratified microstructure of microbial communities. These three biofilms composing the dark grey laminae tend to be organized in a specific pattern that repeats through the stromatolite vertical section. Iron and manganese are distributed differently along the dark and light grey laminae; X-ray absorption and luminescence data showed possible different areas with authigenic iron and iron provided from diagenetic infiltration. Cryptocrystalline apatite in the lowermost biofilms in each dark grey laminae may suggest past metabolic activity of sulfide-oxidizing bacteria. These findings suggest that the microorganisms reached a complex metabolic diversification in order to maintain an equilibrium situation between the three different biofilms along the vertical section of the structures, thus benefiting the whole microbial community. This means that the stromatolites from the Conophyton genus may have formed as a result of a greater complexity of interactions between microorganisms, and not only from competition between photosynthesizers.

Estimates of the oxidation states of magmas are important to current investigations of the geochemical characteristics of their source regions and of evolved magmatic series created during differentiation. One means of achieving such estimates is to capitalize on compositions of coexisting cubic and rhombohedral Fe-Ti oxides determined by electron microprobe. A combination of experimental calibration points and thermodynamic modeling provides a basis for translating such compositions into T-f(O2) values. This has been done until recently by estimating Fe3+/Sigma Fe on the basis of charge balance and stoichiometry by the method of Droop (1987), after matrix corrections of X-ray intensity data have been performed, as EPMA cannot be used routinely to distinguish different elemental valence states, much less accurately quantify abundances of Fe3+ and Fe2+. The traditional approach of undertaking post-data-reduction calculations falls short of attaining the best possible quantitative results. The tactical choice of not accounting for light elements that have not been explicitly analyzed prior to matrix corrections of X-ray intensity data leads to systematic errors in reported oxide abundances for measured elements. This article addresses one such issue, the oxygen associated with Fe3+ (hereafter "excess oxygen"), on the basis of coexisting Fe-Ti oxides from Andean lavas. A new software routine in probe for EPMA (PFE) uses an iterative calculation scheme to calculate amounts of excess oxygen that would not be considered if all iron were assumed to be ferrous and then applies this excess oxygen during matrix corrections. The PFE approach reveals that Fe-concentrations have been underestimated, universally, in these minerals because O atoms absorb FeKa radiation: discrepancies increase as total Fe and Fe3+/Fe2+, hence excess oxygen, increase. Analyses of the most Fe-rich cubic oxide compositions in this data set have similar to 6 wt% excess oxygen and similar to 1 wt% more FeO+Fe2O3 than would be reported without incorporating the impact of excess oxygen in matrix corrections. Minor to negligible differences in other elements are also observed. These effects are not because excess oxygen is directly attributed to these elements, although some may be present in multiple valence states, as matrix corrections are undertaken on the basis of the conventional assumptions that they occur as Cr3+, V3+, Mn2+, Mg2+, Ca2+, and Si4+. Rather, variably small increases in total Fe propagate through the matrix corrections for other elements, and these differences may be recorded as minor increases or decreases in some concentrations, depending on the particular element and the amount of change in Fe-concentration. Fe3+/Sigma Fe in analyses produced with the PFE routine are essentially identical to those determined in the traditional mode, as cation proportions calculated on the basis of charge balance and stoichiometry, with the method of Droop (1987), is a necessary step. The new method: (1) provides more accurate concentrations, mainly for Fe and Ti; (2) is applicable to any mineral containing ferric iron (subject to stoichiometric constraints); (3) provides more accurate analytical totals, which can be advantageous for evaluating analytical quality; and (4) does not impact estimates of oxidation state.

Five Type A CAIs from three CV3 chondrites (Vigarano, Northwest Africa 3118, Allende), which differ in age by no more than -105 years, show mineralogical and textural evidence of gradual transition into Type Bs, indicating that Type B inclusions formed by evolution of Type A CAIs in the solar nebula. This model differs from the conventional condensation model in which aggregates of condensate grains form different kinds of CAIs depending on the relative populations of different kinds of grains. In our model the pyroxene forms nearly isochemically by reaction of perovskite with melilite under highly reducing conditions. Anorthite requires the addition of silica from the gas, and originally forms as veins and reaction rims on gehlenitic melilite within Fluffy Type As. Later partial re-melting of these assemblages results in the formation of poikilitic pyroxene and anorthite that enclose rounded (partially melted) tablets of melilite. Oxygen isotopes in four of the CAIs support the formation of Ti-rich 16O-depleted pyroxene from 16O-depleted perovskite, but not in the fifth CAI. An alternative possibility is that Ti-rich 16O-depleted pyroxene is the result of later solid-state exchange that preferentially affects the most Ti-rich pyroxene. Regardless of the origin of the 16O-depleted pyroxene, we give a model for nebular reservoir evolution based on sporadic FU-Orionis flare-ups in which the 16O-rich region near the proto-Sun fluctuated in size depending on whether the protoSun was in flare-up stage or quiescent. Published by Elsevier Ltd.

Measurements of trace element chemistry, mineralogy, and isotope geochemistry are rarely combined with known environmental data to provide a more complete story about how environmental conditions are recorded in biomineral carbonates. Here, cultured (farmed) pearls serve as relatively pristine time capsules to study these geochemical and mineral-based proxies. Cathodoluminescence (CL) imaging and Raman spectroscopic mapping on the & mu;m-scale reveals that heterogeneous crystal bonding environments, geochemistry, and organic contents across the growth history of a freshwater pearl reflect environmental shifts in Kentucky Lake, TN, USA. A major CL peak at 551 nm aligns with increased manganese and organic contents and correlates with lake conductivity and alkalinity data. These CL features are temporally offset from previous 10-& mu;m-scale aragonite nacre oxygen isotope measurements (& delta;18OArg) that record periods of minimum lake temperatures in winter seasons. Thus, we suggest that these trace element and organic features represent autumn or spring rainy seasons that experience lake turnover events and more land runoff rich in Mn and nutrients, increasing aragonite-bound Mn and organic contents in the nacre. This 551 nm signal is absent in Mn-poor saltwater pearl nacre. A second CL peak at 444 nm shows different heterogeneous features likely due to crystal structure shifts, as evidenced by correlations to a Raman map of translational (T): librational (L) mode height ratios typically signaling changes in nacre tablet orientation. Thus, we show that & mu;m-scale CL and Raman mapping may serve as complementary environmental proxies to novel SIMS-based & delta;18OArg temperature proxies in order to capture additional information about local lake environments at seasonal to sub-seasonal temporal resolutions.

Determination of alkali elements is important to Earth scientists, yet suitable and reliable microanalytical reference materials are lacking. This paper proposes a new albite reference material and evaluates the potential for future K-feldspar reference materials. The proposed Piz Beverin albite reference material from Switzerland yields a homogeneous composition at the centimetre- to micrometre-scale for Si, Al and Na with < 2000 mu g g(-1) total trace elements (mostly heterogeneously distributed Ca, K and Sr). EPMA and LA-ICP-MS measurements confirm a composition of 99.5(2)% albite component, which is supported further by bulk XRF measurements. A round robin evaluation involving nine independent EPMA laboratories confirms its composition and homogeneity for Si, Al and Na. In addition, a set of five distinct clear K-feldspar samples was evaluated as possible reference materials. The first two crystals of adular and orthoclase yield unacceptable inhomogeneities with > 2% relative local variations of Na, K and Ba contents. The three other investigated sets of K-feldspar crystals are yellow sanidine crystals from Itrongay (Madagascar). Despite distinct compositions, EPMA confirms they are each homogeneous at the centimetre to micrometre scale for Si, Al and K and have no apparent inclusions; further investigation to find larger amounts of these materials is therefore justified.