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541.
Mauro Lo Cascio Yan Liang Nobumichi Shimizu Paul C. Hess 《Contributions to Mineralogy and Petrology》2008,156(1):87-102
The grain-scale processes of peridotite melting were examined at 1,340°C and 1.5 GPa using reaction couples formed by juxtaposing
pre-synthesized clinopyroxenite against pre-synthesized orthopyroxenite or harzburgite in graphite and platinum-lined molybdenum
capsules. Reaction between the clinopyroxene and orthopyroxene-rich aggregates produces a melt-enriched, orthopyroxene-free,
olivine + clinopyroxene reactive boundary layer. Major and trace element abundance in clinopyroxene vary systematically across
the reactive boundary layer with compositional trends similar to the published clinopyroxene core-to-rim compositional variations
in the bulk lherzolite partial melting studies conducted at similar P–T conditions. The growth of the reactive boundary layer takes place at the expense of the orthopyroxenite or harzburgite and
is consistent with grain-scale processes that involve dissolution, precipitation, reprecipitation, and diffusive exchange
between the interstitial melt and surrounding crystals. An important consequence of dissolution–reprecipitation during crystal-melt
interaction is the dramatic decrease in diffusive reequilibration time between coexisting minerals and melt. This effect is
especially important for high charged, slow diffusing cations during peridotite melting and melt-rock reaction. Apparent clinopyroxene-melt
partition coefficients for REE, Sr, Y, Ti, and Zr, measured from reprecipitated clinopyroxene and coexisting melt in the reactive
boundary layer, approach their equilibrium values reported in the literature. Disequilibrium melting models based on volume
diffusion in solid limited mechanism are likely to significantly underestimate the rates at which major and trace elements
in residual minerals reequilibrate with their surrounding melt.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
542.
543.
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
544.
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
545.
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
546.
Deep dissolution affects great part of soluble rocks (e.g. gypsum and anhydrite) of the Western Italian Alps. The related superficial phenomena (sinkholes, gravity-induced processes and a local worsening of geomechanical rock properties) are not limited to typical karsts landscape and cause slope instability also affecting populated sites and infrastructures. The paper aims to describe general characteristic of dissolution phenomena, to interpret their conditioning factors and evolutionary stages and to assess possible hazards due to their superficial effects.The search for evidences of deep dissolution leads to the selection of representative sites in the central part of the Western Italian Alps (Piemonte and Valle d'Aosta Region). Detailed geological and geomorphological studies have been used to classify the selected sites by type, size and variable state of activity. Very different evolutionary stages of dissolution phenomena have been interpreted by comparison of case-studies: some are early “embryonic”; others are more evolved, up to typical sinkholes, or even remodelled by other phenomena. Some cases show an extreme complexity in the interactions between corrosion phenomena and other geomorphic processes: slope deformations, from one side, and karst, fluvial and glacial phenomena, to the other. A wide range of movement rates on slope instabilities induced by deep dissolution have been estimated by topographic and geomorphic data. Geochemical data on removed rocks by dissolution indicate 0.4 mm/year values for local subsidence. Historical and technical data indicate low frequency of major dissolution-induced collapses, but highlight widespread damages to tunnels, roads and buildings, especially along slopes. 相似文献
547.
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
548.
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
549.
David M. Sherman Caroline L. Peacock Christopher G. Hubbard 《Geochimica et cosmochimica acta》2008,72(2):298-310
Sorption of U(VI) to goethite is a fundamental control on the mobility of uranium in soil and groundwater. Here, we investigated the sorption of U on goethite using EXAFS spectroscopy, batch sorption experiments and DFT calculations of the energetics and structures of possible surface complexes. Based on EXAFS spectra, it has previously been proposed that U(VI), as the uranyl cation , sorbs to Fe oxide hydroxide phases by forming a bidentate edge-sharing (E2) surface complex, >Fe(OH)2UO2(H2O)n. Here, we argue that this complex alone cannot account for the sorption capacity of goethite (α-FeOOH). Moreover, we show that all of the EXAFS signal attributed to the E2 complex can be accounted for by multiple scattering. We propose that the dominant surface complex in CO2-free systems is a bidentate corner-sharing (C2) complex, (>FeOH)2UO2(H2O)3 which can form on the dominant {101} surface. However, in the presence of CO2, we find an enhancement of UO2 sorption at low pH and attribute this to a (>FeO)CO2UO2 ternary complex. With increasing pH, U(VI) desorbs by the formation of aqueous carbonate and hydroxyl complexes. However, this desorption is preceded by the formation of a second ternary surface complex (>FeOH)2UO2CO3. The three proposed surface complexes, (>FeOH)2UO2(H2O)3, >FeOCO2UO2, and (>FeOH)2UO2CO3 are consistent with EXAFS spectra. Using these complexes, we developed a surface complexation model for U on goethite with a 1-pK model for surface protonation, an extended Stern model for surface electrostatics and inclusion of all known UO2-OH-CO3 aqueous complexes in the current thermodynamic database. The model gives an excellent fit to our sorption experiments done in both ambient and reduced CO2 environments at surface loadings of 0.02-2.0 wt% U. 相似文献
550.
We have determined the metallographic cooling rates for 13 IVA irons using the most recent and most accurate metallographic cooling rate model. Group IVA irons have cooling rates that vary from 6600 °C/Myr at the low-Ni end of the group to 100 °C/Myr at the high-Ni end of the group. This large cooling rate range is totally incompatible with cooling in a mantled core which should have a uniform cooling rate. Thermal and fractional crystallization models have been used to describe the cooling and solidification of the IVA asteroid. The thermal model indicates that a metallic body of 150 ± 50 km in radius with less than 1 km of silicate on the outside of the body has a range of cooling rates that match the metallographic cooling rates in IVA irons in the temperature range 700-400 °C where the Widmanstätten pattern formed. The fractional crystallization model for Ni with initial S contents between 3 and 9 wt% is consistent with the measured variation of cooling rate with bulk Ni and the thermal model. New models for impacts in the early solar system and the evolution of the primordial asteroid belt allow us to propose that the IVA irons crystallized and cooled in a metallic body that was derived from a differentiated protoplanet during a grazing impact. Other large magmatic iron groups, IIAB, IIIAB, and IVB, also show significant cooling rate ranges and are very likely to share a similar history. 相似文献