Garnet inclusions in diamond and the state of the upper mantle

Temperature and pressure estimates for Earth's upper mantle generally are based on indirect information derived from phase equilibria studies and the measurement of temperature and pressure dependent physical and chemical properties for relevant mantle materials. This paper describes an alternative approach, based on solid-inclusion piezothermometry, which utilizes the thermoelastic properties of direct mantle derived mineral samples. In particular, this study provides the theoretical development, based on the Murnaghan equation of state for solids, for a simple method of calculating isomeke lines for host and inclusion minerals of cubic symmetry which may be extrapolated accurately to upper mantle pressure and temperature conditions. The method is demonstrated for the particular case of garnet inclusions in diamond, for which adequate laboratory thermoelastic data are available. A specific application is made in the evaluation of the depth of formation of the D1 garnet-diamond inclusion system described by Harris et al. (1970). The pressure and temperature conditions of inclusion formation lie along the calculated isomeke line within the range constrained by recent graphite-diamond phase equilibria data. However, because the isomeke line for the garnet-diamond system and the graphite-diamond phase transition are very similar in slope, a further constraint is required. Assuming, therefore, that temperature in the upper mantle is bounded by the “Oceanic” and “Shield” geotherms of Clark and Ringwood (1964), the present results indicate that the D1 garnet-diamond system formed within the depth range 138 to 155 km (about 45 to 53 kbar pressure). This result, which relates to the genesis of kimberlite xenoliths, is generally consistent with the results of other studies which utilize phase equilibria data.     

The role of sidewall processes in gully development; some N.S.W. examples

Little attention has been paid to the role of sidewall processes in gully development. Simple estimates of linear incision/sidewall erosion ratios at two localities in New South Wales, Australia suggest that sidewall erosion is responsible for more than half the gully volume. Studies at three localities where overland flow and throughflow are limited to only one sidewall indicate that these processes are responsible for 10-30 per cent of gully volume. These observations have important implications for gully management.     

Non-cylindrical, flexural slip folding in the Ardwell flags—a statistical approach

Non-cylindrical, flexural slip folding is described from the well-known coastal section between Ardwell Bay and Kennedy's Pass, near Girvan, southwest Scotland. Bedding plane slip is recognised by the ubiquitous slickenside striations on bedding surfaces, and these linear elements define the ac kinematic plane of fold formation. This intersects the axial surfaces of folds (ab) in the common direction a which is the movement direction during fold propagation. Statistical treatment of the orientation of structural and kinematic elements yields important conclusions concerning fold formation: the Ardwell folds are markedly non-cylindrical and this is a primary feature amplified during the Ardwell Fold Phase.     

The Late Proterozoic ophiolite of Sol Hamed,NE Sudan

The Sol Hamed complex, a sequence from ultramafics, through gabbros, thin sheeted dykes, to pillow lavas, is an ophiolite. It was obducted, tilted nearly to vertical, eroded and covered unconformably by the Nafirdeib Series. This is a volcaniclastic series, dated at 712 ± 58 Ma, the lower part of which includes conglomerates with ophiolitic clasts, olistostromes, one with large oolitic limestone slabs, felsitic and dacitic tuffs, turbidites and black shales, with andesites above. Some of the structures in the ophiolite are attributed to sub-oceanic deformation. Deformation after the deposition of the Nafirdeib Series produced folds and cleavage trending about NE-SW and, more locally, a cleavage trending NW in a shear zone where the ophiolite was thrust northeastwards over the Nafirdeib Series. The ophiolite and the Nafirdeib Series were intruded by the batholithic granite dated elsewhere at ca. 669 Ma. The ophiolite is thought to occur on a suture with the Nafirdeib Series representing part of an island arc sequence.     

Impacts of the Jessica oil spill on intertidal and shallow subtidal plants and animals

Densities of fishes, invertebrates and plants at rocky intertidal and shallow subtidal sites were censused 1-2 days prior to the Jessica oil spill and compared with information obtained for the same sites one month after the spill, both for sites impacted by oil and unaffected reference sites. While the availability of pre-spill data made this analysis one of the most powerful to date for testing impacts of oil on shoreline environments, no clear changes attributable to oiling could be identified. Discharged oil appeared to cause very little impact in the intertidal and shallow subtidal zones, with such impacts lying within the range of natural spatial and temporal variation at sites investigated. Factors considered to minimize impact in Galápagos included paucity of fully sheltered shores in spill path, moderate wave action, warm temperature, high levels of sunlight, and mixing of bunker oil with diesel.     

The effect of the Jessica grounding on subtidal invertebrate and plant communities at the Galápagos wreck site

Impacts of the grounding of the oil tanker Jessica off San Cristóbal island, Galápagos, included both effects of oil on biota and also mechanical effects associated with a shallow furrow 50 m x 30 m gouged across the rocky seafloor and wreckage strewn over 7500 m(2). The wreckage represented a minor but potentially chronic source of pollution to the surrounding environment through delayed releases of oil, antifouling compounds and other toxic chemicals, and a possible source of exotic marine taxa. Investigation at the wreck site indicated that impacts on subtidal plant and macro-invertebrate communities were largely confined within 100 m of the wreck site itself. Observed population effects included significant increases in cover of opportunistic algae (filamentous green algae, filamentous red algae and Ulva sp.) and the hydroid Ectopleura media adjacent to the wreck, while densities of the green sea urchin Lytechinus semituberculatus significantly decreased from 0.3 m(-2) adjacent to the wreck to 11 m(-2) at 100 m distance.     

Effects of the Jessica oil spill on artisanal fisheries in the Galápagos

In contrast to local perceptions, the impact on the local Galápagos artisanal fishery of the 16 January 2001 grounding of the Jessica and subsequent oil spill was relatively minor. No significant changes in fishing effort, total fishing catches or catch-per-unit effort were detected after the spill based on analyses of fisheries monitoring data. Nevertheless, large boats tended to move away from sites near the path of the spill following the grounding in 2001, with no fishing recorded from the oil-affected regions of Floreana and southern Isabela in February 2001. The total fishing effort of small boats operating from the Jessica-grounding island of San Cristóbal also declined immediately after the spill, probably in part because such boats were used in clean up operations. During 2001, prices paid to fishers remained stable at levels higher than in 2000, with the notable anomaly that prices fell precipitously to 30% of previous levels during a 1-2 week period in early February 2001. Fish exports remained at similar levels for the years 2000 and 2001; however, as in the previous year, little fish product was exported from Galápagos in the month following the spill, with most fish product dried and stored for up to two months prior to transport to the continent.     

Binary upscaling—the role of connectivity and a new formula

Although recognized as important, measures of connectivity (i.e. the existence of high-conductivity paths that increase flow and allow for early solute arrival) have not yet been incorporated into methods for upscaling hydraulic conductivities of porous media. We present and evaluate a binary upscaling formula that utilizes connectivity information. The upscaled hydraulic conductivity (K) of binary media is determined as a function of the proportions and conductivities of the two materials, the geometry of the inclusions, and the mean distance between them. The use of a phase interchange theorem renders the formula equally applicable to two-dimensional media with inclusions of low K and high K as compared with the matrix. The new upscaling formula is tested on two-dimensional binary random fields spanning a broad range of spatial correlation structures and conductivity contrasts. The computed effective conductivities are compared to what is obtained using self-consistent effective medium theory, the coated ellipsoids approximation, and to a streamline approach. It is shown that, although simple, the proposed formula performs better than available methods for binary upscaling. The use of connectivity information leads to significantly improved behavior close to the percolation threshold. The proposed upscaling formula depends exclusively on parameters that are obtainable from field investigations.     

Helium isotope composition of the early Iceland mantle plume inferred from the Tertiary picrites of West Greenland

Picrites from the 61 million year old Vaigat Formation of the Nuussuaq Peninsula in West Greenland have ³He/⁴He ratios trapped in olivine phenocrysts which range up to 30 times the atmospheric ratio. These high values, measured during gas extraction by crushing in vacuum, are similar to the highest magmatic ³He/⁴He ratios found in young terrestrial volcanic rocks. By analogy with young basalts, in which crushing selectively extracts magmatic helium, any significant cosmogenic ³He appears to be absent in these picrites. Additional evidence for the absence of cosmogenic helium is provided by fusion results on the crushed olivine powders and by a single stepwise crushing experiment, in which only magmatic and radiogenic helium components are resolvable. The West Greenland picrites have Pb, Nd and Sr isotope compositions which overlap those found in picrites from Iceland and in basalts from Loihi Seamount, localities which today also have high ³He/⁴He ratios. Isotopic variations in He, Pb, Nd and Sr for the West Greenland picrites are interpreted to largely result from interaction of the early Iceland mantle plume with the upper mantle during plume ascent and dispersion beneath the continental lithosphere. The presence of high ³He/⁴He ratios in West Greenland, and the onset of magmatism across the North Atlantic Volcanic Province near 62 Ma, supports the hypothesis for very rapid dispersion (>1 m/year) of mantle plume head material during the earliest stages of plume impact, as predicted in recent numerical simulations of plume behavior during thermal mantle convection with non-Newtonian rheology.