Ultra-high temperatures from oxygen isotope thermometry of a coesite-sanidine grospydite

The oxygen isotope compositions of coesite, sanidine, kyanite, clinopyroxene and garnet were measured in an ultra-high pressure-temperature grospydite from the Roberts Victor kimberlite, South Africa. The ¹⁸O values (per mil v. SMOW) of each phase and (1 ) are as follows: coesite, 8.62 (0.31); sanidine, 8.31 (0.02); kyanite, 7.98 (0.08); pyroxene, 7.63 (0.11); garnet, 7.53 (0.03). In situ analyses of the coesite with the laser extraction system are ¹⁸O=9.35 (0.08), n=4, demonstrating that the coesite is homogeneous. The coesite has partially inverted to polycrystalline quartz and the pyroxene is extensively altered during uplift. The larger scatter for the mineral separate coesite and pyroxene data may be due to partial reequilibration between the decompression-related breakdown products of these two phases. The anomalously high ¹⁸O value of the grospydite (¹⁸O_wholerock=7.7) is consistent with altered oceanic crust as a source rock. Temperature estimates from a linear regression of all the data to three different published calibrations correspond to an equilibrium temperature of 1310±80°C. The calculated isotopic pressure effect is to lower these estimates by about 40°C at 40 kb. The estimated temperature based on Al–Si disorder in sanidine is 1200±100°C and that from Fe–Mg exchange thermometry between garnet and clinopyroxene is 1100±50°C. Given the large errors associated with thermometry at such high temperatures, it is concluded that the xenolith equilibrated that 1200±100°C. Pressure estimates are 45±5 kb, based on dilution of the univariant equilibria albite = jadeite + coesite and 2 kyanite + 3 diopside = grossular + pyrope + 2coesite. Zoning in the outer 20 m of the feldspar from Ab_0.8 to Ab₁₆ indicates rapid decompression to 25 kb or less. The isotopic temperature estimates are the highest ever obtained and combined with the high degree of Al–Si disorder in sanidine require rapid cooling from ultra-high temperatures. It is inferred that the xenolith was sampled at the time of equilibration, providing a point on the upper Cretaceous geotherm in the mantle below South Africa.     

Coastal sedimentation in East Sussex during the Holocene

Lithostratigraphic and biostratigraphic evidence is presented which suggests that shoreward movement of sediment, resulting in the formation of a barrier beach, has been the dominant method of accretion in the Eastbourne area, with longshore drift probably being of only secondary importance. A model of coastal evolution for this area has been constructed. It appears that distinct phases of accretion have occurred during the Holocene as well as a change in the type of sediment transported into this area. Originally of silty clay then sand and finally of gravel, a distinct coarsening-upwards in the sedimentary sequence is apparent. A loss of sediment since the 18th century has resulted in the erosion of Eastbourne's beaches. This current episode of erosion may have important implications for land use. The study of Iron Age forest clearance in the Combe Haven valley of East Sussex exemplifies the inter-relationship of sedimentation between river and coastal systems. At the heart of this relationship is the hypothesis that forest clearance resulted in a substantial release of colluvial material, and a concurrent increase in river discharge. Biostratigraphic evidence, supported by radiocarbon dating, suggests that the development of the floodplain in the Combe Haven valley may be directly linked to forest clearance, and that the estuary of this river was modified by the increase in discharge due to reduced evapotranspiration and increased run off, causing a marine incursion into the lower course of the Combe Haven valley. The results from the two areas discussed suggest cyclic erosion phases associated with a change from one sediment type to another.     

Organic petrology and source rock potential of sediments in the Eromanga Basin, South Australia

Cuttings and cores from the Poolowanna 1 well, Eromanga Basin, South Australia (in which oil was discovered in Lower Jurassic reservoirs) and the Macumba 1 well (no oil) have been analyzed petrographically to assess the nature of the coals and dispersed organic matter present. The Jurassic and Cretaceous coals have medium to high vitrinite contents, low to relatively high exinite, and medium to low inertinite contents. The dispersed organic matter has comparatively less vitrinite, more exinite and/or more inertinite than the associated coals. The microlithotype compositions of the coals indicate that the original vegetation was largely woody in character and was buried before much oxidation had occurred.The Jurassic sediments contain up to 2% dispersed organic matter by volume, 0–75% of which is exinite, including alginite. Vitrinite reflectances range from 0.5 to 0.7%. Where sufficiently mature, the Jurassic sediments are good potential source rocks for hydrocarbons.Statistical testing of the analytical results for the Jurassic Poolowanna Formation using Kendall's τ as a measure of dependence shows that there is a significant association between the macerals in coal and dispersed organic matter. The ratio of exinite to inertinite in dispersed organic matter is reasonably well predicted by the corresponding ratio in the associated coal.     

Effect of enstatite activity and oxygen partial pressure on the point-defect chemistry of olivine

The variation of the point-defect concentrations in olivine with changes in enstatite activity and oxygen partial pressure are derived from formal thermodynamic arguments. Two models for these variations are constructed; the models differ in the choice of the approximate charge-neutrality condition in the near-stoichiometric regime. Under favorable circumstances, the effect of the oxygen partial pressure on olivine's point-defect chemistry can provide a diagnostic technique. Activity differences within the upper mantle may have a significant effect on olivine's transport properties because of their effect on the point-defect concentrations. The activity of enstatite plays an important role in the point-defect chemistry of olivine.     

Intertidal organisms of an industrialized estuary

A study of the intertidal organisms of the Clyde Estuary is being undertaken to assess the effects of changing levels of pollution and to relate to these and other changes the distribution of important winter flocks of waders and ducks.     

The crystal chemistry of armalcolites from Apollo 17*

X-ray single-crystal studies have been made of armalcolites from the 2–5-mm fraction of Apollo 17 soils 75082 and 78502. Two types of armalcolite, “ortho” and “para” have previously been distinguished on the basis of optical properties and crystal habit in the Apollo 17 samples. “Ortho”-armalcolite has the space group of pseudobrookite, Bbmm, and has cell dimensions: a = 9.743 (5); b = 10.001 (5); and c = 3.728 (2)Å. The crystal structure, refined from 582 symmetry-independent intensities, is ordered with Ti in the site with m symmetry and Mg and Fe in the site with mm symmetry. “Para”-armalcolite has space group Bbmm, cell dimensions: a = 9.712 (20), b = 9.997 (20), and c 3.735 (8)Å; and is structurally identical with “ortho”-armalcolite. The two crystals studied are similar but not identical in composition, but the slight compositional differences and identical structures do not warrant the use of separate mineral names.     

Trace element crystal chemistry of mantle eclogites

Rare earth element (REE) concentrations have been measured using instrumental neutron activation analysis on clean separates of primary minerals from 11 eclogite samples from the Bobbejaan and Roberts Victor kimberlites, South Africa. Samples were selected to reflect minimal secondary alteration and represent a broad range of eclogite compositions from coesiteand corundum-grospydites through magnesian bimineralic eclogites. Correlations between REE concentrations and major-element compositions suggest that garnet and clinopyroxene crystal chemistry are the dominant control on REE distribution and that these approach solidstate equilibrium distributions. Reconstructed wholerock REE concentration variation with whole-rock major-element compositions are consistent with an origin by high-pressure igneous fractionation followed by reequilibration to lower temperatures at pressures in excess of three GPa.     

Organic petrological composition of Triassic source rocks and their clastic depositional environments in some Australian sedimentary basins

Kerogen from terrestrial plant debris (type III) has commonly been considered to be a good source for hydrocarbon gas, but not for oil, compared with types I and II kerogen from marine and lacustrine sediments. The Gippsland Basin, Australia, contains giant oil fields producing from organic matter of land plant origin. Clearly some terrestrial paleodepositional environments have produced organic matter of land plant origin with the potential to generate large volumes of oil. An attempt has been made here to identify some environments that contain organic matter of terrestrial origin with the potential to generate oil.The dispersed organic matter (DOM) in sediments from various paleodepositional environments in the Northern Carnarvon, Clarence-Moreton, Simpson Desert, Bowen and Gunnedah Basins of Australia has been analysed petrographically. To reduce variations in organic matter type due to differences in geological age, examples of Triassic age only have been compared. DOM with relatively high contents of liptinite, which is widely accepted as having a better potential to generate oil than vitrinite and inertinite, was found in the following environments: fluvio-deltaic (Bowen Basin), proximal lacustrine (Gunnedah Basin) and fluvio-deltaic (Northern Carnarvon).Relationships between Triassic DOM types and paleodepositional environments found in one basin did not necessarily hold true for other basins. It is not valid to infer a unique paleodepositional environment from DOM type, but within a given basin, DOM type may be predicted from environment.     

Reply to comments by Taylor and Neal on “Trace element crystal chemistry of mantle eclogites”

Conclusion In summary, there are no existing clinopyroxene-liquid REE distribution data obtained under appropriate conditions to permit numerical modeling of trace elements to constitute a critical test of the pyroxene fractionation process. There are, however, numerous clinopyroxeneliquid partitioning data in the literature that give qualitatively similar patterns to those observed in grospydites. Further, the high pressure fractionation model accounts for the major-element and exsolution phenomena in a manner directly consistent with experimental petrologic studies, and can qualitatively account for the trace element and isotopic characteristics of the rocks in a manner consistent with the crystal chemistry of the phases. We therefore continue to prefer the high-pressure clinopyroxene fractionation model over a low-pressure plagioclase fractionation model for the origin of the grospydites.