Proterozoic Granulites from the Rauer Group, East Antarctica. I. Decompressional Pressure-Temperature Paths Deduced from Mafic and Felsic Gneisses

Granulites from the Rauer Group, East Antarctica, were metamorphosedat 860?40?C during a high-grade tectonothermal episode youngerthan 1400 Ma and probably close to 1000 Ma in age. A spatialvariation of pressures of metamorphism at the thermal peak iscalculated for felsic and mafic granulites preserving garnet-orthopyroxene-plagioclaseassemblages with or without additional clinopyroxene and quartz.Pressures of 6 to 7.5 kb are derived for the northern partsof the Rauer Group, whereas 7–8?5 kb pressures are calculatedfor similar granulites some 10–20 km further south. Post-deformational reaction textures including orthopyroxene-plagioclasesymplectites after garnet in basic granulites and plagioclasemoats or rims on garnet and orthopyroxene in felsic granulitesindicate a decompressional pressure-temperature-time evolution(P-T-t) which is confirmed by garnet-orthopyroxene-plagioclase-quartzand garnet-orthopyroxene barometry of zoned and regrown minerals.A pervasive decompression through c. 2 to 3–5 kb in thenorthern Rauer Group and to 5–6 kb in the southern partof the region occurred at temperatures above 700?C and probablyin excess of 750?C. This P-T evolution, which indicates a uniformunroofing of some 6–9 km while quite high mid- to lower-crustaltemperatures only decreased by c. 100?C, is consistent withthe later stages of a prolonged collision-related thermal evolution.Comparisons of the P-T-t paths of the late Proterozoic granulitesfrom the Rauer Group and elsewhere in East Antarctica with calculatedP-T paths for simple collisional models where erosion terminatesthe heating phase show that externally- derived magmatic additionsand an enhanced total heat budget are necessary to produce theobserved high-temperature evolution.     

Mingling of Immiscible Dolomite Carbonatite and Trachyte in Tuffs from the Massif Central, France

A pyroclastic tuff from the Velay volcanic province in the FrenchMassif Central contains blocks up to 30 cm long of local basementrocks, lava clasts, coarse-grained cumulates and pyroclasticfragments, with more or less diffuse boundaries with the hosttuff, which probably represent more consolidated parts of thetuff. All of the pyroclastics examined and approximately 10%of the cumulate xenoliths contain carbonates in variable amounts,textures and mineralogy. In some of the tuff samples, dolomiteoccurs in large amounts (up to 57%), principally as immiscibleglobules in trachytic melt (now glass), and represents the firstoccurrence of carbonatite reported from the Massif Central.The other carbonates, magnesiosiderite in the mafic cumulatesor occasionally in some tuffs, and calcite in the felsic cumulates,are always associated with a silicate glass of trachytic composition.Coexisting feldspars and carbonates in the various types ofsample are approximately in Sr isotopic equilibrium with aninitial ratio of about 0·7042. C- and O-isotopic compositionsof the carbonates covary and cover a very wide range of compositionfrom -2·9 to 3·9     

Particle size distribution analysis of sand-sized particles by laser diffraction: an experimental investigation of instrument sensitivity and the effects of particle shape

Laser diffraction is now widely used for particle size distribution analysis of sediments and soils. The technique can be very precise, and offers advantages of speed and cost over many other methods when used to analyse mixtures of sand, silt and clay. This study presents the results of an experimental investigation that examined the sensitivity of the Beckman-Coulter LS230 instrument to mixtures of different grain populations and differences in particle shape. The instrument was found to have high sensitivity to coarse particles in a finer matrix (detection threshold 1–2%), but much lower sensitivity to finer particles in a coarser mixture (detection threshold 12–17%). Experiments using near-spherical ballotini showed that laser analysis provides very similar values to dry sieving for the mean, median and mode, but for a range of natural sand samples values for the mean, median and mode were offset by 8–21%, with an average of ca 15%, compared with sieving. Analysis using a Beckman-Coulter RapidVUE instrument, which provides both size and shape information, provided support for the hypothesis that the differences between laser analysis and sieving are partly attributable to the effects of particle shape. However, an additional factor is the way in which the laser software interprets the optical diffraction data. The software predicts a high degree of log-normality in the size distribution, such that highly skewed, truncated or bimodal samples are poorly represented. Experiments using sieved fractions of ballotini indicated that, even with near-perfectly spherical particles, the particle size distribution predicted by the laser software includes a relatively large percentage of particles outside the sieve class limits.     

Experimental evidence for rapid biotic and abiotic reduction of Fe (III) at low temperatures in salt marsh sediments: a possible mechanism for formation of modern sedimentary siderite concretions

Fe (III) reduction is a key component of the global iron cycle, and an important control on carbon mineralization. However, little is known about the relative roles and rates of microbial (biotic) iron reduction, which utilizes organic matter, versus abiotic iron reduction, which occurs without carbon mineralization. This paper reports on the capacity for salt marsh sediments, which typically are rich in iron, to support abiotic reduction of mineral Fe (III) driven by oxidation of sulphide. Sediment was reacted with amorphous FeS under strictly anaerobic conditions at a range of temperatures in biotic and abiotic microcosm experiments. Fe (III) reduction driven by sulphide oxidation occurs abiotically at all temperatures, leading to Fe (II) and elemental sulphur production in all abiotic experiments. In biotic experiments elemental sulphur is also the oxidized sulphur product but higher bicarbonate production leads to FeCO₃ precipitation. Abiotic reduction of Fe (III) occurs at rates that are significant compared with microbial Fe (III) reduction in salt marsh sediments. The solid phases produced by coupled abiotic and biotic reactions, namely elemental sulphur and FeCO₃, are comparable to those seen in nature at Warham, Norfolk, UK. Furthermore, the rates of these processes measured in the microcosm experiments are sufficient to generate siderite concretions on the rapid time scales observed in the field. This work highlights the importance of abiotic Fe (III) reduction alongside heterotrophic reduction, which has implications for iron cycling and carbon mineralization in modern and ancient sediments.     

Refractory inclusions in the unique carbonaceous chondrite Acfer 094

Abstract– Acfer 094 is an unshocked, nearly unaltered carbonaceous chondrite with an unusual suite of refractory inclusions. The refractory inclusions in a newly prepared thin section and a small aliquot of disaggregated material were studied to compare the population with previous work, and to report new or unusual inclusion types. A total of 289 Ca‐, Al‐rich inclusions in the thin section and 67 among the disaggregated material, having a total of 31 different mineral assemblages, were found. Inclusions are largely free of secondary alteration products, and are typically ≤200 μm across. The most common are gehlenitic melilite+spinel±perovskite, spinel+perovskite, and spinel with a thin, silicate rim, typically melilite±diopside. Such rims and (thicker) mantles are very common among Acfer 094 inclusions, and they exhibit a variety of zoning patterns with respect to åkermanite and FeO contents. In the thin section, about 13% of the inclusions contain hibonite and approximately 5% are grossite‐bearing; in the disaggregated material, the percentages are 14 and 9, respectively, comparable to previous work. Among the unusual inclusions are a fine‐grained, porous, Ti‐rich hibonite+spinel+perovskite+melilite inclusion with a compact, coarse, Ti‐poor hibonite+spinel+melilite clast; two inclusions in which hibonite has reacted to form grossite; two inclusions with FeO‐rich spinel; and a small object consisting of fassaite enclosing euhedral spinel, the first fragment of a Type B inclusion reported from Acfer 094. Inclusions similar to those found in CM or CV chondrites are rare; Acfer 094 contains a distinctive population of inclusions. The population, dominated by small, melilite‐bearing inclusions, is most similar to that of CO chondrites. A distinguishing feature is that in Acfer 094, almost every phase in almost every refractory inclusion contains 0.5–1.5 wt% FeO. A lack of diffusion gradients and the pristinity of the matrix imply that the inclusions experienced prolonged exposure to FeO‐bearing fluid prior to accretion into the Acfer 094 parent body. There are no known nebular conditions under which the refractory phases found in the present samples could acquire FeO enrichments to the observed levels. The most likely setting is therefore in an earlier, FeO‐rich parent body. The inclusions were ejected from this parent body, mixed with typical CAIs, chondrules, amoeboid olivine aggregates, and amorphous material, and incorporated into the Acfer 094 parent body.