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651.
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
652.
David M. Hirsch 《Contributions to Mineralogy and Petrology》2008,155(4):401-415
Garnet-bearing schists from the Waterville Formation of south-central Maine provide an opportunity to examine the factors
governing porphyroblast size over a range of metamorphic grade. Three-dimensional sizes and locations for all garnet porphyroblasts
were determined for three samples along the metamorphic field gradient spanning lowest garnet through sillimanite grade, using
high-resolution X-ray computed tomography. Comparison of crystal size distributions to previous data sets obtained by stereological
methods for the same samples reveals significant differences in mode, mean, and shape of the distributions. Quantitative textural
analysis shows that the garnets in each rock crystallized in a diffusion-controlled nucleation and growth regime. In contrast
to the typical observation of a correlation between porphyroblast size and position along a metamorphic field gradient, porphyroblast
size of the lowest-grade specimen is intermediate between the high- and middle-grade specimens’ sizes. Mean porphyroblast
size does not correlate with peak temperatures from garnet-biotite Fe-Mg exchange thermometry, nor is post-crystallization
annealing (Ostwald Ripening) required to produce the observed textures, as was previously proposed for these rocks. Robust
pseudosection calculations fail to reproduce the observed garnet core compositions for two specimens, suggesting that these
calc-pelites experienced metasomatism. For each of these two specimens, Monte Carlo calculations suggest potential pre-metasomatism
bulk compositions that replicate garnet core compositions. Pseudosection analyses allow the estimation of the critical temperatures
for garnet growth: ∼481, ∼477, and ∼485°C for the lowest-garnet-zone, middle-garnet-zone, and sillimanite-zone specimens,
respectively. Porphyroblast size appears to be determined in this case by a combination of the heating rate during garnet
crystallization, the critical temperature for the garnet-forming reaction and the kinetics of nucleation. Numerical simulations
of thermally accelerated, diffusion-controlled nucleation, and growth for the three samples closely match measured crystal
size distributions. These observations and simulations suggest that previous hypotheses linking the garnet size primarily
to the temperature at the onset of porphyroblast nucleation can only partially explain the observed textures. Also important
in determining porphyroblast size are the heating rate and the distribution of favorable nucleation sites. 相似文献
653.
M. I. Holloway F. Bussy T. W. Vennemann 《Contributions to Mineralogy and Petrology》2008,155(1):111-121
Migmatites produced by low-pressure anatexis of basic dykes are found in a contact metamorphic aureole around a pyroxenite–gabbro
intrusion (PX2), on Fuerteventura. Dykes outside and inside the aureole record interaction with meteoric water, with low or
negative δ18O whole-rock values (+0.2 to −3.4‰), decreasing towards the contact. Recrystallised plagioclase, diopside, biotite and oxides,
from within the aureole, show a similar evolution with lowest δ18O values (−2.8, −4.2, −4.4 and −7.6‰, respectively) in the migmatite zone, close to the intrusion. Relict clinopyroxene phenocrysts
preserved in all dykes, retain typically magmatic δ18O values up to the anatectic zone, where the values are lower and more heterogeneous. Low δ18O values, decreasing towards the intrusion, can be ascribed to the advection of meteoric water during magma emplacement, with
increasing fluid/rock ratios (higher dyke intensities towards the intrusion acting as fluid-pathways) and higher temperatures
promoting increasing exchange during recrystallisation.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
654.
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
655.
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
656.
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
657.
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:
658.
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
659.
Imad A.M. Ahmed Neil M.J. Crout Scott D. Young 《Geochimica et cosmochimica acta》2008,72(6):1498-1512
Time-dependent sorption and desorption of Cd on calcite was studied over 210 days utilizing 109Cd as a tracer to distinguish between ‘labile’ and ‘non-labile’ forms of sorbed Cd. Stabilizing the calcite suspensions for 12 months under atmospheric PCO2 and controlled temperature was necessary to reliably follow Cd dynamics following initial sorption. Results revealed time-dependant Cd sorption and marked desorption hysteresis by calcite under environmentally relevant conditions. Data obtained were fitted to a first-order kinetic model and a concentric shell diffusion model. Both models described the progressive transfer of Cd2+ to a less reactive form within calcite and subsequent desorption of Cd subject to different initial contact times. The kinetic model provided a better fit to the combined sorption and desorption data (R2 = 0.992). It differentiates between two ‘pools’ of sorbed Cd2+ on calcite, ‘labile’ and ‘non-labile’, in which labile sorbed Cd is in immediate equilibrium with the free Cd2+ ion activity in solution whereas non-labile Cd is kinetically restricted. For the diffusion model (R2 = 0.959), the rate constants describing Cd dynamics in calcite produced a half-life for Cd desorption of ∼175 d, for release to a ‘zero-sink’ solution. Results from this study allow comment on the likely mechanisms occurring at the calcite surface following long-term Cd sorption. 相似文献
660.
Silicon isotope fractionation in bamboo and its significance to the biogeochemical cycle of silicon 总被引:2,自引:0,他引:2
A systematic investigation on silica contents and silicon isotope compositions of bamboos was undertaken. Seven bamboo plants and related soils were collected from seven locations in China. The roots, stem, branch and leaves for each plant were sampled and their silica contents and silicon isotope compositions were determined. The silica contents and silicon isotope compositions of bulk and water-soluble fraction of soils were also measured. The silica contents of studied bamboo organs vary from 0.30% to 9.95%. Within bamboo plant the silica contents show an increasing trend from stem, through branch, to leaves. In bamboo roots the silica is exclusively in the endodermis cells, but in stem, branch and leaves, the silica is accumulated mainly in epidermal cells. The silicon isotope compositions of bamboos exhibit significant variation, from −2.3‰ to 1.8‰, and large and systematic silicon isotope fractionation was observed within each bamboo. The δ30Si values decrease from roots to stem, but then increase from stem, through branch, to leaves. The ranges of δ30Si values within each bamboo vary from 1.0‰ to 3.3‰. Considering the total range of silicon isotope composition in terrestrial samples is only 7‰, the observed silicon isotope variation in single bamboo is significant and remarkable. This kind of silicon isotope variation might be caused by isotope fractionation in a Rayleigh process when SiO2 precipitated in stem, branches and leaves gradually from plant fluid. In this process the Si isotope fractionation factor between dissolved Si and precipitated Si in bamboo (αpre-sol) is estimated to be 0.9981. However, other factors should be considered to explain the decrease of δ30Si value from roots to stem, including larger ratio of dissolved H4SiO4 to precipitated SiO2 in roots than in stem. There is a positive correlation between the δ30Si values of water-soluble fractions in soils and those of bulk bamboos, indicating that the dissolved silicon in pore water and phytoliths in soil is the direct sources of silicon taken up by bamboo roots. A biochemical silicon isotope fractionation exists in process of silicon uptake by bamboo roots. Its silicon isotope fractionation factor (αbam-wa) is estimated to be 0.9988. Considering the distribution patterns of SiO2 contents and δ30Si values among different bamboo organs, evapotranspiration may be the driving force for an upward flow of a silicon-bearing fluid and silica precipitation. Passive silicon uptake and transportation may be important for bamboo, although the role of active uptake of silicic acid by roots may not be neglected. The samples with relatively high δ30Si values all grew in soils showing high content of organic materials. In contrast, the samples with relatively low δ30Si values all grew in soil showing low content of organic materials. The silicon isotope composition of bamboo may reflect the local soil type and growth conditions. Our study suggests that bamboos may play an important role in global silicon cycle. 相似文献