Ca Al Silicates Research Articles

Paragenesis of secondary Ca–Al silicates during hydrothermal alteration of Proterozoic granitic rocks (SE Sweden)

The texture and chemical compositions of secondary Ca–Al silicates (prehnite, pumpellyite, epidote, and titanite) and of related minerals (chlorite, sericite, K‐feldspar, albite, Fe‐oxides, fluorite, and calcite) are used to discuss their formation conditions during interaction of Proterozoic granitic rocks from SE Sweden with hydrothermal fluids. The paragenetic sequence of the Ca–Al silicates is suggested to have been accomplished at ca. 200–350°C and was controlled by small‐scale variations in the chemistry of the hydrothermal fluids, such as aCa2+/aH+ ratio and fO2 and fCO2. These variations in fluid chemistry occurred mostly in closed systems, which involved the redistribution of elements released from the alteration of the magmatic minerals, primarily plagioclase, biotite, and amphibole and inducing porosity modifications. Accordingly, results obtained in this study should have implications for the role of hydrothermal alterations on element redistribution in the continental crust and for suitability of granitic rocks as repositories for radioactive wastes.

Iron-nickel metallic components bearing silicate-melts and coesite from Ramgarh impact structure, west-central India: Possible identification of the impactor

The Ramgarh structure (rim-to-rim diameter ~2.4 km) in the Vindhyan Supergroup of sedimentary rocks (including sandstone, shale and minor limestone) of the Mesoproterozoic age in the west-central India, is India’s third confirmed asteroid impact crater. This eroded structure is roughly rectangular in shape and resembles to the Barringer Crater, USA. The presence of central peak and its current crater diameter/depth ratio of ~12 well corroborate the range (10–20) of terrestrial complex asteroid impact craters. The mm-sized, iron-rich (FeO ~50 wt.% in average), spherule-like particles, recovered from the alluvium inside the Ramgarh structure, have internal morphology similar to those of the accretionary lapilli described in known impact craters. The in-situ LA-ICP-MS analyses also suggested high Co–Ni (up to 13,000 and 2500 ppm, respectively)-rich areas locally within these spherules/lapilli. A few non-in-situ, mm-sized particles, recovered from the rim of the structure show the presence of coesite, one of the diagnostic indicators of shock metamorphism. A few fragments of iron-rich, Ca–Al–silicate glasses recovered from the soil inside the structure and outside of the western crater rim include the presence of dendritic magnetite with occasional inclusions of relict native iron. Our microprobe analyses confirm that these metallic irons contain high proportions of Co (~350–3000 ppm), Ni (~200–4000 ppm) and Cu (~2200–7000 ppm) and possibly could be the relict component of a Cu-rich iron meteorite impactor. The field observation and relative enrichment of compatible and incompatible trace elements in the spherule-like substance (recovered from the alluvium inside the Ramgarh structure) as compared to target rocks suggests that hydrothermal activity played an important role in the evolution of the crater.

Hydrothermal Chromitites from the Oman Ophiolite: The Role of Water in Chromitite Genesis

The role of water-rich solutions in the formation of chromitites has been the matter of controversy. We found small chromite concentrations (chromitites) in diopsidites, precipitated from high-temperature hydrothermal fluids, in the mantle to the crust of the Oman ophiolite. Here, we present petrologic characteristics of the hydrothermal chromitites to understand their genesis. In the chromitites, the chromite is associated with uvarovite in the crust and diopside + grossular in the mantle. They are discriminated from the magmatic podiform chromitite by dominance of the Ca-Al silicates in the matrix. The fluids responsible for chromite precipitation are possibly saline, being derived from the seawater circulated into the mantle through the crust. The saline fluids precipitate chromite to form chromite upon decompression and cooling, and transport platinum-group elements (especially Pt and Pd). The fluids obtain Ca and Al from the crustal rocks and Cr from the mantle rocks during circulation. Saline fluids are also supplied from the slab to the mantle wedge, and can metasomatically precipitate chromite and pyroxenes within peridotites. They re-distribute Cr and chromite in peridotites along with circulation of saline fluids in the mantle wedge.

In-situ Sr-Pb isotope geochemistry of lawsonite: A new method to investigate slab-fluids

Lawsonite is a hydrous Ca-Al silicate mineral which is broadly stable under modern style subduction conditions. In order to understand the elemental fractionation and the sources of subducted materials, lawsonite crystals in lawsonite-eclogite facies basaltic and sedimentary rocks from South Motagua Mélange of Guatemala were investigated using LA-ICPMS and LA-MC-ICPMS. Mass balance calculation based on in-situ trace element and modal compositions in an eclogite with a low-variance mineral assemblage confirmed that lawsonite hosts most of the light rare earth element (LREE), Sr, Pb, Th and U in the bulk. In-situ Sr-Pb isotope analyses of eclogite-facies lawsonite crystals revealed isotopic variations reflecting their protoliths. Isotopic zoning was also detected in some crystals. Lawsonite crystals in a phengite-rich eclogite have relatively low 87Sr/86Sr = 0.70335–0.70355 with variations in 207Pb/206Pb = 0.8401–0.8512 and 208Pb/206Pb = 2.0757–2.0911. Lawsonite crystals from another eclogite are remarkably zoned. The cores have 87Sr/86Sr = 0.70558–0.70601 and the rims have relatively elevated 87Sr/86Sr = 0.70636–0.70662, exhibiting MORB-like 207Pb/206Pb = 0.843–0.844 at the cores and more enriched 207Pb/206Pb = 0.839–0.841 at the rims. The radiogenic Sr isotope composition would have been derived from seafloor alteration before subduction. In fact, lawsonite crystals in a metachert have higher 87Sr/86Sr = 0.70697–0.70757 with 208Pb/206Pb = ∼2.07. However, the isotopically zoned metabasaltic lawsonite crystals with MORB-like core compositions suggest that metasomatism by an external fluid from sediment protolith occurred during overgrowth of their rims. Our study indicates that lawsonite crystals record both the isotopic compositions of the protoliths and metasomatic fluids from different protoliths. The in-situ Sr-Pb isotope analysis has a potential to reveal records of the complicated metamorphic processes.

Firing-induced transformations in Copper Age ceramics from NE Romania

The Copper Age Cucuteni-Tripolye (Trypillian) Cultural Complex extended from central and northern present-day Romania to the east, into Moldavia and Ukraine. Elegant-shaped light red to orange pots, painted with white, dark red and brown-black geometric patterns are characteristic of this culture. We studied 46 potsherds from Ruginoasa, 60 km west of Iasi (Romania) in terms of mineralogy and petrography. Polarized-light microscopy shows a red, porous body, with fine to semifine microstructure and more or less oriented microtexture. The ceramic consists of a microcrystalline to amorphous/vitreous matrix and non-plastic clasts e.g . quartz, plagioclase, K-feldspar, muscovite, some chloritized biotite, heavy minerals and various lithoclasts, including calcareous ones. Backscattered electron (BSE) images revealed, in addition to large non-plastic clasts, a highly inhomogeneous and porous matrix, consisting of fine-grained clasts of quartz, feldspar and muscovite, some CaAl silicates and glass, all embedded in a fine-grained mass of clay minerals. Electron microprobe analyses (EMPA) indicate that several processes took place on a small scale during the firing. Upon softening and possible partial melting, quartz rims incorporate variable amounts of Al, K, Fe, Mg, Na, Ca and Ti. The alkali feldspar ranges from pure K-feldspar to albite, including anorthoclase. Plagioclases are rare. Several unusual compositions that plot in the feldspar immiscibility domain point to newly-formed phases. Frequently, former clasts of albite or plagioclase are replaced by K- or NaK-feldspars at the rims or inside the grains. Muscovite ranges from Al-rich composition to Si-rich phengite and is locally partially melted. The clay-rich part of the matrix is characterized by a loss of K and incorporation of Ca leading to compounds between K-illite and a hypothetical Ca-illite. The transformation processes, assisted by a fluid phase, indicate a temperature interval between 850 °C and 950 °C for the firing. Our results show that the thermal reactions did not reach equilibrium, most likely due to the short time of firing. At a small scale the resultant material consists of an inhomogeneous mix of various micro-domains.

Chloritization in Proterozoic granite from the Äspö Laboratory, southeastern Sweden: record of hydrothermal alterations and implications for nuclear waste storage

AbstractHydrothermal alteration of Proterozoic granitic rocks in the Äspö underground laboratory, southestern Sweden, resulted in the formation of chlorite with large variations in textural and chemical characteristics, which reflect differences in formation temperatures, fluid composition, and reaction mechanisms. The mineral assemblage associated with chlorite, including Ca-Al silicates (prehnite, pumpellyite, epidote, and titanite), Fe-oxides, calcite, albite and K-feldspar, suggests that chloritization occurred at temperatures of between 200–350°C during various hydrothermal events primarily linked to magmatism and rock deformation. Petrographic and electron microprobe analyses revealed that chlorite replaced biotite, amphibole and magnetite, and hydrothermal chlorite phases filled fractures and vugs in the granitic rocks. While fracture-filling chlorite reduces fracture permeability, chloritization reactions in the host granite resulted in the formation of new localized microporosity that should thus be taken into consideration when evaluating the safety of the granitic basement rocks as a repository for nuclear waste. It is also important to take into account that similar alteration reactions may occur at the site of stored nuclear waste where temperatures in excess of 100°C might be encountered.

Late Alpine brittle faulting in the Rotondo granite (Switzerland): deformation mechanisms and fault evolution

The unlined Bedretto tunnel in the Central Swiss Alps has been used to investigate in detail the fault architecture and late Alpine brittle faulting processes in the Rotondo granite on macroscopic and microscopic scales. Brittle faults in the late Variscan Rotondo granite preferentially are situated within the extent of preexisting ductile shear zones. Only in relatively few cases the damage zone extends into or develops in the previously undeformed granite. Slickensides suggest a predominant (dextral) strike-slip movement along these steeply dipping and NE–SW-striking faults. Microstructures of these fault rocks illustrate a multi-stage retrograde deformation history from ductile to brittle conditions up to the cessation of fault activity. In addition these fabrics allow identifying cataclastic flow, fluid-assisted brecciation and chemical corrosive wear as important deformation mechanisms during this retrogressive deformation path. Based on the analysis of zeolite microfabrics (laumontite and stilbite; hydrated Ca–Al- and Na–Ca–Al–silicate, respectively) in fault breccias, cataclasites and open fractures we conclude, that the main phase of active brittle faulting started below 280°C and ceased ca. 14 Ma ago at temperatures slightly above 200°C. This corresponds to a depth of approx. 7 km.

Mass transfer and porosity evolution during low temperature water–rock interaction in gneisses of the simano nappe: Arvigo, Val Calanca, Swiss Alps

Late Alpine fissures and fractures in amphibolite-facies basement gneisses at Arvigo (Val Calanca, Swiss Alps) show distinct cm-sized reaction selvages parallel to the fracture walls that composed of subgreenschist facies assemblages produced by the interaction of water present in the fracture porosity with the old high-grade gneiss assemblages. The process of selvage or reaction-vein formation occurred in the brittle deformation regime and at temperatures characteristic of, first the prehnite-pumpellyite facies and then later of the zeolite facies. The vein formation occurred during uplift and cooling at very late stages of the Alpine orogeny. The reaction veins are composed of a selvage of altered gneiss on both sides of the central fracture and a central zone with fissure minerals that have been growing in the open fracture pore space. The central zone of the Arvigo veins contains an early assemblage with epidote, prehnite and chlorite and a late succession sequence of various species of zeolite. The veins of the Arvigo quarry are convincing evidence that fracture fluids in gneiss and granite have the potential to precipitate Ca–zeolite. This is an important find because many fluids recovered from deep continental drill holes and from geothermal energy exploration are found to be oversaturated in respect to a number of Ca–zeolite species. Vein formation during late uplift and cooling of the Alps occurred at continuously decreasing T and at hydrostatic pressure: (1) coexisting prehnite/epidote records temperatures of 330–380°C, (2) chlorite formation at temperature of 333 ± 32°C and (3) formation of zeolites <250°C. In the selvages the prime reaction is the replacement of plagioclase by albite along a sharp reaction front that separates the selvage from unaltered gneiss. In addition to albitisation, chloritisation of biotite is the second important reaction in the alteration process. The reactions release components for the formation of Ca–Al silicates. The water–rock interaction is associated with a depletion of Al, Si, Ca, Fe and K in the altered wall rock. The overall reaction is associated with an increase in porosity of up to 14.2 ± 2.2% in the selvage zone (altered wall rock), caused by the volume decrease during albitisation and the removal of biotite. The propagation of the sharp reaction front through the gneiss matrix occurred via a dissolution-reprecipitation mechanism. Zeolite formation is tied to the plagioclase alteration reaction in the rock matrix, which releases components for zeolite formation to a CO2-poor aqueous liquid.

Near vein metasomatism along propylitic veins in the Baksa Gneiss Complex, Pannonian Basin, Hungary

In many parts of the metapelitic (gneiss, mica schist) rock section of the Baksa Complex, significant wall-rock alteration is observable along the Ca-Al silicate veins, which show a di → ep ± czo →sp → ab ± kfs → chl → adu → prh → py → cal mineral sequence (FINTOR et al., 2009). These alterations appear as narrow (few cm thick) bleached margins beside thin veins, and broad alteration bands along thick veins where detailed epidotization and chloritization of the adjacent rock are recognizable. Based on petrographic and mineralogical examination of the altered wallrocks, metasomatic zones with characteristic mineral paragenesis can be distinguished: Zone 1 (ab + ttn ± ep), Zone 2 (ep + chl+ ttn + ab ± ser), Zone 3 (chl + ep + ser + rt ± ttn), Zone 4 (ser ± chl). Bulk rock chemical analyses were made from the different metasomatic zones. The results show that fluid circulated in the propylitic veins caused metasomatic alteration of the wall-rock, with transport of considerable amount of Ca2+ toward the adjacent rocks. The hydrothermal leaching almost totally removed the K, Fe, Mg, and Mn ions from the wall rock. The main alteration processes are the epidotization and chloritization of biotite, and albitization of micas (muscovite + biotite) content of metapelites. Based on mobilization of different cations alteration was due to? to a near neutral fl uid (~pH 5–7). The pervasive hydrothermal leaching caused significant secondary porosity (cavities) in the altered domains, which were partially filled by epidote. Fluid inclusions of cavity filling epidote indicate a similar character (Th: 180–360 °C; Salinity: 0.2–1.6 mass% eq. NaCl) to that can be found in Ca-Al silicate veins. The alteration most probably occurred in the 360–480 °C temperature range as products of 'near vein metasomatism' and the altered rock can be related to the propylite metasomatic family.

Ca-Al hydrous silicates in the chlorite-grade pelitic schists in Sanbagawa metamorphic belt and a petrogenetic analysis in the model mixed-volatile system

Lawsonite, pumpellyite and epidote coexist in the chlorite zone pelitic schists of the Sanbagawa metamorphic belt. As these pelitic schists contain calcite ubiquitously, the phase relations of hydrous Ca-Al silicate minerals are examined by mixed volatile equilibria in the K2O-Na2O-CaO-MgO-Al2O3-SiO2-CO2-H2O system with excess quartz, calcite and a fluid phase. XCO2 of the stability field of pumpellyite is lower than that of lawsonite. An idealized geometry of an isobaric T-XCO2 diagram including mixed volatile reactions shows that the stable hydrous Ca-Al mineral changes from lawsonite through pumpellyite to epidote for pelitic schists in the chlorite zone with progressive increase in both temperature and XCO2 of the fluid phase, though ferric iron expands the stability of epidote throughout the chlorite zone. The fluid phase released by the metamorphic reactions in the chlorite zone is high in H2O content even in the presence of CaCO3. The proposed mixed volatile reactions, instead of the pure dehydration reactions, are applicable directly to the phase relations of CaCO3-bearing Sanbagawa basic schists and to the interpretation of the mineral sequence of the hydrous Ca-Al silicates of other high P/T metamorphic belts.

Palaeohydrogeology: A methodology based on fracture mineral studies

Two sites on the east coast of Sweden (Forsmark and Laxemar/Simpevarp) are currently being investigated as potential geologic hosts for a deep repository isolating high-level nuclear waste. In this paper, a methodology for fracture mineral studies is suggested with focus on the variation in depth of the fresh/saline water interface and location of the redox front in the bedrock. The most commonly precipitated fracture minerals in crystalline rocks are chlorite, calcite, quartz, K-feldspar, Ca–Al-silicates like epidote, prehnite and laumontite, sulphides and Fe-oxides. Of these, calcite is the mineral best suited for palaeohydrological studies since it precipitates during a wide range of conditions including low-temperature conditions during the Pleistocene and Holocene epochs. Sulphides and Fe-oxides/hydroxides provide information on the position of the redox front. In order to carry out palaeohydrological studies, a number of prerequisites are required such as; high quality drill core material, geological knowledge of the sequence of fracture mineralizations; the post-glacial (Holocene) evolution in the area; high quality groundwater chemistry, including stable isotopes; and a conceptual model of the hydrogeochemistry that is to be tested. The choice of methods used here is based on the fact that both the Forsmark and Laxemar/Simpevarp sites are situated in Palaeoproterozoic crystalline rocks with reactivation of fractures over at least 1.5 Ga, and they have been exposed to glaciations/deglaciations and transgressions/regressions of the Baltic Sea during the Quaternary. This has resulted in a palaeohydrology with a range of groundwaters of quite different chemistry and stable isotopic composition. The suggested scheme for solving the variation in depth of the fresh/saline water interface focuses on fracture calcite. It includes a step-by-step procedure with; (1) Initial δ 18 O and δ 13 C, analyses and complementary petrographic studies of thin sections and crystal morphology followed by (2) 87 Sr/ 86 Sr and trace element analyses, and finally (3) Fluid inclusion analyses and cathodoluminescence (CL) studies of zoning. For redox front investigations, the following methods are suggested: (1) Study of the distribution of redox sensitive minerals; (2) U-series measurements; (3) Trace element analyses of fracture fillings; (4) Fe(III)/Fe(II) analyses +/− Fe isotope analyses. It is concluded that fracture mineral studies can reveal a palaeohydrological record in crystalline rock that is essential to understand the stability or evolution of the groundwater system over a time scale that is relevant to performance assessment for a spent nuclear fuel repository. It is also concluded that the suggested methodology for palaeohydrogeological studies is site-specific.

Cycling of B, Li, and LILE (K, Cs, Rb, Ba, Sr) into subduction zones: SIMS evidence from micas in high- P/ T metasedimentary rocks

The Catalina Schist (California) affords a unique opportunity to evaluate the effects of varying prograde P– T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting sediments. Analysis of a suite of trace elements (Li, B, Rb, Sr, Cs, Ba) in micas obtained by secondary ion mass spectrometry (SIMS) complements previously published whole-rock data for metasedimentary rocks in the Catalina Schist and circumvents problems associated with chemical/isotopic overprinting during retrogression, further elucidating trace element redistribution during devolatilization along prograde P– T paths. SIMS mineral analyses confirm the inference from the whole-rock data that white mica (and in a few higher-grade samples, also biotite) is the dominant reservoir for B, Rb, Ba, and Cs. The mineral residency of Li and Sr is more complex, as Li is also highly concentrated in chlorite (where present) and in some amphiboles, and Sr whole-rock budgets are strongly influenced by the presence or absence of Ca-rich phases (e.g., lawsonite, clinozoisite, minor carbonate and titanite). Changes of concentrations of B and Cs, in white micas, with increasing metamorphic grade parallel the whole-rock trends, further demonstrating the loss of these elements from recrystallizing white micas during prograde devolatilization reactions. Trace element data for other minerals in the metasedimentary samples (Ca–Al silicates, plagioclase, chlorite, biotite, garnet, kyanite) confirm expected concentrations as related to ionic radius and charge and demonstrate element redistribution during prograde (and retrograde) reactions. For amphibolite-facies metasedimentary rocks, multiple generations of white mica, including a finer-grained, higher-Si generation related to retrogradation, have distinct trace element compositions. In pegmatites produced by partial melting at amphibolite-facies conditions, B, Li, and the LILE are similarly concentrated in coarse-grained muscovite, indicating inheritance of trace element compositions from their metasedimentary sources. These SIMS data, combined with the whole-rock data for the same samples, demonstrate that the deep transfer of relatively “fluid-mobile” trace elements is strongly dependent upon the prograde P– T path experienced by the subducting sediments. Efficient retention of B, Cs, and N in micas, to depths of > 40 km, is evident from study of these and other forearc metasedimentary suites that experienced low- T prograde paths representative of most modern subduction zones, and is consistent with models invoking additions of these elements from the slab into arc source regions, perhaps to contribute to the across-arc chemical trends (Be/Be, B/La, Cs/Th, Ba/Th) observed in some margins.

Ocean‐floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker

AbstractThe Limousin ophiolite is located at the suture zone between two major thrust sheets in the western French Massif Central. This ophiolitic section comprises mantle‐harzburgite, mantle‐dunite, wehrlites, troctolites and layered gabbros. It has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and/or pargasite‐bearing chloritites. With various thermobarometric methods, it is possible to show that the different varieties of amphibole have registered low‐P (c. 0.2 GPa) conditions with temperature ranging from high‐T, late‐magmatic conditions to greenschist–zeolite metamorphic facies. The abundance of undeformed metamorphic rocks (which is typical of the lower oceanic crust), the occurrence of Ca–Al (–Mg) metasomatism illustrated by the growth of Ca–Al silicates in veins or replacing the primary magmatic minerals, the P–T conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from Ocean Drilling Program and worldwide ophiolites are the main arguments for an ocean‐floor hydrothermal metamorphism in the vicinity of a palaeo‐ridge. Among the West‐European Variscan ophiolites, the Limousin ophiolites constitute an extremely rare occurrence that has not been involved in any HP (subduction‐related) or MP (orogenic) metamorphism as observed in other ophiolite occurrences (i.e. France, Spain and Germany).

Material degradation during isothermal ageing and thermal cycling of hybrid mica seals under solid oxide fuel cell exposure conditions

Hybrid phlogopite mica seals with glass interlayers were evaluated in terms of materials degradation in a combined ageing and thermal cycling test. Three glass interlayers were investigated: a standard Ba–Ca–Al silicate glass (G18), a modified Ba–Ca–Al silicate glass with a nucleation agent added (G18m), and a borosilicate glass (G6). The hybrid phlogopite mica seals were aged at 800 °C for ∼500 to ∼1000 h in moist, dilute hydrogen fuel, and then subjected to short-term thermal cycling between ∼100 and 800 °C. Seals with G18 and G18m glass interlayers showed very poor thermal cycle stability after isothermal ageing. Seals with borosilicate glass interlayers showed very good thermal cycle stability with constant, low leakage (<0.01 sccm cm −1 at 0.2 psi) after ageing for 508 h followed by 56 thermal cycles. Post-mortem analyses showed degradation of the mica due to interaction with the interlayer glasses. Leak paths were identified and correlated to the measured leak rates.

Fluid inclusion characteristics of the Uti gold deposit, Hutti-Maski greenstone belt, southern India

The Uti gold deposit occurs within amphibolites of the late Archaean Hutti-Maski greenstone belt of southern India. Gold mineralization is associated with intense silicification, biotite-actinolite- and sulfide-rich alteration zones in between small anastomozing shear zones. Characteristic biotite-K-feldspar alteration and arsenopyrite thermometry constrain the mineralizing event to above 400 °C, which is concordant with evidence of dynamic recrystallization in mineralized quartz veins. Shearing, concomitant alteration and Au-mineralization took place at post-peak metamorphic conditions. A post-mineralization, sub-greenschist facies assemblage of prehnite/pumpellyite and hydrogarnet later imprinted the rocks. Fluid inclusion microthermometric studies on mineralized and barren quartz veins reveal similar fluid chemistry, P– T and, surprisingly, a complete absence of carbonic inclusions. Fluid inclusion textures and the shape of T h histograms point towards moderate reequilibration during a phase of isothermal decompression. The microthermometric data, coupled with stability relations of the hydrous Ca–Al silicates, indicate post-mineralization isobaric cooling followed by a near-isothermal decompression. The observed temperature–salinity variation is schematically explained by simple cooling of a low salinity heated meteoric fluid, followed by isothermal mixing with a high salinity granitic fluid. However, fluid mixing was later than, and unrelated to mineralization. On the other hand, formation of auriferous sulfide-bearing quartz veins at Uti was essentially due to remobilization and attendant fluid–rock interaction involving suitable host rocks that contained primary gold.

Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO 2 sink in the Early Archean

Early Archean (3.46 Ga) hydrothermally altered basaltic rocks exposed near Marble Bar, eastern Pilbara Craton, have been studied in order to reveal geological and geochemical natures of seafloor hydrothermal carbonatization and to estimate the CO 2 flux sunk into the altered oceanic crust by the carbonatization. The basaltic rocks are divided into basalt and dolerite, and the basalt is further subdivided into type I, having original igneous rock textures, and type II, lacking these textures due to strong hydrothermal alteration. Primary clinopyroxene phenocrysts are preserved in some part of the dolerite samples, and the alteration mineral assemblage of dolerite (chlorite + epidote + albite + quartz ± actinolite) indicates that the alteration condition was typical greenschist facies. In other samples, all primary minerals were completely replaced by secondary minerals, and the alteration mineral assemblage of the type I and type II basalts (chlorite + K-mica + quartz + carbonate minerals ± albite) is characterized by the presence of K-mica and carbonate minerals and the absence of Ca-Al silicate minerals such as epidote and actinolite, suggesting the alteration condition of high CO 2 fugacity. The difference of the alteration mineral assemblages between basalt and dolerite is probably attributed to the difference of water/rock ratio that, in turn, depends on their porosity. Carbonate minerals in the carbonatized basalt include calcite, ankerite, and siderite, but calcite is quite dominant. The δ 13C values of the carbonate minerals are −0.3 ± 1.2‰ and mostly within the range of marine carbonate, indicating that the carbonate minerals were formed by seafloor hydrothermal alteration and that carbonate carbon in the altered basalt was derived from seawater. Whole-rock chemical composition of the basaltic rocks is essentially similar to that of modern mid-ocean ridge basalt (MORB) except for highly mobile elements such as K 2O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K 2O, Rb, and Ba, and are depleted in Na 2O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO 2 enrichment is recognized in the basalt due to the ubiquitous presence of carbonate minerals, but there was essentially neither gain nor loss of CaO. This suggests that the CO 2 in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO 2 ratios of the basalt are generally the same as that of pure calcite, indicating that Ca in the basalt was almost completely converted to calcite during the carbonatization, although Mg and Fe were mainly redistributed into noncarbonate minerals such as chlorite. The carbon flux into the Early Archean oceanic crust by the seafloor hydrothermal carbonatization is estimated to be 3.8 × 10 13 mol/yr, based on the average carbon content of altered oceanic crust of 1.4 × 10 -3 mol/g, the alteration depth of 500 m, and the spreading rate of 1.8 × 10 11 cm 2/yr. This flux is equivalent to or greater than the present-day total carbon flux. It is most likely that the seafloor hydrothermal carbonatization played an important role as a sink of atmospheric and oceanic CO 2 in the Early Archean.

Epidote Group Minerals in Low-Medium Pressure Metamorphic Terranes

Research Article| January 02, 2004 Epidote Group Minerals in Low–Medium Pressure Metamorphic Terranes Rodney H. Grapes; Rodney H. Grapes Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Search for other works by this author on: GSW Google Scholar Paul W. O. Hoskin Paul W. O. Hoskin Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Search for other works by this author on: GSW Google Scholar Author and Article Information Rodney H. Grapes Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Paul W. O. Hoskin Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Publisher: Mineralogical Society of America First Online: 03 Mar 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2004) 56 (1): 301–345. https://doi.org/10.2138/gsrmg.56.1.301 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Rodney H. Grapes, Paul W. O. Hoskin; Epidote Group Minerals in Low–Medium Pressure Metamorphic Terranes. Reviews in Mineralogy and Geochemistry 2004;; 56 (1): 301–345. doi: https://doi.org/10.2138/gsrmg.56.1.301 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Epidote group minerals are common in metamorphosed mafic to intermediate igneous rocks, quartzofeldspathic sediments and calc-alumina silicate (marl) rocks of higher grade zeolite to medium grade amphibolite facies of low–medium pressure contact and regional metamorphic terranes (i.e., pressure and temperature conditions below the calcite-to-aragonite transition). Within any one rock, epidote composition in terms of Fe3+/(Fe3+ + Al) can be variable, but in general, is limited by whole-rock composition, such that epidote group minerals in metabasite lithologies are more Fe-rich than those in marls that tend to be more Al-rich and typically include zoisite. Because of their wide... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Hydrothermal alteration, fluid flow and volume change in shear zones: the layered mafic–ultramafic Kettara intrusion (Jebilet Massif, Variscan belt, Morocco)

AbstractDuring emplacement and cooling, the layered mafic–ultramafic Kettara intrusion (Jebilet, Morocco) underwent coeval effects of deformation and pervasive fluid infiltration at the scale of the intrusion. In the zones not affected by deformation, primary minerals (olivine, plagioclase, clinopyroxene) were partially or totally altered into Ca‐amphibole, Mg‐chlorite and CaAl‐silicates. In the zones of active deformation (centimetre‐scale shear zones), focused fluid flow transformed the metacumulates (peridotites and leucogabbros) into ultramylonites where insoluble primary minerals (ilmenite, spinel and apatite) persist in a Ca‐amphibole‐rich matrix. Mass‐balance calculations indicate that shearing was accompanied by up to 200% volume gain; the ultramylonites being enriched in Si, Ca, Mg, and Fe, and depleted in Na and K. The gains in Ca and Mg and losses in Na and K are consistent with fluid flow in the direction of increasing temperature.When the intrusion had cooled to temperatures prevailing in the country rock (lower greenschist facies), deformation was still active along the shear zones. Intense intragranular fracturing in the shear zone walls and subsequent fluid infiltration allowed shear zones to thicken to metre‐scale shear zones with time. The inner parts of the shear zones were transformed into chlorite‐rich ultramylonites. In the shear zone walls, muscovite crystallized at the expense of Ca–Al silicates, while calcite and quartz were deposited in ‘en echelon’ veins. Mass‐balance calculations indicate that formation of the chlorite‐rich shear zones was accompanied by up to 60% volume loss near the centre of the shear zones; the ultramylonites being enriched in Fe and depleted in Si, Ca, Mg, Na and K while the shear zones walls are enriched in K and depleted in Ca and Si. The alteration observed in, and adjacent to the chlorite shear zones is consistent with an upward migrating regional fluid which flows laterally into the shear zone walls. Isotopic (Sr, O) signatures inferred for the fluid indicate it was deeply equilibrated with host lithologies.

The chemistry and mineralogy of the Negev oil shale ashes

The characteristic feature of the oil shale ashes produced by fluidized bed combustion (700–800 °C) in PAMA's demonstration power plant is the large amount of amorphous phases, Ca–Al-silicates and Al-silicates, together with anhydrite and lime. Practically all the S and heavy metals in the oil shales are retained in the ash, which, from an ecological point of view, is important. Two kinds of ashes were examined: industrial ashes produced at PAMA's demonstration plant and ashes produced in laboratory experiments. Three different types of ash are produced at the demonstration plant. Ash Cooler (AC), which is comparable to bottom ash in coal power plants. This ash is produced from oil shale subjected to the lowest temperatures and is the most coarse-grained. It contains relatively larger quantities of unaltered minerals (calcite, clays, apatite, etc.) than the other two. The two other ashes Boiler Bank (BB) and Fly Ash (FAS) are not much different from each other and both may be compared to fly ash in coal power plants. Both BB and FAS ashes contain more authigenic (formed in the boiler) phases than AC. The results of the laboratory experiments show that the main factor in the raw material controlling the mineralogy and chemistry of the oil shale ashes is the Al 2O 3 concentration (clay content), and not the organic matter concentration.

Retrograde P–T evolution and high temperature–low pressure fluid circulation in relation to late Hercynian intrusions: a mineralogical and fluid inclusion study of the Charroux‐Civray plutonic complex (north‐western Massif Central, France)

AbstractThe calc‐alkaline plutonic complex from Charroux‐Civray (north‐western part of the French Massif Central) displays multiphase hydrothermal alteration. Plutonic rocks, as well as early retrograde Ca–Al silicate assemblages, which have crystallized during cooling and uplifting of the plutonic series, are affected by multiphase chlorite–phengite–illite–carbonate alteration linked to intense pervasive fluid circulation through microfractures. The petrographic study of alteration sequences and their associated fluid inclusions in microfissures of the plutonic rocks, as well as in mineral fillings of the veins, yields a reconstruction of the P–T–X evolution of the Hercynian basement after the crystallization of the main calc‐alkaline plutonic bodies. This reconstruction covers the uplift of the basement to its exposure and the subsequent burial by Mesozoic sediments.Cooling of the calc‐alkaline plutonic series started at solidus temperatures (∼650°C), at a pressure of about 4 kbar (1 bar = 105 N m−2), as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data. Cooling continued under slightly decreasing pressure during uplift down to 2–3 kbar at 200–280°C (prehnite–pumpellyite paragenesis). Then, a hot geothermal circulation of CO2‐bearing fluids was induced within the calc‐alkaline rocks leading to the formation of greisen‐like mineralizations. During this stage, temperatures around 400–450°C were still high for the inferred depths (∼2 kbar). They imply abnormal heat flows and thermal gradients of 60–80°C km−1. The hypothesis of the existence of one large or a succession of smaller peraluminous plutons at depth, supported by geophysical data, suggests that localized heat flows were linked to concealed leucogranite intrusions. As uplift continued, greisen mineralization was subsequently affected by the chlorite–phengite–dolomite assemblage, correlated with aqueous and nitrogen‐bearing fluid circulations in the temperature range of 400–450°C. In a later stage, a continuous temperature decrease at constant pressure (∼0.5 kbar) led to the alteration of the dolomite–illite–chlorite type in the 130–250°C temperature range.