Primodial retention of carbon by the terrestrial planets     

John S. Lewis  Stephen S. Barshay  Barbara Noyes 《Icarus》1979,37(1):190-206

Chemical equilibrium calculations on the stability of pure and dissolved graphite and cohenite (Fe₃C), several other carbides, and several carbonates have been carried out for a system with solar elemental abundances over a very wide range of temperature and pressure. The calculated abundances of condensed carbon compounds are similar to the observed inventories on Earth and Venus, but fully 10 times smaller than the minimum carbon abundance found in ordinary chondrites. The total carbon content of most iron meteorites is compatible with their origin as a cooling FeNiCSP solution which was saturated with dissolved carbon at the solidus, such as would be produced by melting an ordinary chondrite, not by direct condensation from or equilibrium with the primitive solar nebula. It is argued that the carbon content of Mars need not be appreciably greater than that of the Earth. Material with even lower formation temperatures than Mars, such as the primitive material in the asteroid belt, may retain substantially more carbon as disequilibrium polymeric organic matter, possibly by the Fischer-Tropsch mechanism favored by Anders. Carbonates are not found as equilibrium products in a solar-composition system, and are probably secondary alteration products. CaCO₃ might, however, persist in a solar-composition gas at temperatures below 460°K and pressures below 10^?6.6 bar. The most stable condensed carbon compounds are found to be graphite, Fe₃C, and possibly TiC, all in solid solution in the metal phase.  相似文献   

Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists   总被引：1，自引：1，他引：0  

Harold A. Smith  Barbara Barreiro 《Contributions to Mineralogy and Petrology》1990,105(5):602-615

A study of the occurrence of and relations between rare-earth element (REE) minerals in pelitic schists indicates that monazite forms at or near the P and T of the staurolite isograd. Samples at staurolite grade from the Silurian Perry Mountain Formation in the Rumford quadrangle of Maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites. The bulk chemistry of the metapelites, as seen in the major element abundances and REE patterns, does not vary significantly across the study area. Thus the appearance and disappearance of REE phases is assumed to reflect changes in metamorphic grade. In a sample from the biotite zone, scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale. The texture suggest that metastable detrital monazite breaks down, distributing its REE components to allanite. From samples below staurolite grade in which monazite is not present, our observations suggest that REEs are partitioned into allanite. At or near the staurolite isograd monazite forms as a metamorphic mineral, initiating its role as a geochronometer. Garnet-biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite: 525° C±25°C at 3.1±0.25 kbar. A total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated. Each date is remarkably concordant, even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes. Calculations indicate insignificant Th disequilibrium in these monazites. The conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade. Two distinct metamorphic events are resolved, one at around 400 Ma and one at about 370 Ma. The latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 Ma. This work suggests that in addition to dating plutonism and high-grade metamorphism, monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists.  相似文献   

Gas maturity and alteration systematics across the Western Canada Sedimentary Basin from four mud gas isotope depth profiles   总被引：1，自引：0，他引：1  

Barbara Tilley  Karlis Muehlenbachs 《Organic Geochemistry》2006,37(12):1857

Carbon isotope and molecular compositions of Mississippian to Upper Cretaceous mud gases have been examined from four depth profiles across the Western Canada Sedimentary Basin (WCSB). The profiles range from the shallow oil sands in the east (R₀ = 0.25) to the very mature sediments in the overthrust zone to the west (R₀ = 2.5). In the undisturbed WCSB, δ¹³C₁–δ¹³C₂ and δ¹³C₂–δ¹³C₃ cross-plots show three maturity and alteration trends: (1) pre-Cretaceous gas sourced from type II kerogen; (2) Cretaceous Colorado Group gas; and (3) Lower Cretaceous Mannville Group biodegraded gas. A fourth set of distinctly different maturity trends is recognized for Lower Cretaceous gas sourced from type III kerogen in the disturbed belt of the WCSB. Displacement of these latter maturity trends to high δ¹³C₂ values suggests that the sampled gas was trapped after earlier formed gas escaped, probably as a result of overthrusting. Unusually ¹³C-enriched gas (δ¹³C₁ = −34‰, δ¹³C₂ = −13‰, and δ¹³C₃ = 0‰), from the Gething Formation in the disturbed belt, is the result of late stage gas cracking in a closed system. In general, gas maturity is consistent with the maturity of the host sediments in the WCSB, suggesting that migration and mixing of gases was not pervasive on a broad regional and stratigraphic scale. The ‘Deep Basin’ portion of the WCSB is an exception. Here extensive cross-formational homogenization of gases has occurred, in addition to updip migration along the most permeable stratigraphic units.  相似文献   

Book reviews     

Anne Marie de Grosbois 《Environmental Geology》2006,49(4):642-643

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S-wave velocity structure, mantle xenoliths and the upper mantle beneath the Kaapvaal craton   总被引：1，自引：0，他引：1  

Angela Marie Larson  J. Arthur Snoke  David E. James 《Geophysical Journal International》2006,167(1):171-186

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Book Reviews     

Jörg Matschullat  Anne Marie de Grosbois  Fakhry A Assaad 《Environmental Geology》2006,49(8):1224-1225

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Contrasting sources for lead in Cu-polymetallic and Zn–Pb mineralisation in Ireland: constraints from lead isotopes     

Judith A. Kinnaird  Robert A. Ixer  Barbara Barreiro  Paul A. Nex 《Mineralium Deposita》2002,37(5):495-511

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Late-Glacial and Early Holocene Environmental and Climatic Change at Lake Tambichozero, Southeastern Russian Karelia     

Barbara Wohlfarth  Ludmila Filimonova  Ole Bennike  Leif Bjrkman  Lars Brunnberg  Nadja Lavrova  Igor Demidov  Gran Possnert 《Quaternary Research》2002,58(3):261

High-resolution lithostratigraphy, mineral magnetic, carbon, pollen, and macrofossil analyses, and accelerator mass spectrometry ¹⁴C measurements were performed in the study of a sediment sequence from Lake Tambichozero, southeastern Russian Karelia, to reconstruct late-glacial and early Holocene aquatic and terrestrial environmental changes. The lake formed ca. 14,000 cal yr B.P. and the area around the lake was subsequently colonized by arctic plants, forming patches of pioneer communities surrounded by areas of exposed soil. A minor rise in lake productivity and the immigration of Betula pubescens occurred ca. 11,500 cal yr B.P. The rise in summer temperatures probably led to increased melting of remnant ice and enhanced erosion. The distinct increase in lake productivity and the development of open Betula-Populus forests, which are reconstructed based on plant macrofossil remains, indicate stable soils from 10,600 cal yr B.P. onward. Pinus and Picea probably became established ca. 9900 cal yr B.P.  相似文献   

Iterative resolution estimation in least‐squares Kirchhoff migration     

Sergey Fomel  James G. Berryman  Robert G. Clapp  Marie Prucha 《Geophysical Prospecting》2002,50(6):577-588

We apply iterative resolution estimation to least‐squares Kirchhoff migration. Reviewing the theory of iterative optimization uncovers the common origin of different optimization methods. This allows us to reformulate the pseudo‐inverse, model resolution and data resolution operators in terms of effective iterative estimates. When applied to Kirchhoff migration, plots of the diagonal of the model resolution matrix reveal low illumination areas on seismic images and provide information about image uncertainties. Synthetic and real data examples illustrate the proposed technique and confirm the theoretical expectations.  相似文献   

Orbitolinidae and Alveolinidae (Foraminiferida) from the uppermost Albian-lower Cenomanian of Monti d'Ocre (Abruzzi, Italy)     

Anna Mancinelli  Maurizio Chiocchini  Barbara Coccia 《Cretaceous Research》2003,24(6):729-741

A biostratigraphic study carried out in the Monti d'Ocre area, Abruzzi, Central Apennines, allowed us to recognize Orbitolina (Conicorbitolina) moulladei ‘Strata 5 (1985) 1’, Praealveolina iberica Reichel and Praealveolina simplex Reichel in uppermost Albian–lower Cenomanian shelf-edge deposits of the Fossato Machè succession. These foraminifers have now been found for the first time in the Apennines of central Italy; their finding is quite important from a palaeobiogeographic viewpoint, as it contributes to the improvement of our knowledge on facies distribution in the circum-Mediterranean regions during the Cretaceous Period. In the study area, the coeval Monte Rotondo and Monte Orsello sections also crop out; these are characterized by bauxite deposits and stratigraphic gaps reflecting episodes of emergence on the carbonate platform. The Monte Rotondo and Monte Orsello sections accumulated in a platform back-reef environment; consequently, in this sector of the Monti d'Ocre area, the depositional environment shifted from a back-reef westward and southward to a shelf-edge northward, during the latest Albian–early Cenomanian. Owing to synsedimentary tectonics, the area investigated underwent differential subsidence: westward and southward, wide areas were uplifted and subjected to emergence, karstification and bauxite accumulation, whereas sedimentation continued in a shelf-edge environment in the north-eastern area.  相似文献