ALBERT

Notes: 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.

Notes: Previous models of hydrodynamics in contact metamorphic aureoles assumed flow of aqueous fluids, whereas CO2 and other species are also common fluid components in contact metamorphic aureoles. We investigated flow of mixed CO2–H2O fluid and kinetically controlled progress of calc-silicate reactions using a two-dimensional, finite-element model constrained by the geological relations in the Notch Peak aureole, Utah. Results show that CO2 strongly affects fluid-flow patterns in contact aureoles. Infiltration of magmatic water into a homogeneous aureole containing CO2–H2O sedimentary fluid facilitates upward, thermally driven flow in the inner aureole and causes downward flow of the relatively dense CO2-poor fluid in the outer aureole. Metamorphic CO2-rich fluid tends to promote upward flow in the inner aureole and the progress of devolatilization reactions causes local fluid expulsion at reacting fronts. We also tracked the temporal evolution of P-T-XCO2conditions of calc-silicate reactions. The progress of low- to medium-grade (phlogopite- to diopside-forming) reactions is mainly driven by heat as the CO2 concentration and fluid pressure and temperature increase simultaneously. In contrast, the progress of the high-grade wollastonite-forming reaction is mainly driven by infiltration of chemically out-of-equilibrium, CO2-poor fluid during late-stage heating and early cooling of the inner aureole and thus it is significantly enhanced when magmatic water is involved. CO2-rich fluid dominates in the inner aureole during early heating, whereas CO2-poor fluid prevails at or after peak temperature is reached. Low-grade metamorphic rocks are predicted to record the presence of CO2-rich fluid, and high-grade rocks reflect the presence of CO2-poor fluid, consistent with geological observations in many calc-silicate aureoles. The distribution of mineral assemblages predicted by our model matches those observed in the Notch Peak aureole.

Notes: Fluid compositions and bedding-scale patterns of fluid flow during contact metamorphism of the Weeks Formation in the Notch Peak aureole, Utah, were determined from mineralogy and stable isotope compositions. The Weeks Formation contains calc-silicate and nearly pure carbonate layers that are interbedded on centimetre to decimetre scales. The prograde metamorphic sequence is characterized by the appearance of phlogopite, diopside, and wollastonite. By accounting for the solution properties of Fe, it is shown that the tremolite stability field was very narrow and perhaps absent in the prograde sequence. Unshifted oxygen and carbon isotopic ratios in calcite and silicate minerals at all grades, except above the wollastonite isograd, show that there was little to no infiltration of disequilibrium fluids. The fluid composition is poorly constrained, but X(CO2)fluid must have been 〉0.1, as indicated by the absence of talc, and has probably increased with progress of decarbonation reactions.The occurrence of scapolite and oxidation of graphite in calc-silicate beds of the upper diopside zone provide the first evidence for limited infiltration of external aqueous fluids. Significantly larger amounts of aqueous fluid infiltrated the wollastonite zone. The aqueous fluids are recorded by the presence of vesuvianite, large decreases in δ18O values of silicate minerals from c. 16‰ in the diopside zone to c. 10‰ in the wollastonite zone, and extensive oxidation of graphite. The carbonate beds interacted with the fluids only along margins where graphite was destroyed, calcite coarsened, and isotopic ratios shifted.The wollastonite isograd represents a boundary between a high aqueous fluid-flux region on its higher-grade side and a low fluid-flux region on its lower-grade side. Preferential flow of aqueous fluids within the wollastonite zone was promoted by permeability created by the wollastonite-forming reaction and the natural tendency of fluids to flow upward and down-temperature near the intrusion-wall rock contact.

Notes: Abstract The 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 δ 18O, δ 13C, and δD. The δ 13C and δ 18O values for the limestones remain constant at about 0.5 (PDB) and 20 (SMOW), respectively, with increasing metamorphic grade. The whole rock δ 18O values of the argillites systematically decrease from 19 to as low as 8.1, and the δ 13C values of the carbonate fraction from 0.5 to −11.8. The change in δ 13C values can be explained by Rayleigh decarbonation during calcsilicate reactions, where calculated $$\Delta ^{13} {\text{C}}_{\left( {{\text{CO}}_{\text{2}} - {\text{cc}}} \right)}$$ 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 δ 18O to the values observed in the argillites. The low δ 18O values close to the intrusion suggest interaction with magmatic water that had a δ 18O value of 8.5. The extreme lowering of δ 13C by fractional devolatilization and the lowering of δ 18O 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.

Notes: Abstract Clinopyroxene with exsolved orthopyroxene and coexisting orthopyroxene with exsolved clinopyroxene (inverted pigeonite) in metaigneous rocks from the Adirondacks, New York, were experimentally homogenized at temperatures near those inferred for their original crystallization. The purposes were several: (1) to test the graphical two-pyroxene geothermometer of Lindsley (1983); (2) to test the hypothesis of Bohlen and Essene (1978) that these were originally igneous pyroxenes; and (3) to test whether modal recombination of complexly exsolved pyroxenes yields realistic compositions. Experiments on Fe-rich compositions at 930° and 870° C (1 GPa) are compatible with the graphical thermometer of Lindsley (1983); however, this graphical thermometer yields apparent temperatures approximately 50° C too high for experiments at 1050° C and 1100° C (0 MPa). This suggests that at intermediate Mg/Fe the augite isotherms for these temperatures lie at lower wollastonite compositions than shown by Lindsley. The results are, however, in good agreement with isotherms derived from the solution model of Davidson (1985). When these isotherms are applied to a variety of terrestrial and lunar igneous rocks and the metaigneous rocks from the Adirondacks, temperatures given by augite and pigeonite compositions from coexisting pairs are similar. Comparison of the experimentally homogenized compositions with modally recombined compositions of Bohlen and Essene (1978) show that discrepancies between augite and pigeonite temperatures may nevertheless arise if pyroxene grains formed by granular exsolution are not correctly reintegrated.

Notes: Abstract The mineralogy, petrology and geochemistry of the Proterozoic Harney Peak Granite, Black Hills, South Dakota, were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare-element pegmatites. The granite was emplaced at 3–4 kbar as multiple sills and dikes into quartz-mica schists at the culmination of a regional high-temperature, low-pressure metamorphic event. Principally along the periphery of the main pluton and in satellite intrusions, the sills segregated into granite-pegmatite couplets. The major minerals include quartz, K-feldspar, sodic plagioclase and muscovite. Biotite-{Mg No. [Molar MgO/(MgO+FeO)]=0.32-0.38} is the predominant ferromagnesian mineral in the granite's core, whereas at the periphery of the main pluton and in the satellite intrusions tourmaline (Mg No.=0.18–0.48) is the dominant ferromagnesian phase. Almandine-spessartine garnet is also found in the outer intrusions. There is virtually a complete overlap in the wide concentration ranges of SiO2, CaO, MgO, FeO, Sr, Zr, W of the biotite- and tourmaline-bearing granite suites with no discernable differentiation trends on Harker diagrams, precluding the derivation of one suite from the other by differentiation following emplacement. This is consistent with the oxygen isotope compositions which are 11.5 ± 0.6‰ for the biotite granites and 13.2 ± 0.8‰ for the tourmaline granites, suggesting derivation from different sources. The concentrations of TiO2 and possibly Ba are higher and of MnO and B are lower in the biotite granites. The normative Orthoclase/Albite ratio is extremely variable ranging from 0.26 to 1.65 in the biotite granites to 0.01–1.75 in the tourmaline granites. Very few sample compositions fall near the high-pressure, watersaturated haplogranite minima-eutectic trend, indicating that the granites for the most part are not minimum melts generated under conditions with $$a_{{\text{H}}_{\text{2}} {\text{O}}}$$ =1. Instead, most biotite granites are more potassic than the water-saturated minima and eutectics and in analogy with experimentally produced granitic melts, they are best explained by melting at ∼6 kbar, $$a_{{\text{H}}_{\text{2}} {\text{O}}}$$ 〈1 and temperatures ∼800°C. Such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals (Nabelek et al. 1992). Several of the tourmaline granite samples contain virtually no K-feldspar and have oxygen isotope equilibration temperatures 716–775°C. Therefore, they must represent high-temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water-saturated haplogranite minima or during fractionation of intruded melts into granite-pegmatite couplets accompanied by volatile-aided differentiation of the alkali elements. The indicated high temperatures, $$a_{{\text{H}}_{\text{2}} {\text{O}}}$$ 〈1, the relatively high TiO2 and Ba concentrations and the relatively low $$\partial ^{18} {\text{O}}$$ values of the biotite granites suggest that they were generated by high-extent, biotite-dehydration melting of an immature Archean metasedimentary source. The ascent of the hot melts may have triggered low-extent, muscovite-dehydration melting of schists higher in the crust producing the high-B, low-Ti melts comprising the periphery of the main pluton and the satellite intrusions. Alternatively, the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse, with the low- $$\partial ^{18} {\text{O}}$$ biotite granites generated at a deeper, hotter region and the high- $$\partial ^{18} {\text{O}}$$ tourmaline granites at a higher, cooler region of the crust. The low-Ti and high-B concentrations in the high- $$\partial ^{18} {\text{O}}$$ melts resulted in the crystallization of tourmaline rather than biotite, which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions.

Notes: Abstract Failure to correct for fission products of235U is shown to result in significant errors in the measured concentrations of La, Sm, Nd, Ba, Zr, and Mo by Instrumental Neutron Activation Analysis of high uranium rocks. Measured and calculated correction factors are presented as the ratio of the fission product to parts per million by weight of uranium in the rock. Potential errors in petrogenetic interpretations of uncorrected data are outlined.