Standard chemical‐based tephra extraction methods significantly alter the geochemistry of volcanic glass shards     

Claire L. Cooper  Ivan P. Savov  Graeme T. Swindles 《第四纪科学杂志》2019,34(8):697-707

The chemical compositions of tephra shards are widely utilised in a myriad of disciplines, including volcanology, petrology, tephrochronology, palaeoecology and climate studies. Previous research has raised concerns over the possible chemical alteration of microscopic (<100 µm) volcanic glass shards through standard extraction procedures, such as the widely used acid digestion method. This study subjects 10 samples of well‐characterised volcanic glasses ranging from basalt to rhyolite to three common methods used in the extraction of volcanic material from lake sediments and peats. The major element geochemistry of each sample was analysed and compared with a control group. The results of this test indicate that basaltic and andesitic glasses are highly susceptible to chemical alteration, particularly to the concentrated corrosive materials used in acid and base digestion techniques. PERMANOVA analysis of the variation within groups suggests that the oxides most susceptible to variation are alkalis from groups I and II (K₂O, Na₂O, CaO, MgO) and SiO₂, and the most stable oxides are Al₂O₃ and FeO. Felsic glasses are considerably less susceptible to alteration by both acidic (HCl, HNO₃, H₂SO₄) and alkaline (KOH) digestions. Our findings have important implications for interpreting the geochemistry of volcanic glasses. Copyright © 2019 John Wiley & Sons, Ltd.  相似文献   

On the occurrence of fossil microspores,pollen grains,and microplankton in bottom sediments of the Ross Sea,Antarctica     

Graeme J. Wilson 《新西兰海洋与淡水研究杂志》2013,47(3):381-389

Palynomorphs from off‐shore glacial sediments provide information on the nature and age of strata buried beneath the Antarctic ice. Four mud samples, collected between 530 and 1,300 m, from widely separated areas of the Ross Sea floor contain palynomorphs of Lower Tertiary and Permo‐Triassic ages. The Lower Tertiary species, which include both marine and non‐marine taxa, can be correlated directly with similar forms occurring in Tertiary erratics in moraines in the McMurdo Sound region. This could indicate a widespread occurrence of Tertiary rocks beneath the Ross Sea, Ross Ice Shelf, and adjacent ice‐covered areas. The Permo‐Triassic species show considerable variation in degree of carbonisation. They do not include marine forms and have probably been derived, either directly or indirectly, from various horizons and facies of the Beacon Group.  相似文献   

Astrometry of Southern radio sources     

Graeme L. White  David L. Jauncey  Bruce R. Harvey  Ann Savage  Samuel Gulkis  Robert A. Preston  Bruce A. Peterson  John E. Reynolds  George D. Nicolson  D. F. Malin 《Astrophysics and Space Science》1991,177(1-2):79-83

An overview is presented of a number of astrometry and astrophysics programs based on radio sources from the Parkes 2.7 GHz catalogues. The programs cover the optical identification and spectroscopy of flat-spectrum Parkes sources and the determination of their milliarc-second radio structures and positions. Work is also in progress to tie together the radio and Hipparcos positional reference frames. A parallel program of radio and optical astrometry of southern radio stars is also under way.  相似文献   

Lithodiversity and Its Spatial Association with Metallic Mineral Sites,Great Basin of Nevada     

Mihalasky  Mark J.  Bonham-Carter  Graeme F. 《Natural Resources Research》2001,10(3):209-226

Geographical information system (GIS) techniques were used to investigate the spatial association between metallic mineral sites and lithodiversity in Nevada. Mineral site data sets include various size and type subsets of about 5,500 metal-bearing occurrences and deposits. Lithodiversity was calculated by counting the number of unique geological map units within four sizes of square-shaped sample neighborhoods (2.5-by-2.5, 5-by-5, 10-by-10, and 20-by-20 km) on three different scales of geological maps (national, 1:2,500,000; state, 1:500,000; county, 1:250,000). The spatial association between mineral sites and lithodiversity was observed to increase with increasing lithodiversity. This relationship is consistent for (1) both basin-range and range-only regions, (2) four sizes of sample neighborhoods, (3) various mineral site subsets, (4) the three scales of geological maps, and (5) areas not covered by large-scale maps. A map scale of 1:500,000 and lithodiversity sampling neighborhood of 5-by-5 km was determined to best describe the association. Positive associations occurred for areas having >3 geological map units per neighborhood, with the strongest observed at approximately >7 units. Areas in Nevada with more than three geological map units per 5-by-5 km neighborhood contain more mineral sites than would be expected resulting from chance. High lithodiversity likely reflects the occurrence of complex structural, stratigraphic, and intrusive relationships that are thought to control, focus, localize, or expose mineralization. The application of lithodiversity measurements to areas that are not well explored may help delineate regional-scale exploration targets and provide GIS-supported mineral resource assessment and exploration activity another method that makes use of widely available geological map data.  相似文献   

Classifying mineral potential in support of land-use policy decisions in British Columbia,Canada     

Graeme P. McLaren 《Natural Resources Research》1992,1(1):85-96

British Columbia covers a vast segment of the Cordillera Mountain system that is richly endowed with a diversity of resources. British Columbia's historic patterns of resource development increasingly have been in conflict with demands for greater environmental protection. To avoid such conflicts, a recently legislated process that provides for detailed mineral resource assessments in candidate park areas has stimulated the creation of a mineral potential classification system for use in land-use planning and policy decisions.A mineral potential study of the Chilko Lake Planning Area provided three unique categories of field data on which to build the classification system. These categories are geological setting, geochemistry, and mineral occurrences. Data in each category were compiled independently to provide indicators of mineral potential. The field data were used to develop a widely understood classification of mineral potential. The classification is based on two factors: favorability and degree of confidence.  相似文献   

Reviews     

A. Hubaux  Frank Simpson  Graeme Bonham-Carter 《Mathematical Geosciences》1970,2(4):415-420

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From compassion to compliance? Trends in refugee and humanitarian migration in Australia     

Graeme Hugo 《GeoJournal》2002,56(1):27-37

Australia has a long history of offering protection to refugees. In the post-World War II period it has predominantly been involved in an offshore refugee settlement program whereby it has assisted international efforts to assist refugees and resettled almost 600,000 refugees in Australia. In recent years, however, substantial numbers of asylum seekers have arrived on Australia's shores and this has seen the development of an onshore refugee program which is seen as quite separate from the offshore program. This has involved the detention of asylum seekers while their claims for protection are assessed and when they are assessed as being qualified for protection they are given different status to offshore determined refugee settlers. The paper summarises the development of refugee policy in Australia and the evolution of the offshore and onshore policies. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

Major active faults determine the location of the Tongonan geothermal field: Evidence provided by rock alteration and stable isotope geochemistry     

Graeme L. Scott 《Island Arc》2004,13(2):370-386

Abstract The influence of major active faults on rock alteration and stable isotope geochemistry is described for the Tongonan geothermal field, Leyte, the Philippines. In the Pliocene, acid alteration with characteristic iron enrichment (3 g/100 g) and calcium depletion (2 g/100 g) occurred along a Riedel shear fault in the Malitbog sector, and initial minor acid alteration also occurred along a similar shear in the Mahiao sector. Later, sodium metasomatism (5 g/100 g) coincided with the highest aquifer chloride (10 000 mg/kg) as a result of dissociation of saline magmatic fluids discharging through the reservoir rocks in the Upper Mahiao. The incursion of magmatic fluids (possibly δD 35‰, δ¹⁸O +7‰) set up a vigorous convection cell of meteoric water, which focused around low‐angle (L) shears centered in the Sambaloran sector. Meteoric water (δD ?35 to ?40‰, δ¹⁸O ?6 ± 1‰) depleted the reservoir in silica (6 g/100 g) and potassium (1–2 g/100 g). It also completely exchanged oxygen isotopes rapidly (within months) at high temperatures (300–400°C), and now does so continuously with fractured isotopically fresh or incompletely altered rock at small scales (centimeters or less) exposed by a 2 cm/year creep around the L shears to form a new component called geothermal water. Geothermal water mixes with meteoric water at lower temperatures (<300°C) to create the characteristic shift in δ¹⁸O of 6‰ at near constant δD (?35 ± 5‰). The 10‰ variation in δD is due to groundwater recharge derived from rain falling on steep terrain (5‰) and to enrichment of deuterium in boiling saline solutions (5‰); it is not due to two‐component mixing of meteoric with magmatic water. The low (～1) isotopic water/rock (W/R) ratios calculated from oxygen isotopes in previously published reports are meaningless, because the water contains four components (predominantly geothermal and meteoric water; <10% magmatic and rock water). W/R ratios of up to 1500 calculated from spring and rock chemistry are more realistic and, with a flow rate of approximately 50 L/s through a 30 km³ reservoir, can account for the estimated 3 My age of the system.  相似文献   

Aerodynamic entrainment thresholds and dislodgement rates on impervious and permeable beds     

Jon J. Williams  Graeme R. Butterfield  David G. Clark 《地球表面变化过程与地形》1990,15(3):255-264

In the literature it has been suggested that on permeable, granular beds, both the threshold and rate of aerodynamic entrainment may be affected significantly by seepage flows into and out of the bed induced by fluctuating pressures in the overlying turbulent boundary layer. Using a range of grain sizes and flow conditions, the series of laboratory experiments reported here compares directly the aerodynamic entrainment of loose grains overlying fixed permeable sediment beds with that occuring over fixed impervious beds. For a given granular material, no significant differences in entrainment dynamics on the two types of bed were observed and in the range of flow conditions examined both the threshold shear velocity (U _*T) and the aerodynamic entrainment coefficient (k) were found to be independent of bed permeability.  相似文献   

The effect of soil solution chemistry on the weathering rate of a Histic Andosol     

Bergur Sigfusson  Sigurdur R. Gislason  Graeme I. Paton 《Journal of Geochemical Exploration》2006,88(1-3):321

Histic Andosol in Western Iceland was studied using laboratory based repacked microcosms conjointly with sampling of field soil solution. The main primary phase of the 205 cm thick soil profile was basaltic glass, allophane content ranged from 2 to 22 wt.% and the soil carbon content ranged from 11 to 42 wt.%. At constant temperature, the dissolution rate of the basaltic glass, and probably allophane and imogolite, was dictated by the a_H+³ / a_Al3+ activity ratio only, which in turn is governed by the pH, total dissolved Al and the anions capable of complexing Al³⁺; SO₄²⁻, F⁻ and organic anions (DOC). Dissolution rate was slowed down by up to 20% by decreasing undersaturation in the field. Dissolution rate of basaltic glass was stable after an initial flushing event at the beginning of microcosm experiments. Predicted dissolution rates increased up to a factor of 7 and 30 by speciating Al³⁺ with oxalate in field and microcosms respectively. Speciation with oxalate generally had more effect in shallow horizons than deep horizons.  相似文献