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1.
Oxygen isotope fractionation factors between calcium carbonates and water have been applied to ancient marine geochemistry
principally for the purpose of geothermometry. The problem was encountered, however, with respect to the direction and magnitude
of oxygen isotope fractionation between calcite and aragonite at thermodynamic equilibrium. This basically involves sound
understanding of both thermodynamics and kinetics of oxygen isotope fractionation between inorganically precipitated carbonate
and water at low temperatures. Thus the crucial issues are to acknowledge the processes of chemical reaction and isotopic
exchange during precipitation of CaCO3 minerals in solution, the kinetic mechanism of isotope equilibrium or disequilibrium, the effect of polymorphic transition
from metastable aragonite to stable calcite under hydrous or anhydrous conditions, and the presence or absence of isotope
salt effect on oxygen isotope exchange between carbonate and water in response to the hydrous or anhydrous conditions at thermodynamic
equilibrium. Because good agreements exist in carbonate–water oxygen isotope fractionation factors between theoretical calculations
and experimental determinations, it is encouraging to applying the thermodynamic and kinetic data to isotopic paleothermometry
and geochemical tracing. 相似文献
2.
Oxygen isotope exchange between minerals during metamorphism can occur in either the presence or the absence of aqueous fluids. Oxygen isotope partitioning among minerals and fluid is governed by both chemical and isotopic equilibria during these processes, which progress by intragranular and intergranular diffusion as well as by surface reactions. We have carried out isotope exchange experiments in two- and three-phase systems, respectively, between calcite and tremolite at high temperatures and pressures. The two-phase system experiments were conducted without fluid either at 1 GPa and 680 °C for 7 days or at 500 MPa and 560 °C for 20 days. Extrapolated equilibrium fractionations between calcite and tremolite are significantly lower than existing empirical estimates and experimental determinations in the presence of small amounts of fluid, but closely match calculated fractionations by means of the increment method for framework oxygen in tremolite. The small fractionations measured in the direct calcite–tremolite exchange experiments are interpreted by different rates of oxygen isotope exchange between hydroxyl oxygen, framework oxygen and calcite during the solid–solid reactions where significant recrystallization occurs. The three-phase system experiments were accomplished in the presence of a large amount of fluid (CO2+H2O) at 500 MPa and 560 °C under conditions of phase equilibrium for 5, 10, 20, 40, 80, 120, 160, and 200 days. The results show that oxygen isotope exchange between minerals and fluid proceeds in two stages: first, through a mechanism of dissolution-recrystallization and very rapidly; second, through a mechanism of diffusion and very slowly. Synthetic calcite shows a greater rate of isotopic exchange with fluid than natural calcite in the first stage. The rate of oxygen diffusion in calcite is approximately equal to or slightly greater than that in tremolite in the second stage. A calculation using available diffusion coefficients for calcite suggests that grain boundary diffusion, rather than volume diffusion, has been the dominant mechanism of oxygen transport between the fluid and the mineral grains in the later stage.Editorial responsibility: T.L. Grove 相似文献
3.
矿物稳定同位素地球化学研究 总被引：36，自引：2，他引：34
通过测定矿物中元素H ,C ,O和S的同位素比值 ,认识矿物体系中的同位素效应 ,不仅能够确定矿物之间和矿物与流体之间的同位素平衡关系 ,而且能够了解影响矿物平衡和动力学同位素性质的因素。文中评述了稳定同位素分馏系数校准的理论计算、实验测定和经验估计方法 ,讨论温度、压力、化学成分和晶体结构等对矿物同位素性质的影响。由于同位素效应取决于矿物的物理和化学性质 ,因此应用稳定同位素来作为示踪剂不仅能够追索各种矿物学反应的路径 ,而且能够提供证据来阐明矿物晶体结构的某些细节。 相似文献
4.
Abstract Oxygen and hydrogen isotope analyses have been made of coexisting quartz, ilmenite, muscovite, and biotite from Late Precambrian metapelitic rocks, staurolite-kyanite to K-feldspar-muscovite-sillimanite zones, from Mica Creek, British Columbia. The δ18 O and †D values of these minerals are generally uniform and do not decrease significantly with increasing metamorphic grade. This implies that there has not been significant infiltration of deep crustal, possibly magmatic, fluids into the metapelites that has been suggested for other high-grade metamorphic terranes. The uniformity of oxygen isotope compositions of the Mica Creek metapelite rocks may reflect isotopic uniformity in the sedimentary protolith rather than widespread exchange with an isotopically homogeneous metamorphic pore fluid.
Temperature estimates based upon18 O exchange thermometry for samples below the sillimanite zone are in reasonable agreement with the results of garnet-biotite Fe–Mg exchange thermometry. In the higher grade rocks, the oxygen isotope and garnet-biotite thermometry yield results which disagree by about 100°C. The highest temperatures recorded by oxygen isotope thermometry, 595°C, are at least 60°C below the minimum temperatures required by phase equilibria. These discrepancies appear to result from pervasive equilibrium retrograde exchange of oxygen isotopes between coexisting minerals. In addition, there are problems with calibration of garnet-biotite thermometry at higher temperatures. Retrograde oxygen isotope exchange may be a general characteristic of high-grade metamorphic rocks and oxygen isotope thermometry may not usually record peak metamorphic temperatures if they significantly exceed 600°C. 相似文献
Temperature estimates based upon
5.
Experimental hydrogen isotope studies III: Diffusion of hydrogen in hydrous minerals,and stable isotope exchange in metamorphic rocks 总被引：4，自引：0，他引：4
Colin M. Graham 《Contributions to Mineralogy and Petrology》1981,76(2):216-228
Diffusion parameters for hydrogen diffusion in epidote-group minerals and micas have been measured under hydrothermal conditions, or calculated from existing experimental data, for bulk hydrogen isotope exchange experiments between hydrous minerals and water. Activation energies in the range 14 to 31 kcals/g-atom H are comparable to those derived by application of kinetic theory to experimental hydrogen isotope exchange data, and to those for oxygen diffusion in minerals under hydrothermal conditions. Diffusion of hydrogen in epidote is about four orders of magnitude faster than in muscovite, and about two orders of magnitude faster than in zoisite. Hydrogen diffusion in micas is about five orders of magnitude faster than oxygen diffusion, and hydrogen transport occurs dominantly parallel to the layers rather than parallel to the c-axis as for oxygen.Rapid hydrogen transport in minerals may proceed by hydrolysis of Si-O and Al-O bonds, followed by exchange of hydrolyzed oxygens with slower-diffusing (OH) or H2O. Water appears to be essential for stable isotope exchange between minerals in slowly cooling metamorphic rocks.Stable isotope data for regional metamorphic mineral assemblages suggests that water is usually present in small amounts during cooling of prograde regional metamorphic systems, and estimated closure temperatures for cessation of stable isotope exchange are often more comparable to those calculated from diffusion data than to likely temperatures of metamorphism.Alpine deformation of the Hercynian Monte Rose Granite (Frey et al. 1976) permitted access of water and initiated stable isotope exchange amongst coexisting minerals. The diffusional behaviour of species in relict Hercynian muscovites is consistent with available experimental diffusion data. 相似文献
6.
Clay minerals record chemical data about the past, acting like natural computer memory chips. To retrieve the data we must understand how they are stored. To achieve this we have examined the isotopic information revealed by two trace elements, lithium and boron, that are incorporated into the common clay minerals illite-smectite (I-S) during diagenesis. We used hydrothermal experiments at 300°C, 100 MPa, to speed up the reaction of smectite to illite that normally occurs during slow (10-100 Ma) sediment burial. During illitization, Li substitutes into the octahedral sites and B enters the tetrahedral sites of the silicate framework. Both Li and B are also adsorbed in the interlayer of smectite, but Li is preferred over B in the exchange sites. To determine the equilibrium isotope fractionation of the two trace elements it is important to remove these adsorbed interlayer species. By measuring the isotopic composition of Li and B in the silicate framework during reaction, we can address the relative timing of element exchange in the different crystallographic sites. Furthermore, because illitization of smectite is a crystal growth process (not an isomorphous replacement) we have examined the effect of crystal size on the isotope fractionation.The results show that Li and B approach an isotopic steady state when R1 ordering occurs, long before oxygen isotopes equilibrate with the fluid. The isotopic fractionation (αmineral-water) for Li (0.989) is similar to that for B (0.984) at 300°C. However, when separated into <0.2, 0.2-2.0, and >2.0 μm fractions, there are significant differences in measured isotope ratios by as much as 9‰. Crystal growth mechanisms and surface energy effects of nanoscale crystals may explain the observed isotopic differences. The fact that different crystals equilibrate at different rates (based on size) may be applied to natural samples to reveal the changing paleofluid history, provided we understand the conditions of equilibrium. This has very important implications for the interpretation of diagenetic environments, fluid flow, and surficial geochemical cycling. 相似文献
7.
Peter Deines 《Geochimica et cosmochimica acta》1977,41(12):1709-1730
A compilation of 18O analyses of minerals separated from about 400 igneous and metamorphic rocks from published investigations reveals regularity in the fractionation of 18O among associated minerals, suggesting that an approach to isotopic equilibrium may be common. However, for only a minority of terrestrial rocks are these regularities sufficiently systematic to be compatible with the actual attainment and preservation of isotopic equilibrium among three minerals. Fractionations among triplets of quartz, calcite, feldspar, muscovite, and magnetite show some correspondence to those expected on the basis of experimental calibrations; however, there are also considerable deviations. The variability of natural data is such that less than half of the rocks analyzed to date would yield concordant 18O-derived temperatures. Of the additional 52 mineral triplets studied, plagioclase-pyrox-ene-ilmenite, plagioclase-pyroxene-magnetite, plagioclase-pyroxene-olivine, quartz-amphibole-garnet, pyroxene-ilmenite-magnetite, muscovite-biotite-magnetite, and quartz-muscovite-amphibole show the most systematic oxygen isotope fractionations. For 12 other mineral triplets a defined isotope fractionation relationship may be postulated to underlie the data; however for these a close approach to isotopic equilibrium is not commonly observed. For 33 of the mineral triplets an approach to isotopic equilibrium can be noted; however, the scatter of the available data is such that a systematic influence of a factor, such as temperature, on the size of the 18O fractionation could not be detected. In the past, regularities of oxygen isotope fractionations among three minerals have been used to establish secondary isotope geothermometers. Before this can be done with any reliability, however, the effects of possible retrograde isotope exchange and spurious correlation must be accounted for. 相似文献
8.
矿物水体系氢同位素平衡分馏系数和动力分馏系数是同位素地球化学研究中的重要参数。这些参数大多由实验测定。氢同位素分馏的实验研究主要包括矿物水体系氢同位素交换实验,交换实验前后矿物、水的氢同位素分析及分馏机理、平衡分馏、动力分馏理论研究。为确保氢同位素分馏系数和一系列动力学参数的准确可靠,实验中防止氢透过容器壁扩散,避免空气中水汽污染样品,正确控制实验温度等都很重要。本研究以石英管代替前人常用的金（银、铂）管作反应容器,建立了一套实验研究羟基矿物水体系氢同位素平衡分馏和动力分馏的新方法,并开展了电气石水、黑柱石水体系氢同位素分馏的实验研究。所得一系列参数的精度明显好于国外报道的资料。此研究方法可广泛应用于羟基矿物水体系的氢同位素分馏的实验研究。 相似文献
9.
耦合的质量传输和动力学限制的同位素交换模型综合考虑了平流、扩散、热液弥散 ,以及岩石与水之间的不平衡同位素交换等项因素。把耦合模型应用于 2个构造环境下的古热液系统 ,进而解释稳定同位素数据。对于造成环绕浅成侵入岩体分布的18O亏损环带的流体的流动几何学 ,以及浅部正断层流体流动的几何学 ,耦合模型提供了不同于单一同位素交换模型的解释。耦合模型也提供了关于同位素相对交换速率和同位素交换机制的信息。结果表明 ,断层带的动态重结晶促进了表面反应 ,进而便利了同位素交换 ;在化学不反应性和未变形的矿物中 ,同位素交换可能受制于固态条件下的扩散。 相似文献
10.
The influence of NaCl, CaCl2, and dissolved minerals on the oxygen isotope fractionation in mineral-water systems at high pressure and high temperature was studied experimentally. The salt effects of NaCl (up to 37 molal) and 5-molal CaCl2 on the oxygen isotope fractionation between quartz and water and between calcite and water were measured at 5 and 15 kbar at temperatures from 300 to 750°C. CaCl2 has a larger influence than NaCl on the isotopic fractionation between quartz and water. Although NaCl systematically changes the isotopic fractionation between quartz and water, it has no influence on the isotopic fractionation between calcite and water. This difference in the apparent oxygen isotope salt effects of NaCl must relate to the use of different minerals as reference phases. The term oxygen isotope salt effect is expanded here to encompass the effects of dissolved minerals on the fractionations between minerals and aqueous fluids. The oxygen isotope salt effects of dissolved quartz, calcite, and phlogopite at 15 kbar and 750°C were measured in the three-phase systems quartz-calcite-water and phlogopite-calcite-water. Under these conditions, the oxygen isotope salt effects of the three dissolved minerals range from ∼0.7 to 2.1‰. In both three-phase hydrothermal systems, the equilibrium fractionation factors between the pairs of minerals are the same as those obtained by anhydrous direct exchange between each pair of minerals, proving that the use of carbonate as exchange medium provides correct isotopic fractionations for a mineral pair.When the oxygen isotope salt effects of two minerals are different, the use of water as an indirect exchange medium will give erroneous fractionations between the two minerals. The isotope salt effect of a dissolved mineral is also the main reason for the observation that the experimentally calibrated oxygen isotope fractionations between a mineral and water are systematically 1.5 to 2‰ more positive than the results of theoretical calculations. Dissolved minerals greatly affect the isotopic fractionation in mineral-water systems at high pressure and high temperature. If the presence of a solute changes the solubility of a mineral, the real oxygen isotope salt effect of the solute at high pressure and high temperature cannot be correctly derived by using the mineral as reference phase. 相似文献
11.
A generalized approach for retrieving equilibrium isotope fractionations from natural rocks is proposed in which models of
prograde reaction histories and retrograde diffusional exchange are used to identify coexisting minerals with similar isotope
closure temperatures. Examples using literature data and new analyses from 32 natural amphibolite-facies schists demonstrate
both the feasibility and limitations of obtaining equilibrium oxygen isotope fractionations from minerals in natural rocks.
By screening samples according to the theoretical models, natural data are shown to have highly consistent mineral fractionations
(±2σ reproducibilities of ±0.16 to 0.54‰) that within uncertainty reproduce experimental determinations among the minerals
quartz, biotite, muscovite, and calcic amphibole. This correspondence indicates that the proposed theoretically-based selection
criteria improve the likelihood of measuring equilibrium fractionations. The new data further corroborate the expected progressive
enrichment of δ18O in the orthosilicates with increasing Al+Si relative to Fe+Mg: Δ(Ky-Grt) ∼1.05‰, Δ(St-Grt) ∼0.6‰, and Δ(St-Cld) ∼0.3‰ at
525–575 °C. In contrast, typical samples that fail to satisfy screening criteria exhibit fractionations involving quartz,
biotite, and amphibole that are strongly disequilibrium because of exchange during cooling. Theoretical screening of samples
prior to isotope analysis allows robust, independent assessment of theoretical and experimental determinations of equilibrium
isotope fractionations.
Received: 14 January 1997 / Accepted: 9 March 1998 相似文献
12.
The assumption of oxygen isotope and major element equilibrium during prograde metamorphism was tested using staurolite‐grade pelitic schists that have undergone sequential porphyroblast growth and multiple episodes of recrystallization of matrix minerals and foliation development. Textural relationships are used to infer a metamorphic history that involves garnet growth followed by staurolite growth, with each porphyroblast growth event followed by at least one period of recrystallization of matrix minerals. Conventional geothermobarometry using Qtz–Grt–Pl–Ms–Bt ± St equilibria yields peak P–T conditions of c. 625 °C at 9–11 kbar, consistent with KMnFMASH petrogenetic grid predictions for stability of the assemblage Grt + St + Bt. Qtz–Grt oxygen isotope fractionations yield apparent temperatures of c. 590 °C and Qtz–St fractionations yield an apparent temperature of c. 595 °C. Diffusional modelling indicates that quartz isotopic compositions were reset by c. 30 °C via retrograde isotopic diffusional exchange with micas. The isotopic temperatures appear to be in excellent agreement with one another, and suggest oxygen isotope equilibrium was attained between garnet and staurolite at c. 625 °C. However, the agreement of Qtz–Grt and Qtz–Str isotopic temperatures is not consistent with petrographic observations (garnet grew before staurolite) and petrogenetic grid constraints that predict that garnet grows over a temperature interval of c. 525–550 °C. Given that: (i) oxygen diffusion rates in staurolite and garnet are slow enough to render an individual porphyroblast effectively closed to exchange after it forms; and (ii) matrix minerals are able to exchange isotopes via recrystallization during each period of deformation; garnet and staurolite could not have simultaneously achieved oxygen isotope equilibrium with each other or with minerals in the recrystallized matrix. Thus, the Qtz–Grt fractionations, which yield apparent temperatures that are in apparent agreement with peak metamorphic temperature and apparent temperatures for Qtz–St fractionations, cannot be fractionations resulting from equilibrium isotopic exchange. Instead, they are apparent fractionations between porphyroblasts formed at different temperature and times in the prograde P–T–D path, and quartz that recrystallized and exchanged with micas and plagioclase during several phases of deformation. 相似文献
13.
The granulites of the Fraser Range are assumed to have formed in a carbon-rich fluid, and are generally devoid of hornblende, and lack obvious hydrous retrograde features. In these granulites, pyroxene, garnet, plagioclase and quartz are the minerals most likely to retain the oxygen isotope ratios fixed at an early stage of initial granulite metamorphism. Temperature estimates using these minerals commonly suggest that oxygen isotopic exchange ceased in the range 600 to 680°C. The peak metamorphic temperature was probably ～ 850°C as based on the stability fields of the coexisting minerals and some cation temperatures from coexisting pyroxenes in these rocks. Ilmenite may be slightly out of isotopic equilibrium with the other minerals. Thus, grains of quartz, feldspar, pyroxene and ilmenite have suffered considerable oxygen isotopic exchange during the retrogressive phase of the metamorphism, in spite of the fact that very little water was present in these granulites. The observed deviation from the peak metamorphic temperatures can be explained by essentially closed system solid-state diffusion (on at least a scale of centimetres) during slow cooling of the rocks from ~850 to 650°C, followed by more rapid cooling down to ～ 300°C. Such an explanation is not at variance with the radiometric data available for rocks from the area, which suggest that the latter phase could have involved uplift rates of ?0.5 mm/yr for a period of about 40 Ma. Wholerock δ18O values on non-quartzose mafic granulites, about 7.2%., fall within the range of basalts affected by seafloor weathering. 相似文献
14.
Rhian H Jones Laurie A Leshin Zachary D Sharp Alan J Schilk 《Geochimica et cosmochimica acta》2004,68(16):3423-3438
We report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the Mokoia, oxidized CV3 chondrite. Bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination, and oxygen isotope ratios of individual grains, mostly olivine, were obtained in situ on polished mounts using secondary ion mass spectrometry (SIMS). Our results can be compared with data obtained previously for the oxidized CV3 chondrite, Allende. Bulk oxygen isotope ratios of Mokoia chondrules form an array on an oxygen three-isotope plot that is subparallel to, and slightly displaced from, the CCAM (carbonaceous chondrite anhydrous minerals) line. The best-fit line for all CV3 chondrite chondrules has a slope of 0.99, and is displaced significantly (by δ17O ∼ −2.5‰) from the Young and Russell slope-one line for unaltered calcium-aluminum-rich inclusion (CAI) minerals. Oxygen isotope ratios of many bulk CAIs also lie on the CV-chondrule line, which is the most relevant oxygen isotope array for most CV chondrite components. Bulk oxygen isotope ratios of most chondrules in Mokoia have δ18O values around 0‰, and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values. In general, it appears that chondrule mesostases have higher δ18O values than olivines in the same chondrules. Our bulk chondrule data spread to lower δ18O values than any ferromagnesian chondrules that have been measured previously. Two chondrules with the lowest bulk δ18O values (−7.5‰ and −11.7‰) contain olivine grains that display an extremely wide range of oxygen isotope ratios, down to δ17O, δ18O around -50‰ in one chondrule. In these chondrules, there are no apparent relict grains, and essentially no relationships between olivine compositions, which are homogeneous, and oxygen isotopic compositions of individual grains. Heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates, followed by solid-state chemical diffusion without concomitant oxygen equilibration. Alternatively, oxygen isotope exchange between an 16O-rich precursor and an 16O-poor gas may have taken place during chondrule formation, and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains. If this is the case, we can infer that the earliest nebular solids from which chondrules formed had δ17O and δ18O values around -50‰, similar to those observed in refractory inclusions. 相似文献
15.
碾子山晶洞碱性花岗岩矿物-水氧同位素交换反应动力学 总被引：4，自引：0，他引：4
对黑龙江碾子山碱性花岗岩的全岩及其主要单矿物进行了氧同位素分析，结果表明，全岩和单矿物不仅δ^18O 值变化范围较大（全岩－2．4－2．0‰，石英0．0－5．8‰，碱性长石－3．8－0．1‰，磁铁矿－8．5－1．0‰），而且强烈亏损^18O。共生矿物之间表现出明显不平衡的氧同位素分馏特征，指示在花岗岩侵位之后与水之间发生了同位素交换，根据锆石和现代大气降水的氧同位素组成，对岩石与外来流体的δ^18O值进行了估计，多维矿水－岩反应时限约为0．3－3Ma，水／岩比（氧摩尔比）介于0．11－1．02之间。水－岩反应温度较高（约400度）和反应时间较长是导致石英δ^18O值降低的主要原因。 相似文献
16.
《Geochimica et cosmochimica acta》1999,63(11-12):1781-1786
Oxygen isotope exchange between calcite and forsterite were investigated in the presence of a large amount of supercritical fluid. The experiments were conducted in standard cold-seal pressure vessels at 680°C and 500 MPa in the T-P-XCO2 stability field of the calcite-forsterite assemblage for 2, 5, 10, 20, 40 and 80 days, respectively. The weight ratio of mineral to fluid in the starting mixture is 1.46; the fluid was a mixture of H2O and CO2 with the mole fraction of CO2 being 0.1. The results show that the oxygen isotope exchange between the minerals was accomplished via mineral-fluid exchange by a dual-mechanism, i.e. initial rapid exchange due to Ostwald ripening of both calcite and forsterite, followed by a slower diffusion-controlled process. Furthermore, for the given fluid composition, calcite shows a greater rate of dissolution-recrystallization and oxygen isotope exchange with fluid than forsterite. As a result, oxygen isotope fractionations between calcite and forsterite and between the minerals and the fluid can simply pass the equilibrium fractionations with time and even lead to crossover behavior. Once diffusion becomes a primary mechanism for further isotope exchange in the three-phase system, the rate of oxygen diffusion in calcite is equal to, or slightly less than that in forsterite. 相似文献
17.
《Geochimica et cosmochimica acta》1999,63(3-4):351-361
It is often argued that the δ18O value of oceanic water was maintained close to 0‰ for hundreds of millions of years, as a consequence of oxygen isotope exchange between oceanic crust and seawater. However, for several decades, the interpretation of the biosedimentary oxygen isotope record has conflicted with the igneous record because, with increasing age, a general trend of decreasing δ18O values (about 6‰) is observed in most carbonates, cherts and phosphates, especially for the Paleozoic and early Mesozoic. We developed a dynamical model of seawater-crust interaction that computes the δ18O value in these two reservoirs as function of time. This model takes into account the continuous production of crust at oceanic ridges, its expansion rate, the permeability profile with space and time, the mineralogical mode of the crust, and the kinetics of oxygen isotope exchange between rock-forming minerals and seawater. The model indicates that the δ18O value of seawater may vary by ±2‰ with a time response ranging from 5 to 50 Ma for expansion rates of 1 to 10 cm.a−1. The variation of ±2‰ is fixed by both integrated water-rock ratio and closure time of the seawater-crust system by sediments. Variations in the oxygen isotope ratio of seawater through time have important implications for the interpretation of the systematically low δ18O values of pre-Jurassic marine sediments. According to our model, marine paleotemperatures could be up to 10°C lower than those expected when applying the classical hypothesis of an ice-free ocean with a δ18O value of −1‰. 相似文献
18.
D. A. Orsoev 《Geology of Ore Deposits》2010,52(7):543-550
The oxygen isotopic composition was studied in minerals and rocks from the critical zone of the Ioko-Dovyren layered pluton.
The δ18O values vary from +5.4 to +6.1‰ in rocks, from +4.8 to +5.8‰ in olivine, from +5.5 to 6.5‰ in pyroxene, and from +5.8 to
+6.9‰ in plagioclase and fall into the interval of mantle values for continental mafic and ultramafic rocks. A decrease in
δ18O could have been caused by penetration of meteoric water. Postmagmatic (retrograde) oxygen isotopic redistribution in the
slowly cooling rocks is responsible for disturbance of oxygen isotope equilibria in the coexisting minerals, which were crystallized
from the same magma at a high temperature. The nonequilibrium oxygen isotopic composition in the associated minerals and calculated
temperature of the final isotopic equilibration do not contradict the model of “fluid” formation of low-sulfide PGE mineralization
in the Ioko-Dovyren layered pluton. 相似文献
19.
Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate ‘clumped isotope’ analysis. The CO2 recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H2CO3 reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of 13C-18O-containing isotopologues (‘clumped’ isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72‰, 0.220‰, 0.137‰, 0.593‰ for, respectively, 18O/16O ratios (1000 lnα∗) and three indices that measure proportions of multiply-substituted isotopologues . We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of 13C-18O bonds (the measured change in Δ47 = 0.23‰) during phosphoric acid digestion of calcite at 25 °C.We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H2CO3 interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of 13C-18O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of for different carbonate minerals. Combined with previous theoretical evaluations of 13C-18O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H2CO3. 相似文献
20.
G. R. T. JENKIN C. M. FARROW A. E. FALLICK D. HIGGINS 《Journal of Metamorphic Geology》1994,12(3):221-235
Retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model, which predicts both the zonation profile of δ18 O within grains, and the bulk δ18 O value of each mineral in the rock. Apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the δ18 O values of each mineral pair within the rock. In systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate, such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values ('slow cooling'), the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the Dodson closure temperatures ( T c ) for the two phases and the mode of the rock. Adding a third phase into a system which undergoes 'slow' cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two-phase system. In some systems oxygen isotope reversals can be developed. If cooling is not 'slow', then the mineral δ18 O values resulting from cooling will be partly dependent upon the initial temperature of the system concerned. The model successfully simulates the mineral δ18 O values that are often observed in granitic rocks. Application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings, and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems. 相似文献