Paleozoic Accretion-Collision Events and Kinematics of Ductile Deformation in the Eastern Part of the Southern-Central Tianshan Belt, China

The Tianshan range could have been built by both late Early Paleozoic accretion and Late Paleozoic collision events. The late Early Paleozoic Aqqikkudug-Weiya suture is marked by Ordovician ophiolitic melange and a Silurian flysch sequence, high-pressure metamorphic relics, and mylonitized rocks. The Central Tianshan belt could principally be an Ordovician volcanic arc; whereas the South Tianshan belt, a back-arc basin. Macro- and microstructures, along with unconformities, provide some kinematic and chronological constraints on 2-phase ductile deformation. The earlier ductile deformation occurring at ca. 400 Ma was marked by north-verging ductile shearing, yielding granulite-bearing ophiolitic melange blocks and garnet-pyroxene-facies ductile deformation, and the later deformation, a dextral strike-slip tectonic process, occurred during the Late Carboniferous-Early Permian. Early Carboniferous molasses were deposited unconformably on pre-Carboniferous metamorphic and ductilely sheared rocks, implying t     

Formation of metal in Grosvenor Mountains 95551 and comparison to ordinary chondrites

Siderophile element abundances in individual metal grains in the ungrouped chondrite Grosvenor Mountains (GRO) 95551 and in the ordinary chondrites Tieschitz H3.6, Soko-Banja LL4, and Allegan H5 were measured with laser ablation-inductively coupled plasma mass spectrometry. Matrix metal in GRO 95551 falls into two distinct compositional groups, a high-Ni group with 7.2 ± 0.4 wt% Ni and a low-Ni group with 3.7 ± 0.1 wt% Ni, indicating that kamacite/taenite equilibration at ∼1020 K was followed by rapid cooling. The nonrefractory siderophile elements P, Co, Cu, Ga, Ge, As, Pd, and Au also partition between the high-Ni and low-Ni metal in a manner consistent with kamacite/taenite fractionation, but the refractory siderophiles Ru, Re, Os, Ir, and Pt show correlated variations that are unrelated to kamacite/taenite partitioning and indicate that variations in refractory components of the metal were not completely erased during equilibration at ∼1020 K. The Ni-normalized bulk metal composition of GRO 95551 is refractory depleted and volatile rich relative to Bencubbin and related metal-rich chondrites but bears strong similarities to equilibrated ordinary chondrite metal. GRO 95551 represents a new chondrite type with chemical affinity to the ordinary chondrites. Individual metal grains in unequlibrated ordinary chondrites also have correlated variations in refractory siderophile contents that cannot be produced by redox processes alone; these variations span three orders of magnitude and diminish with increasing metamorphic grade of the ordinary chondrites.     

A reconstruction of Archean biological diversity based on molecular fossils from the 2.78 to 2.45 billion-year-old Mount Bruce Supergroup, Hamersley Basin, Western Australia

Bitumens extracted from 2.7 to 2.5 billion-year-old (Ga) shales of the Fortescue and Hamersley Groups in the Pilbara Craton, Western Australia, contain traces of molecular fossils. Based on a combination of molecular characteristics typical of many Precambrian bitumens, their consistently and unusually high thermal maturities, and their widespread distribution throughout the Hamersley Basin, the bitumens can be characterized as ‘probably of Archean age’. Accepting this interpretation, the biomarkers open a new window on Archean biodiversity. The presence of hopanes in the Archean rocks confirms the antiquity of the domain Bacteria, and high relative concentrations of 2α-methylhopanes indicate that cyanobacteria were important primary producers. Oxygenic photosynthesis therefore evolved > 2.7 Ga ago, and well before independent evidence suggests significant levels of oxygen accumulated in the atmosphere. Moreover, the abundance of cyanobacterial biomarkers in shales interbedded with oxide-facies banded iron formations (BIF) indicates that although some Archean BIF might have been formed by abiotic photochemical processes or anoxygenic phototrophic bacteria, those in the Hamersley Group formed as a direct consequence of biological oxygen production. Biomarkers of the 3β-methylhopane series suggest that microaerophilic heterotrophic bacteria, probably methanotrophs or methylotrophs, were active in late Archean environments. The presence of steranes in a wide range of structures with relative abundances like those from late Paleoproterozoic to Phanerozoic sediments is convincing evidence for the existence of eukaryotes in the late Archean, 900 Ma before visible fossil evidence indicates that the lineage arose. Sterol biosynthesis in extant eukaryotes requires molecular oxygen. The presence of steranes together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least ∼1% of the present atmospheric level (PAL) and may have been sufficient to support aerobic respiration.     

The origin of organic matter in the solar system: evidence from the interplanetary dust particles

The detailed examination of meteorites and interplanetary dust particles provides an opportunity to infer the origin of the organic matter found in primitive Solar System materials. If this organic matter were produced by aqueous alteration of elemental (graphitic or amorphous) carbon on an asteroid, then we would expect to see the organic matter occurring preferentially in interplanetary materials that exhibit evidence of aqueous activity, such as the presence of hydrated silicates. On the other hand, if the organic matter were produced either during the nebula phase of Solar System evolution or in the interstellar medium, we might expect this organic matter to be incorporated into the dust as it formed. In that case pre-biotic organic matter would be present in both the anhydrous and the hydrated interplanetary materials. We have performed carbon X-ray absorption near-edge structure spectroscopy and infrared spectroscopy on primitive anhydrous and hydrated interplanetary dust particles (IDPs) collected by NASA from the Earth's stratosphere. We find that organic matter is present in similar types and abundances in both the anhydrous and the hydrated IDPs, and, in the anhydrous IDPs some of this organic matter is the “glue” that holds grains together. These measurements provide the first direct, experimental evidence from the comparison of extraterrestrial samples that the bulk of the pre-biotic organic matter occurs in similar types and abundances in both hydrated and anhydrous samples. This indicates that the bulk of the pre-biotic organic matter in the Solar System did not form by aqueous processing, but, instead, had already formed at the time that primitive, anhydrous dust was being assembled. Thus, the bulk of the pre-biotic organic matter in the Solar System was formed by non-aqueous processing, occurring in either the Solar nebula or in an interstellar environment. Aqueous processing on asteroids may have altered this pre-existing organic matter, but such processing did not affect in any substantial way the C=O content of the organic matter, the aliphatic C-H abundance, or the mean aliphatic chain length.     

From geometry to dynamics of microstructure: using boundary lengths to quantify boundary misorientations and anisotropy

The microstructure of a quartzite experimentally deformed and partially recrystallised at 900 °C, 1.2 GPa confining pressure and strain rate 10⁻⁶/s was investigated using orientation contrast and electron backscatter diffraction (EBSD). Boundaries between misoriented domains (grains or subgrains) were determined by image analysis of orientation contrast images. In each domain, EBSD measurements gave the complete quartz lattice orientation and enabled calculation of misorientation angles across every domain boundary. Results are analysed in terms of the boundary density, which for any range of misorientations is the boundary length for that range divided by image area. This allows a more direct comparison of misorientation statistics between different parts of a sample than does a treatment in terms of boundary number.The strain in the quartzite sample is heterogeneous. A 100×150 μm low-strain partially recrystallised subarea C was compared with a high-strain completely recrystallised subarea E. The density of high-angle (>10°) boundaries in E is roughly double that in C, reflecting the greater degree of recrystallisation. Low-angle boundaries in C and E are produced by subgrain rotation. In the low-angle range 0–10° boundary densities in both C and E show an exponential decrease with increasing misorientation. The densities scale with exp(−θ/λ) where λ is approximately 2° in C and 1° in E; in other words, E has a comparative dearth of boundaries in the 8–10° range. We explain this dearth in terms of mobile high-angle boundaries sweeping through and consuming low-angle boundaries as the latter increase misorientation through time. In E, the density of high-angle boundaries is larger than in C, so this sweeping would have been more efficient and could explain the relative paucity of 8–10° boundaries.The boundary density can be generalised to a directional property that gives the degree of anisotropy of the boundary network and its preferred orientation. Despite the imposed strain, the analysed samples show that boundaries are not, on average, strongly aligned. This is a function of the strong sinuosity of high-angle boundaries, caused by grain boundary migration. Low-angle boundaries might be expected, on average, to be aligned in relation to imposed strain but this is not found.Boundary densities and their generalisation in terms of directional properties provide objective measures of microstructure. In this study the patterns they show are interpreted in terms of combined subgrain rotation and migration recrystallisation, but it may be that other microstructural processes give distinctive patterns when analysed in this fashion.     

Hydrogeochemical characterization of major factors affecting the quality of shallow groundwater in the coastal area at Kimje in South Korea

This study has evaluated the main hydrogeochemical characteristics that affect the quality of shallow groundwater in the coastal area at Kimje in South Korea. In this area, the chemical composition of groundwater is mostly classified into Na⁺?Cl^? and Ca²⁺+Mg²⁺?NO₃^-+Cl^? types. These types are affected by three major factors: seawater intrusion, fertilizers and redox processes. The Na⁺?Cl^? type, which is generally localized in the coastal area, shows typical characteristics of groundwater affected not only by modern seawater but also by old seawater residing in the reclaimed land. In contrast, the Ca²⁺+Mg²⁺?NO₃^-+Cl^? type, which is usually found inland, is very contaminated by fertilizers. As groundwater flows from the inland area to the coast, the redox condition changes from an oxic condition to a suboxic/anoxic condition. In addition, the reclamation has caused a large amount of underlying organic matter. These effects provide optimum conditions for the occurrence of redox processes in the coastal area.     

Evaluation of remedial alternatives for a petroleum contaminated unconfined aquifer with fluctuating groundwater level

This study evaluated remedial alternatives for a petroleum-contaminated site where an unconfined aquifer composed of a sandy layer of about 3–3.5 m thickness is covered by alluvial deposits and reclaimed soil of about 1.5 m thickness. Precambrian gneiss, of low permeability, lies below the sandy layer. The shallow water table is about 3 m below the surface, but shows high fluctuations of up to 1.5 m in response to precipitation events. The unsaturated soil near the water table and the groundwater are highly contaminated with petroleum hydrocarbons, especially toluene, which have apparently leaked from storage tanks. Selection of the remedial alternatives required consideration of the relevant laws on soil and groundwater conservation in Korea, the results of risk analysis and the hydrogeological conditions. The contaminated area has been divided into zones in which different remediation goals are set based on risk analysis and the degree of natural attenuation. It is estimated that the clean-up goal can be achieved in two years by the combined use of a trench drain and well point pumping to collect the contaminated groundwater for treatment, and a dual air injection system for the contaminated soil.