Sample records for paragonite

  1. The paragonite-muscovite solvus: II. Numerical geothermometers for natural, quasibinary paragonite-muscovite pairs

    NASA Astrophysics Data System (ADS)

    Blencoe, James G.; Guidotti, Charles V.; Sassi, Francesco P.


    Three parametric (nonthermodynamic) equations have been developed to calculate final equilibration temperatures for natural, quasibinary paragonite-muscovite (Pg-Ms) pairs (mole fraction margarite in Pg < 0.05, Si/formula unit ≤ 6.2 and/or Σ(Mg + Fe 2+ + Fe 3+) ≤ 0.35 for coexistent Ms). The first two equations are paragonite-based and muscovite-based, because they yield equilibration temperatures that depend on the Na-K compositions of K-saturated Pg and Na-saturated Ms, respectively. The third equation is closure-based, because it yields equilibration temperatures that vary with the width of the gap between the Na-K compositions of coexisting Pg and Ms (i.e., the degree of solvus closure). Equilibration temperatures can be estimated, using just one of the geothermometers, or by averaging the temperatures obtained from any combination of the three equations. Comparison of the three calculated temperatures yields information on the mutual consistencies of the Na-K compositions of coexisting Pg and Ms with respect to their utility in Pg-Ms solvus thermometry. Geothermometric inconsistencies in the Na-K compositions of quasibinary Pg-Ms pairs can be caused by (1) inaccuracies in determinations of the Na-K compositions of the coexisting micas, (2) H 3O + → K + and vacancy → K + substitution in muscovite and/or paragonite (3) crystallization and preservation of quasibinary Pg-Ms pairs with metastable Na-K compositions. Due to the effects of pressure on the stability relations of Pg-Ms pairs and the K/Na ratios of Nasaturated muscovites, practical applications of Pg-Ms solvus thermometry are restricted to quasibinary Pg-Ms pairs that equilibrated at pressures between approximately 2 and 8 kbar. Within this pressure range, the utility of Pg-Ms solvus thermometry is limited thermally to approximately 300 ≤ T ≤ 700° C by (1) the decomposition of K-saturated paragonite at temperatures between approximately 580 and 700°C, and (2) steepening of the solvus limbs

  2. Paragonite in marbles from the Tauern Window, Austria: Compositional and thermobaric controls

    NASA Astrophysics Data System (ADS)

    Droop, G. T. R.


    Paragonite coexists with phengite, chlorite, zoisite, tremolite and quartz in dolomitic marbles from near Döllach in the Sonnblick area of the SE Tauern Window, Austria, prompting an investigation into the petrogenetic significance of paragonite in carbonate rocks. Thermodynamic calculations on pure-end-member reactions indicate that the assemblages paragonite + CaCO3 + quartz and paragonite + dolomite + quartz are stable over wide ranges of X(CO2) and have P-T stability fields almost as wide as that of paragonite + quartz. P-T pseudosection calculations spanning 0.35-1.4 GPa and 400-590 °C in the model system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-TiO2-H2O-CO2 yield wide fields for paragonite-bearing assemblages in the Döllach paragonite-marbles at low X(CO2). The bulk compositions of these rocks are characterised by high values of molar Na/(Na + K) and Al/(Na + K) and these parameters are influential in stabilising paragonite-bearing assemblages in marbles. A large proportion of published limestone and dolostone whole-rock compositions would be capable of supporting paragonite at P-T conditions within the greenschist, blueschist or epidote-amphibolite facies. Paragonite is probably much more common in low- and medium-grade marbles than the rarity of reports of its occurrence suggest. Apart from zoisite, the hydrous silicates in the Döllach marbles are characterised by modest fluorine contents, with XF values of coexisting minerals decreasing in the order Tr > Phe > Pa ≈ Chl. Calcite-dolomite thermometry and pseudosection calculations indicate peak-metamorphic conditions for the Döllach marbles of T = 510 ± 20 °C, P > 0.77 GPa and X(CO2) < 0.065. The P-T data agree with previous estimates for the area.

  3. Genesis and solvus relations of submicroscopically intergrown paragonite and phengite in a blueschist from northern California

    NASA Astrophysics Data System (ADS)

    Shau, Yen-Hong; Feather, Melanie E.; Essene, Eric J.; Peacor, Donald R.


    Electron microbeam techniques have been used to examine submicroscopically intergrown paragonite, phengite and chlorite from the South Fork Mountain Schist of the Franciscan Terrane of northern California, which was subjected to blueschist facies metamorphism. The sample also contains quartz, albite, lawsonite, and rutile. The subassemblage albite-lawsonite-rutile requires metamorphic conditions on the low-temperature side of the equilibrium albite+lawsonite+rutile=paragonite+sphene+quartz+H2O (T<200° C and P<7.4 kbars based on thermodynamic data of Holland and Powell 1990). The white micas appear to be optically homogeneous, but back-scattered electron images can distinguish two different micas by their slight difference in contrast. Electron microprobe analyses (EMPA) of micas show Na/(Na+K) ranging from 0.2 to 0.8. The two micas are resolved by transmission electron microscopy (TEM) as packets of phengite and paragonite that range from 20 to several hundred nm in thickness. The compositions, determined by analytical electron microscopy (AEM), constrain the limbs of the phengite-paragonite solvus to values of Na/(Na+K)=<0.02 and 0.97, representing less mutual solid solution than ever reported by EMPA. The textural relations imply that the sheet silicates were derived from reactions between fluids and detrital clays and that they are in an intermediate stage of textural development. We caution that microprobe analyses of apparently homogeneous sheet silicates may yield erroneous data and lead to faulty conclusions using phengite barometry and paragonite-muscovite thermometry, especially in fine-grained rocks that formed at relatively low temperatures.

  4. Solubility of Albite + Paragonite +/- Quartz in H2O at 1 GPa, 580° C: Implications for Metamorphic Fluids

    NASA Astrophysics Data System (ADS)

    Antignano, A.; Manning, C.


    One of the most common mineral assemblages in crustal metamorphism is feldspar + quartz; however, little is known about the solubility of this assemblage in metamorphic fluids. We measured the solubility of albite and albite+quartz in H2O at 580° C and 1.0 GPa using a piston cylinder apparatus. Experiments were conducted using Amelia albite in NaCl-graphite assemblies. Experiments were conducted using a double capsule arrangement. Inner capsules consisting of perforated 1.6 mm OD Pt capsules containing a single albite crystal were load in 3.5mm OD outer capsules containing ultra pure H2O +/- quartz. Solubility was determined by the weight loss of single albite and quartz grains after 8 hr runs. Time series experiments on this system show no variation in fluid composition or solubility after 4hrs. Albite exhibited incongruent dissolution, yielding paragonite as a husk that mantles the albite grain. The composition of the fluid was determined by mass balance as determined by the weights of albite, quartz, and paragonite. In the albite-only experiments, the concentration of total dissolved solids (TDS) were 0.628 molal, with Na, Al and Si in the fluid of 0.137, 0.081, and 0.41 molal, respectively, and Na/Al of 1.691. In the presence of quartz, there is an increase in the overall TDS to 0.777 molal. Albite-quartz experiments produce an increase in Si concentration to 0.628 molal, with lower Na and Al concentrations of 0.098 and 0.051 molal, respectively, and Na/Al of 1.922. The aqueous Si concentrations in the albite-only experiments are higher than those in fluid equilibrated with quartz (Manning, 1994, GCA, 58, 4831) at the same conditions. This is consistent with the data of Anderson and Burnham (1983, Am. J. Sci., 283-A, 283) on albite. In the quartz-present experiments results show still higher Si concentration in the fluid phase. Our results demonstrate enhanced solubility of silica in the presence of albite and albite+quartz, relative to quartz alone

  5. Albite-Paragonite-Quartz Solubility: Experimental Constraints on the Thermodynamic Behavior of Si, Al, and Na in Aqueous Fluids at 0.5-1.0 GPa from 350-500C

    NASA Astrophysics Data System (ADS)

    Lin, H. A.; Manning, C. E.


    The solubility silica, sodium, and aluminum was measured under subduction zone conditions through experiments in pure H2O and S-bearing solutions in equilibrium with albite-paragonite-quartz at 350-500C, 0.5-1.0 GPa in a piston-cylinder apparatus (sulfur was included for another study, and has no impact on measured solubilities). Starting materials were natural Amelia albite, Brazillian quartz, and microcrystalline paragonite synthesized from finely ground Amelia albite, Al2O3 and H2O at 0.5GPa and 400C for 138 hours. Solutions were extracted in 5% HNO3 and analyzed by ICP-AES. Silica is the most soluble element ( 30 to 217 milli-molal), followed by sodium (9 to 108 milli-molal), and then aluminum ( 3-40 milli-molal). The results agree well with experimental trends of Woodland and Walther (1987, GCA, v. 51, p.365) and with the predicted values at 0.5 GPa calculated using SUPCRT92. Na-Al complexing were not required by the measured concentrations. Comparison with the results of Manning (1994, GCA v.58, p. 4831) and Woodland and Walther (1987) indicate that the solubilities of Si, Na, and Al increase sharply with increasing pressures and more moderately at higher pressures. These results provide the first measurements of alkali-silica-aluminum solubilities under high pressure conditions. The solubilities measured demonstrate that sodium, and by inference other alkalis, are very soluble at high pressures. Aluminum is also extremely soluble. Pressure enhances the solubilities of major rock-forming elements in pure H2O. This indicates that metamorphic and subduction zone fluids can profoundly metasomatize the rocks they interact with as they migrate toward the surface.

  6. L'aspidolite fluorée : rôle des évaporites dans la genèse du rubis des marbres de Nangimali (Azad-Kashmir, Pakistan)

    NASA Astrophysics Data System (ADS)

    Garnier, Virginie; Ohnenstetter, Daniel; Giuliani, Gaston


    Ruby-bearing marbles from Nangimali, in the Azad-Kashmir, Pakistan, contain, besides phengite, different mica intergrowths: paragonite, phlogopite and aspidolite (sodium phlogopite). Both phlogopites, intimately linked and coexisting with paragonite, are fluorine rich, contrary to phengite and paragonite. F-enriched aspidolite is described for the first time. Phengite is either associated with phlogopite or could be isolated. The presence of aspidolite in the ruby-bearing marbles, together with other arguments such as salt solid inclusions and presence of anhydrite, suggest the implication of evaporites in the genesis of gem corundums. To cite this article: V. Garnier et al., C. R. Geoscience 336 (2004).

  7. Mineralogy, paragenesis and textures associated with metasomatic- hydrothermal processes, Qatruyeh area, Sanandaj- Sirjan zone, SW Iran

    NASA Astrophysics Data System (ADS)

    Asadi, S.; Rajabzadeh, M. A.


    The Qatruyeh area is located at about 40 Km northeastern of Neyriz region, in the eastern edge of the high P- Low T metamorphic Zagros orogenic belt. The studied area principally includes outcrops of green schist facies metamorphic rocks that are thrusted over the Neotethyan ophiolites. Hydrothermal activities occurred episodically in the Mesozoic era . The textural relationships, mineral assemblages and X- Ray diffractions have recognized two different stages of hydrothermal alteration during mineralization. The Mineralization was largely controlled by striking faults and host rock layers. Three different types of magnetite are distinguished in microscopic investigation. The First is euhedral to subhedral, partly replacement by martite with exsolved ilmenites. The Second reveals myrmekite like textures and the last type includes layered magnetite. All of the types are related to replacement textures such as psedomorphism, widening of a fracture filling, irregulare or vermicular intergrowths, islands of unreplaced host rock, cusp or caries, nonmatching walls or borders of a fracture and rims of one mineral penetrating another along its crystallographic direction. X- Ray analyses indicate oxide minerals (Magnetite, Hematite, Maghemite, Specularite, Goethite, Limonite and Ilmenite), Silicate minerals (Tourmaline-shorl and dravite-, Epidote, Chlorite, Actinolite, Titanite, Paragonite, Talc, Muscovite and Quartz), Carbonate minerals (Siderite and Malachite) and Sulfide minerals (Pyrite and Chalcopyrite-minor-) as major phases. The mineral paragenesis and textures show two different stages of metasomatic- hydrothermal alteration. The first stage alteration (Sodic- Calsic) accompanying with mineral paragenesis of Magnetite+ Tourmaline+ Titanite+ Paragonite and the second stage of alteration (Solfidation- Oxidation) follows with Magnetite+ Hematite+ Quartz

  8. Zoning and recrystallization of phengitic micas: implications for metamorphic equilibration

    NASA Astrophysics Data System (ADS)

    Dempster, T. J.


    White micas (phengites) in the metasediments of the Scottish Dalradian display a large range of compositions within single samples. The variations in the composition of these phengites are strongly controlled by their structural age, with early fabrics containing a paragonite-poor, celadonite-rich phengite whereas in later fabrics the micas are generally paragonite-rich and celadonite-poor. Retrograde phengite growth, identified using back scattered electron imaging, occurs as celadonite-rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica-plagioclase grain boundaries. The presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient. It is proposed that diffusion-controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible. This closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism. Many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation/recrystallization controlled equilibria.

  9. Wall-rock metasomatism of carbonaceous terrigenous rocks in the Lena gold district

    NASA Astrophysics Data System (ADS)

    Rusinov, V. L.; Rusinova, O. V.; Kryazhev, S. G.; Shchegol'Kov, Yu. V.; Alysheva, E. I.; Borisovsky, S. E.


    The Lena gold district is situated in the fold-and-shear belt of the southern framework of the Siberian Platform. The gold deposits are hosted in the Riphean-Vendian Khomolkho and Aunakit formations, revealing the strict control of ore mineralization by folding and shearing. The microstructure of metasomatically altered ore-bearing carbonaceous sedimentary rocks at the Sukhoi Log, Golets Vysochaishy, and Verninsky deposits (the latter includes the Pervenets vein zone) testifies to parallelism in the development of shearing, foliation, and ore-forming metasomatism. The local pressure gradients are marked by removal of silica from pressured zones into opened cleavage fractures and pockets. Two metasomatic stages are recognized: (1) early sodic metasomatism, which is characterized by the assemblage of magnesian siderite and paragonite, and (2) late potassic metasomatism, with formation of muscovite in association with sideroplesite and ankerite. The rocks altered at the early stage are distinguished by elevated Ni, Cr, and probably PGE contents. The second stage, close in age to the emplacement of Hercynian granitic plutons, was accompanied by the gain of chalcophile metals and deposition of the bulk of gold. In mineral composition, the metasomatic rocks are close to beresites, but the alteration differed in somewhat elevated alkalinity, so that microveinlets of albite and potassium feldspar occur in the ore zone together with muscovite. The ratio of modal muscovite to paragonite contents in orebodies is substantially higher than in the surrounding metasomatized rocks. This ratio directly depends on the degree of rock permeability and the intensity of the flow of ore-forming solutions. Carbonaceous matter (CM) in the ore zone underwent reworking and redeposition. CM is graphitized to a lesser extent than in the rocks affected by regional metamorphism. The spatial distribution of CM containing nitro and amino groups indicates more oxidizing conditions in the zone of

  10. Origin of peak and retrograde assemblages during Grenvillian orogeny from garnet-staurolite bearing mica schist of Bhilwara Supergroup, NW India: constraints from pseudosection modelling

    NASA Astrophysics Data System (ADS)

    Prakash, Abhishek; Saha, Lopamudra; Sarkar, Saheli


    Fractionation of components due to formation of garnet porphyroblasts during prograde metamorphism, have been constrained from pseudosection analyses. Such fractionation process leads to changes in the effective bulk composition within the rock, which can be modelled with well-preserved growth zonation patterns in garnet porphyroblasts. On the contrary, textures and mineralogy in metamorphic rocks can be far more complex with different textural domains within a single rock preserving assemblages formed along different segments of the P-T paths or during different metamorphic events. Examples of such textures include pseudomorphs, reaction rims or coronae, symplectites formed by breakdown of both cores and rims of porphyroblasts. Apart from pressure and temperature, availability of fluids during metamorphic reactions plays important roles in defining mineral assemblages and textures. In this study we have constrained formation of garnet porphyroblasts and paragonite-albite-sillimanite-quartz-staurolite bearing domains within the mica schist from the Rajpura-Dariba sequence of the Bhilwara Supergroup in NW India. The mica schist is inter-layered with calc-silicates and quartzite and together the units form a NE-SW trending Grenvillian orogenic belt in southern part of Bhilwara Supergroup sequence. Within the mica schist, three distinct textural domains have been observed: (i) muscovite-biotite-quartz-feldspar bearing matrix foliation, (ii) garnet porphyroblasts within the matrix foliation, (iii) staurolite-paragonite-albite-staurolite-sillimanite-quartz bearing domains. Paragonite, albite and sillimanite occur exclusively in the pseudomorph domains. Garnet porphyroblasts show variation in compositions from cores (Spessartine0.14 Grossular0.10 Pyrope0.12 Almandine0.72) to rims (Spessartine0.09Grossular0.15Pyrope0.12Almandine0.75). The average XMg contents of staurolite and matrix biotite are 0.21 and 0.57 respectively. Pseudosections have been constructed from the

  11. Pressure-temperature and deformational evolution of high-pressure metapelites from Variscan NE Sardinia, Italy

    NASA Astrophysics Data System (ADS)

    Cruciani, Gabriele; Franceschelli, Marcello; Massonne, Hans-Joachim; Carosi, Rodolfo; Montomoli, Chiara


    Chloritoid schists crop out north of the village of Lula in the Inner Zone of the Variscan chain of Sardinia consisting of a variety of metamorphic rocks. The S1 and S2 foliations in these schists are defined by the orientation of muscovite, paragonite, and chloritoid. Chlorite is an additional mineral oriented along S2. Late margarite grew at the expense of chloritoid included in garnet. Garnet porphyroblasts, enclosing quartz, chloritoid, rutile, Fe-oxide, apatite and paragonite, show a progressive decrease of spessartine component from 17 to 7 mol% and an increase of pyrope component from 4 to 6 mol% from core to rim. The grossular content firstly increases from the inner (Grs~ 21) to the outer core (Grs~ 27) and then decreases towards the outermost rim (Grs~ 15). Compositional mapping of white mica also revealed zoning and a wide range in Si content (from 6.0 to 6.6 pfu). The highest Si content is related to the highest Fe and Mg contents and the lowest Na content. P-T pseudosections were calculated in the system Na2O-K2O-CaO-FeO-MnO-MgO-Al2O3-TiO2-SiO2-H2O for compositions of chloritoid schists. The highest Si contents of K-white mica and the garnet core composition suggest pressures close to 1.8 GPa and temperatures of 460-500 °C. The garnet rim composition and low Si contents in K-white mica are compatible with re-equilibration at 540-570 °C and 0.7-1.0 GPa. These results suggest an HP-metamorphic imprint during the D1 deformation phase which occurred before the Barrovian amphibolite-facies metamorphism of NE Sardinia. D2 folding and shearing occurred at decreasing P-T conditions during the exhumation of the metamorphic complex.

  12. Trace elements in metapelitic schists from Alpe Sponda (Central Alps, Switzerland)

    NASA Astrophysics Data System (ADS)

    Redler, Charlotte; Irouschek, Alfred; Jeffries, Teresa; Gieré, Reto


    Lenses of kyanite-cordierite-garnet-bearing paragonite-biotite schist were found in paragonite-free metapelitic rocks at Pizzo Forno, near Alpe Sponda (Simano Nappe, Central Alps). These lenses are mineralogically distinct, as they lack both quartz and staurolite, but they contain large amounts of tourmaline (up to 20-25 vol.%). Tourmaline shows strong colour zoning, which in combination with the major and trace element data indicates at least two stages of crystal growth during prograde metamorphism. Mineral equilibrium modelling based on the bulk-rock compositions yielded peak metamorphic conditions of amphibolite-facies grade (T = 590-660 °C and P = 6.5-7.8 kbar), which is consistent with the P-T conditions estimated previously for this part of the Central Alps. The light elements lithium and boron are strongly enriched in the Alpe Sponda samples, which therefore show unusual bulk-rock concentrations. Lithium is mainly distributed to cordierite (450 ppm) and to a lower extent to muscovite (250 ppm), biotite (180 ppm) and paragonite; all other minerals show only minor amounts of lithium. As commonly expected, boron is partitioned into tourmaline and to a much lesser amount into muscovite, whereas all other minerals have low boron contents. In addition, cordierite has a relatively high Be content, underlining the hypotheses of the presence of Na-Be cordierite. The analysis of rare earth elements (REE) shows that all samples are enriched in low REE (LREE; La-Eu) relative to the heavy REE (HREE: Gd-Lu), and have a small negative Ce anomaly, as well as a pronounced negative Eu anomaly, which can be illustrated in a chondrite-normalised diagram. Furthermore, all minerals that do not show a distinct positive Eu anomaly (i.e. garnet, biotite, muscovite, cordierite, kyanite, chlorite and rutile) show a marked negative Ce anomaly. In addition, plagioclase and tourmaline exhibit a pronounced positive Eu anomaly but no Ce anomaly. Especially in the case of tourmaline the

  13. Petrology of HP/LT metapelitic rocks of an accretionary wedge, the Central Pontides, Turkey: evidence for tectonic stacking and syn-subduction exhumation

    NASA Astrophysics Data System (ADS)

    Aygül, Mesut; Okay, Aral; Oberhänsli, Roland


    Cretaceous HP/LT chloritoid-bearing metapelitic rocks crop out widely in the central part of the Pontides, northern Turkey. They are associated with eclogite to blueschist-facies metabasites and represent deep levels of an underplated subduction-accretion complex, which is tectonically overlain by low-grade HP phyllite/metasandstone intercalation along an extensional shear zone. Close to the shear zone, the footwall micaschists consist of quartz, phengite, paragonite, chlorite, rutile with syn-kinematic albite porphyroblast formed by pervasive shearing during exhumation. This type of micaschists is tourmaline-bearing and their retrograde nature suggests high-fluid flux along shear zones. Peak metamorphic assemblages are partly preserved in the chloritoid-micaschist farther away from the shear zone. Three peak metamorphic assemblages are identified and their PT conditions are constrained by pseudosections produced by Theriak-Domino and by Raman spectra of carbonaceous material: 1) garnet-chloritoid-glaucophane with lawsonite pseudomorphs (P: 17.5 ± 1 Kbar, T: 390-450 °C) 2) chloritoid with glaucophane pseudomorphs (P: 16-18 Kbar, T: 475 ± 40 °C) and 3) relatively high-Mg chloritoid (17%) with jadeite pseudomorphs (22-25 Kbar; T: 440 ± 30 °C) in addition to phengite, paragonite, quartz, chlorite, rutile. The last mineral assemblage is interpreted as transformation of the chloritoid + glaucophane assemblage to chloritoid + jadeite paragenesis with increasing pressure. Absence of tourmaline suggests that the chloritoid-micaschist did not interact with B-rich fluids during zero strain exhumation. Peak metamorphic assemblages and PT estimates suggest tectonic stacking within wedge with different depths of burial. 40Ar/39Ar phengite age of a pervasively sheared footwall micaschist is constrained to 100.6 ± 1.3 Ma and that of a chloritoid-micaschist is constrained to 91.8 ± 1.8 Ma suggesting exhumation during on-going subduction. Coupling and exhumation of the

  14. White micas with mixed interlayer occupancy: a possible cause of pitfalls in applying illite Kübler index for the determination of metamorphic grade

    NASA Astrophysics Data System (ADS)

    Árkai, P.; Livi, K. T.; Frey, M.; Brukner-Wein, A.; Sajgó, Cs.


    Integrated X-ray powder diffraction (XRPD) modal composition, illite Kübler index and chlorite "crystallinity" determinations and vitrinite reflectance measurements were carried out on marly slates from selected profiles of the Helvetic domain of the Central Alps, Switzerland. In some of the localities studied illite Kübler index values were anomalously high, indicating only diagenetic conditions, while in these same samples chlorite "crystallinity" and vitrinite reflectance showed anchi- and epizonal metamorphic grades. XRPD observations indicated subordinate amounts of swelling (smectitic) interstratifications in white mica. In addition to the dominant white K-mica, traces of paragonitic and tobelitic impurities could also be detected using higher-order basal reflections. The amount of NH_4^+ fixed in inorganic phases could be estimated on the basis of: (1) the organic maturity assumed from vitrinite reflectance, (2) probable partitioning of N and H between organic and inorganic phases, and (3) results of elemental (C, H, N, S) analyses carried out on the <2μm fraction decarbonated and oxidized samples. Small, but consistent absorption bands between 1400 and 1440 cm-1 in the FTIR spectra unequivocally proved the presence of NH_4^+ in the rocks studied. Electron energy loss spectroscopy revealed small amounts of N in the mica structures, confirming that NH_4^+ is fixed in the interlayer site position. X-ray maps generated by scanning transmission electron microscope energy dispersive spectroscopy exhibited heterogeneities in the interlayer cation occupancies. Although K was always dominant, local irregular domain-like enrichments of Na could be seen, but without strong segregation into paragonite. Thus, instead of mixed-layering with homogeneous interlayer site compositions, a dioctahedral mica structure with irregularly varying interlayer occupancy, combined also with subordinate amounts of swelling mixed-layers, seems to be a reliable model to describe the

  15. Barium-rich fluids and melts in a subduction environment (La Corea and Sierra del Convento mélanges, eastern Cuba)

    NASA Astrophysics Data System (ADS)

    Blanco-Quintero, Idael Francisco; Lázaro, C.; García-Casco, A.; Proenza, J. A.; Rojas-Agramonte, Y.


    Whole-rock compositions of muscovite-bearing amphibolite, trondhjemite, pegmatite and quartz-muscovite rocks from Sierra del Convento and La Corea mélanges (eastern Cuba), as well as mineralogy, record complex circulation of Ba-rich fluids and melts in the subduction environment. Partial melting of fluid-fluxed, MORB-derived amphibolite produced trondhjemite magmas that crystallized at depth, in some cases evolving into pegmatites. Qtz-Ms rocks probably crystallized from primary fluids derived from subducted sediments. All these rocks have elevated concentrations of large-ion lithophile elements, especially Ba (up to 11,810 ppm), presumably released from slab sediments by fluids and/or melts. Fluid-rock interaction produced crystallization of phengite in parental amphibolites. The phengite crystallized in all types of rocks is rich in Ba, with concentric zoning, characterized by Ba-rich cores and Ba-poor rims, indicating a compatible behaviour of Ba in the studied systems. Zoning in phengite is governed primarily by the celadonite (tschermak) exchange vector ((Mg,Fe)Si-(Al)-2), with more moderate contributions of the celsian (BaAl-(KSi)-1) and paragonite (NaK-1) exchange vectors. Late remobilization of Ba at relatively low temperature formed retrograde celsian. The compatible behaviour of Ba in the studied rocks strengthens the importance of the stability of phengite for the transfer of LIL elements from the subduction to the volcanic arc environments.

  16. Aluminium phosphate and phosphate-sulphate minerals in kyanite schists of the Ichetuyskoye area, West Transbaikalia, Russia: crystal chemistry and evolution

    NASA Astrophysics Data System (ADS)

    Izbrodin, Ivan A.; Ripp, German S.; Doroshkevich, Anna G.


    Aluminium phosphate and aluminium phosphate-sulphate (APS) minerals occur as disseminated crystals and fine-grained aggregates in kyanite schists near Ichetuyskoye, in the Dzhida basin (West Transbaikal region, Russia). Petrographical, mineralogical and geochemical data suggest a metamorphic evolution through prograde and retrograde stages. Lazulite-scorzalite and trolleite in association with kyanite, quartz, muscovite, paragonite, topaz, rutile, magnetite and hematite formed during the prograde stage. More than 25 minerals identified in schists are genetically related to the retrograde stage. Some phosphates and all of the APS minerals belong to the alunite, beudantite and plumbogummite groups. Electron-microprobe data reveal the presence of wide compositional variations and complex solid-solution series among the members. The main crystal-chemical variations of the APS solid-solution series in the alunite supergroup concern the relative proportions of svanbergite, woodhouseite, goyazite, crandallite, florencite-(Ce) and natroalunite. Chemical analyses of APS minerals indicate extremely high amounts of LREE, Sr, Ba, Ca and Na. In some cases, APS minerals have compositions of Ba and Ca,Ba phosphates- sulphates.

  17. Garnet cannibalism provides clues to extensive hydration of lower crustal fragments in a subduction channel (Sesia Zone, Northwestern Alps)

    NASA Astrophysics Data System (ADS)

    Giuntoli, Francesco; Lanari, Pierre; Engi, Martin


    The extent to which granulites are transformed to eclogites is thought to impose critical limits on the subduction of continental lower crust. Although it is seldom possible to document such densification processes in detail, the transformation is believed to depend on fluid access and deformation. Remarkably complex garnet porphyroblasts are widespread in eclogite facies micaschists in central parts of the Sesia Zone (Western Italian Alps). They occur in polydeformed samples in assemblages involving phengite+quartz+rutile ±paragonite, Na-amphibole, Na-pyroxene, chloritoid. Detailed study of textural and compositional types reveals a rich inventory of growth and partial resorption zones in garnet. These reflect several stages of the polycyclic metamorphic evolution. A most critical observation is that the relict garnet cores indicate growth at 900 °C and 0.9 GPa. This part of the Eclogitic Micaschist Complex thus derived from granulite facies metapelites of Permian age. These dry rocks must have been extensively hydrated during Cretaceous subduction, and garnet records the conditions of these processes. Garnet from micaschist containing rutile, epidote, paragonite and phengite were investigated in detail. Two types of garnet crystals are found in many thin sections: mm-size porphyroclasts and smaller atoll garnets, some 100 µm in diameter. X-ray maps of the porphyroclasts show complex zoning in garnet: a late Paleozoic HT-LP porphyroclastic core is overgrown by several layers of HP-LT Alpine garnet, these show evidence of growth at the expense of earlier garnet generations. Textures indicate 1-2 stages of resorption, with garnet cores that were fractured and then sealed by garnet veins, rimmed by multiple Alpine overgrowth rims with lobate edges. Garnet rim 1 forms peninsula and embayment structures at the expense of the core. Rim 2 surrounds rim 1, both internally and externally, and seems to have grown mainly at the expense of the core. Rim 3 grew mainly at

  18. Paragenesis and chemical characteristics of the celsian-hyalophane-K-feldspar series and associated Ba-Cr micas in barite-bearing strata of the Mesoarchaean Ghattihosahalli Belt, Western Dharwar Craton, South India

    NASA Astrophysics Data System (ADS)

    Raith, Michael M.; Devaraju, Tadasore C.; Spiering, Beate


    The upper greenschist - lower amphibolite facies, argillaceous to chemical-exhalative metasedimentary sequence of the Mesoarchaean Ghattihosahalli Schist Belt (GHSB), southern India, has been examined with a special focus on the paragenesis and solid solution characteristics of barian feldspars and associated dioctahedral Ba-Cr-bearing micas. Barian feldspars occur as untwinned porphyroblasts in a recrystallized finely banded matrix of barite, quartz and minor white mica. Idioblastic celsian (Cls98-76Or2-20Ab1-8) and hyalophane (Cls55-39Or35-51Ab10) predate the greenschist-facies foliation, whereas xenoblastic hyalophane (Cls44-35Or45-59Ab8-17) and mantles on celsian (Cls45-35Or42-60Ab13-5) as well as xenoblastic barian K-feldspar (Cls6Or90Ab2) postdate the last fabric-defining event. The preservation of extremely complex zoning patterns down to the micron-scale shows that diffusional homogenization did not operate at fluid-present low to medium-grade conditions (350-550 °C, 3-5 kb). Microstructures indicate that at these conditions barian feldspars deform exclusively by brittle fracturing and do not undergo recrystallization. Barian feldspar compositions confirm the positive correlation of Na-content with temperature and the existence of a narrow asymmetric compositional gap (Cls90-85↔Cls55, ~350 °C) which probably closes at lower amphibolite facies conditions (Xc ~Cls75; Tc ~550 °C). White micas are solid solutions of the end-members muscovite, ganterite (Ba0.5 K0.5)Al2(Al1.5Si2.5)O10(OH)2, paragonite, celadonite with a significant substitution of [VI]Al by Cr. Zoning is a common feature with cores being enriched in Ba. The data document extensive Ba substitution for K from muscovite to ganterite, exclusively controlled by the coupled substitution [XII]K + [IV]Si ↔ [XII]Ba + [IV]Al and strongly dependent on bulk composition. The extent of solid solution from (Ms+Gnt) towards paragonite and celadonite end-members is controlled by the miscibility gap in the

  19. Coupled alkai fieldspar dissolution and secondary mineral precipatation in batch systems-2: New experiments with supercritical CO2 and implications for carbon sequestration

    SciTech Connect

    Lu, Peng; Fu, Qi; Seyfried, William E. Jr.; Hedges, Sheila W.; Soong, Yee; Jones, Kyle; Zhua, Chen


    In order to evaluate the extent of CO{sub 2}–water–rock interactions in geological formations for C sequestration, three batch experiments were conducted on alkali feldspars–CO{sub 2}–brine interactions at 150–200 °C and 300 bars. The elevated temperatures were necessary to accelerate the reactions to facilitate attainable laboratory measurements. Temporal evolution of fluid chemistry was monitored by major element analysis of in situ fluid samples. SEM, TEM and XRD analysis of reaction products showed extensive dissolution features (etch pits, channels, kinks and steps) on feldspars and precipitation of secondary minerals (boehmite, kaolinite, muscovite and paragonite) on feldspar surfaces. Therefore, these experiments have generated both solution chemistry and secondary mineral identity. The experimental results show that partial equilibrium was not attained between secondary minerals and aqueous solutions for the feldspar hydrolysis batch systems. Evidence came from both solution chemistry (supersaturation of the secondary minerals during the entire experimental duration) and metastable co-existence of secondary minerals. The slow precipitation of secondary minerals results in a negative feedback in the dissolution–precipitation loop, reducing the overall feldspar dissolution rates by orders of magnitude. Furthermore, the experimental data indicate the form of rate laws greatly influence the steady state rates under which feldspar dissolution took place. Negligence of both the mitigating effects of secondary mineral precipitation and the sigmoidal shape of rate–ΔG{sub r} relationship can overestimate the extent of feldspar dissolution during CO{sub 2} storage. Finally, the literature on feldspar dissolution in CO{sub 2}-charged systems has been reviewed. The data available are insufficient and new experiments are urgently needed to establish a database on feldspar dissolution mechanism, rates and rate laws, as well as secondary mineral

  20. Trace element characteristics of clinozoisite pseudomorphs after lawsonite in talc-garnet-chloritoid schists from the Makbal UHP Complex, northern Kyrgyz Tian-Shan

    NASA Astrophysics Data System (ADS)

    Orozbaev, Rustam; Hirajima, Takao; Bakirov, Apas; Takasu, Akira; Maki, Kenshi; Yoshida, Kenta; Sakiev, Kadyrbek; Bakirov, Azamat; Hirata, Takafumi; Tagiri, Michio; Togonbaeva, Asel


    Polyphase mineral aggregates (PMAs) composed of clinozoisite + kyanite + quartz ± chlorite ± paragonite ± phengite have been found within garnet and in the matrix of talc-garnet-chloritoid schists from the Makbal ultrahigh-pressure complex in the northern Kyrgyz Tian-Shan. These mineral textures are interpreted as pseudomorphs after lawsonite, and we reconstructed the compositions of PMAs of clinozoisite + kyanite + quartz, consistent with lawsonite. Petrological study demonstrated that lawsonite was stable during the prograde to the UHP peak stage (P = 28-33 kbar and T = 530-580 °C) and decomposed to the PMAs during isothermal decompression around P = 16-20 kbar and T = 510-580 °C. Trace element characteristics of the clinozoisite grains in the PMAs (former lawsonite) show a flat rare earth element (REE) chondrite-normalized pattern, comparable with the typical reported REE pattern of lawsonite, although the abundance of REE varied from sample to sample. Thus, the REE content of clinozoisite in the PMAs included in garnet was likely inherited from the former lawsonite as the decomposition reaction took place isolated from the matrix. Discrete clinozoisite grains in the matrix have high light REE enrichment over heavy REE in the chondrite-normalized pattern, consistent with the typical epidote pattern. Our results indicate that the talc-garnet-chloritoid schists in the Makbal complex were buried to great depth (> 100 km) with a low geothermal gradient (< 6 °C/km) during the Early Paleozoic (480-509 Ma). Lawsonite decomposition and clinozoisite-forming reactions accompany fluid release during the isothermal decompression stage, implying that the fluids can be generated not only during subduction, but also during exhumation of ultrahigh-pressure rocks in cold subduction settings.

  1. Titanium in phengite: a geobarometer for high temperature eclogites

    NASA Astrophysics Data System (ADS)

    Auzanneau, Estelle; Schmidt, M. W.; Vielzeuf, D.; D Connolly, J. A.


    Phengite chemistry has been investigated in experiments on a natural SiO2-TiO2-saturated greywacke and a natural SiO2-TiO2-Al2SiO5-saturated pelite, at 1.5-8.0 GPa and 800-1,050°C. High Ti-contents (0.3-3.7 wt %), Ti-enrichment with temperature, and a strong inverse correlation of Ti-content with pressure are the important features of both experimental series. The changes in composition with pressure result from the Tschermak substitution (Si + R2+ = AlIV + AlVI) coupled with the substitution: AlVI + Si = Ti + AlIV. The latter exchange is best described using the end-member Ti-phengite (KMgTi[Si3Al]O10(OH)2, TiP). In the rutile-quartz/coesite saturated experiments, the aluminoceladonite component increases with pressure while the muscovite, paragonite and Ti-phengite components decrease. A thermodynamic model combining data obtained in this and previous experimental studies are derived to use the equilibrium MgCel + Rt = TiP + Cs/Qz as a thermobarometer in felsic and basic rocks. Phengite, rutile and quartz/coesite are common phases in HT-(U)HP metamorphic rocks, and are often preserved from regression by entrapment in zircon or garnet, thus providing an opportunity to determine the T- P conditions of crystallization of these rocks. Two applications on natural examples (Sulu belt and Kokchetav massif) are presented and discussed. This study demonstrates that Ti is a significant constituent of phengites that could have significant effects on phase relationships and melting rates with decreasing P or increasing T in the continental crust.

  2. Very low-temperature metamorphism in Ordovician metasedimentary rocks above and below the Sardic unconformity, SW Sardinia, Italy

    NASA Astrophysics Data System (ADS)

    Franceschelli, M.; Battaglia, S.; Cruciani, G.; Pasci, S.; Puxeddu, M.


    In the Iglesiente region, the Cabitza and Monte Argentu Formations are separated by an angular unconformity known as the Sardic unconformity. This is related to an early Ordovician mild compressional phase, known as the "Sardic phase". The pelitic samples from the structurally lower Cabitza Formation consist of alternating reddish phyllosilicate-rich and whitish phyllosilicate-poor, sialic layers, whose S0 bedding plane is parallel to a pre-Variscan S1P schistosity overprinted by the Variscan S1V schistosity. Pelitic samples from the Monte Argentu Formation are characterized by a Variscan S1V axial plane schistosity. Samples from the two formations consist of quartz and phyllosilicates. The latter are potassic white mica, chlorite, paragonite, locally kaolinite, and pyrophyllite. The illite crystallinity values determined for the Cabitza samples are 0.25-0.31, with an average of 0.29; meanwhile, the Monte Argentu samples produce values of 0.33-0.38, with an average of 0.35. The chlorite crystallinity and b0 of potassic white mica values show greater heterogeneity in the Cabitza than the Monte Argentu samples. The b0 values and P-T pseudosections allow us to confirm that there is no significant difference in the P-T metamorphism conditions between the Cabitza and Monte Argentu samples. The Iglesiente region, which is considered to be the rift zone behind the Middle Ordovician Sarcidano-Barbagia volcanic arc, underwent the "Sardic phase", giving rise to E-W folds. These were first overprinted by weak E-W, and then by stronger N-S-oriented Variscan deformation events.

  3. The Flin Flon paleosol and the composition of the atmosphere 1.8 BYBP

    NASA Technical Reports Server (NTRS)

    Holland, H. D.; Feakes, C. R.; Zbinden, E. A.


    Within the 1800 to 1900 my old Flin Flon-Snow Lake greenstone belt, Amisk Group volcanics are overlain by Missi Group fluvial sediments. Several localities along the Missi-Amisk contact, the volcanics show evidence of subaerial weathering. Field relationships, mineralogical evidence, and chemical analyses confirm that this alteration zone is a paleosol. Pedogenic fabrics and mineralogy were somewhat obscured by greenschist-grade metamorphism associated with the Hudsonian orogeny (1750 my). This is especially true in the upper meter of the paleosol, where metamorphic paragonite and sericitic micas developed in a crenulated fabric. This metamorphism did not, however, obliterate the imprint of weathering on the Amisk volcanics. Features characteristic of well-drained modern soils are evident in the paleosol. Corestones of spheroidally weathered pillow lavas occur at depth within the paleosol (Cr horizon). The corestones decrease in size upward and eventually disappear into a hematite-rich horizon at the top of the paleosol. These macroscopic changes are accompanied by a decrease in CaO and MgO and by an increase in Al2O3, TiO2, and total iron toward the paleosol-Missi contact. Ferrous iron decreases upward toward the contact; FeO was apparently oxidized to ferric iron and retained within the paleosol during weathering. The oxidation and retention of iron within the Flin Flon paleosol indicates that PO2 was probably > or = 10(-2) P.A.L. at the time of weathering. The behavior of iron in the Flin Flon paleosol contrasts sharply with its behavior in the 2200 my Hekpoort paleosol, which is strongly depleted in iron. This difference suggests that a significant increase in the ratio of PO2/PCO2 in the atmosphere took place between 2200 and 1800 mybp.

  4. Laboratory Far-infrared Spectroscopy Of Terrestrial Phyllosilicates To Support Analysis Of Cosmic Dust Spectra.

    NASA Astrophysics Data System (ADS)

    Yesiltas, Mehmet; Brusentsova, T.; Peale, R.; Maukonen, D.; Figueiredo, P.; Harlow, G. H.; Ebel, D. S.; Nissinboim, A.; Sherman, K.; Lisse, C. M.


    Poster Abstract: 219th AAS Meeting M. Yesiltas1, T. Brusentsova1, R. E. Peale1, D. Maukonen1, P. Figueiredo1, G. E. Harlow2, D. S. Ebel2, A. Nissinboim2, K. Sherman2, and C. M. Lisse3 Remote spectral detection of hydrated minerals is of general interest in the solar system and dusty circumstellar disks. This paper presents spectroscopy of terrestrial phyllosilicate minerals in the wavelength range 15 - 250 µm to support interpretation of returned data from far-IR space-missions such as the Herschel Space Observatory. The far-IR spectral region beyond 15 micron wavelength is especially diagnostic of mineral composition and crystal structure. Relatively little far-IR spectral data exists in the literature on suitably-characterized naturally-occurring phyllosilicate minerals in the wavelength range 60-210 microns corresponding to the PACS instrument of Herschel Space Observatory. Extending the database of laboratory far-IR spectra of terrestrial mineral analogs is therefore desirable and timely. Seventeen phyllosilicate minerals expected in various astronomical environments were sampled from the American Museum of Natural History for diversity and astrophysical relevancy, based on their identification in Stardust, in stratospheric IDP samples, or in meteorites. These include serpentines (Antigorite and Chrysotile), smectites (Talc, Pyrophyllite, Vermiculite, Montmorillonite, Beidellite, Saponite, Nontronite and Hectorite), chlorites (Clinochlore), micas (Muscovite, Paragonite, Margarite, Clintonite, Biotite and Illite), and kaolinites (Dickite, Nacrite, Kaolinite, Halloysite, Attapulgite and Sepiolite). Spectra of micron-sized powder suspensions in polyethelyne pellets reveal prominent and characteristic far-IR features, which differ significantly in some cases from already published spectra, where available. Acknowledgements : This research was supported by NASA-JPL Contract # 1327221. 1Department of Physics, University of Central Florida, Orlando FL 32816 USA2

  5. A thermodynamic analysis of the system LiAlSiO4-NaAlSiO4-Al2O3-SiO2-H2O based on new heat capacity, thermal expansion, and compressibility data for selected phases

    NASA Astrophysics Data System (ADS)

    Fasshauer, Detlef W.; Chatterjee, Niranjan D.; Cemic, Ladislav

    Heat capacity, thermal expansion, and compressibility data have been obtained for a number of selected phases of the system NaAlSiO4-LiAlSiO4-Al2O3-SiO2-H2O. All Cp measurements have been executed by DSC in the temperature range 133-823K. The data for T>=223K have been fitted to the function Cp(T)=a+cT -2+dT -0.5+fT -3, the fit parameters being The thermal expansion data (up to 525°C) have been fitted to the function V0(T)=V0(T) [1+v1 (T-T0)+v2 (T-T0)2], with T0=298.15K. The room-temperature compressibility data (up to 6 GPa) have been smoothed by the Murnaghan equation of state. The resulting parameters are These data, along with other phase property and reaction reversal data from the literature, have been simultaneously processed by the Bayes method to derive an internally consistent thermodynamic dataset (see Tables 6 and 7) for the NaAlSiO4-LiAlSiO4-Al2O3-SiO2-H2O quinary. Phase diagrams generated from this dataset are compatible with cookeite-, ephesite-, and paragonite-bearing assemblages observed in metabauxites and common metasediments. Phase diagrams obtained from the same database are also in agreement with the cookeite-free, petalite-, spodumene-, eucryptite-, and bikitaite-bearing assemblages known to develop in the subsolidus phase of recrystallization of lithium-bearing pegmatites. It is gratifying to note that the cookeite phase relations predicted earlier by Vidal and Goffé (1991) in the context of the system Li2O-Al2O3-SiO2-H2O agree with our results in a general way.

  6. Example of trondhjemite genesis by means of alkali metasomatism: Rockford Granite, Alabama Appalachians

    SciTech Connect

    Drummond, M.S.; Wesolowski, D.; Ragland, P.C.; Ragland, J.


    An alternative model for trondhjemite genesis is proposed where granite is transformed to trondhjemite via infiltration by a Na-rich metamorphic fluid. The Rockford Granite serves as the case example for this process and is characterized as a synmetamorphic, peraluminous trondhjemite-granite suite. The major process operative in the conversion of granite to trondhjemite involves cation exchange of Na for K in the feldspar and mica phases through a volatile fluid medium. Whole-rock delta/sup 18/O values for the trondhjemites are negatively correlated with the atomic prop. K/Na ratio indicating a partial reequilibration of the altered granitoids with a Na- and /sup 18/O-rich metamorphically derived fluid. Biotite decomposition to an Al-epidote-paragonitic muscovite-quartz assemblage is also associated with the Na-metasomatism, as are apatite replacement by Al-epidote and secondary zircon crystallization. The replacement of magmatic phases by metasomatic phases exemplifies the chemical changes produced during infiltration metasomatism where the trondhjemites are depleted in all REE's. The timing of the infiltration metasomatism is thought to have occurred during regional metamorphism, producing a discrete fluid phase in the surrounding amphibolite-grade metasediments. Foliation planes in the granitoid apparently served as conduts for fluid flow with reaction enhanced permeability accompanying the 8% molar volume reduction during Na for K exchange in the feldspars. A model is proposed where metamorphic fluids migrated updip and along strike from their source and were buffered by the presence or absence of two feldspars in the metasediments.

  7. Carbonate dissolution and transport in H2O fluids during subduction revealed by diamond-bearing rocks from the Alps

    NASA Astrophysics Data System (ADS)

    Frezzotti, M.; Selverstone, J.; Sharp, Z. D.; Compagnoni, R.


    Here we discuss the fate of subducted carbonates and its implications for recycling of crustal carbon. Thermodynamic models predict little decarbonation along most subduction geotherms, and the mechanisms by which carbon is transferred from the subducting slab to the overlying mantle remain poorly constrained. Diamond-bearing fluid inclusions in garnet in oceanic metasedimentary rocks from Lago di Cignana (western Alps) represent the first occurrence of diamond from a low-temperature subduction complex of clearly oceanic origin (T ≤600°C; P ≥3.5 GPa). The presence of diamonds in and associated with fluid inclusions provides clear evidence of carbon transport by fluids at depths that are directly relevant to slab-mantle fluid transfer during subduction. At room temperature, the fluid inclusions contain aqueous fluid, a vapor bubble, and multiple solid daughter crystals. Daughter crystals identified by Raman spectroscopy and microprobe analysis include ubiquitous Mg-calcite/calcite and rutile, and less common diamond, quartz, paragonite, dawsonite, rhodochrosite, dypingite, and pentahydrite. Molecular CO2 is absent or in trace amounts. The aqueous liquid phase contains ≥0.2 wt%, HCO3-, CO32-, and SO42- ions. In Raman spectra, broad peaks at 773 and 1017 cm-1 point to the presence of both Si(OH)4(aq) and deprotonated monomers (e.g., SiO(OH)3-(aq), and SiO2(OH)22-(aq)), indicative of alkaline solutions. The absence of CO2 in the vapor, and the presence of carbonate daughter minerals, CO32-(aq), and HCO3-(aq) also show that the trapped fluids are alkaline at ambient conditions. High activities of aqueous carbon species reveal that carbonate dissolution is an important mechanism for mobilizing slab carbon at sub-arc depths (100-200 km) during oceanic subduction. Our results imply that the magnitude of carbon release and transport from the slab at sub-arc depths is greater than experimentally predicted on the basis of decarbonation reactions alone.

  8. Compositions of hydrothermal dravitic tourmalines from some deposits in the Urals, Russia

    NASA Astrophysics Data System (ADS)

    Baksheev, I.; Kudryavtseva, O.


    We have studied compositions of tourmaline from the phlogopite schist and other alterations in the Emerald mines, propylitic- and listwaenitic-type alterations and related veins at the Berezovskoe Au and the Shabrovskoe talc deposits in the Central Urals and Syrostanskoe talc deposit in the South Urals. In the Emerald mines tourmaline associates with hornblende and different phyllosilicates, including phlogopite, paragonite, margarite and chlorite. All phyllosilicates are F-bearing. Based on calculations, however, they are characterized by different values of the Fe/(Fe+Mg) ratio (f) and F, OH, and O in the W site. Tourmaline from the earliest albite-hornblende alteration is OH dominated (0.67-0.96 apfu) with the f value ranging from 0.15 to 0.22. Tourmaline from usual phlogopite schists is OH dominated (0.44-0.80 apfu) with F and O ranging from 0.02 to 0.38 apfu and from 0 to 0.24 apfu. The f value ranges from 0.12 to 0.25. Tourmaline associated with the Fe-free phlogopite and paragonite cementing brecciated chromium spinel crystals is F or OH dominated ranging from 0 to 0.75 apfu and from 0 to 0.83 apfu, respectively. The f value is very low ranging from 0 to 0.01. Tourmaline associated with paragonite and topaz is also F or OH dominated ranging from 0 to 0.52 apfu and from 0.18 to 0.76 apfu, respectively. The f value varies from 0.24 to 0.32. Tourmaline from margarite veinlets cutted phlogopite schists is OH dominated (0.54-0.80 apfu). The f value ranges from 0.19 to 0.29. Tourmalines from the phlogopite-chlorite alteration and carbonate-muscovite-chlorite veinlets are F-poor and are characterized by different amounts of OH and O, ranging from 0.29 to 0.97 apfu and from 0 to 0.59 apfu. The f values of these tourmalines range from 0.17 to 0.29 and from 0.25 to 0.37, respectively. Based on the Moesbauer data the Fe3+/Fetotal ratio in the Emerald mines tourmalines is low and ranges from 0.04 to 0.08. All above tourmalines are Na dominated in the X site. According

  9. Record of Variscan Low-P/High-T metamorphism in Alpine medium-P rocks of the Pelagonian Zone, Greece

    NASA Astrophysics Data System (ADS)

    Mposkos, E.; Krohe, A.; Perraki, M.


    In the Early Cretaceous, obduction of the ophiolites from the Vardar ocean over the eastern Pelagonian continental margin caused medium-P metamorphism in the pre-Alpine basement and the Alpine volcanosedimentary cover series of the Pelagonian Zone (Yarwood &Dixon, 1977, Schermer et al., 1990, Perraki et al., 2003). Representative mineral assemblages are garnet-chloritoid-chlorite-kyanite-white K-mica-paragonite-rutile in Permo-Triassic pelitic metasediments from Voras and Vermion Mountains (Western Macedonia) and hornblende-garnet-clinozoisite-albite-quartz-rutile in metabasites. Pervasive deformation associated with this metamorphism completely converted Variscan granitoids and pelitic migmatites into mylonites with the syn-mylonitic mineral assemblage quartz-albite-microcline-phengite (Si up to 3.5 atoms p.f.u.)-garnet-clinozoisite-biotite on granitoid mylonites. Pelitic migmatites from Voras and Vernon Massifs are converted to garnet-chlorite-biotite-muscovite schists, garnet-chloritoid-kyanite-chlorite-muscovite schists and garnet-kyanite-staurolite-chlorite-muscovite schists. Less deformed domains of pelitic migmatites locally preserve Pre-Alpine mineral assemblages characteristic for a low-P/high-T metamorphism. In the NE Vernon Massif, SE Othris and N. Euboea island sillimanite-biotite-garnet-muscovite-K-feldspar-plagioclase-quartz indicate P~4.5 kbar and T~640 0C. In western Vernon and Voras Massifs the mineral assemblages andalusite-cordierite-biotite-muscovite-sillimanite-plagioclase-quartz, andalusite-sillimanite-cordierite-K-feldspar-plagioclase-muscovite-quartz evidence even lower pressures of 2.5 kbar and ~600-620 0C. This metamorphism reflects heat transfer by intrusion of large volumes of granitoid magmas in a Variscan magmatic arc setting. Klippen and tectonic slivers of LP/HT metamorphic rocks, also, occur in the Cyclades and the ophiolite melange of Crete, where they are conventionally considered as Upper Cretaceous magmatic arc. In contrast we

  10. Elemental responses to subduction-zone metamorphism: Constraints from the North Qilian Mountain, NW China

    NASA Astrophysics Data System (ADS)

    Xiao, Yuanyuan; Niu, Yaoling; Song, Shuguang; Davidson, Jon; Liu, Xiaoming


    Subduction zone metamorphism (SZM) and behaviors of chemical elements in response to this process are important for both arc magmatism and mantle compositional heterogeneity. In this paper, we report the results of our petrographic and geochemical studies on blueschist and eclogite facies rocks of sedimentary and basaltic protoliths from two metamorphic sub-belts with different metamorphic histories in the North Qilian Mountain, Northwest China. The protolith of low-grade blueschists is basaltic in composition and is most likely produced in a back-arc setting, while the protoliths of high-grade blueschists/eclogites geochemically resemble the present-day normal and enriched mid-oceanic ridge basalts plus some volcanic arc rocks. The meta-sedimentary rocks, including meta-graywacke, meta-pelite, meta-chert and marble, show geochemical similarity to global oceanic (subducted) sediments. Assuming that high field strength elements (HFSEs) are relatively immobile, the correlated variations of rare earth elements (REEs) and Th with HFSEs suggest that all these elements are probably also immobile, whereas Pb and Sr are mobile in rocks of both basaltic and sedimentary protoliths during SZM. Ba, Cs and Rb are immobile in rocks of sedimentary protoliths and mobile in rocks of basaltic protolith. The apparent mobility of U in rocks of basaltic protolith may be inherited from seafloor alterations rather than caused by SZM. On the basis of in situ mineral compositional analysis (both major and trace elements), the most significant trace element storage minerals in these subduction-zone metamorphic rocks are: lawsonite, pumpellyite, apatite, garnet and epidote group minerals for REEs, white micas (both phengite and paragonite) for large ion lithophile elements, rutile and titanite for HFSEs. The presence and stability of these minerals exert the primary controls on the geochemical behaviors of most of these elements during SZM. The immobility of REEs, Th and U owing to their

  11. Metamorphic evolution of the high-pressure metamorphic rocks from the Kemer area (Biga Peninsula, NW Turkey): Implications for the accretionary continental growth and closure of the Intra-Pontide Ocean

    NASA Astrophysics Data System (ADS)

    Aygül, M.; Topuz, G.; Satır, M.


    The northwestern part of the Biga Peninsula, NW Turkey, corresponds to the so-called Intra-Pontide suture, separating the Rhodope-Strandja zone to the north and Sakarya zone to the south. This suture zone is marked by the exposures of high-pressure metamorphics, oceanic accretionary complexes and ophiolites, which are widely covered by Eocene to Miocene volcanics and volcano-clastics, and crosscut by Eocene to Miocene granites. The Kemer area is one of the key-areas where high-pressure metamorphics and an ophiolitic mélange were widely exposed. This contribution deals with the metamorphic evolution of the Kemer high-pressure rocks, and their implications for the geodynamic evolution. The Kemer high-pressure rocks comprise predominantly micaschist, calcschist, marble and minor metabasite and serpentinite. The micaschists contain mineral assemblages involving garnet, phengite (3,30-3,44 c.p.f.u.), paragonite, epidote, chlorite, albite and titanite, and the metabasites consists of garnet, barrosite, albite, chlorite, epidote, albite and titanite. The equilibrium conditions are poorly constrained as 550 ± 50 °C temperature and >8-10 kbar pressure by Fe-Mg partitioning between garnet and phengite, and phengite-barometry, respectively. Timing of the high-pressure metamorphism is constrained as 84-64 Ma by Rb-Sr phengite-whole rock dating on four samples. Although the obtained age values display a wide scatter, they are consistent with geochronological data from the neighboring high-pressure areas: 86 ± 2 Ma from the Şarköy blueschists (Topuz et al. 2008) and 65-69 Ma from the Çamlıca garnet-micaschists (Okay and Satır, 2000). These data in conjunction with those from the literature suggest that Late Cretaceous represent a time of substantional accretionary continental growth related to the northward subduction of the Intra-Pontide Ocean. The closure of the Intra-Pontide Ocean is constrained between Late Cretaceous and Eocene, because Eocene volcanics and volcano

  12. Pressure-temperature evolution of eclogites from the Kechros complex in the Eastern Rhodope (NE Greece)

    NASA Astrophysics Data System (ADS)

    Mposkos, E.; Baziotis, I.; Proyer, A.


    The Rhodope Domain in NE Greece consists of different tectonometamorphic complexes involved in the Alpine collisional history between the Eurasian and African plates. In the Kechros Complex, which is the lowermost tectonic unit in the East Rhodope, a lense of kyanite eclogite occurs within orthogneiss and common eclogites are found between serpentinized peridotite and underlying pelitic gneisses. In kyanite eclogite, the high-pressure (HP) mineral assemblage is Grt + Omp (Jd35-55) + Ky + Ph + Qz + Rt + (indirectly inferred Tlc + Law); a Na-rich tremolite and zoisite formed at or near peak metamorphic conditions. In common eclogites, the HP mineral assemblage is Grt + Omp (Jd29-41) + Rt and, with less certainty, Amp (Gln-rich + Brs + Wnc + Hbl) ± Czo. The inclusions in garnet are glaucophane, actinolite, barroisite, hornblende, omphacite, clinozoisite, titanite, rutile and rarely paragonite and albite. In kyanite eclogite, peak P- T conditions are constrained at 2.2 GPa and 615°C using garnet-omphacite-phengite geothermobarometry and very similar values of 585 ± 32°C and 2.17 ± 0.11 GPa with the average P- T method, by which conditions of formation could also be narrowed down for the common eclogite (619 ± 53°C and 1.69 ± 0.17 GPa) and for a retrogressed eclogite (534 ± 36°C and 0.77 ± 0.11 GPa). Ages for the HP metamorphism in the Kechros Complex are not yet available. A Rb-Sr white mica age of 37 Ma from orthogneiss records a stage of the exhumation. The HP event may be coeval with the Eocene HP metamorphism (49-55 Ma) recorded in the Nestos Shear Zone in Central Rhodope and in the Attic-Cycladic crystalline belt, where it is interpreted as the result of subduction and final closure of the Axios/Vardar ocean and subsequent subduction of the Apulian continental crust (a promontory of the Africa continent) under the southern margin of the European continent in the late Cretaceous and early Tertiary.

  13. Archaeological jade mystery solved using a 119-year-old rock collection specimen

    NASA Astrophysics Data System (ADS)

    Harlow, G. E.; Davies, H. L.; Summerhayes, G. R.; Matisoo-Smith, E.


    In a recent publication (Harlow et al. 2012), a ~3200-year old small stone artefact from an archaeological excavation on Emirau Island, Bismarck Archipelago, Papua New Guinea was described and determined to be a piece of jadeite jade (jadeitite). True jadeitite from any part of New Guinea was not previously known, either in an archaeological or geological context, so this object was of considerable interest with respect to its geological source and what that would mean about trade between this source and Emirau Island. Fortuitously, the artefact, presumably a wood-carving gouge, is very unusual with respect to both pyroxene composition and minor mineral constituents. Pyroxene compositions lie essentially along the jadeite-aegirine join: Jd94Ae6 to Jd63Ae36, and without any coexisting omphacite. This contrasts with Jd-Di or Jd-Aug compositional trends commonly observed in jadeitites worldwide. Paragonite and albite occur in veins and cavities with minor titanite, epidote-allanite, and zircon, an assemblage seen in a few jadeitites. Surprisingly, some titanite contains up to 6 wt% Nb2O5 with only trace Ta and a single grain of a Y-Nb phase (interpreted as fergusonite) is present; these are unique for jadeitite. In a historical tribute to C.E.A. Wichmann, a German geologist who taught at Utrecht University, the Netherlands, a previously unpublished description of chlormelanite from the Torare River in extreme northeast Papua, Indonesia was given. The bulk composition essentially matches the pyroxene composition of the jade, so this sample was hypothesized as coming from the source. We were able to arrange a loan from the petrology collection at Utrecht University of the specimen acquired by Wichmann in 1893. In addition we borrowed stone axes from the Natural History Museum - Naturalis in Leiden obtained from natives near what is now Jayapura in eastern-most Papua. Petrography and microprobe analysis of sections of these samples clearly show that (1) Wichmann's 1893

  14. Metamorphic evolution of eclogites at Qinglongshan: modeling in system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3

    NASA Astrophysics Data System (ADS)

    Yan, R.; Yang, J.; Zhang, Z.


    Eclogite at Qinglongshan in NE Jiangsu province is one of the research objects which are investigated frequently and products are remarkable in ultra-high pressure (UHP) metamorphic petrology and geochemistry nearly twenty years. Though scholars in China and abroad have made abundant petrological work, there are opposite perspectives to some important petrological questions such as peak assemblage, crystalization time and condition of epidote porphyroblasts which contain coesite, peak assemblage contains talc or not, lawsonite has occurred in the rock or not. This paper choose eclogites in Qinglongshan. Besides traditional petrological work, we applied phase diagram modeling to mineral assemblage and chemical composition, quantitatively investigating the change with temperature and pressure. With the help of petrographic observation the petrological questiones can be solved. Eclogites in Qinglongshan can be divided into three categories: porphyroblastic foliated eclogites, granular massive eclogites and banded eclogites. The peak assemblage in three kinds of eclogites is garnet + omphacite + phengite + kyanite +rutile + coesite. The minerals in the peak assemblage show shape preferred orientation defining the rock foliation in porphyroblastic eclogites. The peak assemblage defined by garnet compositional isopleths in the calculated phase diagram is garnet +omphacite + phengite + kyanite + rutile + coesite + lawsonite + talc in porphyroblastic eclogites, and is garnet + omphacite + phengite + kyanite + rutile +coesite + lawsonite in banded eclogites, both inconsistent with the petrographic observation. This discrepancy probably resulted from the assumption of pure water as the ultrahigh-pressure (UHP) fluid phase. Three stages of metamorphism are established for Qinglongshan eclogites. Prograde inclusions such as amphibole, epidote, plagioclase, chlorite, muscovite, paragonite and albite coexist in the area of < 0.8GPa and < 575. It can infer from mineral assemblage

  15. Mineralogical, IR-spectral and geochemical monitoring of hydrothermal alteration in a deformed and metamorphosed Jurassic VMS deposit at Arroyo Rojo, Tierra del Fuego, Argentina

    NASA Astrophysics Data System (ADS)

    Biel, C.; Subías, I.; Acevedo, R. D.; Yusta, I.; Velasco, F.


    -Chlorite ± Sericite, (2) Quartz-Chlorite, (3) Chlorite ± Quartz-Sericite-Calcite, (4) Quartz-Chlorite ± Calcite and (5) Sericite + Quartz ± Chlorite ± Calcite. Magnesium-chlorite and phengitic white mica typically occur in the vicinity of the Arroyo Rojo ore lenses. To provide field criteria for exploration vectoring, the chemical composition of chlorite and the phengitic and paragonitic content of the white mica were determined and correlated with PIMA Fe-OH and Al-OH absorption wavelengths, respectively, relative to their proximity to the mineralized lenses. The results of this study can be used to help identify (1) felsic proximal facies associations, (2) ore horizons and (3) favorable hydrothermal alteration zones in other parts of the Fin del Mundo district.

  16. Thermobaric structure and metamorphic evolution of the Iratsu eclogite body in the Sanbagawa belt, central Shikoku, Japan

    NASA Astrophysics Data System (ADS)

    Ota, Tsutomu; Terabayashi, Masaru; Katayama, Ikuo


    The Iratsu body is the largest eclogitic body in the Sanbagawa belt, central Shikoku, composed of schists with pumpellyite-actinolite, through blueschist/greenschist transition and epidote-amphibolite, up to eclogite-facies grades. The Iratsu body and the underlying Higashi-Akaishi peridotite body are situated in the intermediate structural level of the Sanbagawa belt which occurs as a subhorizontal thin slab. In the Iratsu body, eclogites and eclogitic metagabbros (garnet-clinopyroxenites) contain garnet+omphacite+epidote+quartz+rutile±phengite±glaucophane±paragonite±barroisite±kyanite, and garnet+sodic augite+rutile±epidote±zoisite±quartz, respectively, as mineral assemblages at the peak P- T conditions. Textural relations between these minerals and secondary minerals indicate that the eclogites were retrograded by hydration under eclogite through epidote-amphibolite to greenschist-facies conditions. Geothermobarometry for the peak metamorphism of the eclogites yielded conditions of 1.4-2.5 GPa and 500-800 °C in the Iratsu body; the estimated P- T conditions gradually change across the body. Eclogites with higher P- T estimates are predominant in the southern part of the body, and the eclogites near a contact with the Higashi-Akaishi body show the highest P- T conditions. Integration of these results with previously published P- T estimates for various rock types suggests a sandwiched thermobaric structure. The highest P- T area is situated in an intermediate structural level, i.e., in the upper part of the Higashi-Akaishi body near the boundary with the Iratsu body, and the metamorphic conditions systematically decrease towards the upper and lower levels not only within the Iratsu body but also in the surrounding lower grade schists. Our thermobaric structure implies that the Iratsu and Higashi-Akaishi composite mass is not a tectonic block but is the integral part of the Sanbagawa belt, which has been formed together with the surrounding schists under

  17. White mica K-Ar geochronology of HP-UHP units in the Lago di Cignana area, western Alps, Italy: Tectonic implications for exhumation

    NASA Astrophysics Data System (ADS)

    Gouzu, Chitaro; Yagi, Koshi; Thanh, Ngo Xuan; Itaya, Tetsumaru; Compagnoni, Roberto


    High-pressure and ultra-high pressure (HP-UHP) blueschist- and eclogite-facies metabasaltic and metasedimentary rocks occur in four different tectonic units near Lago di Cignana, western Alps. We have determined K-Ar ages for white micas (matrix phengite and paragonite) from the Lago di Cignana UHP unit (LCU; 39-41 Ma); the lower and upper units of the Zermatt-Saas meta-ophiolite (LU and UU; 37-38 Ma and 38-41 Ma respectively), and the Combin unit (CU; 36-40 Ma). These K-Ar ages overlap with single-grain Ar-Ar plateau ages (36-42 Ma) previously determined for phengites from LCU metasediments. Matrix white micas have been severely deformed during exhumation, and their chemistries differ from those of micas included in garnet. Although individual mica grains in the matrix could have experienced different degrees of deformation which have reset their K-Ar systems, "bulk" white mica separates provide the average age of all the individual grains in the separate. The similarity of ages determined for white micas from the LCU, LU, UU and CU units, regardless of rock type and mineral species, suggests that these four units were metamorphosed together as part of a single metamorphic sequence in the Piemonte-Liguria paleosubduction zone and were subsequently exhumed together. However, present-day structural relationship among those units and the limited occurrence of UHP minerals in LCU suggests that the exhumation of LCU was more rapid than that for LU, UU and CU. The age gaps between the youngest value of white mica K-Ar ages in each unit and the inferred timing of the metamorphic peak (U-Pb age: 44 Ma) is 5, 7, 6 and 8 Myr for LCU, LU, UU and CU, respectively. These intervals are considerably shorter than that determined for the Sanbagawa HP metamorphic belt of Southwest Japan (> 31 Myr). The short interval observed for the Lago di Cignana units that we have studied is consistent with the model of rapid exhumation of the UHP-bearing metamorphic domain, suggesting the

  18. Chemical and isotopic composition of the lower crust beneath the Meguma Lithotectonic Zone, Nova Scotia: evidence from granulite facies xenoliths

    NASA Astrophysics Data System (ADS)

    Eberz, G. W.; Clarke, D. B.; Chatterjee, A. K.; Giles, P. S.


    Granulite-facies xenoliths from an Upper Devonian lamprophyre dyke near Tangier, Nova Scotia, provide new information about the lower crust in the Meguma Zone. Two mineralogically and chemically distinet groups of xenoliths occur. Both groups contain quartz+feldspar+biotite+Fe-Ti oxides+sulfides. In addition, group A contains garnet+graphite±[aluminosilicates+spinel±sapphirine (hight Al2O3 subgroups A1 and A2)] or [clinopyroxene+sphene+apatite (high CaO sub-group A3)]. Group B has highly variable proportions of orthopyroxene (B1), clinopyroxene (B2), and amphibole (B3). Trace-element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other “undepleted” granulite-facies rocks. Low P-T sedimentary assemblages of quartz-chlorite-clay minerals-calcite-albite or paragonite can account for the compositions of group A xenoliths. Within group B, a high-MgO (MgO>13 wt%) subgroup with high transition-metal contents, and low-MgO (MgO<9 wt%) sub-groups with higher LIL (large-ion-lithophile) element contents exist. Although the rare-earth and high-field-strength elements indicate a tholeiitic or low-K calc-alkaline chemistry, the LIL elements are as high as those from high-K calc-alkaline volcanic are rocks. Isotopically, group A ranges from ɛNdt=-2.56 to-0.80 and87Sr/86Sr t =0.7046 to 0.7182 for t=370 Ma. For group B these values are +1.45 to +5.33 and 0.7028 to 0.7048, respectively. Model ages (TCHUR) are correspondingly low and are tightly constrained (544±52 Ma). These “young” ages contrast with the middle Proterozoic Nd model ages of the overlying Meguma Zone low-grade flysch. This time-inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous Tangier lower crust (≡Avalon), and an older allochthonous Meguma Group upper crust.

  19. Tectono-Metamorphic Evolution of the Eastern Rif of Morocco (External Zone of Rif Chain, Morocco) during Mesozoic and Tertiary Times

    NASA Astrophysics Data System (ADS)

    Vázquez-Vílchez, M.; Asebriy, L.; Azdimousa, A.; Jabaloy, A.; Booth-Rea, G.; Barbero, L.; Mellini, M.


    In this work we have determine the characteristic of low grade metamorphism in metapelites from Temsamane and Ketama Units from the eastern Rif and the mineralogical assemblages of serpentinites of peridotites from Beni-Malek. The Tánger-Ketama unit has an estratigraphic sequence from the Lower Jurassic to the Cenozoic and it is composed by metapelitic and quartzitic rocks, and marbles. The mineralogical composition determined by XRD of the samples is quartz, calcite, chlorite and muscovite. The Crystallinity Index (IC) of mica was determined in the ethilenglycol and natural samples. In some samples, the IC was different between the ethilenglycol and natural samples, which indicate the presence of the R3 illite/smectite. For these samples we have used the 5 Å peak of mica in order to determine the metamorphism grade. The Lower Cretaceous rocks have an anquizone grade (0.331-0.496 ° 2 ?). The Jurassic samples present diagenetic conditions (0.567 ° 2 ?). Azdimousa et al. (1998) determine the age of two events metamorphic with fission track analysis that occurred during the Cretaceous period. The first age is Barriasian to Hauterivian (126.6 ± 2.1 to 131.9 ± 3.2 Ma) and it is related with an important extension period related with the exhumation of the peridotites from Beni-Malek and the second one was Santonian to Campanian (78.0 ± 1.2 to 81.6 ± 4.0 Ma). The Temsamane Unit forms a mountains chain of ENE-OSO directions and is composed by marbles, quartzites, and phyllites from Palaeozoic to Aptian-Albian times. The mineralogical composition of the samples studied is quartz, calcite, plagioclase, chlorite, muscovite, and paragonite. The kaolinite is present in some samples. The IC of mica indicates a late anquizone to epizone grade (0.300- 0.159 ° 2 ?). In according to Guidotti and Sassi (1986), the b parameter of mica indicates intermediate pressure (at less 3 Kbar). The presence of paragonite indicates a metamorphism temperature higher of 200° C. However

  20. Rutile solubility in H2O-NaAlSi3O8 fluids at High T and P: Implications form HFSE mobility in Subduction zones

    NASA Astrophysics Data System (ADS)

    Antignano, A.; Manning, C. E.


    The trace element signatures of arc magmas are characterized by HFSE depletion relative to the LILE. Rutile, a common accessory phase in high-pressure assemblages, is an important reservoir for the HFSE and is often invoked to explain the HFSE depletion of arc magmas. This model is in part based on experimental studies, which show that rutile has very low solubility in pure H2O. However, rutile is also a common accessory to eclogite-facies vein assemblages of albite, paragonite and quartz, which likely precipitated from slab-derived fluids. This observation requires either that fluid fluxes were unrealistically high, or that current estimates of Ti solubility are too low. A possible solution to this problem is that dissolved silicate components can enhance Ti solubility via complexing. To test this, we measured the solubility of rutile in H2O-NaAlSi3O8 (albite) bearing fluids at high T and P. Experiments were conducted using a piston-cylinder apparatus with NaCl-graphite furnaces. A single synthetic rutile crystal was loaded into a 1.6 mm OD Pt inner capsule, which was lightly crimped and then placed in a 3.5 OD Pt outer capsule with ultra pure H2O and powdered Amelia albite. Solubility was determined by the weight loss of the rutile grain after 10 hrs. A time series demonstrates that equilibrium is achieved after 8-10 hrs. Preliminary results at 800°C, 1.0 GPa, show that rutile solubility rises with increasing NaAlSi3O8 concentration from 1.15(12) millimolal at 2.18 wt% NaAlSi3O8 to 3.77(13) at 8.80 wt% NaAlSi3O8. Corundum mats + fluid are observed in 3.4-8.80 wt% NaAlSi3O8 and are interpreted to be the result of incongruent dissolution of albite. Quenched melt spheres where observed in an experiment containing ~15 wt% NaAlSi3O8, but not at 8.80 wt%. At 8.80 wt% NaAlSi3O8, rutile solubility is higher by a factor of 6 relative to that in pure H2O. Our results suggest that TiO2 solubility is increased by complexing with Na-Al-Si-bearing fluid components. It has

  1. Near-Solidus Solubility of Albite+Quartz in H2O at 1 GPa: Implications for Crustal Magmas and Fluids

    NASA Astrophysics Data System (ADS)

    Manning, C. E.; Antignano, A.


    The solubility and melting of albite (Ab, NaAlSi3O8) ± quartz (Qz, SiO2) in H2O has been used to gain insight into the nature of magmas and metamorphic fluids in the continental crust. For example, the Ab+H2O and Ab+Qz+H2O solidi are widely used as guides to crustal melting. However, some studies suggest that Ab dissolves incongruently in H2O at near-solidus conditions, which implies that these solidi are not likely to be encountered in nature. To address this problem, we carried out experiments on the stability and solubility of the assemblage Ab+Qz in H2O at 1 GPa, 580-640°C. Starting materials were natural Amelia Ab and Brazilian Qz. Ab crystals were placed in a 1.6 mm OD Pt capsule, which was lightly crimped and then loaded with ultrapure H2O and a quartz crystal into a 3.5 mm OD Pt capsule, which was sealed by arc welding. Experiments were conducted for 24 h in a piston-cylinder apparatus using 1" NaCl- graphite furnaces. Quenched run products were evaluated to assess stable assemblages, and solubilities were determined from crystal weight changes. Hydrous melt (L) became stable at 630±10°C. Paragonite (Pg, NaAl3Si3O10(OH)2) was present in all run products, indicating that Ab+Qz and L+Qz dissolve incongruently in H2O, and that Ab+Qz+L+H2O is metastable at 1 GPa. The bulk solubility of the assemblage Ab+Pg+Qz in H2O doubles from 4.0 to 8.0 wt% as the solidus is approached (580- 630°C), with corresponding increases Si, Al, and Na molalities. Over the same T range, Na/Al and Na/Si (molar) respectively decrease (2.1 to 1.5) and increase (0.153 to 0.175), indicating that fluids are Na- and Si-rich peralkaline solutions far from Ab stoichiometry. At 580°C, measured Al solubility agrees well with that predicted from extrapolation of thermodynamic data for Al and Na-Al species; however, measured Al solubility at the solidus is much greater than predicted. Similar observations hold for Na and Si. This implies that there is a strong, pre-melting increase in the

  2. The uncertainty of the standard entropy and its effect on petrological calculations

    NASA Astrophysics Data System (ADS)

    Dachs, E.; Geiger, C. A.; Benisek, A.


    Internally consistent thermodynamic data bases are now routinely used in the Earth sciences for investigating various petrological, geochemical and geophysical problems. In this regard, the central question is to what level of precision the standard entropy So should be known for petrological calculations. We have addressed this issue and have chosen as study cases the dehydration reaction muscovite = K-feldspar + corundum + H2O, the Mg-Fe exchange reaction between garnet and biotite, the GASP-barometer (grossular = 3 anorthite + 2 kyanite + quartz) and the continuous reaction muscovite /paragonite + quartz = K-feldspar/albite + sillimanite + H2O, and we have used Monte Carlo simulation to calculate the effect of uncertainties in So on reaction positions in P-T space. It turns out that the standard entropy should be precise in the 0.1 percent range in order that uncertainty envelopes of calculated reaction positions are not too broad and phase diagrams would become meaningless. Current calorimetric methods are able to deliver entropies with that quality (e.g. Dachs and Benisek, 2011). So in internally consistent thermodynamic data bases is sometimes not sufficiently precise. In the case of mineral end-members where So has not been measured, entropy estimation schemes are sometimes used (e.g. in THERMOCALC). However, deviations from calorimetric values can be several percent and even up to ~10% in some cases. One example is spessartine. Recent relaxation calorimetric measurements on synthetic spessartine give S° = 335.3 J/mole/K (Dachs et al., 2009), whereas previous estimates were considerably removed from this value (S° = 367 J/mole/K, Holland and Powell, 1998). Entropy estimation techniques are not able to deliver reliable entropies and they are no substitute for calorimetric measurements. A further issue is the extrapolation of heat capacities to high temperatures outside the range where calorimetric data are available. This is important when e.g. calculating

  3. On the relative roles of carbonate and molecular CO2 in subduction zones: implications for Earth's deep carbon cycle (Invited)

    NASA Astrophysics Data System (ADS)

    Manning, C. E.; Kavner, A.; Chopelas, A.


    Subduction-zone volcanism returns oxidized carbon (chiefly CO2) to Earth’s surface reservoirs. However, the mechanism by which this carbon is transferred from the slab to the arc-magma source region is uncertain. Phase equilibrium studies of subduction zones indicate that in the absence of fluids carbonate minerals are quite stable along slab P-T paths, so their simple breakdown cannot be a significant mechanism for CO2 loss, at least to subarc depths. Alternatively, CO2 can be generated from slab carbonates by infiltration of an H2O-rich fluid, driving reactions that can be modeled by aragonite + quartz = wollastonite + CO2. However, the high P and low T of subduction zones causes maximum CO2 concentrations to remain low at depths <100 km. But such calculations assume that dissolved carbon is present as molecular CO2, whereas carbon liberated by dissolution of CaCO3 in H2O at slab P and T is predominantly carbonate, dominantly H2CO3. Using the equation of Caciagli and Manning (2003, CMP, 146, 275) for the solubility of CaCO3 in H2O, we computed dissolved carbonate content in fluids at the P and T of the Cascadia slab mantle interface. In H2O equilibrated with CaCO3 at depths shallower than ~80 km, more carbon is present as carbonic acid than as molecular CO2. Molecular CO2 is 10x lower than carbonic acid over most of the depth interval. It rises rapidly at 70-80 km, where the two species are present in subequal concentrations (~2000 ppm). At greater depths, carbon in the fluid is predicted to be dominantly molecular CO2. Where carbonate species predominate in slab fluids, the capacity for carbon transport will depend strongly on pH. For example, at 1.8 GPa, the Cascadia slab-mantle interface is predicted to be 500°C. At this P and T, the pH of pure H2O equilibrated with CaCO3 is 6.7 (neutral pH is 3.8). However, the pH buffered by model mineral assemblages of the slab (e.g., jadeite-paragonite-quartz) or mantle wedge (talc-olivine) is 4-6. Because CaCO3

  4. Coupled deformation and metamorphism: Strain localization facilitated by solution mass transfer

    NASA Astrophysics Data System (ADS)

    Hunter, R. A.; Andronicos, C. L.


    The Vallecito Conglomerate of the Needle Mountains, Colorado, is a Paleoproterozoic conglomerate with interbedded layers of aluminous schist that experienced deformation and contact metamorphism associated with emplacement of a pluton with a U/Pb age of ~1.435 Ga. The well-defined contact aureole includes sillimanite close to the pluton, andalusite and sillimanite together in a diffuse zone ~2 km from the pluton, and andalusite alone at greater distance from the pluton. Samples bearing both andalusite and sillimanite show a high degree of strain partitioning along discrete shear zones. Samples of both high strain (N09-5A) and low strain (N09-5) regimes were collected. The high strain sample, N09-5A, is characterized by shear bands of sillimanite separated by lower strain regions of quartz and andalusite. Samples N09-5 and N09-5A contain identical mineral assemblages of muscovite, paragonite, chloritoid, quartz, andalusite, sillimanite, rutile and ilmenite and were collected from the same outcrop. Based on their mineral assemblages and proximity of the two samples, it was assumed that N09-5A and N09-5 initially had the same chemical composition. An isoconcentration diagram was created using whole rock chemical analyses. This diagram shows that N09-5A is enriched in Ti, Zr, Th and La - all high field strength elements (HFS) - when compared to N09-5. Under the assumption that HFS are immobile, we interpret this result as evidence for depletion of the major elements from N09-5A relative to N09-5. Electron microprobe X-ray element intensity maps support the interpretation of the isoconcentration diagram. X-ray maps of sample N09-5A cover high strain sillimanite shear bands and low strain regions of quartz and andalusite. Quantitative analysis of the abundance of elements in each region shows a significant depletion of K, Al, and Si in high strain regions relative to low strain regions. Textural observations made at the thin section scale suggest that strain localization

  5. Modelling metamorphism in the Hoosac Schist, Western Massachusetts: new approaches to a New England problem

    NASA Astrophysics Data System (ADS)

    Bidgood, Anna; Waters, Dave; Gardiner, Nick


    Along the western margin of the metamorphic Appalachians in New England, Taconic (Ordovician) tectonism and metamorphism are overprinted towards the east by Acadian (Devonian) structures and metamorphism. The Hoosac Schist, a probable correlate of the well-known Gassetts Schist of Vermont, lies in the region of overprinting. It forms a narrow N-S-trending tectonically-bound zone crossing several Barrovian mineral-assemblage zones from garnet to kyanite grade. Highly aluminous units containing cm-sized garnets (Cheney & Brady, 1992) are noted for the occurrence of textural unconformities within the garnets, separating inclusion-rich cores from inclusion-poor rims. Matrix domains contain both paragonite and muscovite. Muscovite is present in at least two compositionally distinct generations, with broad later laths cutting across a microfolded earlier fabric. Rutile is restricted to inclusions in garnet, whereas the matrix Ti- phase is ilmenite. These features suggest a polymetamorphic history, potentially recording the superimposition of Acadian metamorphism on Taconic, but it has not yet proved possible to demonstrate the presence of two metamorphic cycles. This study aims to test and employ the new and revised activity models recently developed for metapelites in the full system MnNCKFMASHTO (White et al, 2014), for use with the Holland & Powell data-set 6. Features that can now be more explicitly modelled include garnet zonation in relation to its inclusion suites and microstructural features, the occurrence, texture and distribution of Ti-bearing accessory minerals, and the assemblages and compositional trends in white micas. Preliminary modelling, correlated with microstructural observation, indicates (1) some confirmation of the concern expressed by White et al (2014) that the stability of margarite-bearing assemblages may be somewhat overestimated, (2) that apart from this, the early growth history of garnet is consistent with its suite of trapped inclusions

  6. Phengite-hosted LILE enrichment in eclogite and related rocks: Implications for fluid-mediated mass transfer in subduction zones and arc magma genesis

    USGS Publications Warehouse

    Sorensen, Sorena S.; Grossman, J.N.; Perfit, M.R.


    Geochemical differences between island arc basalts (LAB) and ocean-floor basalts (mid-ocean ridge basalts; MORB) suggest that the large-ion lithophile elements (LILE) K, Ba, Rb and Cs are probably mobilized in subduction zone fluids and melts. This study documents LILE enrichment of eclogite, amphibolite, and epidote ?? garnet blueschist tectonic blocks and related rocks from melanges of two subduction complexes. The samples are from six localities of the Franciscan Complex, California, and related terranes of Oregon and Baja California, and from the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Most Franciscan blocks are MORB-like in their contents of rare earth elements (REE) and high field strength elements (HFSE); in contrast, most Samana blocks show an LAB signature of these elements. The whole-rock K2O contents of both groups range from 1 to 3 wt %; K, Ba, Rb, and Cs are all strongly intercorrelated. Many blocks display K/Ba similar to melasomatized transition zones and rinds at their outer margins. Some transition zones and rinds are enriched in LILE compared with host blocks; others are relatively depleted in these elements. Some LILE-rich blocks contain 'early' coarse-grained muscovite that is aligned in the foliation defined by coarse-grained omphacite or amphibole grains. Others display 'late' muscovite in veins and as a partial replacement of garnet; many contain both textural types. The muscovite is phengite that contains ???3??25-3??55 Si per 11 oxygens, and ???0??25-0??50 Mgper 11 oxygens. Lower-Si phengite has a significant paragonite component: Na per 11 oxygens ranges to ???0??12. Ba contents of phengite range to over 1 wt % (0??027 per 11 oxygens). Ba in phengite does not covary strongly with either Na or K. Ba contents of phengite increase from some blocks to their transition zones or rinds, or from blocks to their veins. Averaged KlBa ratios for phengite and host samples define an array which describes other subsamples of

  7. Reconstruction of in situ composition of sedimentary formation waters

    NASA Astrophysics Data System (ADS)

    Palandri, James L.; Reed, Mark H.


    Chemical equilibrium calculations on sedimentary formation waters show that the waters, as analyzed, cannot be in equilibrium with diagenetic minerals in their host rocks at the formation temperature. However, if alkalinity is corrected to account for organic acid anions, and if the pH and bicarbonate are corrected for CO 2 loss from the sample, chemical equilibrium between formation waters and host rock diagenetic minerals can be clearly shown for systems in the temperature range of 75 to 160°C. Compositional reconstruction of some formation waters from published analyses is complicated by lack of analytical data for aluminum, silica, and organic acid anions. Missing aluminum and silica can be estimated by assuming equilibrium with an aluminum silicate (K-feldspar, muscovite) and quartz or chalcedony. pH, CO 2, and organic acid anions can be reconstructed by fixing CO 2 to exactly saturate calcite at the formation temperature because the fast kinetics of calcite precipitation makes it almost certain that calcite saturation is more likely than the strong supersaturation that is otherwise observed. Results from the equilibrium calculations are evaluated by using graphs of the saturation states of diagenetic minerals vs. temperature, for each of many sedimentary brines. If the diagenetic minerals selected as diagnostic of equilibrium (from qz, chalcedony, mus, paragonite, k-sp, alb, kaol, ca, and dol) are not saturated at or near a single temperature, the missing or erroneous quantities of components are adjusted to obtain agreement in the saturation temperature. Composition data for fluids from four locations are used in the calculations: Kettleman North Dome, California, offshore Norway, the Texas Gulf Coast, and offshore Texas. The calculations suggest that in most cases, control of silica concentration shifts from chalcedony to quartz with increasing temperature near 100°C. In some fluids, silica concentration may approach chalcedony saturation to temperatures

  8. Paragenetic Relationships among Staurolite and Aluminosilicate Polymorphs in the Barrovian Metapelites of the Imjingang Belt, Central Korea

    NASA Astrophysics Data System (ADS)

    Kim, Y.; Cho, M.


    are poorly constrained. The absence of paragonite and the low celadonite component of muscovite (less than ~3.1 Si / 11 O atoms) argue against the supply of fluid via the breakdown of hydrous minerals. On the other hand, Triassic igneous activities in the vicinity of the Imjingang belt, recently dated at 237-232 Ma, might have provided heat and fluid necessary for the growth of staurolite and monazite at mid- crustal depths. In summary, our result suggests that the Imjingang belt has experienced a near-isothermal decompression from ~11 to 6.5 kbar at ~252-238 Ma, and the formation of staurolite porphyroblasts enclosing biotite, kyanite and garnet is possibly attributed to the thermal flux associated with Middle Triassic magmatic activity.

  9. Unraveling eclogite-facies fluid-rock interaction using thermodynamic modelling and whole-rock experiments: the in-situ eclogitization of metapelites from Val Savenca (Sesia Zone, Western Alps)

    NASA Astrophysics Data System (ADS)

    Jentsch, Marie; Tropper, Peter


    A common feature of HP and UHP terranes is the subduction of crustal rocks to great depths. Previous investigations have shown that this process is triggered by fluids present during an eclogite-facies metamorphic overprint. An examples is exposed in the metapelites at Val Savenca in the Sesia-Lanzo Zone, Italy where Alpine eclogite-facies metamorphism and fluid flow led to partial transformation of Variscan amphibolite-eclogite facies metapelites (garnet + biotite + sillimanite + K-feldspar + plagioclase + quartz) to zoisite ± jadeite + kyanite + phengite + quartz. This transformation took place under P-T conditions of 1.7 - 2.1 GPa at 600°C and low a(H2O) of 0.3-0.6. The replacement of plagioclase by jadeite + zoisite + kyanite + quartz takes place also along former fractures. Biotite is replaced by the assemblage phengite + omphacite ± kyanite adjacent to former plagioclase, otherwise by phengite + rutile/titanite. Garnet and clinopyroxene show variable compositions depending in which micro-domain (plagioclase or biotite) they grew. The extreme development of microdomains can best be studied by thermodynamic pseudosection modelling of individual microdomains using stoichiometric mixtures of protolith minerals from this domain and the program DOMINO (De Capitani & Petrakakis, 2010). The aim of these calculations was: 1.) to reproduce the observed mineral assemblage and 2.) to provide constraints on the amount of fluid present in the transformation. The results so far indicate that the amount of fluid was very low, otherwise paragonite would have formed instead of jadeite and reproduction of the observed mineral assemblage has only been partly successful so far since biotite is still stable in the calculations. In addition to understand the role of fluids in the mineralogical and textural transformation piston-cylinder experiments with a fresh, natural orthogneiss granulite from the Moldanubic Unit in upper Austria with the assemblage garnet + biotite + K

  10. Mid-Ocean Ridge Hydrothermal Vent Fluid Chemistry at Ultrafast Spreading Rates: Control by Phase Separation and Water-Rock Equilibrium

    NASA Astrophysics Data System (ADS)

    O'Grady, K. M.; Von Damm, K. L.


    results of EQ3/6 equilibrium modeling support the conclusion that the SouEPR Area reaction zone is both hotter and deeper than that in the N. Hump Area. The vent fluid chemistry from the N. Hump Area appears to be controlled by equilibrium (or steady-state) with aluminosilicate alteration minerals (most likely phyllosilicates such as muscovite, paragonite and margarite) representative of a greenschist metamorphic facies. All of the vent fluids modeled from the SouEPR area, with the exception of Jasmine vent, appear to be in equilibrium (or steady-state) with an alteration mineral assemblage typical of a higher grade metamorphic facies as the fluids have low Li- and K-to-Cl ratios and are in equilibrium with a Ca-rich plagioclase, a K-poor alkali feldspar and epidote. The vent fluid compositions and geochemical modeling results therefore suggest that on a finer scale, tectonically dominated areas may allow the penetration of fluids to greater depths below the seafloor, which can translate into the fluids achieving equilibrium or steady-state with a higher grade alteration mineral assemblage.

  11. Phase-equilibrium modelling of blueschists from the Vestgötabreen Complex (SW Svalbard)

    NASA Astrophysics Data System (ADS)

    Kośmińska, Karolina; Majka, Jarosław; Manecki, Maciej; Lorenz, Henning; Kozub, Gabriela


    In Svalbard Archipelago, blueschists are known from Motalafjella area (Oscar II Land). They belong to the Vestgötabreen Complex, which is divided into a Lower (LU) and Upper Unit (UU). The former is composed of high pressure-low temperature (HP-LT) metasediments. The latter consists mainly of blueschists and eclogites. Various radiometric dating yielded an age of c. 470 Ma for the HP-LT metamorphism in the Motalafjella area. The pressure-temperature (P-T) conditions for carpholite-bearing schists from LU have been estimated to c. 16 kbar and 330-450°C (Agard et al., 2005), whereas eclogites from UU indicate peak conditions of 18-24 kbar and 580-640°C (Hirajima et al., 1988). During the fieldwork in 2011, blueschists were also discovered at the western coast of Nordenskiöld Land. They form isolated bodies enclosed within metasedimentary units, but their tectonic position is still under debate. Preliminary P-T estimates indicate peak pressure conditions of c. 17 kbar and 480°C (Kośmińska et al., in revision). The age of metamorphism is unknown, however P-T conditions as well as metamorphic assemblage suggest that the blueschists from Nordenskiöld Land may be an equivalent of these in the Vestgötabreen Complex. Samples of blueschists from UU have been collected on Skipperryggen. They consist mainly of glaucophane, garnet, white micas (phengite and paragonite), rutile, lawsonite and chlorite. The garnet typically forms euhedral to subhedral porphyroblasts which contain voluminous inclusions. Its composition varies from Alm63Prp13Grs22Sps2 in the cores to Alm60Prp19Grs20Sps1 in the rims. The change in chemical zoning is rather gradual. The garnet shows bowl-shaped pyrope profiles and opposite almandine trends. The P-T conditions were estimated using phase equilibrium modeling. Preliminary modeling in the NCKFMMnASHTO system yields peak pressure conditions at c. 20 kbar and 520°C. The estimated P-T conditions for the blueschists from Skipperryggen are in

  12. Supercritical aqueous fluids in subduction zones carrying carbon and sulfur: oxidants for the mantle wedge?

    NASA Astrophysics Data System (ADS)

    Sverjensky, Dimitri; Manning, Craig


    Much speculation surrounds the nature of aqueous fluids in subduction zones. Aqueous fluids likely trigger partial melting in the mantle wedge, influencing the chemistry of the magmas that erupt in island arcs. They also may play a role in transporting elements that could metasomatize and oxidize the overlying mantle wedge, most importantly C, S and Fe. However, full coupling of aqueous fluid chemistry with the silicate, carbonate, C, sulfide and sulfate minerals has remained limited to pressures of 0.5 GPa because of limitations on the HKF aqueous ion equation of state. Recent progress in developing a Deep Earth Water model (Sverjensky et al., 2014), calibrated with new experimental data, now enables a detailed evaluation of the evolution of aqueous fluid chemistry to a pressure of 6 GPa, well into subduction zone conditions. We report aqueous speciation models for eclogitic aqueous fluids constrained by model mineral assemblages that give preliminary indications of the solubilities of elements that could contribute to mass transfer and redox changes in the mantle wedge. For example, at 600 °C and 2.5 GPa, an aqueous fluid in equilibrium with jadeite, paragonite, muscovite, quartz, lawsonite, almandine, talc, magnesite and pyrite at QFM oxidation state with 0.1 molal total Cl, contains 5.5 molal C, 0.04 molal S, and 9 micromolal Fe. The fluid has a pH of 4.7, much greater than the neutral pH of 3.3; the predominant species and molalities are CO2 (5.0), Na+ (0.44), Si(OH)4 (0.36), HCO3- (0.26), H3SiO4- (0.23), CaHCO3+ (0.18), silica dimer (0.10), Cl- (0.09), K+ (0.08), HCOO- (0.06), H2S (0.03). Calculations for model eclogitic fluids at the higher pressures and temperatures of subarc conditions also show that the solubility of C is much greater than either S or Fe at QFM. However, in subarc eclogitic fluids of higher oxidation state (QFM +3 to +4) in equilibrium with hematite, anhydrite, jadeite, kyanite, phlogopite, coesite, lawsonite, almandine-pyrope, and