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Stability Field (stability + field)
Selected AbstractsBog Iron Ores and their Potential Role in Arsenic Dynamics: An Overview and a "Paleo Example"ENGINEERING IN LIFE SCIENCES (ELECTRONIC), Issue 6 2008A. Banning Abstract Bog iron ores (BIOs), i.e. terrestrial accumulations of iron (Fe) minerals forming within the zone of groundwater oscillation, have been described in several regions in Germany and other countries. Since BIOs are composed of a variety of Fe minerals, primarily amorphous Fe hydroxides, they are likely to have an influence on the arsenic (As) dynamics of an area, as these minerals represent important natural As sources and sinks. In this study, mineralogical research results (XRD, microscopy) of altered BIOs of Tertiary age ("paleo" BIOs or PBIOs), occurring within Cretaceous sands in an area of North Rhine-Westphalia, are briefly presented. Genesis and mineralogical evolution of the categorized five different types of PBIOs, along with hydrogeochemical data from the literature, are discussed and compared to studies describing Holocene BIOs from other areas. In doing so, striking similarities (depositional environment, substratum, Fe source and its transport, geochemical evolution, and mineralogy) became evident. Differences in mineralogical and chemical composition can be attributed to the longer period of oxidation that the PBIOs have undergone (Fe hydroxide "aging"). This process is still ongoing (most of the groundwaters in the area plot in the goethite stability field) and leads to a higher stability of the Fe phases and thus, a stronger As retention. The known impact of the PBIOs on the As budget of the study area (they represent the source for elevated As concentrations in soils) can be transferred to more recent environments fostering BIO formation. These are likely to be even more important As sinks , and sources , as they contain higher Fe concentrations, higher shares of potentially mobile As and highly variable redox conditions which might lead to an As output from the BIOs into groundwater, soils and plants. Therefore, BIOs and their potential role in As behaviour are not only of scientific, but also of public interest. [source] The mechanism of fluid infiltration in peridotites at Almklovdalen, western NorwayGEOFLUIDS (ELECTRONIC), Issue 3 2002O. Kostenko Abstract A major Alpine-type peridotite located at Almklovdalen in the Western Gneiss Region of Norway was infiltrated by aqueous fluids at several stages during late Caledonian uplift and retrogressive metamorphism. Following peak metamorphic conditions in the garnet,peridotite stability field, the peridotite experienced pervasive fluid infiltration and retrogression in the chlorite,peridotite stability field. Subsequently, the peridotite was infiltrated locally by nonreactive fluids along fracture networks forming pipe-like structures, typically on the order of 10 m wide. Fluid migration away from the fractures into the initially impermeable peridotite matrix was facilitated by pervasive dilation of grain boundaries and the formation of intragranular hydrofractures. Microstructural observations of serpentine occupying the originally fluid-filled inclusion space indicate that the pervasively infiltrating fluid was characterized by a high dihedral angle (, > 60°) and ,curled up' into discontinuous channels and fluid inclusion arrays following the infiltration event. Re-equilibration of the fluid phase topology took place by growth and dissolution processes driven by the excess surface energy represented by the ,forcefully' introduced external fluid. Pervasive fluid introduction into the peridotite reduced local effective stresses, increased the effective grain boundary diffusion rates and caused extensive recrystallization and some grain coarsening of the infiltrated volumes. Grain boundary migration associated with this recrystallization swept off abundant intragranular fluid inclusions in the original chlorite peridotite, leading to a significant colour change of the rock. This colour change defines a relatively sharp front typically located 1,20 cm away from the fractures where the nonreactive fluids originally entered the peridotite. Our observations demonstrate how crustal rocks may be pervasively infiltrated by fluids with high dihedral angles (, > 60°) and emphasize the coupling between hydrofracturing and textural equilibration of the grain boundary networks and the fluid phase topology. [source] Talc-phengite-albite assemblage in piemontite-quartz schist of the Sanbagawa metamorphic belt, central Shikoku, JapanISLAND ARC, Issue 1 2000J. Izadyar Abstract The talc (Tlc) + phengite (Phn) + albite (Ab) assemblage is newly confirmed in MnOtotal -rich (1.65 wt% in average) piemontite-quartz schists from the intermediate- and high-grade part of the Sanbagawa belt, central Shikoku, Japan. Talc is in direct contact with Phn, Ab and chlorite (Chl) with sharp boundaries, suggesting that these four phases mutually coexist. Other primary constituents of the Tlc-bearing piemontite-quartz schist are spessartine, braunite, hematite (Ht), crossite/barroisite and dolomite. Phlogopite (Phl) rarely occurs as a later stage mineral developing along the rim of Phn. The studied piemontite-quartz schist has mg# (= Mg/(Mg + Fe2+)) ~ 1.0, because of its high oxidation state. Schreinemakers' analysis in the KNMASH system and the mineral assemblage in the Sanbagawa belt propose a possible petrogenetic grid, in which the Tlc,Phn assemblage is stable in a P-T field surrounded by the following reactions: lower-pressure limit by Chl + Phl + quartz (Qtz) = Phn + Tlc + H2O as proposed by previous workers; higher-pressure limit by glaucophane + Qtz = Tlc + Ab + H2O; and higher-temperature limit by Tlc + Phn + Ab = Phl + paragonite + Qtz + H2O. Thermodynamic calculation based on the database of Holland & Powell (1998), however, suggests that the Tlc,Phn stability field defined by these reactions is unrealistically limited around 580,600 °C at 11.6,12.0 (± 0.7) kbar. Schreinemakers' analysis in the KNMA-Fe3+ -SH system and the observed mineral assemblage predict that Chl + crossite = Tlc + Ab + Ht + H2O is a preferable Tlc-forming reaction in the intermediate-grade part of the Sanbagawa belt and that excess Ab + hematite narrows the stability field of the Tlc,Phn assemblage. [source] Reaction-induced nucleation and growth v. grain coarsening in contact metamorphic, impure carbonatesJOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2010A. BERGER Abstract The understanding of the evolution of microstructures in a metamorphic rock requires insights into the nucleation and growth history of individual grains, as well as the coarsening processes of the entire aggregate. These two processes are compared in impure carbonates from the contact metamorphic aureole of the Adamello pluton (N-Italy). As a function of increasing distance from the pluton contact, the investigated samples have peak metamorphic temperatures ranging from the stability field of diopside/tremolite down to diagenetic conditions. All samples consist of calcite as the dominant matrix phase, but additionally contain variable amounts of other minerals, the so-called second phases. These second phases are mostly silicate minerals and can be described in a KCMASHC system (K2O, CaO, MgO, Al2O3, SiO2, H2O, CO2), but with variable K/Mg ratios. The modelled and observed metamorphic evolution of these samples are combined with the quantification of the microstructures, i.e. mean grain sizes and crystal size distributions. Growth of the matrix phase and second phases strongly depends on each other owing to coupled grain coarsening. The matrix phase is controlled by the interparticle distances between the second phases, while the second phases need the matrix grain boundary network for mass transfer processes during both grain coarsening and mineral reactions. Interestingly, similar final mean grain sizes of primary second phase and second phases newly formed by nucleation are observed, although the latter formed later but at higher temperatures. Moreover, different kinetic processes, attributed to different driving forces for growth of the newly nucleated grains in comparison with coarsening processes of the pre-existing phases, must have been involved. Chemically induced driving forces of grain growth during reactions are orders of magnitudes larger compared to surface energy, allowing new reaction products subjected to fast growth rates to attain similar grain sizes as phases which underwent long-term grain coarsening. In contrast, observed variations in grain size of the same mineral in samples with a similar T,t history indicate that transport properties depend not only on the growth and coarsening kinetics of the second phases but also on the microstructure of the dominant matrix phase during coupled grain coarsening. Resulting microstructural phenomena such as overgrowth and therefore preservation of former stable minerals by the matrix phase may provide new constraints on the temporal variation of microstructures and provide a unique source for the interpretation of the evolution of metamorphic microstructures. [source] Using estimated thermodynamic properties to model accessory phases: the case of tourmalineJOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2007V. J. VAN HINSBERG Abstract Accessory phases and minor components in minerals are commonly ignored in thermodynamic modelling. Such an approach seems unwarranted, as accessory phases can represent a significant element reservoir and minor components can substantially change their host mineral's stability field. However, a lack of thermodynamic data prohibits assessment of these effects. In this contribution, the polyhedron method is used to estimate the thermodynamic properties of tourmaline, a common and widespread accessory phase, stable over a range of P,T,X conditions. The polyhedron method allows ,H, S, V, CP and Vm(T,P) properties to be estimated from a linear stoichiometric summation over the fractional properties of its polyhedron constituents. To allow for estimates of tourmaline, fractional thermodynamic properties for BIII and BIV polyhedra were derived. Mixing contributions to molar volume were evaluated and symmetrical mixing parameters derived for Al-Mg, Al-Fe and Al-Li interaction on tourmaline's Y-site and T-site Al-Si interaction. Evaluation of the estimated properties using experimental and natural equilibria between tourmaline and melts, minerals and hydrothermal fluids, shows that reliable semi-quantitative results are obtained. The boron contents in fluids coexisting with tourmaline are calculated to within an order of magnitude of measured content, and where anchor-points are available, agreement improves to within a factor of 2. Including tourmaline in petrogenetic modelling of metamorphic rocks indicates that its presence leads to disappearance of staurolite and garnet, among others, and modifies the XMg of coexisting phases, in line with observations on natural rocks. [source] Petrogenesis of lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, TurkeyJOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2006P. B. DAVIS Abstract The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre- to centimetre-scale eclogite pods occur in blueschist facies metabasalt, marble and quartz-rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P,T conditions: ,26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15,16 kbar, 480,500 °C (lawsonite-epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P,T conditions and paths. For example, in some lawsonite-bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P,T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P,T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P,T conditions were tectonically juxtaposed as kilometre-scale slices and associated high- P pods, although all shared the same exhumation path from ,9,11 kbar, 300,400 °C. Within the tectonic slices, alternating millimetre,centimetre-scale layers of eclogite and blueschist formed together at the same P,T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO2 or other chemical factors. [source] Controls on low-pressure anatexisJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2006C. C. GERBI Abstract Low-pressure anatexis, whereby rocks melt in place after passing through the andalusite stability field, develops under more restricted conditions than does low-pressure metamorphism. Our thermal modelling and review of published work indicate that the following mechanisms, operating alone, may induce anatexis in typical pelitic rocks without inducing wholesale melting in the lower crust: (i) magmatic advection by pervasive flow; (ii) crustal-scale detachment faulting; and (iii) the presence of a high heat-producing layer. Of these, only magmatic advection by pervasive flow and crustal-scale detachment faulting have been shown quantitatively to provide sufficient heat to cause widespread melting. Combinations of the above mechanisms with pluton-scale magmatic advection, shear heating, removal of the lithospheric mantle, or with each other provide additional means of developing suitable high temperatures at shallow crustal levels to generate low-pressure anatexis. [source] P,T modelling of the andalusite,kyanite,andalusite sequence and related assemblages in high-Al graphitic pelites.JOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2001Prograde, retrograde paths in a late kyanite belt in the Variscan Iberia Abstract The exceptional andalusite,kyanite,andalusite sequence occurs in Al-rich graphitic slates in a narrow pelite belt on the hangingwall of a ductile normal fault in NW Variscan Iberia. Early chiastolite is replaced by Ky,Ms,Pg aggregates, which are overgrown by pleochroic andalusite near granites intruded along the fault. Slates plot in AKFM above the chloritoid-chlorite tie-line. Their P,T grids are modelled with Thermocalc v2.7 and the 1998 databases in the NaKFMASH and KFMASH systems. The univariant reaction Ctd + And/Ky = St + Chl + Qtz + H2O ends at progressively lower pressure as F/FM increases and A/AFM decreases, shrinking the assemblage Cld,Ky,Chl, and opening a chlorite-free Cld,Ky trivariant field on the low temperature reaction side. This modelling matches the observed absence of chlorite in high F/FM rocks, which is restricted to low pressure in the andalusite stability field. The P,T path deduced from modelling shows a first prograde event in the andalusite field followed by retrogression into the kyanite field, most likely coupled with a slight pressure increase. The final prograde evolution into the andalusite field can be explained by two different prograde paths. Granite intrusion caused the first prograde part of the path with andalusite growth. The subsequent thermal relaxation, together with aH2O decrease, generated the retrograde andalusite,kyanite transformation, plus chlorite consumption and chloritoid growth. This transformation could have been related to folding in the beginning, and aided later by downthrowing due to normal faulting. Heat supplied by syntectonic granite intrusion explains the isobaric part of the path in the late stages of evolution, causing the prograde andalusite growth after the assemblage St,Ky,Chl. Near postectonic granites, a prograde path with pressure decrease originated the assemblage St,And,Chl. [source] Sulfur Isotope Study and Re-examination of Ore Mineral Assemblage of the Hol Kol and the Tul Mi Chung Skarn-type Copper,Gold Deposits of the Suan Mining District, Korean PeninsulaRESOURCE GEOLOGY, Issue 4 2000Akira Imai Abstract: Ore specimens collected by the late Professor Takeo Watanabe from the Hol Kol and the Tul Mi Chung deposits, Suan mining district, Korean peninsula, were examined. In addition, measurements of sulfur isotopic ratio of ores and preliminary fluid inclusion microthermometry were carried out. Ores from the New orebody of the Hol Kol deposit consist mainly of bornite, wittichenite and chalcopyrite presently, which exhibit lamellae intergrowth texture, associated with native bismuth and electrum. Bismuthian bornite solid solution is considered to be a principal initial phases, while native bismuth was nucleated as molten bismuth melt initially. The occurrence of cubanite, miharaite, carrollite, siegenite, hessite and geikielite are recognized from the New orebody. Ores from the Eastern orebody of the Hol Kol deposit consist chiefly of chalcopyrite, occasionally associated with trace amounts of pyrrhotite, pyrite, bismuthinite and rare tellurobismuthite, while an ore specimen from the Western orebody consists mainly of sphalerite associated with chalcopyrite, pyrite and galena. Ores from the Tul Mi Chung deposit consist mainly of chalcopyrite and pyrite, occasionally associated with magnetite, sphalerite, galena and rare molybdenite. Some portions of magnetite are revealed to be silician magnetite. Sulfur fugacity is supposed to be below the stability field of bismuthinite in the New orebody. A reducing condition is suggested by the occurrence of geikielite without Fe3+ content. The sulfur and oxygen fugacities for the Eastern and Western orebodies of the Hol Kol deposit and for the Tul Mi Chung deposit were higher than the New orebody of the Hol Kol deposit. On the other hand, the Suan granite (porphyritic granodiorite) and the Chil Sing Dai granite (biotite granite porphyry) from the Hol Kol area can be classified as weakly magnetic magnetite-series. Polyphase fluid inclusions are observed in gangue diopside associated with Cu ore of two specimens. The dissolution temperatures of daughter crystals are 394±26°C and 442±45°C, while the disappearing temperatures of vapor bubble were 475±25°C and > 500°C. Highly saline fluids were responsible for the mineralization at the Hol Kol deposit. The ,34S values of ore sulfides of the Hol Kol and the Tul Mi Chung deposit range from +11. 5% to +16. 1%, having anomalous lower values mainly from the Tul Mi Chung deposit. Such anomalous lower 634S values can be caused by isotopic fractionation against oxidized sulfur species. The ,34S value of bulk sulfur in the ore solutions responsible for the Hol Kol and the Tul Mi Chung deposit is estimated to be +13.5±2.5,. [source] High Pressure Response of Rutile Polymorphs and Its Significance for Indicating the Subduction Depth of Continental CrustACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2008MENG Dawei Abstract: ,-PbO2 -type TiO2 (TiO2 -II) is an important index mineral for ultrahigh-pressure metamorphism. After the discovery of a natural high-pressure phase of titanium oxide with ,-PbO2 -structure in omphacite from coesite-bearing eclogite at Shima in the Dabie Mountains, China, a nano-scale (<2 nm) ,-PbO2 -type TiO2 has been identified through electron diffraction and high-resolution transmission electron microscopy in coesite-bearing jadeite quartzite at Shuanghe in the Dabie Mountains. The crystal structure is orthorhombic with lattice parameters a = 4.58times10,1 nm, b = 5.42times10,1 nm, c = 4.96times10,1 nm and space group Pbcn. The analysis results reveal that rutile {011}R twin interface is a basic structural unit of ,-PbO2 -type TiO2. Nucleation of ,-PbO2 -type TiO2 lamellae is caused by the displacement of one half of the titanium cations within the {011}R twin slab. This displacement reduces the Ti-O-Ti distance and is favored by high pressure. The identification of ,-PbO2 -type TiO2 in coesite-bearing jadeite quartzite from Shuanghe, Dabie Mountains, provides a new and powerful evidence of ultrahigh-pressure metamorphism at 4,7 GPa, 850°C-900°C, and implies a burial of continental crustal rocks to 130,200 kilometers depth or deeper. The ,-PbO2 -type TiO2 may be a useful indicator of the pressure and temperature in the diamond stability field. [source] P,T,X controls on phase stability and composition in LTMP metabasite rocks , a thermodynamic evaluationJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2010G. PHILLIPS Abstract The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H2O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe2O3 bulk contents on the stability of these phases is illustrated using P,T and P,X phase diagrams. This approach allowed P,T conditions of ,4.0 kbar and ,260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe2O3 contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous,Triassic New England Orogen. P,T and P,X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe2O3 -saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO-rich (10.30 wt%) rocks. With intermediate Fe2O3 (,3.50 wt%) and CaO (,8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P,T conditions. For Fe2O3 -saturated rocks, compositional isopleths for sodic amphibole (Al3+ and Fe3+ on the M2 site), epidote (Fe3+/Fe3+ + Al3+) and chlorite (Fe2+/Fe2+ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al3+ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher- P stability fields where albite is absent, Fe2+ -Mg ratios in chlorite may be dependent on pressure. The Fe3+/Al and Fe2+/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages. [source] False metamorphic events inferred from misinterpretation of microstructural evidence and P,T dataJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2008R. H. VERNON Abstract Geometrical relationships involving inclusions and partial inclusions in metamorphic microstructures can be inadequate for inferring an order of crystallization and hence a metamorphic reaction. Unique spatial and/or chemical relationships need to be defined for mineral inclusions, in the context of a reference paragenesis, commonly the matrix assemblage. Corona microstructures are reliable indicators of metamorphic reactions, but require considerable care when used to infer reactions or changes in P,T conditions, owing to kinetic problems, as well as to changes in the effective reaction volume during changes across relatively broad P,T stability fields of assemblages. Mineral equilibria models, most commonly implemented through P,T pseudosections, may allow the order in which different minerals become stable along a given P,T path to be inferred. However, the order in which two minerals become stable may be different from the order in which two grains of these minerals nucleate. Furthermore, such diagrams cannot make predictions about which minerals will form porphyroblasts and which minerals will form inclusions in porphyroblasts. An evaluation of three examples from the Australian Proterozoic shows that modelling, in combination with inclusion-host relationships, is a powerful tool for understanding the metamorphic evolution of a rock, but involves considerable uncertainty. [source] The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O,FeO,MgO,Al2O3,SiO2,H2O,TiO2,Fe2O3JOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2000White Mineral equilibria calculations in the system K2O,FeO,MgO,Al2O3,SiO2,H2O,TiO2,Fe2O3 (KFMASHTO) using thermocalc and its internally consistent thermodynamic dataset constrain the effect of TiO2 and Fe2O3 on greenschist and amphibolite facies mineral equilibria in metapelites. The end-member data and activity,composition relationships for biotite and chloritoid, calibrated with natural rock data, and activity,composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO2 and Fe2O3 on the stability of these minerals. Thermodynamic models for ilmenite,hematite and magnetite,ulvospinel solid solutions accounting for order,disorder in these phases allow the distribution of TiO2 and Fe2O3 between oxide minerals and silicate minerals to be calculated. The calculations indicate that small to moderate amounts of TiO2 and Fe2O3 in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in metapelites at greenschist to amphibolite facies, compared with those calculated in KFMASH. The addition of large amounts of TiO2 to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; in contrast, rocks rich in Fe2O3 develop a markedly different metamorphic succession from that of common Barrovian sequences. In particular, Fe2O3 -rich metapelites show a marked reduction in the stability fields of staurolite and garnet to higher pressures, in comparison to those predicted by KFMASH grids. [source] | ||||||||||