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1.
The diffusion rate of 18O tracer atoms in anorthite (An97Ab03) under anhydrous conditions has been measured using SIMS techniques. The tracer source was 18O2 98.4% gas at 1 bar, in the temperature range 1300° C–850° C. The measured diffusion constants are D 0=1 –0.6 +1 ×10–9 m2s–1 Q=236±8 kJ mol–1 Comparison of these values with published data for 18O diffusion in anorthite under hydrothermal conditions shows that dry oxygen diffusivities are orders of magnitude lower than equivalent wet values at similar temperatures. The effect of these differences on oxygen isotope equilibration during cooling is discussed.  相似文献   

2.
We report measurements of Pb diffusion in both synthetic (CePO4) and natural monazites run under dry, 1-atm conditions. Powdered mixtures of prereacted CePO4 and PbZrO3 were used as the source of Pb diffusant for “in-diffusion” experiments conducted in sealed Pt capsules for durations ranging from a few hours to several weeks. Following the diffusion anneals, Pb concentration profiles were measured with Rutherford Backscattering Spectroscopy (RBS) and supplemented by measurements with secondary ion mass spectrometry (SIMS). In order to evaluate potential compositional effects upon Pb diffusivity and simulate diffusional Pb loss that might occur in natural systems, we also conducted “out-diffusion” experiments on Pb-bearing natural monazites. In these experiments, monazite grains were surrounded by a synthetic zircon powder to act as a “sink.” Monazites from these experiments were analyzed with SIMS. Over the temperature range 1100 to 1350°C, the Arrhenius relation determined for in-diffusion experiments on synthetic monazite is given by:
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3.
Offsets from isotopic equilibrium in biogenic carbonates have complicated paleoclimate reconstructions for decades. A new archive of climate, deep-sea corals, is used to evaluate the calcification processes, independent of photosynthesis, that contribute to these offsets. Carbon and oxygen stable isotope data from six modern deep-sea corals show strong linear trends between δ13C and δ18O. Slopes of these trends between samples are similar and range between 1.9 to 2.6 for Δδ13C/Δδ18O. Linear trends intersect isotopic equilibrium for δ18O and are slightly depleted for δ13C. Variations in the isotopic ratios are strongly correlated with the density banding structure. Isotopically depleted aragonite is associated with light, quickly precipitating bands, whereas isotopically enriched points correspond to slowly accumulating, less dense aragonite. The densest white band at the trabecular center is furthest from isotopic equilibrium for both carbon and oxygen. Data from this region fall off the linear trend between δ18O and δ13C. This deviation, where δ13C remains constant while the δ18O continues to decrease, does not support “vital effect” mechanisms that call upon kinetic fractionation to explain offsets from isotopic equilibrium. We propose a new mechanism for vital effects in these deep-sea corals that is based on a thermodynamic response to a biologically induced pH gradient in the calcifying region.  相似文献   

4.
Basalt contamination by continental crust: Some experiments and models   总被引：1，自引：1，他引：1  
Chemical interaction between molten basalt and felsic minerals of the continental crust (quartz, K-feldspar, and oligoclase) was examined in static and dynamic experiments at 1,200°–1,400° C. Under circumstances of continuous stirring at 1,400°, -quartz dissolves in tholeiite melt at a rate of 3.3×10–6 g/s per cm2 of contact area; at 1,300°, the solution rate is 1.5×10–6 g/cm{cm2}s. The feldspars are molten at the experimental conditions, and interact with contacting basalt melt by diffusion in the liquid state. This is a complex process characterized by rapid initial diffusion of alkalies to establish a distribution between felsic melt and basalt similar to that observed in cases of actual two-liquid equilibrium (both alkalies reach concentrations in the felsic melt 1.5–3 times those in the basalt). Alkali diffusion may be uphill or downhill, depending on which direction of net flux is required to produce a two-liquid type distribution. Once this distribution is attained, subsequent diffusion of all melt species is slow and apparently limited by the diffusivity of SiO2, which is 10–9-l0–10 cm2/s at 1,200° C. Interdiffusion experiments involving molten basalt and synthetic granite confirm the behavior illustrated by the feldspar/basalt results, and give similar SiO2 diffusivities.The solution rates and interdiffusion data can be used to model basalt contamination processes likely to occur in the continental crust. For the restricted case of solid quartzitic xenoliths, the uptake of SiO2 in a well-mixed basalt magma is quite fast: appreciable SiO2 contamination may occur over exposure times of only days to years. If basalt magma induces local melting of crustal rocks, the assimilation process becomes one of liquid-state interdiffusion. In this case, the varying diffusivities of ions and their differing preferences for silicic relative to basaltic melts can produce marked selective contamination effects. Selective contamination of ascending basaltic magmas is particularly likely in the case of K2O, which may be introduced in substantial amounts even when other elements remain unaffected. The Na2O content of mantle-derived magmas is buffered against contamination by crustal materials, and K2O is buffered against further increases once it reaches a level of 1–1.5 wt.%.  相似文献   

5.
Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen‐bearing minerals and the dependence of mineral 18O/16O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10‐μm scale by secondary ion mass spectrometry. The patterns of δ18O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain‐size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage‐Colton Mylonite Zone (CCMZ), a long‐lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090–1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW‐dipping protomylonitic fabric (S1) is crosscut by near‐vertical ultramylonitic shear zones (S2), which are locally reoriented by a NNW‐dipping mylonitic fabric (S3). Texturally early titanite (pre‐S2) shows diffusion‐dominated δ18O zoning that records cooling from peak Ottawan, granulite‐facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed δ18O profiles for cooling rates of 50 ± 20 °C Ma?1, which are considerably faster than the 1–5 °C Ma?1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn‐S2) has higher overall δ18O and shows a transition from diffusion‐dominated to recrystallization‐dominated δ18O zoning, indicating infiltration of elevated‐δ18O fluids along S2 shear zones and continued shearing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The texturally latest titanite (post‐S3) has growth‐dominated δ18O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.  相似文献   

6.
Zr diffusion in titanite   总被引：2，自引：0，他引：2  
Chemical diffusion of Zr under anhydrous, pO2-buffered conditions has been measured in natural titanite. The source of diffusant was either zircon powder or a ZrO2–Al2O3–titanite mixture. Experiments were run in sealed silica glass capsules with solid buffers (to buffer at NNO or QFM). Rutherford Backscattering Spectrometry (RBS) was used to measure diffusion profiles. The following Arrhenius parameters were obtained for Zr diffusion parallel to c over the temperature range 753–1,100°C under NNO-buffered conditions: D Zr = 5.33 × 10−7 exp(−325 ± 30 kJ mol−1/RT) m2 s−1 Diffusivities are similar for experiments buffered at QFM. These data suggest that titanite should be moderately retentive of Zr chemical signatures, with diffusivities slower than those for O and Pb in titanite, but faster than those for Sr and the REE. When applied in evaluation of the relative robustness of the recently developed Zr-in-titanite geothermometer (Hayden and Watson, Abstract, 16th V.M. Goldschmidt Conference 2006), these findings suggest that Zr concentrations in titanite will be less likely to be affected by later thermal disturbance than the geothermometer based on Zr concentrations in rutile (Zack et al. in Contrib Mineral Petrol 148:471–488, 2004; Watson et al. in Contrib Mineral. Petrol, 2006), but much less resistant to diffusional alteration subsequent to crystallization than the Ti-in-Zircon geothermometer (Watson and Harrison in Science 308:841–844, 2005).  相似文献   

7.
The stable isotopes of sulfate are often used as a tool to assess bacterial sulfate reduction on the macro scale. However, the mechanisms of stable isotope fractionation of sulfur and oxygen at the enzymatic level are not yet fully understood. In batch experiments with water enriched in 18O we investigated the effect of different nitrite concentrations on sulfur isotope fractionation by Desulfovibrio desulfuricans.With increasing nitrite concentrations, we found sulfur isotope enrichment factors ranging from −11.2 ± 1.8‰ to −22.5 ± 3.2‰. Furthermore, the δ18O values in the remaining sulfate increased from approximately 50-120‰ when 18O-enriched water was supplied. Since 18O-exchange with ambient water does not take place in sulfate, but rather in intermediates of the sulfate reduction pathway (e.g. ), we suggest that nitrite affects the steady-state concentration and the extent of reoxidation of the metabolic intermediate sulfite to sulfate during sulfate reduction. Given that nitrite is known to inhibit the production of the enzyme dissimilatory sulfite reductase, our results suggest that the activity of the dissimilatory sulfite reductase regulates the kinetic isotope fractionation of sulfur and oxygen during bacterial sulfate reduction. Our novel results also imply that isotope fractionation during bacterial sulfate reduction strongly depends on the cell internal enzymatic regulation rather than on the physico-chemical features of the individual enzymes.  相似文献   

8.
The surface of a crystal in equilibrium with solute-bearing fluid generally has a composition that differs from that of the bulk crystal. If the crystal is growing, the surface composition may be “captured” by the newly formed lattice to a degree that depends upon the growth rate and the mobility of atoms in the near-surface region: rapid growth promotes this growth “entrapment,” high near-surface mobility works against it. Natural calcites may be particularly susceptible to this kind of kinetic disequilibrium, because their precipitation rates from aqueous solution can be relatively high even at near-ambient temperatures, where ion mobility in the critical near-surface region may be limited.Existing laboratory data on trace-element uptake as a function of calcite growth rate are examined here in the context of recent discoveries concerning the structure, chemistry and kinetics of the near-surface region of calcite crystals. Recent demonstrations that ions can be mobile in the outermost few nanometers of the calcite lattice even at room temperature have the greatest potential to affect growth entrapment. The model of Watson and Liang (1995)—which quantifies entrapment efficiency in terms of growth rate, diffusivity and surface-layer thickness—is modified to include a depth-dependent diffusivity and possible depletion (as well as enrichment) of some elements in the near-surface region. With these changes, the model is shown to be qualitatively consistent with the body of experimental data on trace element uptake during calcite precipitation.This apparent success of the model invites application to stable isotopes. Constraining data are few, but available information on oxygen isotope fractionation can be used to show that growth entrapment at ambient temperatures may (depending on model assumptions) produce deviations from calcite/H2O equilibrium of up to several ‰. The preferred choice of 18O/16O for the surface layer is lighter than the lattice equilibrium value, and leads to a reduction in 18O/16O of crystals grown at higher growth rates, mimicking “vital effects.”  相似文献   

9.
Bacterial sulfate reduction is one of the most important respiration processes in anoxic habitats and is often assessed by analyzing the results of stable isotope fractionation. However, stable isotope fractionation is supposed to be influenced by the reduction rate and other parameters, such as temperature. We studied here the mechanistic basics of observed differences in stable isotope fractionation during bacterial sulfate reduction. Batch experiments with four sulfate-reducing strains (Desulfovibrio desulfuricans, Desulfobacca acetoxidans, Desulfonatronovibrio hydrogenovorans, and strain TRM1) were performed. These microorganisms metabolize different carbon sources (lactate, acetate, formate, and toluene) and showed broad variations in their sulfur isotope enrichment factors. We performed a series of experiments on isotope exchange of 18O between residual sulfate and ambient water. Batch experiments were conducted with 18O-enriched (δ18Owater = +700‰) and depleted water (δ18Owater = −40‰), respectively, and the stable 18O isotope shift in the residual sulfate was followed. For Desulfovibrio desulfuricans and Desulfonatronovibrio hydrogenovorans, which are both characterized by low sulfur isotope fractionation (εS > −13.2‰), δ18O values in the remaining sulfate increased by only 50‰ during growth when 18O-enriched water was used for the growth medium. In contrast, with Desulfobacca acetoxidans and strain TRM1 (εS < −22.7‰) the residual sulfate showed an increase of the sulfate δ18O close to the values of the enriched water of +700‰. In the experiments with δ18O-depleted water, the oxygen isotope values in the residual sulfate stayed fairly constant for strains Desulfovibrio desulfuricans, Desulfobacca acetoxidans and Desulfonatronovibrio hydrogenovorans. However, strain TRM1, which exhibits the lowest sulfur isotope fractionation factor (εS < −38.7‰) showed slightly decreasing δ18O values.Our results give strong evidence that the oxygen atoms of sulfate exchange with water during sulfate reduction. However, this neither takes place in the sulfate itself nor during formation of APS (adenosine-5′-phosphosulfate), but rather in intermediates of the sulfate reduction pathway. These may in turn be partially reoxidized to form sulfate. This reoxidation leads to an incorporation of oxygen from water into the “recycled” sulfate changing the overall 18O isotopic composition of the remaining sulfate fraction. Our study shows that such incorporation of 18O is correlated with the stable isotope enrichment factor for sulfur measured during sulfate reduction. The reoxidation of intermediates of the sulfate reduction pathway does also strongly influence the sulfur stable isotope enrichment factor. This aforesaid reoxidation is probably dependent on the metabolic conversion of the substrate and therefore also influences the stable isotope fractionation factor indirectly in a rate dependent manner. However, this effect is only indirect. The sulfur isotope enrichment factors for the kinetic reactions themselves are probably not rate dependent.  相似文献   

10.
Water is an important volatile component in andesitic eruptions and deep-seated andesitic magma chambers. We report an investigation of H2O speciation and diffusion by dehydrating haploandesitic melts containing ?2.5 wt.% water at 743-873 K and 100 MPa in cold-seal pressure vessels. FTIR microspectroscopy was utilized to measure species [molecular H2O (H2Om) and hydroxyl group (OH)] and total H2O (H2Ot) concentration profiles on the quenched glasses from the dehydration experiments. The equilibrium constant of the H2O speciation reaction H2Om+O?2OH, K = (XOH)2/(XH2OmXO) where X means mole fraction on a single oxygen basis, in this Fe-free andesite varies with temperature as ln K = 1.547-2453/T where T is in K. Comparison with previous speciation data on rhyolitic and dacitic melts indicates that, for a given water concentration, Fe-free andesitic melt contains more hydroxyl groups. Water diffusivity at the experimental conditions increases rapidly with H2O concentration, contrary to previous H2O diffusion data in an andesitic melt at 1608-1848 K. The diffusion profiles are consistent with the model that molecular H2O is the diffusion species. Based on the above speciation model, H2Om and H2Ot diffusivity (in m2/s) in haploandesite at 743-873 K, 100 MPa, and H2Ot ? 2.5 wt.% can be formulated as
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11.
Anaerobic incubations of upland and wetland temperate forest soils from the same watershed were conducted under different moisture and temperature conditions. Rates of nitrous oxide (N2O) production by denitrification of nitrate () and the stable isotopic composition of the N2O (δ15N, δ18O) were measured. In all soils, N2O production increased with elevated temperature and soil moisture. At each temperature and moisture level, the rate of N2O production in the wetland soil was greater than in the upland soil. The 15N isotope effect (ε) (product − substrate) ranged from −20‰ to −29‰. These results are consistent with other published estimates of 15N fractionation from both single species culture experiments and soil incubation studies from different ecosystems.A series of incubations were conducted with 18O-enriched water (H2O) to determine if significant oxygen exchange (O-exchange) occurred between H2O and N2O precursors during denitrification. The exchange of H2O-O with nitrite () and/or nitric oxide (NO) oxygen has been documented in single organism culture studies but has not been demonstrated in soils prior to this study. The fraction of N2O-O derived from H2O-O was confined to a strikingly narrow range that differed between soil types. H2O-O incorporation into N2O produced from upland and wetland soils was 86% to 94% and 64% to 70%, respectively. Neither the temperature, soil moisture, nor the rate of N2O production influenced the magnitude of O-exchange. With the exception of one treatment, the net 18O isotope effect (εnet) (product-substrate) ranged from +37‰ to +43‰.Most previous studies that have reported 18O isotope effects for denitrification of to N2O have failed to account for the effect of oxygen exchange with H2O. When high amounts of O-exchange occur after fractionation during reductive O-loss, the 18O-enrichment is effectively lost or diminished and δ18O-N2O values will be largely dictated by δ18O-H2O values and subsequent fractionation. The process and extent of O-exchange, combined with the magnitude of oxygen isotope fractionation at each reduction step, appear to be the dominant controls on the observed oxygen isotope effect. In these experiments, significant oxygen isotope fractionation was observed to occur after the majority of water O-exchange. Due to the importance of O-exchange, the net oxygen isotope effect for N2O production in soils can only be determined using δ18O-H2O addition experiments with δ18O-H2O close to natural abundance.The results of this study support the continued use of δ15N-N2O analysis to fingerprint N2O produced from the denitrification of . The utilization of 18O/16O ratios of N2O to study N2O production pathways in soil environments is complicated by oxygen exchange with water, which is not usually quantified in field studies. The oxygen isotope fractionation observed in this study was confined to a narrow range, and there was a clear difference in water O-exchange between soil types regardless of temperature, soil moisture, and N2O production rate. This suggests that 18O/16O ratios of N2O may be useful in characterizing the actively denitrifying microbial community.  相似文献   

12.
Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2–8 m and precision in the range of 1 (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of 18O at this scale. High precision micro-analyses of this sort will permit wide-ranging new applications in stable isotope geochemistry. The analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite. Magnetites are near to Fe3O4 in composition, and optically and chemically homogeneous. A combination of ion probe plus conventional BrF5 analysis shows that individual grains are homogeneous with 18O=8.9±1 SMOW from the core to near the rim of 0.1–1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that rims are 9 depleted in 18O. These low 18O values increase in smooth gradients across the outer 10 m of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150–350 m diameter magnetites which average 1.2 lower in 18O than coarse magnetites due to low 18O rims. Conventional analysis of coexisting calcite yields °18O=18.19, suggesting that bulk 18O (Cc-Mt)=9.3 and yielding an apparent equilibration temperature of 525° C, over 200° C below the temperature of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for oxygen in calcite under hydrothermal conditions at high P(H2O) indicates that diffusion is faster in magnetite and modelling of the low 18O rims on magnetite would suggest that the Adirondacks experienced slow cooling after Grenville metamorphism, followed by a brief period of rapid cooling, possibly related to uplift. Conversely, the data for calcite at low P(H2O) show slower oxygen diffusion than in magnetite. Modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals, suggesting that the low 18O rims form by exchange with late, low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets. In either case, the ion microprobe results indicate that two distinct events are recorded in the post-metamorphic exchange history of these magnetites. Recognition of these events is only possible through microanalysis and has important implications for geothermometry.  相似文献   

13.
The Pliocene and Quaternary Patagonian alkali basalts of southernmost South America can be divided into two groups. The cratonic basalts erupted in areas of Cenozoic plateau volcanism and continental sedimentation and show considerable variation in 87Sr/86Sr (0.70316 to 0.70512), 143Nd/144Nd (Nd) and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (18.26 to 19.38, 15.53 to 15.68, and 38.30 to 39.23, respectively). These isotopic values are within the range of oceanic island basalts, as are the Ba/La, Ba/Nb, La/Nb, K/Rb, and Cs/Rb ratios of the cratonic basalts. In contrast, the transitional basalts, erupted along the western edge of the outcrop belt of the Pliocene and Quaternary plateau lavas in areas that were the locus of earlier Cenozoic Andean orogenic arc colcanism, have a much more restricted range of isotopic composition which can be approximated by 87Sr/86Sr=0.7039±0.0004, Nd, 206Pb/204Pb=18.60±0.08, 207Pb/204Pb=15.60±0.01, and 208Pb/204Pb=38.50±0.10. These isotopic values are similar to those of Andean orogenic are basalts and, compared to the cratonic basalts, are displaced to higher 87Sr/86Sr at a given 143Nd/144Nd and to higher 207Pb/204Pb at a given 208Pb/204Pb. The transitional basalts also have Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios higher than the cratonic and oceanic island basalts, although not as high as Andean orogenic are basalts. In contrast to the radiogenic isotopes, 18O values for both groups of the Patagonian alkali basalts are indistinguishable and are more restricted than the range reported for Andean orogenic are basalts. Whole rock 18O values calculated from mineral separates for both groups range from 5.3 to 6.5, while measured whole rock 18O values range from 5.1 to 7.8. The trace element and isotopic data suggest that decreasing degrees of partial melting in association with lessened significance of subducted slabderived components are fundamental factors in the west to east transition from arc to back-arc volcanism in southern South America. The cratonic basalts do not contain the slab-derived components that impart the higher Ba/La, Ba/Nb, La/Nb, Cs/Rb, 87Sr/86Sr at a given 143Nd/144Nd, 207Pb/204Pb at a given 208Pb/204Pb, and 18O to Andean orogenic arc basalts. Instead, these basalts are formed by relatively low degrees of partial melting of heterogeneous lower continental lithosphere and/or asthenosphere, probably due to thermal and mechanical pertubation of the mantle in response to subduction of oceanic lithosphere below the western margin of the continent. The transitional basalts do contain components added to their source region by either (1) active input of slab-derived components in amounts smaller than the contribution to the mantle below the arc and/or with lower Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios than below the arc due to progressive downdip dehydration of the subducted slab; or (2) subarc source region contamination processes which affected the mantle source of the transitional basalts earlier in the Cenozoic.  相似文献   

14.
The Kolan geothermal field is located 24 km northwest of the Karakoçan area of Elaz city in southeastern Turkey. The Keban crystallized limestone of Permo-Carboniferous age is the oldest unit in the area. This unit is overlain by the middle-upper Eocene Krkgeçit formation consisting of conglomerate, sandstone, limestone and marl alternations. The youngest unit in the area is the Upper Miocene-Lower Pliocene Karabakr formation consisting of basalt, andesite and tuffs. Thermal Springs in the Kolan field issue along two major fault zones. The temperature of the hot springs is between 38°C and 45°C. Thermal waters in the area are heated by the geothermal gradient. The waters are classified as hot or very hot and as brackish waters. On the basis of International Association of Hydrogeologists Classification (IAH), waters are grouped as Ca–Na-HCO3–CO2-bearing thermal and mineral waters. Chemical analyses indicate a chemical composition of Ca>(Na+K)>Mg - HCO3>SO4>Cl. Saturation calculations reveal that the waters are oversaturated with respect to calcite, dolomite and aragonite while undersaturated with respect to gypsum, anhydrite and halite minerals. On the basis of 18O, 2H and 3H isotope data, Kolan waters are of meteoric origin, recharged from high elevations and then circulated deeply.  相似文献   

15.
The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (350–400°C) to migmatite grade (700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, 18O values of normal metapelites decrease from 14–16 to as low as 9.0, while 18O values of calcite in normal marbles decrease from 22–24 to as low as 13.2 These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (18O=8.4–8.6) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO2 mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in 18O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x109 moles/m2 (105 m3/m2). If fluid flow occurred over 105–106 years, estimated intrinsic permeabilities are 10-20 to 10-16m2. Variations in 18O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO2 values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both 18O and XCO2 values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.This paper is a contribution to IGCP Project 304 Lower Crustal Processes  相似文献   

16.
The transformation from blueschist to greenschist facies forms a major part of the Alpine regional geodynamic evolution of the Cyclades. The transition in metabasic rocks on Sifnos involves the retrogression of eclogites, blueschists and actinolite-bearing rocks from high-pressure conditions which have been estimated at 460±30° C and 15±3 kb. Petrographically observed parageneses are interpreted by a sequence of hydration and carbonation reactions involving the breakdown of omphacite and glaucophane-bearing assemblages to albite+chlorite±actinolite±calcite assemblages. The retrograde processes are calculated to occur at pressures of 10 to 8 kb during the isothermal uplift of the Sifnos units. Oxygen isotope analyses of different rock types show that interlayered lithologies have experienced a large degree of isotopic equilibration during both metamorphic phases. However, chemically equivalent rocks show systematic increases in 18O from lowest values (8 to 11 in metabasics) in the unaltered blueschists in the upper stratigraphic levels to higher values (>15 in metabasics) associated with greenschists in the deepest stratigraphic levels. Relict eclogites enclosed within greenschists have the lower 18O values typical of unaltered blueschist facies rocks. These isotopic gradients and the 13C and 18O compositions of carbonates demonstrate an infiltration mechanism involving the upward movement of 18O-enriched fluids whose compositions were buffered by exchange with marble units. Calculated minimum fluid/rock ratios for the blueschist-to-greenschist transition decrease from 0.4 in the deepest studied level (Central Sifnos) and 0.2 in the intermediate level (Kamares Bay samples) to an assumed value of zero in unaltered blueschists. These ratios may be lower if recycling of fluids occurred between schists and marbles. Infiltration of fluids became inhibited as the transformation advanced as a consequence of sealing effect of the hydration and carbonation reactions. Although infiltration most probably was a pre-requisite for the regional occurrence of the blueschist-to-greenschist transformation in the Cyclades, the evidence on Sifnos does not suggest the introduction of large quantities of fluid.  相似文献   

17.
O18/O16 ratios have been measured for 29 quartz samples, 6 whole-rocks, 3 muscovites, and 1 K-feldspar from two adjacent granitic plutons of vastly different age (about 1660 m.y, and 70 m.y.) intruded into the same type of country rock, the Precambrian Pinal schist. Sample traverses were made across 3 different contact zones of these intrusive bodies. Except for 2 quartz veins with O18=+11.0 and + 12.3, all quartz samples collected more than 15 cm from the margin of the Early Tertiary Texas Canyon pluton are isotopically exceedingly uniform with O18=9.47±0.11. Four quartz samples collected more than 10 m from the margin of the Precambrian Johnny Lyon pluton have O18=10.43±0.08. Compared with previous studies of this type, only relatively minor O18-enrichments have occurred in the border zones of the plutons. This is in part because the original O18 differences between the metasedimentary rocks and the intrusives are relatively small (only 3 to 6), but is mainly due to the lack of H2O in the contact zones during intrusion as a result of the general impermeability and prior dehydration of the schist. There is no isotopic evidence for significant influx of external H2O into either of the plutons during their crystallization and cooling. However, in roof-zones where metasedimentary rocks overlie the plutons there is a strong O18 lowering in the contact metamorphic aureoles, indicating upward expulsion of low-O18 magmatic H2O into these rocks.Contribution No. 2015 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91109.  相似文献   

18.
Late Carboniferous (Hercynian) tectonism in the Pyrenees generated extremely steep thermal gradients at 8–14 km depth in the continental crust, producing andalusite- and sillimanite-grade metamorphism and partial melting of Lower Paleozoic metasediments under water-rich conditions. At the same time, amphibolite- and granulitefacies basal gneisses were equilibrated under dryer conditions at pressures of 4 to 7 kbar (14–25 km depth), beneath these higher-level rocks. We present 95 new oxygen isotopic analyses of samples from the Agly, St. Barthelemy, Castillon and Trois Seigneurs Massifs, highlighting contrasting 18O/16O systematics at different structural levels in the Hercynian crust, here termed Zones 1, 2, and 3. The unmetamorphosed, fossiliferous, Paleozoic shales and carbonates of Zone 1 have typical sedimentary 18O values, mostly in the range +14 to +16 for the pelitic rocks and +20 to +25 for the carbonates. The metamorphosed equivalents of these rocks in Zone 2 all have strikingly uniform and much lower 18O values; the metapelites mostly have 18O=+10 to +12, and interlayered metacarbonates from the Trois Seigneurs Massif have 18O of about +12 to +14. Typically, the Zone 3 basal gneisses are isotopically heterogeneous with variable 18O values ranging from +6 in mafic lithologies to +22 in carbonate-rich lithologies. Steep gradients in 18O (as much as 10 per mil over a few cm) are preserved at the margins of some metacarbonate layers. These data indicate that the Zone 3 gneisses were infiltrated by much smaller volumes of metamorphic pore fluids than were the overlying Zone 2 rocks, and that circulation of surface-derived H2O (either seawater or formation waters, as evidenced by high D values) was mainly confined to the Paleozoic supracrustal sedimentary pile. This is compatible with an overall reduction of interconnected porosity with increasing depth, but perhaps even more important, the extensive partial melting at the base of Zone 2 may have produced a ductile, impermeable barrier to downward fluid penetration.Contribution No. 4287, Publication of the Division of Geological and Planetary Sciences, California Institute of Technology  相似文献   

19.
Carbon and oxygen isotopic profiles around a low pressure metasomatic wollastonite reaction front in a marble of the Hida metamorphic terrain, central Japan, display typical metamorphic fluid-enhanced isotopic zonations. Isotopic profiles obtained from detailed microscale analyses perpendicular to the chemical reaction front in calcite marble show that diffusion-enhanced isotopic exchange may control these profiles. Carbon and oxygen isotopic behaviour in grain boundaries is remarkably different. Oxygen isotopic troughs (18O depleted rims) around the calcite-grain boundaries are widely observed in this contact aureole, demonstrating that diffusion of oxygen in calcite grain boundary dominates over lattice diffusion in calcite. In contrast, no difference is observed in carbon isotopic profiles obtained from grain cores and rims. There is thus no specific role of the grain boundary for diffusion of carbonic species in the metamorphic fluid during transportation. Carbon chemical species such as CO2 and CO3 ions in metamorphic fluid migrate mainly through lattice diffusion. The carbon and oxygen isotope profiles may be modelled by diffusion into a semi-infinite medium. Empirically lattice diffusion of oxygen isotopes is almost six times faster than that of carbon isotopes, and oxygen grain-boundary diffusion is ten times faster than oxygen lattice diffusion. Oxygen isotopic results around the wollastonite vein indicate that migration of the metamorphic fluid into calcite marble was small and was parallel to the aquifer. From the stability of wollastonite and the attainment of oxygen isotopic equilibrium, we suggest that diffusion of oxygen occurred through an aqueous fluid phase. The timescale of formation of the oxygen isotopic profile around the wollastonite vein is calculated to be about 0.76 × 106 years using the experimentally determined diffusion constant. Received: 14 January 1997 / Accepted: 23 April 1998  相似文献   

20.
In order to test their chronometric potential, 40Ar/39Ar stepheating- (and 4He-) analyses have been carried out on five manganese ore minerals of the hollandite-cryptomelane series from three Precambrian manganese deposits (Ultevis/Sweden, Sitapar/India, Bachkoun/Morocco). Samples from the metamorphic occurrences Ultevis and Sitapar yielded Ar ages of 1.8 Ga and 0.95 Ga, interpreted as the age of postmetamorphic cooling (Hollandites/Ultevis) and of an early, K-introducing alteration process subsequent to amphibolite facies metamorphism (cryptomelanes/Sitapar). Both data are consistent with known chronologies of the Svecokarelian and Satpura orogenic cycles. A date of 670 Ma obtained for a hollandite from a volcanogenic vein deposit (Bachkoun), however, contrasts with published extrusion ages of 580–560 Ma for the volcanic host rocks (Ouarzazate Series), probably due to incorporation of excess argon. The use of the 40Ar/39Ar technique, together with multiple isotope systematics, made it nevertheless possible to establish a reasonable estimate of a mineralization age close to 580 Ma. Measurement of fractional Ar losses during vacuum step heating (500–1600 °C), although indicating good Ar retentivities, failed to define model diffusion parameters because of non-linear Arrhenius arrays. Helium diffusion results (200–1200 °C) indicated retention of radiogenic 4He by the samples, corroborated by U/He mineral dates between 0.96 and 0.31 Ga. Potassium-bearing manganese oxides are therefore able to retain argon (possibly also 4He) through geological times and may thus provide ages of ore-forming processes (and perhaps later cooling and alteration stages).  相似文献   

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