Secondary Inclusion Research Articles

Microfracture and crack healing in experimentally deformed peridotite

Secondary CO2 fluid inclusion arrays in peridotite xenoliths from basalt are formed by the healing of CO2‐filled microcracks. We have produced healed microcracks and deformation textures comparable to those in xenoliths by deforming peridotite ifi the presence of excess CO2 fluid at a constant strain rate of 10−5 s−1, a confirming pressure of 1.0–1.5 GPa, and a temperature of 800°–1050°C. Plastic and micro‐clastic deformation textures coexist, and healed microcracks extending out of (100) and (011) olivine deformation lamellae have the crystallographic orientation of microcracks nucleated on pileups of (100)[010] and (011)[100] edge dislocations. Healed microcracks in olivine grains without deformation lamellae tend to be in these orientations or in the orientation of microcracks nucleated on pileups of (110)[001] and (010)[001] edge dislocations. The same crystallographic orientation of healed microcracks is present in peridotite xenoliths from basalt. The steady flow stress data of the experiments without added CO2 fluid fit a power law with a stress exponent n of 3 and a thermal activation coefficient of 410 ± 50 kJ mol−1. The maximum stress that can be supported by samples with added CO2 fluid is 100–300 MPa less than the flow stress of samples without added CO2 fluid. The reduction in niechanical strength correlates with an increase in the concentration of intragranular healed microcracks and an increase in the proportion of healed microcracks inclined to the maximum compressive stress direction. Strength and microcrack characteristics of experiments with CO2 performed at 1.5 GPa are similar to experiments without CO2 at 1.0 GPa. The dependence of strength and microcrack concentration on confining pressure suggests that the principal effect of the CO2 fluid is to reduce the effective confining pressure and to promote microclastic deformation mechanisms. The similarity of experimental textures to xenoliths suggests that CO2 fluid may produce semibrittle mechanical behavior in the regions of the lower lithosphere where xenoliths originate.

Origin of saline groundwaters in the Carnmenellis Granite (Cornwall, England): Natural processes and reaction during Hot Dry Rock reservoir circulation

Saline groundwaters (up to 19,100 mg 1 −1 total mineralisation) issue in tin mines in the Carnmenellis Granite in Cornwall (U.K.) at depths up to 800 m. Their stable-isotope composition rules out seawater as a contributor to salinity. Circulation experiments carried out during Hot Dry Rock (HDR) reservoir development in the same granite also produce return fluids with enhanced salinities. Acid hydrolysis of plagioclase and biotite are proposed as the main sources of salinity in the groundwater. Experimental studies carried out on biotites from a borehole used for HDR evaluation demonstrate the reactivity of the biotite and confirm the hypotheses of the field studies. Mg, Li, K and silica levels in reacted solutions reflect the stoichiometric composition of the biotite. Chloride, Na and Ca in solution, on the other hand, are enriched between 1 and 3 orders of magnitude over that of biotite, reflecting the strongly incongruent nature of the reaction. Quartz and chalcedony saturation of the groundwaters encourages silica (or silicate) deposition rather than dissolution of rock-forming quartz; this argues against fluid inclusions as the source of salinity and suggests that new inclusions might be formed. The proposed model for the genesis of saline water therefore links together, or explains, several processes — groundwater movement, convective heat transport, the chemistry of the water, water-rock interaction, secondary mineral (including kaolinite) formation and fluid inclusion formation and stability.

Fluid inclusion studies in relation to hydrothermal alteration and mineralization at the Koloula porphyry copper prospect, Guadalcanal

The Koloula Igneous Complex on the island of Guadalcanal, part of the British Solomon Islands Protectorate, consists of a calc-alkaline sequence of 26 intrusive phases ranging from leucogabbro through diorites and granodiorites to andesite dikes. The intermediate and felsic rocks of the younger central part of the complex are host to porphyry copper mineralization, characterized by two well-defined separate alteration systems.The earlier or system has five distinguishable concentric zones. The innermost is sulfide free and contains K feldspar-quartz-magnetite-actinolite-pyroxene (salite). It is succeeded by zones containing potassic and propylitic assemblages, with chalcopyrite and bornite associated with the potassic zones. The later, or system, is characterized by four zones in which sericite and lesser amounts of clay minerals predominate.Fluid inclusion assemblages are closely similar to those from porphyry copper deposits of the Bingham type. Three types of inclusions are common: gas-rich, moderately saline, highly saline inclusions which contain NaCl + or - KCl + or - hematite + or - birefringent phases. Homogenization temperatures obtained from moderately and highly saline inclusions range up to approximately 700 degrees C in the A system and up to 480 degrees in the B system. There is a continuous variation in salinity from 2 percent equivalent NaCl in some two-phase inclusions up to approximately 60 percent equivalent NaCl in some highly saline inclusions.The evolution of fluids in the A hydrothermal system has not proved easy to elucidate, but the best development of high-salinity inclusions containing KCl daughter crystals is found in areas of strong potassic alteration. Information derived from the sequence of secondary fluid inclusion planes and from the coincidence of alteration and late-intrusive activity suggests that in the B system high-salinity fluids dominated the early phase of alteration and moderate-salinity fluids dominated the later phases. It is considered that both hydrothermal systems developed predominantly under lithostatic pressure conditions with a cover of about 3 km.Counts of inclusions in primary quartz in standard petrographic thin sections reveal a close relationship between fluid inclusion abundance and the location of Cu soil geochemistry anomalies. Such counts may well prove to be a simple preliminary exploration technique for porphyry copper prospect evaluation.

The kinetics and specificity of electron transfer from cytochromes and copper proteins to P700

The rates of electron transfer to P700 from plastocyanin and cytochrome f have been compared with those from three other c-type cytochromes and azurin, a copper protein resembling plastocyanin. Three different disruptive techniques were used to expose P700; digitonin, Triton X-100 and sonication. The following rate constants were measured at 25 °C, pH 7.0, with digitonin-treated chloroplasts: plastocyanin, 8 · 10 7 M −1 · s −1; red-algal cytochrome c-553, 1.9 · 10 7 M −1 · s −1; Pseudomonas cytochrome c-551, 8 · 10 6 M −1 · s −1; azurin, ⪅ 3 · 10 5 M −1 · s −1; cytochrome f, ⪅ 2 · 10 4 M −1 · s −1; mammalian cytochrome c, ⪅ 2 · 10 4 M −1 · s −1. For electron transfer from plastocyanin, the effects of ionic strength, pH and temperature were also studied, and saturation effects found in earlier work were avoided by a full consideration of the various secondary reactions and inclusion of superoxide dismutase. The relative rates are discussed in relation to photosynthetic electron transport.