Two Disequilibrium Quartz-Feldspar 18O/16O Fractionations within the Aral Granite Batholith, Altay Mountains of China: Evidence for Occurrence of Two Stages of O and H Isotopic Exchange of a Heterogeneous Granite System with Aqueous Fluids   总被引：7，自引：0，他引：7  

LIU  WEI 《Journal of Petrology》2000,41(9):1455-1466

¹⁸O/¹⁶O and D/H isotope compositions are reported for coexistingquartz, feldspar and biotite from four lithological units withinthe Aral granite batholith, Altay Mts of China. The data exhibita reversed and an anomalously large positive quartz–feldspar¹⁸O/¹⁶O fractionation. These two ¹⁸O/¹⁶O fractionations, togetherwith a marked decrease in the D values of biotite, are interpretedin terms of a model involving two stages of isotopic exchangewith aqueous fluids. The first stage of ¹⁸O/¹⁶O exchange withan ¹⁸O-rich aqueous fluid occurred during subsolidus cooling.Kinetic effects of the first stage of ¹⁸O/¹⁶O exchange are characterizedby the reversed quartz–feldspar ¹⁸O/¹⁶O fractionationthat was recorded in the megacrystic coarse-grained granitesof Group I and the megacryst-bearing medium- to coarse-grainedgranites of Group II. Robust parameters for the first stageof exchange are obtained by modelling on the hypothesis of initial¹⁸O/¹⁶O heterogeneity in the granite plus subsolidus ¹⁸O/¹⁶Oexchange. The spread in the measured ¹⁸O values of quartz reflectsthe initial heterogeneity of the granite. Calculated isochronsillustrate that an initially heterogeneous system reacting withan externally buffered fluid can generate arrays that mimicisotherms. The second stage of isotopic exchange with ¹⁸O- andD-depleted meteoric water occurred after magma solidification,resulting in the anomalously large positive quartz–feldspar¹⁸O/¹⁶O fractionation and a marked decrease in the D valuesof biotite. These kinetic effects have been recorded in thefiner-grained granites of Groups III and IV. However, the coarser-grainedgranites of Groups I and II have essentially survived the secondstage of ¹⁸O/¹⁶O exchange. KEY WORDS: Altay; granite; initial ¹⁸O/¹⁶O heterogeneity; pseudoisotherm; quartz–feldspar ¹⁸O/¹⁶O reversal  相似文献   

On the causes of 18O-depletion and 18O/16O homogenization during regional metamorphism; the East Humboldt Range core complex,Nevada     

Mark T. Peters  Stephen M. Wickham 《Contributions to Mineralogy and Petrology》1995,119(1):68-82

 Previous stable isotope studies at Lizzies Basin revealed that metasedimentary rocks are ¹⁸O-depleted relative to protolith values, particularly in the lower parts of the section (Lower Zone) where the rocks are also isotopically homogeneous on a scale of hundreds of meters (quartz δ¹⁸O=+9.0 to +9.6 per mil). In contrast, metasedimentary rocks at higher levels at Lizzies Basin (Upper Zone) are less ¹⁸O-depleted and more heterogeneous in δ¹⁸O. In order to understand more fully the isotopic evolution of this terrane, a series of detailed, meter-scale traverses across various metamorphic and igneous lithologies were completed at Lizzies Basin, and at the structurally higher Angel Lake locality. Traverses in the Lizzies Basin Lower Zone and in the lower parts of Angel Lake (Angel Lake Lower Sequence) across various silicate lithologies, including abundant granitoids, reveal similar degrees of homogeneity, although the average δ¹⁸O values are higher at Angel Lake. In contrast, traverses which include substantial thicknesses of marble and calc-silicate gneiss and very little granitoid have more heterogeneous quartz δ¹⁸O values (+11.9 to +13.4 per mil), and also have a higher average δ¹⁸O (+12.9 per mil), than observed elsewhere. The scale of ¹⁸O/¹⁶O homogeneity in quartz observed at Lizzies Basin and Angel Lake (meters to hundreds of meters) requires fluid-mediated isotope exchange, which accompanied Tertiary metamorphism. There is a correlation between the degree of ¹⁸O-depletion in metasedimentary rocks, ¹⁸O/¹⁶O homogenization between lithologies, and the proportion of granitoids (leucogranites in particular) within any part of the section, and a corresponding anticorrelation with the proportion of marble. This points to a causal relationship, whereby the leucogranites (as well as the Tertiary hornblende diorite and biotite monzogranite) acted as both a relatively low-¹⁸O reservoir and a source of fluids to enhance exchange, while the marbles hindered isotope depletion and homogenization by acting as relatively high-¹⁸O reservoirs and impermeable layers. Material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments. Single-pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations. Fluid-mediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations. The generalized model for the isotopic evolution of the East Humboldt Range core complex provides an excellent opportunity to establish the main causes and controlling factors of ¹⁸O-depletion and ¹⁸O/¹⁶O homogenization during regional metamorphism. Received: 27 July 1993 / Accepted: 1 July 1994  相似文献   

Oxygen isotopic compositions of IVA iron meteorites: implications for the thermal evolution derived from in situ ultraviolet laser microprobe analyses     

Pei-Ling Wang  Douglas Rumble III 《Geochimica et cosmochimica acta》2004,68(5):1159-1171

Oxygen isotopic compositions of silicate inclusions in IVA iron meteorites have been measured with an in situ UV laser microprobe technique. The homogeneity of oxygen isotopic compositions within and among individual mineral grains has also been examined. Oxygen isotope fractionations between coexisting mineral pairs were utilized in oxygen isotope thermometry. Our measured Δ¹⁷O values, ranging from 0.97 to 1.25‰, are characteristic of a single reservoir and fully confirm the oxygen isotopic similarity between IVA irons and L/LL chondrites. Steinbach and São João Nepomuceno, containing inclusions of two silicate minerals in mutual contact, exhibit a mass-dependent fractionation of ¹⁸O/¹⁶O between tridymite and bronzite with apparent oxygen isotopic heterogeneity. The SiO₂-bearing member, Gibeon, gives homogeneous oxygen isotopic compositions without detectable fractionation of ¹⁸O/¹⁶O between tridymite and quartz. Oxygen isotope equilibrium temperatures are estimated for coexisting tridymite and bronzite in the same sample slabs or clusters in Steinbach and São João Nepomuceno. The fractionations of ¹⁸O/¹⁶O between bronzite and tridymite range from 1.6 to 2.3‰ in different sample slabs or clusters. On the basis of the closure temperature concept, cooling rates are estimated at approximately 20 to 1000°C/Myr between 800 and 1000°C, a range of temperatures not accessible to other cooling rate methods. Using the Fast Grain Boundary diffusion model, we have demonstrated that significant oxygen heterogeneity both in tridymite and bronzite is probably due to isotope exchange during cooling between minerals with various grain sizes and mineral abundances in different regions of the samples. The new estimates of cooling rate by oxygen isotope thermometry refine previous cooling curves of IVA irons and support the breakup-reassembly model for the IVA parent body.  相似文献