Anorthite Component Of Plagioclase Research Articles

Experimental alteration of granitic rocks: Implications for the evolution of geothermal brines in the Upper Rhine Graben, Germany

Geothermal energy exploitation in the Upper Rhine Graben (URG) chiefly has targeted faults and fractures within or connected with the crystalline basement, where hot fluids of c. 200 °C circulate at depths of c. 5 km. Formation fluids of the crystalline basement are highly saline, NaCl-dominated brines, whereas shallow crystalline basement water (< 500 m) is only weakly mineralized. In order to contribute to a better understanding of redistribution of elements in fluids and alteration phases during fluid-rock interaction processes we conducted leaching and alteration experiments with granites and monzonites from the Schwarzwald at 70 °C (rock powder, pure water) and 200 °C (texturally intact rock cylinders, 2m-NaCl), respectively, inspired by the P-T-x conditions reported for the URG.The trace element concentrations of the leachates are hereby related to the composition and stability of minerals in the rocks and can be directly linked to the proposed and observed dissolution processes at the different temperatures. In experiments with pure water at 70 °C, representing the recharge or infiltration conditions, water-rock interaction mobilized K, Cl, Fe, Mn, Mg, Cu, Zn, Cr, Sc, V, and Li (dissolution of biotite/chlorite) and Si, Al, K, Ca, Na, Ba, Sc, Pb, Sr, Rb, and Eu (dissolution of plagioclase, K-feldspar, and quartz) into the fluid. According to calculated activity diagrams, alteration at these temperatures may result in the formation of illite, kaolinite, Ca-beidellite, and stilbite. In experiments at higher temperatures of 200 °C, according to reservoir conditions, the net output of most elements was increased, but especially of those contained in feldspar, i.e. K, Ca, Al, Si, Ba, Rb, Cs, and Pb, suggesting that fluid-rock interaction is dominated by feldspar dissolution. This alteration is followed by albitization of feldspar surfaces, chloritization of biotite, and precipitation of illite and hematite. In Si-poor samples, analcime occurs in and around porous reaction zones, in agreement with calculated activity diagrams.The amount of dissolution mainly depends on the temperature and the mineralogy of the rock types, in particular on the anorthite-component of plagioclase. Certain elements (e.g., Na, Fe, Mg, Al) were subsequently incorporated into the solids precipitated during the experiments and thus cannot be considered as conservative.

On the origin of sellaite (MgF2)-rich deposits in Mg-poor environments

Sellaite (MgF2) forms from melts, fluids, and gases under variable temperature, pressure, fO₂, and fluid salinity conditions. It is typically associated with, but much rarer, than fluorite (CaF2). The Clara mine near Oberwolfach (Schwarzwald, Germany) is an extensive hydrothermal vein-type deposit, where sellaite occurs in huge quantities (thousands of tons) in veins of mostly Jurassic/Cretaceous age. The sellaite mineralization, occurring in gneisses altered prior to and during sellaite mineralization, represents a stockwork-like network of veins and fissures, which is overlain and sealed by sediments, preventing the inflow of and fluid-mixing with sedimentary formation waters. The occurrence of sellaite is unique among the more than 1000 hydrothermal vein-type occurrences of the Schwarzwald ore district. The favored formation of sellaite compared to fluorite requires the initial Ca/Mg-ratio of the mineralizing fluid to be unusually low. These conditions are possible if fluids equilibrate with pre-altered rocks that lost some or much of their Ca during an earlier hydrothermal alteration event. Indeed, calculations demonstrate that rock-buffered fluids of pre-altered rocks (i.e., gneiss around the Clara mine altered during prior hydrothermal events) show significantly lower Ca/Mg-ratios than fluids equilibrated with unaltered gneisses, because Ca-phases (e.g., the anorthite component of plagioclase) are more prone to hydrothermal destruction. Due to the network-like structure of the sellaite-bearing portion of the Clara fluorite vein, the fluid is shielded from sedimentary formation water, resulting in fractionation processes of the repeatedly ascending mineralizing fluid. In addition, fluid cooling and formation of water-bearing phases like illite that consume fluids, favor sellaite, and later fluorite precipitation. The rarity of this combination of prerequisites explains the limited occurrence of sellaite in hydrothermal vein-type deposits.

Garnet-bearing basites of the Kuvalorog massif ( Kamchatka Peninsula )

The petrographic, petrochemical, and mineralogical compositions of the Kuvalorog Ni-bearing cortlandite-norite intrusion and endocontact leucodiorites hosting pyrope-almandine garnet are considered. Lamprophyre dikes with scarce pyrope-almandine garnet, first discovered in the massif, and plagioclase-pyroxene symplectites in garnet porphyroblast areas are studied. Judging from the petrography of rocks and the composition of inclusions rich in incompatible elements in the garnet, the mineral was produced by the reaction of orthopyroxene with the anorthite component of plagioclase at the subsolidus stage of formation of endocontact diorites. This reaction was probably favored by the fluid phase abundant in the parental magma of the Kuvalorog intrusion and, especially, in the zones near its contact with relics of terrigenous rocks, where it was produced as a result of the rock dehydration under the thermal effect of the intrusion.

Thermodynamic conditions of framework grain dissolution of clastic rocks and its application in Kela 2 gas field

Feldspar and clastic debris are the most important constituent framework grains of sedimentary clastic rocks and their chemical dissolution plays an essential role in the formation and evolution of the secondary pore in the reservoir rocks. On the basis of thermodynamic phase equilibrium, this study investigates the chemical equilibrium relationships between fluid and various plagioclase and K-feldspar in diagenesis of the sediments, particularly, the impact of temperature and fluid compositions (pH, activity of K+, Na+, Ca2+and so on) on precipitation and dissolution equilibria of feldspars. Feldspar is extremely easily dissolved in the acid pore water with a low salinity when temperature decreases. The dissolution of anorthite end-member of plagioclase is related to the Ca content of the mineral and the fluid, higher Ca either in the mineral or in the fluid, easier dissolution of the feldspar. Moreover, the dissolution of albite end-member of plagioclase is related to Na of both the mineral and fluid, lower Na out of both the mineral and fluid, easier dissolution of the mineral. Similarly, lower-K fluid is more powerful to dissolve K-feldspar than the higher. The anorthite component of plagioclase is most easily dissolved in ground water-rock system, the albite is the secondary, and K-feldspar is the most stable. Selective dissolution of plagioclase occurs in diagenesis because of the plagioclase solid solution that is mainly composed of albite and anorthite end-members, Ca-rich part of which is preferentially dissolved by the pore water, in contrast to the Na-rich parts. Based on investigation of reservoir quality, secondary pore, dissolution structures of feldspar, clay minerals and ground water chemistry of the Kela 2 gas field of Kuqa Depression in the Tarim basin of Western China, we discovered that the secondary pore is very well developed in the highest quality section of the reservoir, and the plagioclase of which was obviously selectively dissolved, in contrast to the overgrowth of K-feldspar. Chemistry of the ground water out of the highest quality section of the reservoir is consistent with the complete dissolution field of anorthite, partial dissolution field of albite component of plagioclase, and the precipitation of K-feldspar on the temperature-logK phase equilibrium diagram drawn from this study. The thermodynamic modeling calculation gave us an insight into formation of the secondary pore within the reservoir, and the dissolution of plagioclase an important cause of the secondary pore. It is assumed that the solvent to dissolve the plagioclase frame grains originated from the origin-enriched compacted fluid acids out of the mudstone buried beneath the top of geopressured zones in the Kuqa Depression.

The Fluid-absent Partial Melting of a Zoisite-bearing Quartz Eclogite from 1middle dot0 to 3middle dot2 GPa; Implications for Melting in Thickened Continental Crust and for Subduction-zone Processes

Fluid-absent melting experiments on a zoisiteand phengite-bearing INTRODUCTION eclogite (omphacite, garnet, quartz, kyanite, zoisite, phengite and Zoisite or clinozoisite is present in many high-pressure rutile) were performed to constrain the melting relations of these eclogites, and many occurrences are known from the hydrous phases in natural assemblages, as well as the melt and Scandinavian Caledonides (e.g. Holsnoy: Austrheim & mineral compositions produced by their breakdown. From 1·0 to Mork, 1988; Jamtveit et al., 1990; Western gneiss region: 3·2 GPa the solidus slopes positively from 1·5 GPa at 850°C to Griffin et al., 1985; Seve Nappe: Kullerud et al., 1990). 2·7 GPa at 1025°C, but bends back at higher pressures to 975°C Indeed, most eclogites worldwide contain minor amounts at 3·2 GPa. The melt fraction is always low and the melt of zoisite or other hydrous phases, according to the compositions always felsic and become increasingly so with increasing compilations of Smith (1988) and Carswell (1990). During pressure. The normative Ab–An–Or compositions of the initial prograde eclogitization, zoisite forms by breakdown of melts vary from tonalites at 1·0 GPa to tonalite–trondhjemites at the anorthite component of plagioclase in the presence 1·5 GPa, adamellites at 2·1 and 2·7 GPa, and to true granites of a hydrous fluid phase. The origin of these hydrous at 3·2 GPa. At pressures 2·5 GPa zoisite and phengite break fluids is controversial. One possibility is that they are down more or less simultaneously. At 3·2 GPa and 1000°C zoisite introduced from below during continental collision, for is unreacted whereas phengite is absent, so that the first formed melt example, if wet continental sedimentary rocks are deeply at these conditions is granitic. Our experiments show that if subducted. During subduction of oceanic crust zoisite sufficiently high temperatures (of the order of 1000°C) are attained, forms by prograde metamorphism of hydrothermally zoisiteand phengite-bearing eclogites can produce small fractions altered oceanic crust, which in its upper levels contains of silicic melts of a wide range of compositions. These melts are low-temperature hydrous Ca-rich phases (e.g. Poli & rich in water and, probably, in Sr and other incompatible elements, Schmidt, 1997). At the gabbro to dyke transition zone so that they can act as metasomatic agents in the mantle wedge. the temperature and pressure are high enough to allow

The Role of Disseminated Calcite in Rates of Calcium Release and CO2 Consumption During the Weathering of Granitoid Rocks

Although Ca and Na reside predominately in plagioclase feldspar in granitoid rocks, recent studies have demonstrated that excess Ca often exits in surface and ground waters relative to plagioclase stochiometry. Explanations advanced to explain the Ca excess have included selective loss of Ca in soils due to watershed acidification (Hyman et al., 1998), selective weathering of the anorthite component of plagioclase (Clayton 1986), and the rapid weathering of trace mineral phases such as calcite (Mast and Drever 1990). Chemical hydrolysis of Ca-silicate rocks is recognized as impacting long term atmospheric CO2 consumption and global climate through silicate hydrolysis reactions (Bemer and Caldeira 1997). In contrast, carbonate mineral dissolution does not have a corresponding impact because the CO2 consumed in weathering is reintroduced back into the atmosphere by precipitation of carbonates in the ocean. Thus, the solute Ca/Na ratio in watersheds and rivers, reflecting proportions of silicate and carbonate weathering, is important in understanding long term global CO2 balances.

Zonation of low grade regional metamorphic rocks, Nambucca slate belt, northeastern New South Wales

Abstract Crystallization of stilpnomelane and pumpellyite, disappearance of prehnite and development of assemblages containing the pair pumpellyite‐actinolite, and disappearance of pumpellyite, define isograds separating four zones of progressive regional metamorphism in the Nambucca Slate Belt of northeastern New South Wales. Mineralogical changes are paralleled by textural changes in metaclastic rocks. Detrital grains are progressively modified, a preferred orientation of material becomes increasingly perfected, and a foliation develops as grade increases. The most important secondary phases are albite, white mica, chlorite, stilpnomelane, actinolite, pumpellyite, prehnite, epidote, clinozoisite, and calcite. White mica shows an increase in ‘crystallinity’ with grade, chlorite becomes more aluminous, and the ferric iron content of epidote‐group minerals decreases. In metabasic rocks hydration reactions involving the anorthite component of plagioclase and calcium‐poor pyroxene (or olivine) probably led t...