High temperature or metamorphic vermiculites

New chemical data from four different geological occurrences and published information are used to define, approximately, the chemistry of vermiculite of high temperature and pressure origin. Geologic occurrence indicates that this mineral is present towards the upper limit of the muscovite-chlorite zones in pelitic rocks. Geologic conditions range from glaucophane-lawsonite metamorphism to hydrothermal zones such as the Salton Sea area.The mineral has a composition distinct from chlorite and biotite, two commonly accompanying phases, being more silica rich than metamorphic chlorites from pelitic rocks and more aluminous than biotites. Vermiculites characteristically show significant but low K₂O and CaO concentrations.     

Geological evolution,regional perspectives and hydrocarbon potential of the northwest Phu Khanh Basin,offshore Central Vietnam

Seismic stratigraphic and structural analyses of the northwest Phu Khanh Basin, offshore Central Vietnam, based on 2-D seismic data, indicate that the initial rifting began during the latest Cretaceous? or Palaeogene controlled by left-lateral transtension along the East Vietnam Boundary Fault Zone (EVBFZ) and northwest–southeast directed extension east of the EVBFZ. Rifting stopped due to transpression during middle Oligocene times but resumed by left-lateral transtension during the Late Oligocene. Thick sequences of lacustrine and alluvial sediments were deposited during the Palaeogene rift periods. The Late Oligocene rifting ended due to inversion, triggered by right-lateral wrenching near the Palaeogene–Neogene boundary. Following the onset of this inversion regional uplift and volcanism took place in the southern half of the study area and contemporaneous subsidence and transgression took place farther north, leading to widespread carbonate deposition. As the right-lateral wrenching decreased during the early Neogene, thermal subsidence and siliciclastic sedimentation became dominant, resulting in the buildup and southward propagation of the shelf slope. Sediment accumulation and subsidence rates increased after the Middle Miocene times due to eastward tilting of Central Vietnam and the adjacent offshore area.     

Ordering in synthetic aluminous serpentines; infrared spectra and cell dimensions

Infrared and X-ray diffraction studies were made on synthetic serpentines (Si₄ to Si₂Al₂ tetrahedral compositions). Changes in cell dimensions and variations in infrared spectra indicate that ordering can occur in the octahedrally coordinated site and possibly the tetrahedral site. Octahedra order into an Mg₃ and/or Mg₂Al configuration depending upon bulk mineral composition. In the latter cases an Al ion preferentially fills only one of the large M₂ sites found in dioctahedral minerals and is thus characteristic of neither di-nor trioctahedral minerals. Ordering in octahedral and possibly tetrahedral sites does not appear to affect the basic higher frequency vibrations (OH stretch, Si-O stretch) by creating new modes but definite band splitting is seen for vibrations of lower frequency. The alumina-free composition was crystallized into monophase products which can be assimilated to natural chrysotile or antigorite forms. Addition of alumina produces lizardite-type serpentines which are very closely related to the chrysotile structure as indicated by cell-dimension and Si-O stretch bands in infrared spectra.     

First discovery of stishovite in an iron meteorite

Abstract— The first occurrence of stishovite in an iron meteorite, Muonionalusta (group IVA), is reported. The mineral occurs intimately mixed with amorphous silica, forming tabular grains up to ?3 mm wide, with a hexagonal outline. It was identified using X‐ray diffraction and Raman microspectroscopy. The unit‐cell parameters of stishovite are a = 4.165(3) Å and c = 2.661(6) Å, and its chemical composition is nearly pure SiO₂. Raman spectra show relatively sharp bands at 231 and 754 cm^?1 and a broad band with an asymmetric shape and a maximum around 500 cm^?1. The rare grains are found within troilite nodules together with chromite, daubreelite, and schreibersite. From their composition and morphology, and by comparisons with silica inclusions in, e.g., the Gibeon IVA iron, we conclude that these rare grains represent pseudomorphs after tridymite. The presence of stishovite in Muonionalusta is suggested to reflect shock metamorphic conditions in the IVA parent asteroid during a cosmic impact event.     

Experimental modelling of shallow magma emplacement: Application to saucer-shaped intrusions

Saucer-shaped dolerite and sandstone intrusions are common in sedimentary basins world-wide. We have conducted a series of scaled experiments simulating the process of magma emplacement in sedimentary basins, with particular attention on the formation of saucer-shaped sills. The model materials were (1) cohesive fine-grained silica flour, representing brittle crust; and (2) molten low-viscosity oil, representing magma. The experiments were performed in both homogeneous and layered models. In all the experiments, oil injection resulted in doming of the surface. In the homogeneous models, the injected oil formed cone sheets and sub-vertical dykes. Cone sheets formed for shallow injection (1–3 cm), and vertical dykes formed for deeper injection (4–5 cm). In layered models, the injected oil always formed saucer-shaped intrusions. Our experimental results show that (1) sill intrusion results in the formation of a dome, with melt erupting at the rim; (2) layering controls the formation of sills and saucer-shaped sills; (3) saucer-shaped sills are fed from the bottom and the fluid flows upward and outward; and (4) the diameter of saucer-shaped sills increase with increasing emplacement depth. The systematic relation between domes and sills and the depth-dependence of sill diameters show that saucer-shaped intrusions result from the interaction between a growing flat-lying shallow sill and doming of the free surface. We conclude that saucer-shaped intrusions represent fundamental geometries formed by shallow magma intrusion in stratified basins.     

The 1995–2002 Long-Term Monitoring of Comet C/1995 O1 (HALE–BOPP) at Radio Wavelength

The bright comet Hale–Bopp provided the first opportunity to follow the outgassing rates of a number of molecular species over a large range of heliocentric distances. We present the results of our observing campaign at radio wavelengths which began in August 1995 and ended in January 2002. The observations were carried out with the telescopes of Nançay, IRAM, JCMT, CSO and, since September 1997, SEST. The lines of nine molecules (OH, CO, HCN, CH₃OH, H₂CO, H₂S, CS, CH₃CN and HNC) were monitored. CS, H₂S, H₂CO, CH₃CN were detected up to r_h= 3–4 AU from the Sun, while HCN and CH₃OH were detected up to 6 AU. CO, which is the main driver of cometary activity at heliocentric distances larger than 3–4 AU, was last detected in August 2001, at r_h= 14 AU. The gas production rates obtained from this programme contain important information on the nature of cometary ices, their thermal properties and sublimation mechanisms.Line shapes allow to measure gas expansion velocities, which, at large heliocentric distances, might be directly connected to the temperature of the nucleus surface. Inferred expansion velocity of the gas varied as r_h ^-0.4 within 7 AU from the Sun, but remained close to 0.4 km s^-1 further away. The CO spectra obtained at large r_hare strongly blueshifted and indicative of an important day-to-night asymmetry in outgassing and expansion velocity. The kinetic temperature of the coma, estimated from the relative intensities of the CH₃OH and CO lines, increased with decreasing r_h, from about 10 K at 7 AU to 110 K around perihelion.     

An Early Pleistocene interglacial deposit at Pingorsuit,North-West Greenland

At the Pingorsuit Glacier in North-West Greenland, an organic-rich deposit that had recently emerged from the retreating ice cap was discovered at an elevation of 480 m above sea level. This paper reports on macrofossil analyses of a coarse detritus gyttja and peaty soil, which occurred beneath a thin cover of till and glacifluvial deposits. The sediments contained remains of vascular plants, mosses, beetles, caddisflies, midges, bryozoans, sponges and other invertebrates. The flora includes black spruce, tree birch, boreal shrubs and wetland and aquatic taxa, which shows that mires, lakes and ponds were present in the area. We describe a new extinct waterwort species Elatine odgaardii. The fossils were deposited in a boreal environment with a mean July air temperature that was at least 9 °C higher than at present. The fossil assemblages show strong similarities with others from Greenland that have been assigned an Early Pleistocene age, and we suggest a similar age for the sediments found at the margin of the Pingorsuit Glacier. At the Pingorsuit Glacier in North-West Greenland, an organic-rich deposit was discovered at an elevation of 480 m above sea level. The sediments contained remains of vascular plants, mosses, beetles, caddisflies, midges, bryozoans, sponges and other invertebrates. The fossils were deposited in a boreal environment with a mean July air temperature that was at least 9 °C higher than at present.     

Geochemical constraints on the Laurentide Ice Sheet contribution to Meltwater Pulse 1A

Planktonic and benthic δ¹⁸O records adjacent to the runoff outlets of the Laurentide Ice Sheet (LIS) indicate that the LIS contributed to the abrupt ～20 m rise in sea level ～14.6 ka, Meltwater Pulse 1A (MWP-1A). However, the magnitude of the LIS contribution still remains unresolved. Here, I use a freshwater runoff–ocean mixing model to calculate the LIS meltwater required to explain the decreases in planktonic and benthic δ¹⁸O observed during MWP-1A at the southern, eastern and northern runoff outlets of the LIS. Maximum LIS contributions in equivalent sea level rise for a 500-year long MWP-1A are 2.7 m discharged into the Gulf of Mexico as a combined hyperpycnal and hypopycnal flow, 2.1 m discharged into the North Atlantic, and 0.5 m into the Arctic Ocean, for a total LIS contribution of ≤5.3 m. A LIS contribution of <30% to MWP-1A supports the hypothesis that a significant component of this MWP was sourced from the Antarctic Ice Sheet.     

Were last glacial climate events simultaneous between Greenland and France? A quantitative comparison using non‐tuned chronologies

Several large abrupt climate fluctuations during the last glacial have been recorded in Greenland ice cores and archives from other regions. Often these Dansgaard–Oeschger events are assumed to have been synchronous over wide areas, and then used as tie‐points to link chronologies between the proxy archives. However, it has not yet been tested independently whether or not these events were indeed synchronous over large areas. Here, we compare Dansgaard–Oeschger‐type events in a well‐dated record from southeastern France with those in Greenland ice cores. Instead of assuming simultaneous climate events between both archives, we keep their age models independent. Even these well‐dated archives possess large chronological uncertainties that prevent us from inferring synchronous climate events at decadal to multi‐centennial time scales. If possible, comparisons between proxy archives should be based on independent, non‐tuned time‐scales. Copyright © 2009 John Wiley & Sons, Ltd.