Theodore C. Labotka

Contrasting fluid/rock interaction between the Notch Peak granitic intrusion and argillites and limestones in western Utah: evidence from stable isotopes and phase assemblages

AbstractThe Jurassic Notch Peak granitic stock, western Utah, discordantly intrudes Cambrian interbedded pure limestones and calcareous argillites. Contact metamorphosed argillite and limestone samples, collected along traverses away from the intrusion, were analyzed for δ18O, δ13C, and δD. The δ13C and δ18O values for the limestones remain constant at about 0.5 (PDB) and 20 (SMOW), respectively, with increasing metamorphic grade. The whole rock δ18O values of the argillites systematically decrease from 19 to as low as 8.1, and the δ13C values of the carbonate fraction from 0.5 to −11.8. The change in δ13C values can be explained by Rayleigh decarbonation during calcsilicate reactions, where calculated

Coupled cation and oxygen-isotope exchange between alkali feldspar and aqueous chloride solution

\Delta ^{13} {\text{C}}_{\left( {{\text{CO}}_{\text{2}} - {\text{cc}}} \right)}

The Notch Peak Contact Metamorphic Aureole, Utah: Petrology of the Big Horse Limestone Member of the Orr Formation

is about 4.5 permil for the high-grade samples and less for medium and low-grade samples suggesting a range in temperatures at which most decarbonation occurred. However, the amount of CO2 released was not anough to decrease the whole rock δ18O to the values observed in the argillites. The low δ18O values close to the intrusion suggest interaction with magmatic water that had a δ18O value of 8.5. The extreme lowering of δ13C by fractional devolatilization and the lowering of δ18O in argillites close to the intrusion indicates oxgen-equivalent fluid/rock ratios in excess of 1.0 and X(CO2)F of the fluid less than 0.2. Mineral assemblages in conjunction with the isotopic data indicate a strong influence of water infiltration on the reaction relations in the argillites and separate fluid and thermal fronts moving thru the argillites. The different stable isotope relations in limestones and argillites attest to the importance of decarbonation in the enhancement of permeability. The flow of fluids was confined to the argillite beds (argillite aquifers) whereas the limestones prevented vertical fluid flow and convective cooling of the stock.

Diffusion of C and O in calcite at 100 MPa

Subtle but progressive changes in the compositions and relative abundances of olivine, pyroxene, and metal with increasing metamorphic grade in equilibrated (types 4–6) ordinary chondrites indicate that these meteorites experienced oxidation of metallic Fe during heating. Oxygen fugacities calculated from these mineral assemblages increase with petrologic type. The hypothesis of oxidation during chondrite metamorphism differs from a commonly accepted idea that chondritic Fe was reduced by graphite. The mineralogies of unequilibrated (type 3) chondrites do not conform to the progressions in equilibrated chondrites, allowing the possibility that Fe may first have been reduced through reaction with graphite at the onset of metamorphism, although there are conflicting observations supporting both oxidation and reduction in these meteorites. As temperatures increased to levels appropriate for types 4–6 chondrites, the activity of graphite may have been lowered by its dissolution in taenite; and, in any case, oxygen fugacities appear to have been well within the stability field of graphite, precluding its reaction. At these temperatures oxidation state was largely controlled by equilibrium between ferromagnesian silicates and metal. Oxygen fugacities calculated from chondrite mineral equilibria are 2–3 log units below previously reported intrinsic ƒO2 measurements. We propose that progressive oxidation during metamorphism was promoted by interaction with small amounts of an oxidizing vapor, derived by heating ices originally accreted into the parent asteroids. Assuming that this vapor was pure H2O, the watenrock weight ratio required to account for the observed oxidation of Fe in H and L chondrites is very modest, less than several per mil. Progressive oxidation in the metamorphic sequence resulted from different degrees of reaction with such a fluid, possibly caused by temperature variations or evolution of the composition of the fluid as it permeated through the body. The presence of even minor amounts of a vapor phase during metamorphism affects interpretations of the volatile trace-element and oxygen isotopic fractionations observed in ordinary chondrites and inferred to have formed by nebular or parent-body processes.

Petrogenesis of Early Proterozoic pelitic schists of the southern Black Hills, South Dakota: Constraints on regional low-pressure metamorphism

Abstract Nanoscale isotope and chemical images of grains of Amelia albite that were reacted with 2 m 18O-enriched solution of KCl show a correspondence between O-isotope exchange and K-Na exchange. Experiments were conducted for 4.6 d at 600 °C and 200 MPa. After 6 d, the 150 µm diameter albite grains had 5.20 µm rims in which Na was nearly completely replaced by K and in which the O was strongly enriched in 18O. The boundary between the core albite and the K-feldspar replacement is sharp and decorated with numerous pores. The distribution of Na and K, determined by electron probe microanalysis, is uniform within the core and rim and has an abrupt discontinuity at the interface. No evidence exists for K-Na interdiffusion at the resolution of electron probe. The NanoSIMS shows that the interface is also sharp in the distribution of 18O and 16O. The NanoSIMS image data and the electron probe data were coregistered; principal components analysis of the merged data set shows that 86% of the total variance in the data result from a single principal component loaded by the replacement of Na by K and 18O. The combined electron probe and NanoSIMS analyses indicate that both cation and isotope exchange occurred during solution and reprecipitation of the feldspar.

The evolution of water in the contact-metamorphic aureole of the Duluth Complex, northeastern Minnesota

Abstract The geochemistry of the Notch Peak contact-metamorphic aureole was examined to determine the dominant source of fluid attendant on metamorphism and to estimate fluid fluxes. Progressive metamorphism of calc-silicate rocks is defined by phlogopite, diopside and wollastonite isograds. Mineral assemblages at wollastonite and diopside grades indicate water-rich conditions during decarbonation reactions. Near the intrusion, the rocks are highly fractured, in both the horizontal and the vertical direction. Theδ18O values of most unmetamorphosed through diopside grade rocks range from 16.3 to 20.2‰, whereas in the wollastonite zone and near a pre-intrusion fault the values approach the 9.5‰ value of the granitic stock. Coincident with the zone of18O depletion are zones of Ba and Zn enrichment. Consideration of solubilities of these elements in magmatic fluids and of published mineral/water distribution coefficients indicates that these elements were advected from the magma. Modelδ18O profiles for up-temperature flow of formation water, assuming variable Damko¨hler numbers and both static and variable thermal gradients, fit poorly most data and do not reproduce the broadened front in the observed profile. In contrast, model profiles for radial, one-dimensional flow of magmatic water with variable Damko¨hler number under static or variable thermal gradients fit the data well, although the model isotopic front remains less distended that the one observed. The integrated radial flux of magmatic water indicated by the position of the isotopic front is∼ 1.8 × 107 mol m−2. Additional broadening of the observed front and the scatter of the data suggest diffusion or hydrodynamic dispersion of oxygen. Regression of the data indicates an effective porosity of ∼ 12% in the inner aureole during fluid flow. The porosity was largely fracture controlled and resulted from decarbonation, sustained exsolution of water from the magma, and up to 30% reaction-produced volume loss in the high-grade rocks. The coincidence of the Ba, Zn andδ18O fronts suggests that bulk of the magmatic water flow was confined to the wollastontie zone. The exception is the proximity of the fault, where fluids emanating from an underlying part of the intrusion may also have flowed upward through lower grade rocks.

Petrogenesis of the contact-metamorphic rocks beneath the Stillwater Complex, Montana

The Upper Cambrian Big Horse Limestone Member of the Orr Formation is contact metamorphosed by the Jurassic Notch Peak quartz monzonite in the central portion of the House Range, Utah. Two lithologic types were sampled from specific stratigraphic horizons within the member over a lateral distance of 6 km, at locations reflecting a range in metamorphic grades. The rocks were metamorphosed at ∼2 kbar, on the basis of estimates of stratigraphic overburden at the time of intrusion. The lithologies are relatively pure dolomitic limestones and impure argillaceous limestones (argillites). The progressive metamorphism has resulted in the successive appearance of talc, tremolite, scapolite, diopside, and forsterite in the meta-limestones and the appearance of biotite, tremolite, diopside, plagioclase, scapolite, vesuvianite, grossular, and wollastonite in the argillites. Mineral assemblages of the limestones compared with isobaric 2 kbar-phase equilibria in the CaO-MgO-SiO 2 -H 2 O-CO 2 system suggest that: (1) fluid buffering by metamorphic reactions has occurred; (2) domains of equilibrium are small; and (3) the limestones behaved as relatively closed systems during metamorphism. Maximum temperatures and X(CO 2 ) composition consistent with assemblages in the limestone are: (see pdf for table). Temperatures estimated by the calcite-dolomite geothermometer are consistent with these temperature estimates. Temperatures estimated from limestone mineral assemblages are used to establish the sequence of prograde reactions in the multi-component argillite system. Compositions of fluids in equilibrium with low-grade argillites are poorly constrained but may have reached a maximum X(CO 2 ) of ∼0.75. Fluid compositions in equilibrium with medium- to high-grade argillites were more H 2 O-rich than X(CO 2 ) = 0.20 indicated primarily by the presence of wollastonite at temperatures between 475 and 600 °C. Therefore, the argillites were either initially more water-rich than limestones at the same grade, or they were more open to H 2 O-rich fluids during metamorphism.

Diffusion of C and O in calcite from 0.1 to 200 MPa

Abstract The diffusivities of C and O in calcite were determined in a pure CO2 atmosphere at 100 MPa and temperatures ranging from 600 to 800 °C. The calcite crystals were preannealed and H2O was excluded from the system to determine the self-diffusion coefficients. The CO2 consisted of 99% 13C and 90% 18O. After heating for 7-147 d, diffusion profiles were measured with the use of secondary ion mass spectrometry. The results indicate that the diffusivity of C is DC = 7.77 × 10-9 exp (-166 ± 16 kJ/mol/RT) cm2/s and of O is DO = 7.5 × 10-3 exp (-242 ± 39 kJ/mol/RT) cm2/s. In comparison with other determinations of diffusivities in calcite, diffusion of O under the experimental conditions is consistent with vacancy migration in the intrinsic region, and diffusion of C seems to occur by diffusion of carbonate anions. Increased pressure appears to reduce the activation energy and the value of D0, and the presence of H2O greatly increases the diffusivity of O without appreciably changing the activation energy. Closure temperatures calculated for isotopic exchange by diffusion predict that C isotope compositions of calcite are preserved during cooling in most geologic environments, but that O isotope compositions in H2O-rich environments are preserved only in rapidly cooling environments, such as contact metamorphic aureoles.