Distribution of Ferric Iron in some Upper-Mantle Assemblages   总被引：16，自引：5，他引：11  

CANIL  DANTE; O'NEILL  HUGH ST. C. 《Journal of Petrology》1996,37(3):609-635

The distribution of ferric iron among the phases of upper-mantlerocks, as a function of pressure (P), temperature (T) and bulkcomposition, has been studied using ⁵⁷Fe Mssbauer spectroscopyto determine the Fe³⁺/Fe ratios of mineral separates from 35peridotite and pyroxenite samples. The whole-rock Fe³⁺ complementof a peridotite is typically shared approximately evenly amongthe major anhydrous phases (spinel and/or garnet, orthopyroxeneand clinopyroxene), with the important exception of olivine,which contains negligible Fe³⁺. Whole-rock Fe³⁺ contents areindependent of the T and P of equilibration of the rock, butshow a well-defined simple inverse correlation with the degreeof depletion in a basaltic component. Fe³⁺ in spinel and inboth pyroxenes from the spinel Iherzolite facies shows a positivecorrelation with temperature, presumably owing to the decreasein the modal abundance of spinel. In garnet peridotites, theFe³⁺ in garnet increases markedly with increasing T and P, whereasthat in clinopyroxene remains approximately constant. The complexnature of the partitioning of Fe³⁺ between mantle phases resultsin complicated patterns of the activities of the Fe³⁺ -bearingcomponents, and thus in calculated equilibrium f_O2, which showlittle correlation with whole-rock Fe³⁺ or degree of depletion.Whether Fe³⁺ is taken into account or ignored in calculatingmineral formulae for geothermobarometry can have major effectson the resulting calculated T and P. For Fe-Mg exchange geothermometers,large errors must occur when applied to samples more oxidizedor reduced than the experimental calibrations, whose f_O2 conditionsare largely unknown. Two-pyroxene thermometry is more immuneto this problem, and probably provides the most reliable P—Testimates. Accordingly, the convergence of P—T valuesderived for a given garnet peridotite assemblage may not necessarilybe indicative of mineral equilibrium. The prospects for thecalculation of accurate Fe³⁺ contents from electron microprobeanalyses by assuming stoichiometry are good for spinel, uncertainfor garnet, and distinctly poor for pyroxenes. KEY WORDS: mantle; oxidation; partitioning; peridotite; thermobarometry ^*Corresponding author. Present address: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, Victoria, B.C., V8W 2Y2, Canada  相似文献   

The Sulfide Capacity and the Sulfur Content at Sulfide Saturation of Silicate Melts at 1400{degrees}C and 1 bar   总被引：2，自引：0，他引：2  

O'NEILL  HUGH ST. C.; MAVROGENES  JOHN A. 《Journal of Petrology》2002,43(6):1049-1087

The solubility of sulfur as S^2– has been experimentallydetermined for 19 silicate melt compositions in the system CaO–MgO–Al₂O₃–SiO₂(CMAS)± TiO₂ ± FeO, at 1400°C and 1 bar, using CO–CO₂–SO₂gas mixtures to vary oxygen fugacity (fO₂) and sulfur fugacity(fS₂). For all compositions, the S solubility is confirmed tobe proportional to (fS₂/fO₂)^1/2, allowing the definition ofthe sulfide capacity (C_S) of a silicate melt as C_S = [S](fO₂/fS₂)^1/2.Additional experiments covering over 150 melt compositions,including some with Na and K, were then used to determine C_Sas a function of melt composition at 1400°C. The resultswere fitted to the equation  相似文献   

THE SCATTERING OF SH-CHANNEL WAVES BY A FAULT IN A COAL SEAM*     

D.J. BUCHANAN 《Geophysical Prospecting》1986,34(3):343-365

The imaging of faults in coal seams by the in-seam seismic method has now become standard practice. In the UK over 300 surveys have been undertaken and the technique is now part of the exploration arsenal of colliery planners. From these users comes the pressure for two major improvements, namely an increase in range and target identification. This paper is directed towards the latter problem. It has long been recognized that the reflected channel waves must contain information on the fault structure that caused the reflection, and model experiments have been undertaken to investigate the reflection process. Only recently, however, have attempts been made to quantify the reflection process. Calculations using both the finite-difference and finite-element techniques have been carried out, and estimates of the reflection coefficient as a function of frequency have been obtained. The object of this paper is to extend these considerations by calculating analytically the scattering matrix of an SH-channel wave after interaction at a fault plane. The scattering matrix is calculated as a function of frequency, hade angle, and fault throw. The method employed is based on the decomposition of the incident SH-channel wave into Fourier components, the calculation of plane wave reflection and transmission coefficients within the constraints of geometrical acoustics, and finally the synthesis of the scattering matrix by application of the Helmholtz-Kirchhoff integral. The calculation throughout is restricted to normal modes.  相似文献   

Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic   总被引：5，自引：1，他引：5  

HUGH C. JENKYNS  CHRISTOPHER J. CLAYTON 《Sedimentology》1986,33(1):87-106

Detailed sampling and analysis of Jurassic pelagic limestones and marls from Italy, Hungary and Switzerland have enabled construction of an isotope stratigraphy across the Pliensbachian-Toarcian boundary with resolution to the zonal level. The oxygen-isotope record is unremarkable. The carbon isotopes, however, show two positive excursions: one, relatively minor, during the Pliensbachian, margaritatus Zone, subnodosus Subzone, the other, more major, during the Toarcian. early falciferum Zone, where a maximum δ¹³C value of 4·52%_PDB is attained. These intervals are known to be favoured periods of organic-rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record. A comparable phenomenon has been documented from the Cenomanian-Turonian boundary in the Cretaceous where it has been referred to the influence of an ‘Oceanic Anoxic Event’. Some Italian sections spanning this Lower Jurassic interval contain organic-rich shales in the falciferum Zone; the isotopic signatures from their included, locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve. Carbon isotopes from the organic carbon itself are extremely negative, falling to –33δ_PDB and, in one section examined in detail, correlate with the calcium-carbonate content of the shales; they may reflect a partial change to a non-calcified planktonic biota during deposition of this lime-poor interval, possibly responding to upwelling and increased fertility of near-surface waters. The onset of upwelling may have been as early as spinatum-tenuicostatum Zone time, that is, at the Pliensbachian-Toarcian boundary.  相似文献   

Tourist Geology     

HUGH PRUDDEN 《Geology Today》1985,1(6):165-166

  相似文献   

Carbon-isotope records of the Early Jurassic (Toarcian) oceanic anoxic event from the Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps) sections: palaeoceanographic and stratigraphic implications     

NADIA SABATINO  RODOLFO NERI  ADRIANA BELLANCA  HUGH C. JENKYNS†  FRANÇOIS BAUDIN‡  GUIDO PARISI§  DANIELE MASETTI¶ 《Sedimentology》2009,56(5):1307-1328

The Toarcian oceanic anoxic event ( ca 183 Ma) coincides with a global perturbation marked by enhanced organic carbon burial and a general decrease in calcium carbonate production, probably triggered by changes in the composition of marine plankton and elevated carbon dioxide levels in the atmosphere. This study is based on high-resolution sampling of two stratigraphic successions, located in Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps), Italy, which represent expressions of the Toarcian oceanic anoxic event in deep-water pelagic sediments. These successions are characterized by the occurrence of black shales showing relatively low total organic carbon concentrations (compared with coeval strata in Northern Europe), generally < 2%, and low hydrogen indices. On this basis, they are similar to other Toarcian black shales described from the Tethyan region. The positive and negative carbon-isotope records from the two localities permit a high-resolution correlation such that ammonite biostratigraphy information from Valdorbia can be transferred to those parts of the Monte Mangart section that lack these fossils. Spectral analyses of δ¹³C_org values and of CaCO₃ percentages from the sedimentary records of both the Valdorbia and Monte Mangart sections reveal a strong cyclic pattern, best interpreted as an eccentricity signal which hence implies a duration of ca 500 kyr for the negative carbon-isotope excursion. Based on the carbon-isotope curves obtained, the high-resolution correlation between the Italian successions and a section in Yorkshire (Northern Europe) confirms the supposition that the apparent mismatch between the dating of the Toarcian oceanic anoxic event in the Boreal and Tethyan realms is an artefact of biostratigraphy.  相似文献   

Ancient oceans and continental margins of the Alpine-Mediterranean Tethys: deciphering clues from Mesozoic pelagic sediments and ophiolites   总被引：1，自引：0，他引：1  

DANIEL BERNOULLI  HUGH C. JENKYNS† 《Sedimentology》2009,56(1):149-190

  相似文献   

The South African polymict eucrite Macibini     

P. C. BUCHANAN  D. J. LINDSTROM  D. W. MITTLEFEHLDT  C. KOEBERL  W. U. REMOLD 《Meteoritics & planetary science》2000,35(6):1321-1331

Abstract— The polymict eucrite Macibini is a fragmental breccia, predominantly composed of eucritic materials with minor proportions (maximum 2 vol%) of diogenitic material. Hence, it is intermediate between the Yamato‐74159‐type polymict eucrites, which contain negligible amounts of magnesian orthopyroxene, and the howardites. The present study provides mineralogical and bulk compositional data for the meteorite breccia and for six clasts. These clasts include both volcanic and igneous rocks and a variety of impact‐generated rocks. A broad range of degrees of postcrystallization metamorphism affected these materials before the final aggregation of the breccia. Clast A is a fragment of unequilibrated eucrite with subophitic texture. The edges of the zoned pyroxenes in this clast are composed of a host of Fe‐rich augite containing vermicules (blebs) and lamellae composed of a mixture of Fe‐rich olivine and silica. Similar features occur as fragments in lunar breccias and are attributed by some workers to the breakdown of pyroxferroite, an Fe‐rich pyroxenoid. However, textures and compositions of these augite‐olivine‐silica intergrowths in clast A suggest that, in this case, they are the result of decomposition in a series of steps of Fe‐rich subcalcic augite. Among the fragments of impact‐generated material in Macibini is clast 2, an earlier‐formed clastic breccia that was lithified before being broken apart and included in the meteorite breccia. Clast 3 is an impact‐melt breccia that is composed of rock and mineral fragments in a devitrified groundmass. Clast C is also an impact‐melt breccia that has a coarser‐grained, hornfelsic groundmass that resulted from extensive metamorphism after formation.  相似文献   

Mineralogy of carbonaceous chondrite clasts in HED achondrites and the Moon     

Michael E. ZOLENSKY  Michael K. WEISBERG  Paul C. BUCHANAN  David W. MITTLEFEHLDT 《Meteoritics & planetary science》1996,31(4):518-537

Abstract The majority of the carbonaceous chondrite clasts found in howardites, eucrites and diogenites are CM2 material, a lesser proportion is CR2 material, and other rare types are present. A single clast that was found on the Moon and called the Bench Crater meteorite is apparently shocked CM1 material. The CM2 clasts are matrix supported mixtures of olivine-pyroxene-phyllosilicate-sulfide bearing aggregates, loose olivines and pyroxenes, sulfides, carbonates, and sinuous spinel-phyllosilicate-diopside calcium-aluminum-rich inclusions (CAIs). Magnetite and metal are rare. Some aggregates have fine-grained rims of material resembling matrix. The opaque, fine-grained matrix consists predominantly of serpentine of extremely variable composition and sulfides; tochilinite is occasionally present. The trace element data for one Jodzie clast from this study and the average of similar clasts from Kapoeta support a CM classification; volatiles are depleted relative to CI and enriched relative to CR material. The CR2 clasts are found (in small numbers) in only four howardites: Bholghati, Jodzie, Kapoeta and Y793497. Petrographically, they are matrix-supported mixtures of olivine aggregates (sometimes containing sulfides), loose olivines, pyrrhotite, pentlandite, low-Ca pyroxene (minor), hedenbergite (rare), kamacite (rare and only found within olivine), Ca-carbonates and abundant magnetite framboids and plaquets. Phyllosilicates are fine-grained and largely confined to matrix; they are mixtures of serpentine and saponite. The matrix of CR2 clasts also contains pyrrhotite, pentlandite, chromite and a significant fraction of poorly-crystalline material with the same bulk composition as matrix phyllosilicate. There is evidence of heating in a substantial number of clasts, both CM2 and CR2, including: (1) corrugated serpentine flakes, (2) pseudomorphs of anhydrous ferromagnesian material after flaky phyllosilicates, and (3) hedenbergite rims on calcite. While the timing of the hedenbergite rims is debatable, the destruction of phyllosilicates clearly occurred at a late stage, plausibly during impact onto the HED asteroid(s) and Moon, and required peak heating temperatures on the order of 400 °C. We note that in general, CM2 material was the most common carbonaceous chondrite lithology impacting the HED asteroids (with howardites and eucrites taken together), as it is for the Earth today. A total of 61 out of 75 carbonaceous chondrite clasts from HED meteorites belong to the CM clan, petrologic grade 2. This is also supported by published siderophile and volatile element data on howardites, eucrites and diogenites that are taken to indicate that CM-like materials were the most common impactors on the HED asteroid(s). The ratio of CR/CM clasts in HED asteroids is essentially the same as for modern falls at Earth. This may indicate that the ratio of disaggregated CM2 to CR2 asteroidal material has been approximately constant through the history of the solar system. Finally, our results are also compatible with type-2 carbonaceous chondrites being equivalent to or from the same source as the material that originally accreted to form the HED asteroid.  相似文献   

Basin architecture and thermal maturation in the strike-slip Deer Lake Basin, Carboniferous of Newfoundland     

RICHARD S. Hyde  HUGH G. Miller  RICHARD N. Hiscott  JAMES A. Wright 《Basin Research》1988,1(2):85-105

Abstract The Deer Lake Basin is an entirely non-marine basin associated with the Cabot fault zone. Structural and stratigraphic evidence strongly suggest dextral strike-slip movements along the fault zone during Tournaisian-Visean time. Two elongated, end-on structural blocks (probable positive flower structures) contain fold axes and second-order faults oriented obliquely to fault traces bounding the blocks, in a manner implying dextral movements. In one part of the basin, the stratigraphic thickness of a long homoclinal section of later basin-fill sediment (Deer Lake Group) greatly exceeds the suggested depth to basement based on gravity measurements, a situation common to strike-slip basins. Formations representing basin fill can be arranged into megasequences (from oldest to youngest: Anguille Group, Wetstone Point and Wigwam Brook Formations, Deer Lake Group, Howley Formation) corresponding to lateral growth stages of the basin. Gravity, magnetic, and seismic data show that depths to basement on either side of the end-on flower structures are comparable, so that the youngest strata in the basin (Howley Formation) are not underlain by earlier basin fill. These geophysical data, therefore, corroborate the geological conclusion of onlapping stratigraphic relations. The geophysical data suggest participation of basement in Carboniferous gravity faulting and show the location of the subsurface extension of the Taylors Brook Fault in the western part of the Deer Lake Basin. Thermal maturation of the Anguille and Deer Lake Groups, as measured by vitrinite reflectance, clay mineral assemblages, illite crystallinity, and Rock-Eval pyrolysis, indicate a much higher level of maturation for the Anguille than for the Deer Lake Group. Palaeotemperatures for the Anguille and Deer Lake Groups are estimated to be around 200 and 100^oC, respectively, suggesting that Anguille Group rocks are overmature whereas Deer Lake Group strata are within the oil-generating window. Onlapping stratigraphic relations and areally homogeneous time/temperature effects, however, have created a situation in which the Deer Lake Group and Howley Formation have similar maturation levels.  相似文献