Finley C. Bishop

Stabilities of synthetic magnesian calcites in aqueous solution: Comparison with biogenic materials

Abstract Free-drift dissolution data and inverse time plots were used to evaluate the stabilities of synthetic and biogenic magnesian calcites in aqueous solutions at 25°C and 1 atm total pressure. Synthetic phases with MgCO 3 concentrations below 6 mole percent have stoichiometric ion activity products that are less than the value for calcite, whereas the values for phases with higher concentrations are greater than that of calcite. For synthetic phases, stability is a smooth function of composition and all phases (up to 15 mole percent MgCO 3 ) have values of ion activity products less than that for aragonite. These results agree with those of Mucci and Morse (1984) derived from precipitation of magnesian calcites in aqueous solutions. “Average” seawater at 25° and 1 atm total pressure is supersaturated with respect to all synthetic phases in the compositional range studied. Biogenic samples are less stable than synthetic phases of similar Mg concentrations and stability is not a smooth function of composition. Biogenic materials with compositions greater than 11–13 mole percent MgCO 3 have ion activity products greater than that for aragonite. The difference in stability between biogenic materials and synthetic phases is due to greater variation in chemical and physical heterogeneities found for the biogenic samples. If it is assumed that the results of the dissolution experiments reflect only differences in Gibbs free energies of formation between synthetic phases and biogenic materials of similar Mg concentration, the biogenic materials are 200–850 j/mol less stable than the synthetic phases. Only the results of synthetic dissolution experiments should be used to model the thermodynamic behavior of the magnesian calcite solid solution. The results for the synthetic phases, however, may not be appropriate to use for interpreting diagenetic reaction pathways for magnesian calcites in modern sediments, except as a basis of comparison with the behavior of natural materials.

The olivine — clinopyroxene geobarometer: experimental results in the CaO-FeO-MgO-SiO2 system

The exchange of Ca and Mg between olivine and clinopyroxene has been studied in the CFMS system. Experiments were conducted in a piston-cylinder apparatus in the temperature range of 1,100–1,300° C and the pressure range 10–30 kbar. Results confirm the previous suggestion (Adams and Bishop 1982) that this reaction has a sufficiently largeΔ V° to be used as a geobarometer in several basic and ultrabasic systems. Experimental results were corrected for compositional effects using recent activity-composition models for quadrilateral pyroxenes and olivines. The corrected results indicate that the exchange reaction has aΔH1 bar of 34,900 J, a ΔS° of -7.25J/deg, and a ΔV° of -0.489 J/bar. Corrected results agree well with calculations based on the thermodynamic properties of the endmembers.Application of the olivine-clinopyroxene geobarometer to many systems will require additional calibration of non CFMS components. Preliminary pressure estimates based on simple assumptions about the activity relations of these components have been made for spinel lherzolites from southwestern United States and coarse and porphyroclastic garnet lherzolites from southern Africa. A geotherm calculated from spinel lherzolites near the Rio Grande rift is consistent with a geophysical geotherm based on near-surface heat-flow measurements of 100 mW/m2 or greater. Results on garnet lherzolites yield a southern African geotherm with no inflection which falls at somewhat higher temperatures than pyroxene geotherms calculated for the same area.

Occurrence and origin of marialitic scapolite in the Humboldt lopolith, N.W. Nevada

The occurrence and origin of marialitic scapolite in the Humboldt lopolith was investigated in the field and in the laboratory using petrographic and experimental techniques. Scapolite occurs in three modes: as a pervasive replacement of plagioclase and other minerals in gabbro, diorite and extrusive rocks; as a poikiloblastic mineral in scapolitite dikes; and as a fracture-filling mineral with analcime, albite and sphene in scapolite veins. Additional secondary minerals associated with scapolite include epidote, prehnite, hornblende and diopside-salite clinopyroxene. Relations with these minerals suggest that most marialitic scapolite grew at temperatures around 400° C. Scapolite composition varies from EqAn12 to EqAn37, containing from 72 to 96 atomic% Cl in the R position. Experiments on systems of similar compositions indicate that NaCl-H2O fluid having more than 40 mol% NaCl is needed to stabilize the scapolite.Variation in scapolite compositions is due to thermal and fluid compositional gradients normal to conduits of hydrothermal fluids, and occurs on a scale up to 100 m. The likely source of Na and Cl is pre-existing evaporites or evaporitic brine derived from the wallrocks. Salinity could have been increased to a level sufficient to stabilize scapolite by hydration of an originally dry magma, possibly aided by hydrothermal boiling. Results may be applied to hydrothermal alteration in areas of rifting or back-arc spreading, and in mid-ocean ridge hydrothermal systems.

Diagenetic stabilization pathways of magnesian calcites

Stabilization of high magnesian calcites (>4 mole% MgCO3) to low magnesian calcite (0–4 mole% MgCO3) and dolomite involves a reduction in the solubility of these phases during diagenetic alteration. The solubility of a magnesian calcite is controlled not only by the Mg concentration, but also other chemical and physical properties of the solid. These other properties include the amount of: 1) trace element diluents other than Mg (e.g., sodium, sulfate, adsorbed or structural water); 2) carbonate ion positional or cation ordering: 3) microstructural and surface defects; and 4) adhered small particles. Crystal size also may affect the solubility of a magnesian calcite. A magnesian calcite may become more stable in the natural environment by a decrease in Mg concentration, by loss of other trace elements and/or changes in its physical properties. Few studies exist of magnesian calcites in sediments and limestones undergoing diagenetic alteration that can be used to document the typical stabilization pathways followed by magnesian calcites. Several stabilization pathways are proposed, based mainly on experimental and theoretical arguments, to encourage further investigation of magnesian calcite diagenesis.

Experimental determination of the chromium-aluminum mixing parameter in garnet

The equilibrium Grossular + Quartz = Anorthite + 2 Wollastonite was used to obtain the mixing properties of Cr and Al in grossular-uvarovite solid solutions by determining displacement of the univariant boundary curve for measured values of x grossgt. Experiments were performed in a piston cylinder apparatus with initial garnet compositions Gr100, Gr80Uv50, Gr50Uv50, and Gr20Uv80. Reversals were obtained in the temperature range 1040°C–1320°C and corrected pressure range 10–17 kbar. Results indicate that grossular-uvarovite solutions can be modeled as nearly ideal two-site solid solutions. WGCr,Al of +200 to + 1000 cal/mole is consistent with our experimental results.