Atmospheric oxygenation caused by a change in volcanic degassing pressure

International audienceThe Precambrian history of our planet is marked by two major events: a pulse of continental crust formation at the end of the Archaean eon and a weak oxygenation of the atmosphere (the Great Oxidation Event) that followed, at 2.45 billion years ago. This oxygenation has been linked to the emergence of oxygenic cyanobacteria1,2 and to changes in the compositions of volcanic gases3,4, but not to the composition of erupting lavas--geochemical constraints indicate that the oxidation state of basalts and their mantle sources has remained constant since 3.5 billion years ago5,6. Here we propose that a decrease in the average pressure of volcanic degassing changed the oxidation state of sulphur in volcanic gases, initiating themodern biogeochemical sulphur cycle and triggering atmospheric oxygenation. Using thermodynamic calculations simulating gas-melt equilibria in erupting magmas, we suggest that mostly submarine Archaean volcanoes produced gases with SO2/H2S,1 and low sulphur content. Emergence of the continents due to a global decrease in sea level and growth of the continental crust in the late Archaean then led to widespread subaerial volcanism, which in turn yielded gases much richer in sulphur and dominated bySO2. Dissolution of sulphur in sea water and the onset of sulphate reduction processes could then oxidize the atmosphere

Morphology and microstructure of chromite crystals in chromitites from the Merensky Reef (Bushveld Complex, South Africa)

The Merensky Reef of the Bushveld Complex consists of two chromitite layers separated by coarse-grained melanorite. Microstructural analysis of the chromitite layers using electron backscatter diffraction analysis (EBSD), high-resolution X-ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals: fine-grained idiomorphic and large silicate inclusion-bearing crystals. The lower chromitite layer contains both populations, whereas the upper contains only fine idiomorphic grains. Most of the inclusion-bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre-existing smaller idiomorphic crystals. Two possible mechanisms have been proposed for sintering of chromite crystals: (1) amalgamation of a cluster of grains with the same original crystallographic orientation; and (2) sintering of randomly orientated crystals followed by annealing into a single grain. The EBSD data show no evidence for clusters of similarly oriented grains among the idiomorphic population, nor for earlier presence of idiomorphic subgrains spatially related to inclusions, and therefore are evidence against both of the proposed sintering mechanisms. Electron backscatter diffraction analysis maps show deformation-related misorientations and curved subgrain boundaries within the large, amoeboidal crystals, and absence of such features in the fine-grained population. Microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers, and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation.An alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals, formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the Merensky Reef. The upper chromite layer formed from a subsequent magma influx, and hence lacked a mechanism to form dendritic chromite. This accounts for the difference between the two layers

Ancestral Inference and the Study of Codon Bias Evolution: Implications for Molecular Evolutionary Analyses of the Drosophila melanogaster Subgroup

Reliable inference of ancestral sequences can be critical to identifying both patterns and causes of molecular evolution. Robustness of ancestral inference is often assumed among closely related species, but tests of this assumption have been limited. Here, we examine the performance of inference methods for data simulated under scenarios of codon bias evolution within the Drosophila melanogaster subgroup. Genome sequence data for multiple, closely related species within this subgroup make it an important system for studying molecular evolutionary genetics. The effects of asymmetric and lineage-specific substitution rates (i.e., varying levels of codon usage bias and departures from equilibrium) on the reliability of ancestral codon usage was investigated. Maximum parsimony inference, which has been widely employed in analyses of Drosophila codon bias evolution, was compared to an approach that attempts to account for uncertainty in ancestral inference by weighting ancestral reconstructions by their posterior probabilities. The latter approach employs maximum likelihood estimation of rate and base composition parameters. For equilibrium and most non-equilibrium scenarios that were investigated, the probabilistic method appears to generate reliable ancestral codon bias inferences for molecular evolutionary studies within the D. melanogaster subgroup. These reconstructions are more reliable than parsimony inference, especially when codon usage is strongly skewed. However, inference biases are considerable for both methods under particular departures from stationarity (i.e., when adaptive evolution is prevalent). Reliability of inference can be sensitive to branch lengths, asymmetry in substitution rates, and the locations and nature of lineage-specific processes within a gene tree. Inference reliability, even among closely related species, can be strongly affected by (potentially unknown) patterns of molecular evolution in lineages ancestral to those of interest

A bimodal alkalic shield volcano on Skiff Bank: Its place in the evolution of the Kerguelen Plateau

A bimodal volcanic sequence of 230 m thickness on Skiff Bank, a western salient of the northern Kerguelen Plateau, was drilled during ODP Leg 183. The sequence comprises three main units: a mafic unit of trachybasalt flows sandwiched between two units of trachytic or rhyolitic flows and volcaniclastic rocks. Although interpretation is complicated by moderate to strong alteration of the rocks, their original chemical character can be established using the least mobile major and trace elements (Al, Th, high field strength elements and rare earth elements). High concentrations of alkalis and incompatible trace elements indicate that both mafic and felsic rocks are alkalic. The felsic rocks may have been derived by partial melting of mafic rocks, followed by fractionation of feldspar, clinopyroxene, Fe-Ti oxides and apatite. The mafic and felsic rocks have similar Nd and Pb isotopic compositions; Pb-206/Pb-204 ratios are low (17.5-18.0) but, like the Nd-143/Nd-144 ratios (0.5125-0.5126), they are comparable with those of basalts from the central and southern Kerguelen Plateau (e.g. Sites 747, 749, 750). The Sr isotopic system is perturbed by later alteration. There is no chemical or isotopic evidence for a continental crustal component. The bimodal alkalic character and the presence of quartz-phyric rhyolites is interpreted to indicate that the sequence forms part of a shield volcano built upon the volcanic plateau. The age of 68 Ma, obtained on Site 1139 rocks by Duncan (A time frame for construction of the Kerguelen Plateau and Broken Ridge, Journal of Petrology 43, 1109-1119, 2002), provides only a minimum age for the underlying flood volcanic rocks. The high age indicates none the less that Skiff Bank is not the present location of the Kerguelen plume

Differentiation, crustal contamination and emplacement of magmas in the formation of the Nantianwan mafic intrusion of the ~260 Ma Emeishan large igneous province, SW China

The Nantianwan mafic intrusion in the Panxi region, SW China, part of the ~260 Ma Emeishan large igneous province, consists of the olivine gabbro and gabbronorite units, separated by a transitional zone. Olivine gabbros contain olivine with Fo values ranging from 83 to 87, indicating crystallization from a moderately evolved magma. They have 0.2 to 0.9 wt % sulfide with highly variable PGE (17-151 ppb) and variable Cu/Pd ratios (1,500-32,500). Modeling results indicate that they were derived from picritic magmas with high initial PGE concentrations. Olivine gabbros have negative εNd(t) values (-1.3 to -0.1) and positive γOs(t) values (5-15), consistent with low degrees of crustal contamination. Gabbronorites include sulfide-bearing and sulfide-poor varieties, and both have olivine with Fo values ranging from 74 to 79, indicating crystallization from a more evolved magma than that for olivine gabbros. Sulfide-bearing gabbronorites contain 1.9-4.1 wt % sulfide and 37-160 ppb PGE and high Cu/Pd ratios (54,000-624,000). Sulfide-poor gabbronorites have 0.1-0.6 wt % sulfide and 0.2-15 ppb PGE and very high Cu/Pd ratios (16,900-2,370,000). Both sulfide-bearing and sulfide-poor gabbronorites have εNd(t) values (-0.9 to -2.1) similar to those for olivine gabbros, but their γOs(t) values (17-262) are much higher and more variable than those of the olivine gabbros. Selective assimilation of crustal sulfides from the country rocks is thus considered to have resulted in more radiogenic 187Os of the gabbronorites. Processes such as magma differentiation, crustal contamination and sulfide saturation at different stages in magma chambers may have intervened during formation of the intrusion. Parental magmas were derived from picritic magmas that had fractionated olivine under S-undersaturated conditions before entering a deep-seated staging magma chamber, where the parental magmas crystallized olivine, assimilated minor crustal rocks and reached sulfide saturation, forming an olivine- and sulfide-laden crystal mush in the lower part and evolved magmas in the upper part of the chamber. The evolved magmas were forced out of the staging chamber and became S-undersaturated due to a pressure drop during ascent to a shallow magma chamber. The magmas re-attained sulfide saturation by assimilating external S from S-rich country rocks. They may have entered the shallow magma chamber as several pulses so that several gabbronorite layers each with sulfide segregated to the base and a sulfide-poor upper part. The olivine gabbro unit formed from a new and more primitive magma that entrained olivine crystals and sulfide droplets from the lower part of the staging chamber. A transitional zone formed along the boundary with the gabbronorite unit due to chemical interaction between the two rock units. © 2012 Springer-Verlag.link_to_subscribed_fulltex