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951.
John F. Rudge Marian B. Holness Graham C. Smith 《Contributions to Mineralogy and Petrology》2008,156(4):413-429
The spatial distribution of grains in a solidifying igneous rock controls the physical properties of the crystal mush, and
in turn is controlled by the rate of crystal growth and accumulation. A predominant non-spherical habit for igneous minerals
brings into question the use of spherical particles in reference packings used for quantification of spatial distribution.
Furthermore, variations of crystal clustering/ordering with length scale require spatial statistics which take into account
the distribution of particles beyond nearest neighbours. Using random close packings of spherocylinders, we demonstrate the
importance of aspect ratio for the aggregation index (usually known as R) and show that packings of spherical particles have more structure than packings of rods. The spatial distribution functions
demonstrate that the plagioclase grains in the colonnade from the Holyoke basalt are clustered on a length scale of 0.5 mm.
Understanding the controls on grain spatial distribution in igneous rocks will depend on the application of these techniques
to well-understood environments. 相似文献
952.
Pedro J. Marenco Frank A. Corsetti Douglas E. Hammond Alan J. Kaufman David J. Bottjer 《Chemical Geology》2008,247(1-2):124-132
The sulfur isotopic composition of carbonate associated sulfate (CAS) has been used to investigate the geochemistry of ancient seawater sulfate. However, few studies have quantified the reliability of δ34S of CAS as a seawater sulfate proxy, especially with respect to later diagenetic overprinting. Pyrite, which typically has depleted δ34S values due to authigenic fractionation associated with bacterial sulfate reduction, is a common constituent of marine sedimentary rocks. The oxidation of pyrite, whether during diagenesis or sample preparation, could thus adversely influence the sulfur isotopic composition of CAS. Here, we report the results of CAS extractions using HCl and acetic acid with samples spiked with varying amounts of pyrite. The results show a very strong linear relationship between the abundance of fine-grained pyrite added to the sample and the resultant abundance and δ34S value of CAS. This data represents the first unequivocal evidence that pyrite is oxidized during the CAS extraction process. Our mixing models indicate that in samples with much less than 1 wt.% pyrite and relatively high δ34Spyrite values, the isotopic offset imparted by oxidation of pyrite should be much less than ? 4‰. A wealth of literature exists on the oxidation of pyrite by Fe3+ and we believe this mechanism drives the oxidation of pyrite during CAS extraction, during which the oxygen used to form sulfate is taken from H2O, not O2. Consequently, extracting CAS under anaerobic conditions would only slow, but not halt, the oxidation of pyrite. Future studies of CAS should attempt to quantify pyrite abundance and isotopic composition. 相似文献
953.
G. Caro V.C. Bennett B. Bourdon T.M. Harrison A. von Quadt S.J. Mojzsis J.W. Harris 《Chemical Geology》2008,247(1-2):253-265
146Sm–142Nd and 147Sm–143Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The study of Finsch inclusions was conducted on a composite sample of 50 peridotitic pyropes with a Nd model age of 3.3 Ga. Analysis of the Jack Hills zircons was performed on 790 grains with ion microprobe 207Pb/206Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 × ?142Nd = ? 6 ± 12 ppm, ?143Nd = ? 32.5, and 147Sm/144Nd = 0.1150. These results do not confirm previous claims for a 30 ppm 142Nd excess in South African cratonic mantle. The lack of a 142Nd anomaly in these inclusions suggests that isotopic heterogeneities created by early mantle differentiation were remixed at a very fine scale prior to isolation of the South African lithosphere. Alternatively, this result may indicate that only a fraction of the mantle experienced depletion during the first 400 Myr of its history. Analysis of the Jack Hills zircon composite yielded 100 × ?142Nd = 8 ± 10 ppm, ?143Nd = 45 ± 1, and 147Sm/144Nd = 0.5891. Back-calculation of this present-day ?143Nd yields an unrealistic estimate for the initial ?143Nd of ? 160 ?-units, clearly indicating post-crystallization disturbance of the 147Sm–143Nd system. Examination of 146,147Sm–142,143Nd data reveals that the Nd budget of the Jack Hills sample is dominated by non-radiogenic Nd, possibly contained in recrystallized zircon rims or secondary subsurface minerals. This secondary material is characterized by highly discordant U–Pb ages. Although the mass fraction of altered zircon is unlikely to exceed 5–10% of total sample, its high LREE content precludes a reliable evaluation of 146Sm–142Nd systematics in Jack Hills zircons. 相似文献
954.
Historical evidence shows block breakdown and collapse are actively occurring in large fault aligned caverns in the Yorkshire Dales karst. Deployment of ground penetrating radar at two such sites has provided detailed images of the sedimentary sequences below the present day cavern floor but no large blocks are imaged within the sediments. Solutional processes must be removing limestone from the sediment to allow continued cavern growth. Possible mechanisms to account for the lack of large blocks within the sediment fill are discussed. 相似文献
955.
A.N. Krot M. Chaussidon H. Yurimoto N. Sakamoto K. Nagashima I.D. Hutcheon G.J. MacPherson 《Geochimica et cosmochimica acta》2008,72(10):2534-2555
In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of 16O in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinel is consistently the most 16O-rich (Δ17O = −25‰ to −15‰), followed by Al,Ti-dioside (Δ17O = −20‰ to −5‰) and anorthite (Δ17O = −15‰ to 0‰). Melilite is the most 16O-depleted primary mineral (Δ17O = −5‰ to −3‰). We conclude that the original melting event that formed Type C CAIs occurred in a 16O-rich (Δ17O −20‰) nebular gas and they subsequently experienced oxygen isotopic exchange in a 16O-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating 16O-poor chondrule fragments. The observation that the pyroxene is 16O-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a 16O-rich solid in a 16O-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is 16O-enriched (Δ17O = −10‰ to −6‰) relative to melilite (Δ17O = −5‰ to −3‰), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism. 相似文献
956.
Daniel J. Schulze Howard G. Coopersmith Lori-Ann Pizzolato 《Geochimica et cosmochimica acta》2008,72(6):1685-1695
Thirty-four silicate and oxide inclusions large enough for in situ WDS electron microprobe analysis were exposed by grinding/polishing of 19 diamonds from the Kelsey Lake Mine in the Colorado-Wyoming State Line Kimberlite district. Eighteen olivines, seven Cr-pyropes, four Mg-chromites, and one orthopyroxene in 15 stones belong to the peridotite (P) suite and three garnets and one omphacite in three stones belong to the eclogite (E) suite. The fact that this suite is dominated by the peridotite population is in stark contrast to the other diamond suites studied in the State Line district (Sloan, George Creek), which are overwhelmingly eclogitic. Kelsey Lake olivine inclusions are magnesian (17 of 18 grains in 9 stones are in the range Fo 92.7-93.1), typical of harzburgitic P-suite stones worldwide, but unlike the more Fe-rich (lherzolitic) Sloan olivine suite. Mg-chromites (wt% MgO = 12.8-13.8; wt% Cr2O3 = 61.4-66.6) are in the lower MgO range of diamond inclusion chromites worldwide. Seven harzburgitic Cr-pyropes in five stones have moderately low calcium contents (wt% CaO = 3.3-4.3) but are very Cr-rich (wt% Cr2O3 = 9.7-16.7). A few stones have been analyzed by SIMS for carbon isotope composition and nitrogen abundance. One peridotitic stone is apparently homogeneous in carbon isotope composition (δ13CPDB = −6.2‰) but with variable nitrogen abundance (1296-2550 ppm). Carbon isotopes in eclogitic stones range from “normal” for the upper mantle (δ13CPDB = −5.5‰) to somewhat low (δ13CPDB = −10.2‰), with little internal variation in individual stones (maximum difference is 3.6‰). Nitrogen contents (2-779 ppm) are lower than in the peridotitic stone, and are lower in cores than in rims. As, worldwide, harzburgite-suite diamonds have been shown to have formed in Archean time, we suggest that the Kelsey Lake diamond population was derived from a block of Archean lithosphere that, at the time of kimberlite eruption, existed beneath the Proterozoic Yavapai province. The mixed diamond inclusion populations from the State Line kimberlites appear to support models in which volumes of Wyoming Craton Archean mantle survive buried beneath Proterozoic continental crust. Such material may be mixed with eclogitic/lherzolitic regimes emplaced beneath or intermingled with the Archean rocks by Proterozoic subduction. 相似文献
957.
Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis 总被引:10,自引:0,他引:10
The voluminous 2.5 Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling. The major Fe-bearing minerals in the Hamersley-Transvaal BIFs, magnetite and siderite, did not form in Fe isotope equilibrium, but instead reflect distinct formation pathways. The near-zero average δ56Fe values for magnetite record a strong inheritance from Fe3+ oxide/hydroxide precursors that formed in the upper water column through complete or near-complete oxidation. Transformation of the Fe3+ oxide/hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of δ56Fe values: (1) addition of marine hydrothermal , (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with excess that had low δ56Fe values and was produced by DIR. Most siderite has slightly negative δ56Fe values of ∼ −0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative δ56Fe values may record DIR. Support for an important role of DIR in siderite formation in BIFs comes from previously published C isotope data on siderite, which may be explained as a mixture of C from bacterial and seawater sources.Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ56Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.5 Ga Hamersley-Transvaal BIFs may record a period of maximum expansion of DIR in Earth’s history. 相似文献
958.
Sofie P. Pasilis 《Geochimica et cosmochimica acta》2008,72(2):277-287
ATR-FTIR spectroscopy is used to understand the adsorption of uranyl-citrate complexes to Al2O3. Spectral data indicate that uranyl-citrate complexes partially dissociate upon adsorption, allowing full or partial hydrolysis of the uranyl ion. Kads values determined for free citrate adsorption are similar to those for citrate in uranyl-citrate complexes, indicating that the complexation of uranyl by citrate does not significantly affect the ability of citrate to bond with the surface. The isotherm data also indicate enhanced citrate adsorption to Al2O3 in the presence of uranyl, suggesting that uranyl may be the central link between two citrate ligands, and that uranyl is associated with the surface through a bridging citrate ligand. Finally, uranyl-citrate complexes interact with citrate adsorbed to Al2O3 through outer sphere interactions. 相似文献
959.
Core formation and the oxidation state of the Earth: Additional constraints from Nb, V and Cr partitioning 总被引:1,自引:0,他引:1
We have combined metal-silicate partitioning data from the literature with new experimental results at 1.5-8 GPa and 1480-2000 °C to parameterize the effects of pressure, temperature and composition on the partitioning of V, Cr and Nb between liquid Fe metal (with low S and C content) and silicate melt.Using information from the steelmaking literature to correct for interactions in the metal phase, we find that, for peridotitic silicate melts, metal-silicate partition coefficients are given by:
960.
J. Filiberto 《Geochimica et cosmochimica acta》2008,72(2):690-701
Most magmas proposed as parental to the Martian SNC meteorites are high in iron and low in alumina. Yet, experiments at low pressures on such liquids have not produced the cumulate or melt-inclusion assemblages seen in the chassignite meteorites. Therefore, elevated pressure experiments under anhydrous and hydrous (water-undersaturated) conditions were conducted on a high-Fe, low-Al liquid proposed to be parental to the Chassigny meteorite. These experiments failed to produce the most magnesian cumulate phases, as well as the olivine hosted kaersutite-bearing melt-inclusion assemblage, of the chassignites. These results suggest that the parental liquid to the chassignite meteorites is both more magnesium and aluminum-rich than the previously considered composition (A∗; Johnson et al., 1991). The proposed composition is similar to the Martian Adirondack class Gusev basalt Humphrey and suggests a link between the Chassigny meteorite and rocks on the surface of Mars. 相似文献