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1.
Quartz–garnet oxygen isotope thermometry of quartz‐rich metasedimentary rocks from the southern Adirondack Highlands (Grenville Province, New York) yields metamorphic temperatures of 700–800 °C, consistent with granulite facies mineral assemblages. Samples from the Irving Pond quartzite record Δ18O(Qtz–Grt) = 2.68 ± 0.21‰ (1 s.d. , n = 15), corresponding to peak metamorphic conditions of 734 ± 38 °C. This agrees well with the estimates from garnet–biotite exchange thermometry. Similar temperature estimates are obtained from Swede Pond (682 ± 47 °C, n = 3) and King's Station (c. 700 °C). The Whitehall area records higher temperatures (798 ± 25 °C, n = 3). All of these temperatures are higher than previous regional temperature estimates. The c. 800 °C temperatures near Whitehall are consistent with preservation of pre‐granulite contact temperatures adjacent to anorthosite. The preservation of peak metamorphic temperatures in garnet of all sizes is consistent with slow oxygen diffusion in garnet, and closure temperatures of at least 730 °C. Peak metamorphic fractionations are preserved in rocks with varying quartz:feldspar ratios, indicating that the modal percentage of feldspar does not affect retrograde oxygen exchange in these rocks. The lack of this correlation suggests slow rates of oxygen diffusion in quartz and feldspar, consistent with the results of anhydrous oxygen diffusion experiments.  相似文献   

2.
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   

3.
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   

4.
Carbon isotope fractionations between calcite and graphite in the Panamint Mountains, California, USA, demonstrate the importance of mass balance on carbon isotope values in metamorphosed carbon-bearing minerals while recording the thermal conditions during peak regional metamorphism. Interbedded graphitic marbles and graphitic calcareous schists in the Kingston Peak Formation define distinct populations on a δ13C(gr)–δ13C(cc) diagram. The δ13C values of both graphite and calcite in the marbles are higher than the values of the respective minerals in the schists. δ13C values in both rock types were controlled by the relative proportions of the carbon-bearing minerals: calcite, the dominant carbon reservoir in the marble, largely controlled the δ13C values in this lithology, whereas the δ13C values in the schists were largely controlled by the dominant graphite. This is in contrast to graphite-poor calcsilicate systems where carbon isotope shifts in carbonate minerals are controlled by decarbonation reactions. The marbles record a peak temperature of 531±30 °C of a Jurassic low-pressure regional metamorphic event above the tremolite isograd. In the schists there is a much wider range of recorded temperatures. However, there is a mode of temperatures at c. 435 °C, which approximately corresponds to the temperatures of the principal decarbonation metamorphic reactions in the schists, suggesting that the carbon exchange was set by loss of calcite and armouring of graphite by newly formed silicate minerals. The armouring may explain the relatively large spread of apparent temperatures. Although the modal temperature also corresponds to the approximate temperature of the Cretaceous retrograde event, retrograde exchange is thought less likely due to very slow exchange rates involving well-crystallized graphite, armouring of graphite by silicates during the earlier event, and because of other barriers to retrograde carbon exchange. Thus, only the calcite–graphite carbon isotope fractionations recorded by the marbles demonstrate the high-temperature conditions of the low-pressure Jurassic metamorphic event that was associated with the emplacement of granitic plutons to the west of the Panamint Mountains.  相似文献   

5.
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 δ18O 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 upon 18O 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   

6.
大别山东南部高压—超高压变质岩的氢氧同位素地球化学   总被引：1，自引：1，他引：1  
本文报道了大别山东南部安徽石马地区柯石英榴辉岩组合和黄镇地区石英榴辉岩组合的全岩及单矿物的氢氧同位素特征。石马榴辉岩的δ18O值在1.0‰～7.4‰之间,云母δD值为-76‰～-61‰;黄镇榴辉岩的全岩δ18O值为-1.1‰～-2.1‰,云母δD值为-89‰～-77‰。矿物之间的氢氧同位素分馏既有处于平衡状态者,也有处于不平衡状态者,反映这些岩石除继承变质前母体的18O亏损特征外,还经历了显著的同位素退化交换作用。石马和黄镇榴辉岩的矿物对氧同位素温度均给出两个峰值380～430℃与绿片岩相变质条件一致,600～640℃与高角闪岩相变质条件一致;给不出峰变质温度(750～800℃)是由于岩石经历了退变质流体的强烈改造作用。大别东南部高压-超高压榴辉岩的南部边界为石英榴辉岩与花岗片麻岩的界线,但氧同位素研究指示两者为原位接触关系。  相似文献   

7.
报道了大别造山带西南部湖北红安榴辉岩和罗田麻粒岩的氧同位素组成，并讨论了氧扩散作用对矿物氧同位素平衡的影响，结果得到，红安榴辉岩的全岩δ^18O值为6．4－7．3‰，罗田黄土岭麻粒岩的全岩δ^18O值为6．6－7．8‰，罗田惠兰山麻粒岩的全岩δ^18O值为3．9‰，这些榴辉岩和麻粒岩全岩的氧同位素组成保持了峰期变质条件下的平衡分馏特征，得到的氧同位素温度对于红安榴辉岩425－620度，对于罗田麻粒岩为740－945度。根据快速颗粒边界扩散模型计算的矿物对氧同位素温度不仅与大多数实测氧同位素温度一致，而且与岩石学测温结果相吻合，因此，这些岩石与东大别榴辉岩一样在形成后经历了快速冷却过程，退变质反应过程中没有外来流体加入。  相似文献   

8.
The Biwabik Iron Formation of Minnesota (1.9 Ga) underwent contact metamorphism by intrusion of the Duluth Complex (1.1 Ga). Apparent quartz–magnetite oxygen isotope temperatures decrease from ∼700°C at the contact to ∼375°C at 2.6 km distance (normal to the contact in 3D). Metamorphic pigeonite at the contact, however, indicates that peak temperatures were greater than 825°C. The apparent O isotope temperatures, therefore, reflect cooling, and not peak metamorphic conditions. Magnetite was reset in δ18O as a function of grain size, indicating that isotopic exchange was controlled by diffusion of oxygen in magnetite for samples from above the grunerite isograd. Apparent quartz–magnetite O isotope temperatures are similar to calculated closure temperatures for oxygen diffusion in magnetite at a cooling rate of ∼5.6°C/kyr, which suggests that the Biwabik Iron Formation cooled from ∼825 to 400°C in ∼75 kyr at the contact with the Duluth Complex. Isotopic exchange during metamorphism also occurred for Fe, where magnetite–Fe silicate fractionations decrease with increasing metamorphic grade. Correlations between quartz–magnetite O isotope fractionations and magnetite–iron silicate Fe isotope fractionations suggest that both reflect cooling, where the closure temperature for Fe was higher than for O. The net effect of metamorphism on δ18O–δ56Fe variations in magnetite is a strong increase in δ18OMt and a mild decrease in δ56Fe with increasing metamorphic grade, relative to the isotopic compositions that are expected at the low temperatures of initial magnetite formation. If metamorphism of Iron Formations occurs in a closed system, bulk O and Fe isotope compositions may be preserved, although re-equilibration among the minerals may occur for both O and Fe isotopes. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

9.
Oxygen isotope exchange and closure temperatures in cooling rocks   总被引：3，自引：0，他引：3  
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 δ18O within grains, and the bulk δ18O value of each mineral in the rock. Apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the δ18O 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 δ18O values resulting from cooling will be partly dependent upon the initial temperature of the system concerned. The model successfully simulates the mineral δ18O 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   

10.
激光探针分析在氧同位素地球化学研究中的应用   总被引：6，自引：0，他引：6  
肖益林  傅斌 《地学前缘》1998,5(2):283-294
简要介绍了氧同位素分析的最新方法———激光探针法的主要特点,并详细评述了激光探针分析在氧同位素地球化学研究中的应用。某些矿物内部存在氧同位素组成的变化,构成氧同位素环带。与其他化学成分环带一样,矿物氧同位素环带也可以分为生长环带（包括次生加大的环带）和扩散环带两种。变质岩中石榴石经常保存有明显的生长环带,而矽卡岩中石榴石、变质花岗质岩石中锆石和蚀变花岗岩中石英等矿物均可以记录岩石形成早期的某些氧同位素特征。由于这种差异与岩石的形成环境有关,因此通过对矿物氧同位素组成的微区分析可揭示岩石的某些成因信息。在缓慢冷却的变质岩中,磁铁矿的氧同位素环带常是扩散作用形成的,由此可以确定岩石的冷却速率。通过对脉石英的激光探针分析,可以研究流体的流动机理。变质岩的氧同位素微区分析为ｐ Ｔ ｔ ｆ轨迹研究提供了有力的武器,而氧同位素示踪可用来解决流体在变质岩形成过程中的作用问题。  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.
 Preservation of high-temperature mineral isotopic compositions is necessary for successful high-temperature isotopic thermometry. Other requirements include large fractionations between constituent minerals, well-calibrated equilibria, carefully designed sampling strategies and data handling techniques that quantitatively account for retrograde exchange. Here, we apply isotopic thermometry and data handling techniques to calculate and contrast mineral-pair apparent temperature data and observed closure temperature data (T c-observed) (cf. Farquhar et al. 1993) for the very high temperature (>900°C), dry granulites of the Taltson Magmatic Zone of Northwestern Canada and the Napier Complex of Enderby Land, Antarctica. The isotopic compositions of garnet grains from both terrains reflect high temperature conditions (>950°C) and point to this mineral as an excellent candidate for isotopic thermometry. The isotopic compositions of quartz, pyroxene, ilmenite and magnetite indicate that they equilibrated to lower temperature conditions (<900°C) due to faster rates of oxygen diffusion in these minerals, possibly enhanced by exsolution and ductile deformation, compared with garnet. Our temperature data for garnet and pyroxene are ≈200°C higher than is possible to explain by existing “wet” diffusion data, but is consistent with “dry” diffusion data, suggesting that the extremely dry nature of these rocks may have played a significant role in the preservation of high-temperature isotopic compositions. Both quartz and magnetite exhibit subgrain features, indicative of ductile deformation. Quartz-magnetite temperatures from the Napier complex are similar to those inferred for a late (D3) deformation and are lower than those predicted by “dry” diffusion data. We infer that the quartz-magnetite isotopic fractionations reflect deformation-enhanced exchange that accompanied D3. Garnet in these same samples did not undergo ductile deformation and did not exchange oxygen with coexisiting phases during cooling. This may reflect strain partitioning between less easily deformed, low abundance garnet and more easily deformed matrix quartz and magnetite. The resistance of garnet to ductile deformation in these rocks is a second reason why garnet is suitable for isotopic thermometry. Received: 6 February 1996 / Accepted: 25 April 1996  相似文献   

13.
郑永飞  赵子福 《岩石学报》2011,27(2):345-364
在特定的地质事件过程中,矿物等时线放射体系是否达到并且保持了平衡是变质岩Sm-Nd和Rb-Sr同位素年代学中的一个重要问题。在这个问题上矿物对O同位素测温与矿物等时线定年相似,因此两者之间可以相互制约。在岩浆岩和变质岩中,矿物中Sm-Nd、Sr和O之间的扩散速率在无水的条件下一般具有可比性,因此矿物之间O同位素的平衡状态可以用来对Sm-Nd和Rb-Sr定年的有效性进行检验。对大别-苏鲁造山带超高压变质岩的Sm-Nd和Rb-Sr等时线矿物进行O同位素测温,得到Sm-Nd等时线有时给出三叠纪年龄,有时给出非三叠纪年龄;对应的矿物O同位素分馏分别处于平衡和不平衡状态。对于引起非三叠纪等时线年龄的原因,一方面可以是由于榴辉岩相变质过程中同位素体系没有达到平衡,另一方面则可能角闪岩相退变质作用打破了平衡。等时线矿物中初始同位素比值的均一化速率主要受慢扩散矿物的影响,而矿物等时线时钟的启动主要受高母/子比值矿物控制。因此在变质作用过程中,只有当高母/子比值矿物同时具有快的放射成因同位素扩散速率,才可能得到有效的矿物等时线来用于变质年龄的测定。根据不同矿物中不同元素在扩散速率上的差异,能够定量估计大陆碰撞过程中榴辉岩相变质的持续时间。应用增量方法和离子孔隙度经验模型,不仅分别能够从理论上准确计算所有固体矿物的氧同位素分馏系数和获得不同矿物中元素的扩散参数,而且分别能够定量预测热力学平衡条件下共生矿物之间的18O富集顺序和相同条件下矿物中元素扩散速率的相对快慢。  相似文献   

14.
The Priest pluton contact aureole in the Manzano Mountains, central New Mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1.42 Ga Priest pluton. Quartz–garnet and quartz–sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 °C, at a distance of 1 km from the pluton, to 690 °C at the contact with the pluton. Comparison of calculated temperature estimates with one‐dimensional thermal modelling suggests that background temperatures between 300 and 350 °C existed at the time of intrusion of the Priest pluton. Fibrolite is found within 300 m of the Priest pluton in pelitic and aluminous schist metamorphosed at temperatures >580 °C. Coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium, suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite‐in isograd is coincident with the staurolite‐out isograd in pelitic schist, and K‐feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite‐bearing pelitic schist. Muscovite‐rich aluminous schist locally preserves the Al2SiO5 polymorph triple‐point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in isotopic equilibrium in the aluminous schist. Co‐nucleation of fibrolite and andalusite at 580 °C in the presence of muscovite and absence of K‐feldspar suggests that univariant growth of andalusite and fibrolite occurred. Kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 °C lower than andalusite and fibrolite growth. Quartz–muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative, suggesting that late isotopic exchange between externally derived fluids and muscovite, but not quartz, occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz (<500 °C).  相似文献   

15.
北大别主簿源花岗岩和片麻岩矿物的   总被引：10，自引：2，他引：10  
对大别造山带北部主簿源中生代花岗岩侵入体及其围岩片麻岩进行了矿物氧同位素分析，同时对同一样品进行了矿物 Rb- Sr内部等时线定年。结果表明，花岗岩和片麻岩矿物的氧同位素温度大小顺序为：角闪石 >磁铁矿 >榍石 >石英 >黑云母 >长石，遵循缓慢冷却条件下扩散控制的氧同位素交换封闭顺序，指示这些岩石没有受到后期热液蚀变的扰动。根据黑云母-长石-磷灰石-全岩内部 Rb- Sr等时线测定，花岗岩的年龄为 (118± 3) Ma，代表了岩浆侵位冷却年龄；片麻岩的年龄为 (122± 1) Ma，代表了片麻岩受大面积燕山期岩浆侵位热烘烤达到高温同位素平衡后的冷却年龄。因此，矿物之间的氧同位素平衡与否 ,能够对矿物 Rb- Sr体系封闭后平衡状态的保存性以及矿物内部等时线定年的有效性予直接制约。  相似文献   

16.
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   

17.
Nineteen samples of metamorphosed carbonate-bearing rocks were analyzed for carbon and oxygen isotope ratios by ion microprobe with a ∼5-15 μm spot, three from a regional terrain and 16 from five different contact aureoles. Contact metamorphic rocks further represent four groups: calc-silicate marble and hornfels (6), brucite marble (2), samples that contain a reaction front (4), and samples with a pervasive distribution of reactants and products of a decarbonation reaction (4). The average spot-to-spot reproducibility of standard calcite analyses is ±0.37‰ (2 standard deviations, SD) for δ18O and ±0.71‰ for δ13C. Ten or more measurements of a mineral in a sample that has uniform isotope composition within error of measurement can routinely return a weighted mean with a 95% confidence interval of 0.09-0.16‰ for δ18O and 0.10-0.29‰ for δ13C. Using a difference of >6SD as the criterion, only four of 19 analyzed samples exhibit significant intracrystalline and/or intercrystalline inhomogeneity in δ13C at the 100-500 μm scale, with differences within individual grains up to 3.7‰. Measurements are consistent with carbon isotope exchange equilibrium between calcite and dolomite in five of six analyzed samples at the same scale. Because of relatively slow carbon isotope diffusion in calcite and dolomite, differences in δ13C can survive intracrystalline homogenization by diffusion during cooling after peak metamorphism and likely represent the effects of prograde decarbonation and infiltration. All but 2 of 11 analyzed samples exhibit intracrystalline differences in δ18O (up to 9.4‰), intercrystalline inhomogeneity in δ18O (up to 12.5‰), and/or disequilibrium oxygen isotope fractionations among calcite-dolomite, calcite-quartz, and calcite-forsterite pairs at the 100-500 μm scale. Inhomogeneities in δ18O and δ13C are poorly correlated with only a single mineral (dolomite) in a single sample exhibiting both. Because of relatively rapid oxygen isotope diffusion in calcite, intracrystalline inhomogeneities in δ18O likely represent partial equilibration between calcite and fluid during retrograde metamorphism. Calcite is in oxygen isotope exchange equilibrium with forsterite in one of four analyzed samples, in equilibrium with dolomite in none of six analyzed samples, and in equilibrium with quartz in neither of two analyzed samples. There are no samples of contact metamorphic rock with analyzed reactants and products of an arrested metamorphic reaction that are in oxygen isotope equilibrium with each other. The degree of departure from equilibrium in analyzed samples is variable and is often related, at least in part, to alteration of δ18O of calcite during retrograde fluid-rock reaction. In situ sub-grain-scale carbon and oxygen isotope analyses of minerals are advisable in the common applications of stable isotope geochemistry to metamorphic petrology. Correlation of sub-mm scale stable isotope data with imaging will lead to improved understanding of reaction kinetics, reactive fluid flow, and thermal histories during metamorphism.  相似文献   

18.
The O18/O16 and D/H ratios have been determined for rocks and coexisting minerals from several granitic plutons and their contact metamorphic aureoles in the Santa Rosa Range, Nevada, and the Eldora area, Colorado, with emphasis on pelitic rocks. A consistent order of O18/O16 and D/H enrichment in coexisting minerals, and a correlation between isotopic fractionations among coexisting mineral pairs are commonly observed, suggesting that mineral assemblages tend to approach isotopic equilibrium during contact metamorphism. In certain cases, a systematic decrease is observed in the oxygen isotopic fractionations of mineral pairs as one approaches the intrusive contacts. Isotopic temperatures generally show good agreement with heat flow considerations. Based on the experimentally determined quartz-muscovite O18/O16 fractionation calibration curve, temperatures are estimated to be 525 to 625° C at the contacts of the granitic stocks studied.Small-scale oxygen isotope exchange effects between intrusive and country rock are observed over distances of 0.5 to 3 feet on both sides of the contacts; the isotopic gradients are typically 2 to 3 per mil per foot. The degree of oxygen isotopic exchange is essentially identical for different coexisting minerals. This presumably occurred through a diffusion-controlled recrystallization process. The size of the oxygen isotope equilibrium system in the small-scale exchanged zones varies from about 1.5 to 30 cm. A xenolith and a re-entrant of country rock projecting into an intrusive have both undergone much more extensive isotopic exchange (to hundreds of feet); they also show higher isotopic temperatures than the rocks in the aureole. The marginal portions of most plutons have unusually high O18/O16 ratios compared to normal igneous rocks, presumably due to large-scale isotopic exchange with metasedimentary country rocks when the igneous rocks were essentially in a molten state. The isotopic data suggest that outward horizontal movement of H2O into the contact metamorphic aureoles is very minor, but upward movement of H2O is important. Also, direct influx and absorption of H2O from the country rock appears to have occurred in certain intrusive stocks. The D/H ratios of biotites in the contact metamorphic rocks and their associated intrusions show a geographic correlation that is similar to that shown by the D/H ratios of meteoric surface waters, perhaps indicating that meteoric waters were present in the rocks during crystallization of the biotites.Except in the exchanged zones, the O18/O16 ratios of pelitic rocks do not change appreciably during contact metamorphism, even in the cordierite and sillimanite grades; this is in contrast to regional metamorphic rocks which commonly decrease in O18 with increasing grade. Thus, contact metamorphic rocks generally do not exchange with large quantities of igneous H2O, but regional metamorphic rocks appear to have done so.Publications of the Division of Geological Sciences, California Institute of Technology, Contribution No. 1565.  相似文献   

19.
郑永飞 《地质科学》1995,30(1):1-11
利用增量方法和同位素交换技术,对角闪石族矿物的氧同位素分馏进行了理论计算和实验测定。理论结果表明,不同化学成分的角闪石之间存在一定的氧同位素分馏,其13O富集顺序为:钠闪石>蓝闪石>铁闪石>阳起石=镁铁门石≥直闪石≥透闪石>普通角闪石>铝直闪石>韭闪石。高温条件下(>500℃),角闪石相对于水亏损18O达1‰至3‰。实验进行在有少量流体存在的条件下,温度为520℃至680℃。所确定的方解石-透闪石氧同位素分馏系数与理论计算值在误差范围内完全一致。理论和实验确定的石英-透闪石分馏曲线均显着低于已知的经验校准曲线,反映了变质岩中含角闪石矿物集合体内部的退化同位素再平衡。  相似文献   

20.
The oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss (biotite gneiss and granitic gneiss) associated with ultrahigh pressure (UHP) eclogites in the Shuanghe district of the eastern Dabie Mountains. There are significant differences in δ18O between the two gneisses: the UHP biotite gneiss varying from −4.3‰ to 10.6‰ similar to the associated eclogites, whereas the non-UHP granitic gneiss ranges only from −3.8‰ to 1.2‰. The δD values are similar in the two gneisses with −37 to −64‰ for epidote/zoisite, −92 to −83‰ for amphibole, and −63 to −109‰ for biotite/phengite. Hydrogen isotope disequilibrium among the coexisting hydroxyl-bearing minerals is ascribed to retrograde exchange subsequent to amphibolite-facies metamorphism. Oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock δ18O. Oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals. The metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss. The isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite-facies metamorphism. However, high temperatures of 550 to 650 °C are obtained from those minerals resistant to retrograde oxygen isotope exchange, implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers. The 18O-depletion of both gneisses is interpreted to result from meteoric-hydrothermal exchange before/during plate subduction. Therefore, the measured δ18O values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the UHP metamorphism. It is inferred that the UHP unit is in foreign contact with the non-UHP unit like a tectonic melange, but both of them experienced the two common stages of geodynamic evolution: (1) 18O-depletion prior to the UHP metamorphism, (2) uplifting since the amphibolite-facies metamorphism. Received: 5 May 1998 / Accepted: 27 August 1998  相似文献   

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