The geochemistry and evolution of early precambrian mantle   总被引：1，自引：0，他引：1  

Stanley R. Hart  Christopher Brooks 《Contributions to Mineralogy and Petrology》1977,61(2):109-128

Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and ⁸⁷Sr/⁸⁶Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:

1. starting with primordial Sr, a short period of relatively rapid ⁸⁷Sr/⁸⁶Sr growth, followed by Rb depletion;
2. a period between ≧ 3.5 b.y. and ～ 1.7 b.y. when closed-system Sr isotope evolution occurred (with Rb/Sr ～ 0.023);
3. development of large-scale Rb/Sr heterogeneities in the mantle at ～ 1.7 b.y., leading to a present-day mantle with ⁸⁷Sr/⁸⁶Sr ranging from 0.7023 to 0.7065 and Rb/Sr ranging from ～ 0 to 0.065.

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Local contributions to the M₂ lunar geomagnetic variations in the South West of England     

A.M. Hart  C.D. Honebon  W.G.V. Rosser 《Physics of the Earth and Planetary Interiors》1983,32(1):60-64

By comparing the M₂ lunar geomagnetic variations of 12.42 h period at Sidmouth and Exeter, it is shown that local concentrations of the electric currents, induced by sea tides in the seas and oceans around the U.K., can give a significant contribution to the M₂ variations at the two observatories.  相似文献   

Nickel partitioning between olivine and silicate melt   总被引：1，自引：0，他引：1  

Stanley R. Hart  Karleen E. Davis 《Earth and Planetary Science Letters》1978,40(2):203-219

Partitioning of Ni between olivine and silicate melt has been determined for compositions in the system Fo-Ab-An (1 atm) for temperatures ranging from 1250°C to 1450°C. Nickel concentrations were determined by electron microprobe; concentration levels in the liquids ranged from 0.1% to 0.5%. Platinum capsules or Pt wire loops were used as containers. Equilibrium was evaluated from kinetic considerations and by variation of run parameters; it was documented in one case by a bracketed reversal. No evidence was found for a dependence of the partition coefficient D (Ni in olivine/Ni in liquid) on Ni concentration. D is strongly dependent on melt composition, varying linearly with (1/MgO) at constant temperature. The intrinsic temperature dependence of D is small; the apparent temperature dependence reported in previous studies is largely related to the variation of melt composition with temperature. Our D values determined in the simple system Fo-An-Ab agree well with those reported by Leeman for natural (Fe-bearing) basalt systems. Overall variation of D in our system (and in natural basalts) can be expressed by the regression: D = (124/MgO) ? 0.9Our data are used to evaluate published Ni-MgO relationships in natural basalt series from Kilauea, Crozet, Cape Verde and Baffin Bay. A combination of olivine accumulation and fractional crystallization processes are sufficient to model these series. Using our data, unique “parental” liquids can be specified for each of these series; the MgO content of these liquids varies from 6% to 13%. Basalts with MgO contents greater than these “parental” liquids must be accumulative. The linear Ni-MgO trends, high absolute Ni concentrations, and large spread of Ni contents for the high-MgO basalts argue convincingly against their being “primary” liquids. Models such as those of O'Hara [6,13] and Clarke [24], based on the assertion of primary high-MgO liquids, must therefore be re-evaluated.Because of the high Si/O ratio and low MgO content of island arc andesites, the Ni partition coefficient D may be quite high. Therefore, the relatively low Ni content of such andesites may not be an argument against their derivation as direct partial melts of the mantle.  相似文献   

Clinopyroxene-matrix partitioning of K,Rb, Cs,Sr and Ba     

S.E. Hart  C. Brooks 《Geochimica et cosmochimica acta》1974,38(12):1799-1806

Extremely pure samples of clinopyroxene phenocrysts from two volcanic rocks have been analyzed for K, Rb, Cs, Sr and Ba. In conjunction with matrix concentrations, partition coefficients are obtained which are in the range 0.001–0.004 for K, Rb, Cs and Ba. These values are lower than those in the literature by factors of 6–100 but are in good agreement with values determined experimentally at pressures of 15–30 kb by Shimizu (Geochim. Cosmochim. Acta38, 1974). Values for partition coefficients measured on separates of impure or cloudy pyroxenes from these same rocks were higher and similar to those in the literature. We suggest this effect is related to ‘trapping,’ during crystal growth, of liquid which is enriched in the larger ions (such as Rb and Cs) due to lack of diffusion equilibrium in the liquid. Partition coefficient values for olivine and plagioclase from one of these same rocks were also determined.  相似文献   

Relationship of chemical fractions of heavy metals with microbial and enzyme activities in sludge and ash-amended acid lateritic soil from India   总被引：3，自引：0，他引：3  

D.?Chaudhuri  S.?TripathyEmail author  H.?Veeresh  M.?A.?Powell  B.?R.?Hart 《Environmental Geology》2003,45(1):115-123

The influence of metals, Cd, Cr, Cu, Ni, Pb and Zn, on the microbial biomass and enzyme activities of an amended acid lateritic soil were investigated under field conditions receiving a one-time application of 52 t ha^–1 of sludge, coal ash and their mixtures at 1:3, 1:1 and 3:1 proportions, and including control and chemical fertilizer treatment at crop-specific recommended doses. Paddies and peanuts were grown in the experimental plots and soil was sampled twice after 6 months and 1 year after amendment application. The heavy metals in the soil were fractionated using sequential extraction and the increments in their concentrations in amended soil with respect to the control were determined. Concentrations of Cd, Ni and Zn were determined to have increased in their mobile fractions and were more pronounced in soil collected during the second sampling, which was associated with a decrease in soil organic carbon. The size of the microbial biomass carbon and the soil enzyme activities increased with the addition of an amendment and was highest at equal proportions of coal ash and sludge. Further increase in the proportion of sludge resulted in a significant decrease in biomass carbon. Simple correlation revealed significant and strong negative relations of mobile fractions of Cd and Ni with the ratio between microbial biomass C and organic carbon in soil, while the organic carbon content and the pH were positively correlated. The microbial activities were determined to be sensitive to the concentrations of some heavy metals in mobile fractions and therefore indicated possibilities of being useful as indicators for evaluation of toxic effects of sludge-borne metals on soil organisms.  相似文献   

The Northern Cordilleran Mid-Cretaceous Plutonic Province: Ilmenite/Magnetite-series Granitoids and Intrusion-related Mineralisation   总被引：1，自引：0，他引：1  

Craig J. R. Hart    Richard J. Goldfarb    Lara L. Lewis  John L. Mair 《Resource Geology》2004,54(3):253-280

Abstract. Twenty-five Early and mid-Cretaceous (145–90 Ma) plutonic suites and belts are defined across Alaska and Yukon, in the northern North American Cordillera, on the basis of lithological, geochemical, isotopic, and geochronometric similarities. These features are combined with aeromagnetic characteristics, magnetic susceptibility measurements, and whole-rock ferric: ferrous ratios to ascertain the distribution of magnetite- and ilmenite-series plutonic belts. Magnetite-series plutonic belts are dominantly associated with the older parts of the plutonic episode and comprise subduction-generated metaluminous plutons that are distributed preferentially in the more seaward localities dominated by primitive tectonic elements. Ilmenite-series plutonic belts comprise slightly younger, slightly peraluminous plutons in more landward localities in pericratonic to continental margin settings. They were likely initiated in response to crustal thickening following terrane collision. The youngest plutonic belt forms a small, but significant, magnetite-series belts in the farthest inboard position, associated with alkalic plutons that were emplaced during weak extension.
Intrusion-related metallogenic provinces with distinctive metal associations are distributed, largely in accord with classical redox-sensitive granite-series. Copper, Au, and Fe mineralisation are associated with magnetite-series plutons and tungsten mineralisation associated with ilmenite-series plutons. However, there are some notable deviations from expected associations, as intrusion-related Ag-Pb-Zn deposits are few, and significant tin mineralisation is rare. Most significantly, many gold deposits and occurrences are associated with ilmenite-series plutons: these form the basis for the newly recognized reduced intrusion-related gold deposit model.  相似文献   

A literary geography of Soweto     

Deborah Hart 《GeoJournal》1986,12(2):191-195

Humanist inspired literary geography offers one avenue of enquiry into the study of place. Its prime intention is to untangle the subjective, personalised construction and connotations of place meaning. African literature offers a rich source of information into the black urban experience. The presentation focuses upon the literary interpretation of Soweto, South Africa's largest black township. The Sowetan environment emerges as a landscape of placelessness in which the overwhelming emphasis by white and black writers alike is upon relating appalling conditions in an austere and hostile environment.  相似文献   

Petrogenesis of a basalt-rhyolite tephra from the west-central Afar,Ethiopia     

Robert C. Walter  William K. Hart  John A. Westgate 《Contributions to Mineralogy and Petrology》1987,95(4):462-480

The Cindery Tuff is an unusual tephra fall deposit that contains evidence for the mixing of basaltic and rhyolitic liquids prior to eruption. It contains clear rhyolitic glass shards together with brown basaltic glass spheres and a broadly bimodal phenocryst assemblage. Brown glasses are ferrobasaltic in composition and are similar to the voluminous Pliocene tholeiites of the surrounding west-central Afar volcanic field; both are enriched in the light rare earth and incompatible elements and possess higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd than MORB. Rhyolitic glasses are subalkaline and, compared to the basaltic glasses, are strongly depleted in the compatible elements and enriched in the incompatible elements. Both glass types have similar incompatible element and isotopic ratios, and with the rhyolite glass showing a 2-fold parallel enrichment in rare earth element abundances over the basaltic glass. These observations suggest that the two glasses are genetically related.Rare glasses with intermediate compositions occur as phenocryst melt inclusions, as mantles on phenocrysts and as free pumice clasts. Their major element contents do not point to an origin by simple hybrid mixing of the basaltic and rhyolitic melts. Rather, major element mixing calculations indicate formation of the intermediate and rhyolite melts by fractionation of the observed phenocryst assemblage, using a starting composition of the observed basaltic glass. Model calculations from trace element data, though lacking from the intermediate glasses, support fractional crystallization. The bimodal mineral assemblage argues against an immiscible liquid origin for the contrasting glass compositions.  相似文献   

An isotopic study of hydrocarbon generation processes     

J.W. Smith  D. Rigby  K.W. Gould  G. Hart  A.J. Hargraves 《Organic Geochemistry》1985,8(5):341-347

The sources of reservoired liquid hydrocarbons in Australia differ substantially from their Northern Hemisphere counterparts. Not only are Australian source rocks largely of terrigenous origin but the concepts accepted as defining the hydrocarbon potential of source rocks have recently been challenged. Although isotopic measurements on Australian source rocks and liquid hydrocarbon products closely parallel those published from elsewhere, new interpretations of these data have been made, particularly with regard to D/H ratios in coals. As interesting as such results may be, the most important findings relate to the origins and histories of methane, the higher gaseous hydrocarbons and carbon dioxide, and the interpretation of these data in terms of hydrocarbon genesis. Simple Northern Hemisphere concepts which characterize gases as being biogenic, oil-associated, etc., on the bases of their isotopic and chemical compositions are clearly not applicable to the Australian scene. Indeed justification of such precise classifications in the Northern Hemisphere on the available evidence is also questioned.Isotopic and compositional data from gases from the Cooper, Bowen, Sydney and Gippsland Basins and from the North West Shelf (Australia) are reviewed, and assessed in terms of generation pathways.  相似文献   

Possible errors in helium analyses of soil gases by mass spectrometers having constant-pressure inlet systems     

M.K.W. Hart  M.J. Gole  C.R.M. Butt 《Journal of Geochemical Exploration》1985,24(1):121-128

Published data suggest that soil gas helium concentrations of 5.28–5.34 ppm v/v over uranium and hydrocarbon deposits are significantly anomalous compared to the ambient atmospheric background of 5.24 ppm. However, analyses for helium by mass spectrometers having constant-pressure inlet systems, from which most of these data are derived, are subject to errors of equivalent magnitude. These errors arise when the major component composition of unknown and standard gases differ, for the different gases have different flow rates through the inlet system — relative rates being O₂ < dry air < water-saturated air < N₂ < CO₂ CH₄. Soil gas compositions can vary greatly and, compared to a dry air standard, the flow-rate of a water-saturated gas containing 10% biogenic CO₂ will increase, enhancing the apparent He content to 5.33 ppm. Accurate helium analyses can be achieved by using a constant-volume inlet and integrating the detector response over the period of the samples' passage through the detector.  相似文献