ALBERT

Notes: Five groups of rats were fed ad libitum a specific level (0.0, 0.02, 0.1, 0.5, 0.75%) of BHT in the diet. After 8 wk, the animals were examined for differences in body weight, liver weight, ileum histology, and bone marrow histology. No significant differences were observed between the controls and the animals fed 0.02% BHT. Rats fed BHT greater than 0.02% of the diet exhibited a significant reduction in body weight, ileal villi height, crypt depth, goblet cell count and a significant increase in liver to body weight ratio. No significant differences in the number of immature erythrocytes between the control and experimental groups were observed.

Notes: In the Proterozoic Mary Kathleen Fold Belt, northern Australia, infiltration of large volumes of externally derived fluid occurred synchronously with regional amphibolite-facies metamorphism and deformation. This paper develops a model of structurally controlled fluid migration by comparing the distribution of fossil fluid pathways with the inferred stress and strain patterns during the deformation. Intense fluid flow was localized within strong, relatively brittle meta-intrusive bodies, and in discrete, veined, brecciated and altered zones around their margins. In metasediments folded in a ductile manner outside these areas, fluid infiltration was negligible. The direct correlation between structural styles and the magnitude of veining and metasomatism suggests control of permeability enhancement, and hence fluid flow, by deformation. Finite difference modelling of a strong body in a weaker matrix has been used to evaluate the variation of stresses during the deformation, from which it is clear that stress and strain heterogeneities have systematically influenced the development and maintenance of metamorphic fluid pathways. Particular regions in which mean stress may be significantly lower than the average lithostatic pressures include the ‘strain shadow’zones adjacent to the strong bodies, other dilatant zones around the bodies, and the bodies themselves. This geometry is favourable not only for localized brittle deformation under amphilobite facies conditions, but also for focused fluid flow in the low mean stress regions, as evidenced by the abundance of veins. Fluid access through these metamorphic aquifers occurred during tensile failure episodes, with particularly large dilations and decimetre-scale veining in areas of strain incompatibility. It appears likely that fluid circulated many times through the Fold Belt, with flow concentrated in the metamorphic aquifers. A model is developed that explains both the structurally focused fluid flow and the postulated multi-pass recirculation by dilatancy pumping, the ‘pump engines’comprising the low mean stress zones.

Notes: Evidence from rock microstructures, mass transfer and isotopic exchange indicates that substantial quantities of aqueous fluids are involved in low- and medium-grade regional metamorphism. Similar conclusions are drawn from many retrograde environments, whereas high-grade metamorphic fluids may be melt dominated. The mobile fluids play essential roles in metamorphic reactions, mass transport and deformation processes. These processes are linked by the mechanical consequences of metamorphic fluid pressures (Pf) generally being greater than or equal to the minimum principal compressive stress. Under such conditions metamorphic porosity comprises grain boundary tubules and bubbles together with continuously generated (and healed) microfractures. Deformation results in significant interconnected porosity and hence enhanced permeability. Lithologically and structurally controlled permeability variations may cause effective fluid channelling.Simple Rayleigh-Darcy modelling of a uniformly permeable, crustal slab shows that convective instability of metamorphic fluid is expected at the permeabilities suggested for the high Pf metamorphic conditions. Complex, large-scale convective cells operating in overpressured, but capped systems may provide a satisfactory explanation for the large fluid/rock ratios and extensive mass transport demonstrated for many low- and medium-grade metamorphic environments. Such large-scale fluid circulation may have important consequences for heat transfer in and the thermal evolution of metamorphic belts.

Notes: Abstract Large calcite veins and pods in the Proterozoic Corella Formation of the Mount Isa Inlier provide evidence for kilometre-scale fluid transport during amphibolite facies metamorphism. These 10- to 100-m-scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 × 10-km Mary Kathleen Fold Belt, despite the isotopic heterogeneity of the surrounding wallrocks. The fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks. The pods have δ13Ccalcite values of –2 to –7% and δ18Ocalcite values of 10.5 to 12.5%. Away from the pods, metadolerite wallrocks have δ18Owhole-rock values of 3.5 to 7%. and unaltered banded calc-silicate and marble wallrocks have δ13Ccalcite of –1.6 to –0.6%, and δ18Ocalcite of 18 to 21%. In the alteration zones adjacent to the pods, the δ18O values of both metadolerite and calc-silicate rocks approach those of the pods. Large calcite pods hosted entirely in calc-silicates show little difference in isotopic composition from pods hosted entirely in metadolerite. Thus, 100- to 500-m-scale isotopic exchange with the surrounding metadolerites and calc-silicates does not explain the observation that the δ18O values of the pods are intermediate between these two rock types. Pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc-silicates. These data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the Corella Formation at the presently exposed crustal level, although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc-silicates, or thermal fluctuations attending deformation and metamorphism. The overall L-shaped trend of the data on plots of δ13C vs. δ18O is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc-silicate rocks. Localization of the vein systems in dilatant sites around metadolerite/calc-silicate boundaries indicates a strong structural control on fluid flow, and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km. The ultimate source for the external fluid is uncertain, but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle. Intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism.

Notes: Abstract APL computer programs for the thermodynamic calculation of devolatilization and solid-solid equilibria operate using stored values for the molar volume and entropy of solids, the free energies of H2O and CO2, and the free energies of formation for 110 geologically-important phases. P-T-X CO 2 calculations of devolatilization equilibria can be made at pressures from 0.2 through 10 kb, and temperatures from 200 through 1,000° C. P-T-X calculations of solid-solid equilibria may be accomplished at pressures to 30 kb and temperatures to 1,000° C. Calculations can be extrapolations from experimental points, or direct calculations from thermochemical data alone. Options are available in these programs to consider effects of: real vs. ideal gas mixing, thermal expansion and compressibility, solid solution, fluid pressure differing from solid pressure, and uncertainties in high-temperature entropies. A collection of thermodynamic data programs accompanies the programs for calculating P-T-X CO 2 equilibria. Over a wide range of physical conditions, the data functions report free energies, entropies, fugacities of H2O and CO2, high temperature entropies of solids, and activities of components in H2O-CO2 mixtures.

Notes: Abstract Two mineralogic geobarometers based on the assemblages olivine-plagioclase-garnet and orthopyroxeneplagioclase-garnet-quartz have been calibrated from the reaction (1) fayalite+anorthite⇋garnet (Gr1Alm2). The reaction boundary has been determined to within 0.2 kbar using piston-cylinder apparatus. It is located at 4.7, 5.1, 5.5, 5.8, 6.2, 6.6, and 7.0 kbar at 750, 800, 850, 900, 950, 1,000, and 1,050° C, respectively. Summation of ΔG for reaction (1) and fayalite +quartz⇋ferrosilite locates to within 0.3 kbar the following model garnet-forming reaction for quartz-saturated granulites: (2) ferrosilite+ anorthite⇋garnet(Gr1 Alm2) + quartz. Geobarometers based on (1) and (2) are widely applicable in granulite terranes and yield precise pressures that are in agreement with other well-calibrated barometers. Pressures of 7–10 kbar are inferred for many granulite terranes requiring the widespread development of 60–70 km thick continental crust by mid-Proterozoic.

Notes: Abstract The melting of phlogopite-quartz and sanidine-quartz under vapor-absent conditions and in the presence of H2O-CO2 vapor have been determined from 5–20 kbar. In the lower crust (P=6–10 kbar), phlogopite + quartz melts incongruently to enstatite + liquid at temperatures as low as 710° C in the presence of H2O. When the activity of water is sufficiently reduced by addition of CO2, phlogopite + quartz undergoes a dehydration reaction to enstatite + sanidine + vapor, for example at 790±10° C, 5 kbar, with $$X_{H_2 O}^V$$ =0.35. In the absence of vapor, phlogopite + quartz is stable up to a maximum temperature of 900° C in the crust; at higher temperatures this assemblage melts incongruently to enstatite + sanidine + liquid. The melting of sanidine-quartz in the presence of H2O-CO2 vapor shows marked topological differences from melting in the system albite-H2O-CO2, and as a result, apparent activity coefficients for water calculated from sanidine-quartz H2O-CO2 are less than those calculated from albite-H2O-CO2 by up to a factor of five. These data shed light on anatexis in the lower crust, but uncertainties related to ordering of Al and Si in natural and synthetic micas forestall a more rigorous analysis. Nevertheless, maximum temperatures for some granulite terranes can be established.