New constraints on UHT metamorphism in the Eastern Ghats Province through the application of phase equilibria modelling and in situ geochronology

High Mg–Al granulites from the Sunki locality in the central portion of the Eastern Ghats Province record evidence for the high-temperature peak and retrograde evolution. Peak metamorphic phase assemblages from two samples are garnet + orthopyroxene + quartz + ilmenite + melt and orthopyroxene + spinel + sillimanite + melt, respectively. Isochemical phase diagrams (pseudosections) based on bulk rock compositions calculated in the chemical system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO) and Al contents in orthopyroxene indicate peak UHT metamorphic conditions in excess of 960 °C and 9.7 kbar. Microstructures and the presence of cordierite interpreted to record the post-peak evolution show that the rocks underwent decompression and minor cooling from conditions of peak UHT metamorphism to conditions of ~ 900 °C at ~ 7.5 kbar. In situ U–Pb isotope analyses of monazite associated with garnet and cordierite using the Sensitive High Resolution Ion Microprobe (SHRIMP) yield a weighted mean ²⁰⁷Pb/²³⁵U age of ca. 980 Ma, which is interpreted to broadly constrain the timing of high-temperature monazite growth during decompression and melt crystallization at ~ 900–890 °C and 7.5 kbar. However, the range of ²⁰⁷Pb/²³⁵U monazite ages (from ca. 1014 Ma to 959 Ma for one sample and ca. 1043 Ma to 922 Ma for the second sample) suggest protracted monazite growth during the high-temperature retrograde evolution, and possibly diffusive lead loss during slow cooling after decompression. The results of the integrated petrologic and geochronologic approach presented here are inconsistent with a long time gap between peak conditions and the formation of cordierite-bearing assemblages at lower pressure, as proposed in previous studies, but are consistent with a simple evolution of a UHT peak followed by decompression and cooling.     

Discussion of “Thermodynamic parameters of CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>-Mg<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>6</Subscript> pyroxenes based on regular solution and cooperative disordering models” by Holland,Navrotsky, and Newton

Phase equilibria in the join CaMgSi₂O₆-CaFeAlSiO₆-CaTiAl₂O₆ have been determined in air at 1 atm by the ordinary quenching method. Clinopyroxene_ss, forsterite, perovskite, magnetite_ss, spinel_ss, hibonite and an unknown phase X are present at liquidus temperatures (ss: solid solution). At subsolidus temperatures the following phase assemblages were encountered; clinopyroxene_ss+perovskite, clinopyroxene_ss +perovskite+spinel_ss, clinopyroxene_ss +perovskite+melilite (+anorthite), clinopyroxene_ss +perovskite+melilite+spinel_ss+anorthite, clinopyroxene_ss +perovskite+anorthite+spinel_ss, and clinopyroxene_ss +perovskite+anorthite+hibonite. At subsolidus temperatures the single phase field of clinopyroxene_ss extends up to 19 wt.% CaTiAl₂O₆. Even in the field of clinopyroxene_ss+perovskite, the TiO₂ content in clinopyroxene_ss continues to increase and attains 9.2 wt.% TiO₂ with 24.8 wt.% Al₂O₃. An interesting fact is that unusual clinopyroxenes which contain more Al^IV than Si^IV are present in the CaFe-AlSiO₆-rich region. The liquid coexisting with pyroxene is richer in Ti, Al, and Fe³⁺ than the coexisting pyroxene. The clinopyroxenes_ss coexisting with liquid contain less TiO₂, Al₂O₃ and Fe₂O₃ than those crystallized at subsolidus temperatures. The petrological significance of the join and the crystallization of Ti- and Al-rich clinopyroxenes are discussed on the basis of the experimental results of the join.     

Structure of H2O-saturated peralkaline aluminosilicate melt and coexisting aluminosilicate-saturated aqueous fluid determined in-situ to 800 °C and ∼800 MPa

The structure of H₂O-saturated silicate melts and of silicate-saturated aqueous solutions, as well as that of supercritical silicate-rich aqueous liquids, has been characterized in-situ while the sample was at high temperature (to 800 °C) and pressure (up to 796 MPa). Structural information was obtained with confocal microRaman and with FTIR spectroscopy. Two Al-bearing glasses compositionally along the join Na₂O•4SiO₂-Na₂O•4(NaAl)O₂-H₂O (5 and 10 mol% Al₂O₃, denoted NA5 and NA10) were used as starting materials. Fluids and melts were examined along pressure-temperature trajectories of isochores of H₂O at nominal densities (from PVT properties of pure H₂O) of 0.85 g/cm³ (NA10 experiments) and 0.86 g/cm³ (NA5 experiments) with the aluminosilicate + H₂O sample contained in an externally-heated, Ir-gasketed hydrothermal diamond anvil cell.Molecular H₂O (H₂O°) and OH groups that form bonds with cations exist in all three phases. The OH/H₂O° ratio is positively correlated with temperature and pressure (and, therefore, fugacity of H₂O, f_H2O) with (OH/H₂O°)^melt > (OH/H₂O°)^fluid at all pressures and temperatures. Structural units of Q³, Q², Q¹, and Q⁰ type occur together in fluids, in melts, and, when outside the two-phase melt + fluid boundary, in single-phase liquids. The abundance of Q⁰ and Q¹ increases and Q² and Q³ decrease with f_H2O. Therefore, the NBO/T (nonbridging oxygen per tetrahedrally coordination cations), of melt is a positive function of f_H2O. The NBO/T of silicate in coexisting aqueous fluid, although greater than in melt, is less sensitive to f_H2O.The melt structural data are used to describe relationships between activity of H₂O and melting phase relations of silicate systems at high pressure and temperature. The data were also combined with available partial molar configurational heat capacity of Qⁿ-species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H₂O content.     

Alteration of the upper oceanic crust,DSDP site 417: mineralogy and chemistry

Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts.

1. Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage.
2. Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H₂O, increase of Fe³⁺/Fe^T, and possibly some losses of Ca and Mg occurred during this stage.
3. Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H₂O, and occasionally P were added to the rocks, Fe³⁺/Fe^T increased, and Ca, Mg, Si and occasionally Al and Na were lost.
4. Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe³⁺/Fe^T and H₂O occurred.
5. Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H₂O, and CO₂ in the rocks.
6. A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage.

Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.     

Partition of Mg and Fe2+ in coexisting pyroxenes

A general theory for the partition of elements between coexisting, multicomponent phases is outlined and applied to data for Ca-rich pyroxene (Cap) — Ca-poor pyroxene (Op) assemblages from the Skaergaard and Bushveld intrusions and from charnockites. The intercrystalline partition of Mg and Fe²⁺ are studied separately rather than through the exchange reaction, MgSiO ₃ ^Cap +FeSiO ₃ ^Op FeSiO ₃ ^Cap +MgSiO ₃ ^Op .The separate distributions for x_MgSiO3> and x_FeSiO3> are quite distinct and demonstrate directly that solutions of both Mg and Fe²⁺ in the two pyroxenes are nonideal.     

Mg,Fe2+ site occupancies in coexisting pyroxenes

The separate distributions for MgSiO₃ and FeSiO₃ in coexisting pyroxenes from the Skaergaard and Bushveld intrusions and charnockites, which were introduced in an earlier communication, indicate directly that significant amounts of both Fe²⁺ and Mg were present in the M(2) site of the Ca-rich pyroxene at the temperature of final intercrystalline equilibration. The calculated Fe²⁺ M(2) site occupancy in the Ca-rich pyroxene increases markedly with decrease in total MgSiO₃ content but the corresponding Mg site occupancy appears largely independent of MgSiO₃. The mean value of the distribution constant for intracrystalline exchange in the Ca-rich pyroxene decreases, away from unity, with decreasing temperature of equilibration. Occupancy of Mg and Fe²⁺ in the M (2) site of the Ca-rich pyroxene effectively compensates for the expected variation in K _D with composition resulting from intracrystalline partition in Ca-poor pyroxene, and this largely accounts for the difference in K _D between igneous and metamorphic pyroxenes. The variation of the augite limb of the pyroxene solvus within the pyroxene quadrilateral is developed as a possible geothermometer.     

Mineralogy,petrology, and phase relations of glaucophane-lawsonite zone blueschists from the Tavşanli Region,Northwest Turkey

Glaucophane-lawsonite facies blueschists representing a metamorphosed sequence of basic igneous rocks, cherts and shales have been investigated northeast of the district of Tav?anli in Northwest Turkey. Sodic amphiboles are rich in magnesium reflecting the generally high oxidation states of the blueschists. Lawsonite has a very uniform composition with up to 2.5 wt.% Fe₂O₃. Sodic pyroxenes show an extensive range of compositions with all the end-members represented. Chlorites are uniform in their Al/(Al+Fe+Mg) ratio but show variable Fe/ (Fe+Mg) ratios. Garnets from metacherts are rich in spessartine (>50%) whereas those from metabasites are largely almandine. Pistacite rich epidote is found in metacherts coexisting with lawsonite. Phengites are distinctly higher in their Fe, Mg and Si contents than those from greenschist facies. Hematites with low TiO₂ are ubiquitous in metacherts. Fe²⁺/Mg partitioning between chlorite and sodic amphibole is strongly controlled by the calcium content of the sodic amphibole and ranges from 1.1 for low calcium substitution to 0.8 for higher calcium substitution. The Al/Fe³⁺ partition coefficient between sodic amphibole and sodic pyroxene is 2.1. A model system has been constructed involving projections from lawsonite, iron-oxide and quartz onto a tetrahedron with Na, Al, Fe²⁺ and Mg at its apices. Calcite is treated as an indifferent phase. The model system illustrates the incompatibility of the sodic pyroxene with chlorite in the glaucophanelawsonite facies; this assemblage is represented by sodic amphibole. Sodic amphibole compositions are plotted in terms of coexisting ferromagnesian minerals. Five major areas on the sodic amphibole compositional field are delineated, each associated with one of the following minerals: chlorite, stilpnomelane, talc, almandine, deerite.     

Reaction mechanisms,physical conditions,and mass transfer during hydrothermal alteration of mica and feldspar in granitic rocks from south-central Maine,USA

Samples of granitic rock from south-central Maine contain primary igneous minerals altered by hydrothermal fluids. The reaction mechanisms (by which the over-all mineralogical change during the alteration was accomplished) involve several different mineral-fluid reactions at different reaction sites in the rock. The reactions involve both molecular and charged species in solution. The different reaction sites correspond to alteration of different primary igneous minerals. Biotite is partially converted to chlorite+sphene; microcline to muscovite; plagioclase to various combinations of muscovite, epidote, and calcite. The different reaction sites are linked by exchange of ions: some reaction sites produce ions consumed at other sites and vice versa. Physical conditions during the hydrothermal event are estimated from mineralogical and thermochemical data: P = 3,500 (±300) bars; T =425 ° (± 25 °)C. The fluid was characterized by X _{CO 2} = 0–0.13; ln([K⁺]/[H⁺ ]) = 10.0; ln([Ca²⁺]/[H⁺]²)=9.1; ln([Na⁺]/[H⁺]) = 10.5; Fe/(Fe+Mg) = 0.95. Amounts of secondary minerals in altered rock, when compared to the inferred mineral reactions that formed them, indicate that small but significant amounts (0.01–0.3mol/ 1,000cm³ altered rock) of CO₂, H₂O, H⁺, and K⁺ were added to the granites by fluids during the alteration, as well as lesser amounts (< 0.01–0.03 mol/1,000cm³ altered rock) of Mg²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Na⁺, and Ti⁴⁺. The sole element leached from the granitic rocks during alteration was Ca in amounts 0.1–0.3 mol/1,000 cm³ rock. By estimating the composition of the hydrothermal fluids before and after reaction with the granites and by measuring the amount of material added to or subtracted from the granites during the alteration, the amount and volume of hydrothermal fluid involved can be calculated. Two independent calculations require minimum volumes in the range 100–1,000 cm³ fluid/1,000cm³ altered rock to participate in the hydrothermal event.     

Occurrence of wide-chain Ca-pyriboles as primary crystals in the Salton Sea Geothermal Field,California, USA

Amphiboles and pyroxenes occurring in the Salton Sea Geothermal Field were found to contain coherent intergrowths of chain silicates with other than double and single chain widths by using transmission and analytical electron microscopy. Both occur in the biotite zone at the temperature (depth) interval of 310° C (1,060 m) to 330° C (1,547m) which approximately corresponds to temperatures of the greenschist facies. The amphiboles occur as euhedral fibrous crystals occupying void space and are composed primarily of irregularly alternating (010) slabs of double or triple chains, with rare quadruple and quintuple chains. Primary crystallization from solution results in euhedral crystals. Clinopyroxenes formed mainly as a porefilling cement and subordinately as prismatic crystals coexisting with fibrous amphiboles. Fine lamellae of double and triple chains are irregularly intercalated with pyroxene. AEM analyses yield formulae (Ca_1.8Mg_2.9Fe_1.9Mn_0.1) Si₈O_21.8(OH)_1.8 (310° C) and (Ca_2.0Fe_2.5Mg_2.3) Si₈O_21.8 (OH)_2.0 (330° C) for amphiboles and (Ca_1.1Fe_0.6Mg_0.3) Si₂O₆ for clinopyroxene. Thermodynamic calculations at P_fluid=100 bar of equilibrium reactions of (1) 3 chlorite +10 calcite + 21 quartz = 3 actinolite + 2 clinozoisite + 8 H₂O + 10 CO₂ and (2) actinolite+ 3 calcite+ 2 quartz = 5 clinopyroxene + H₂O + 3 CO₂ using Mg-end member phases indicate that formation of amphibole and pyroxene require very water-rich conditions at temperatures below 330° C.Contribution No. 420 from the Mineralogical Laboratory, Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan     

Speciation of reduced C-O-H volatiles in coexisting fluids and silicate melts determined in-situ to ∼1.4 GPa and 800 °C

The structure of silicate melts in the system Na₂O·4SiO₂ saturated with reduced C-O-H volatile components and of coexisting silicate-saturated C-O-H solutions has been determined in a hydrothermal diamond anvil cell (HDAC) by using confocal microRaman and FTIR spectroscopy as structural probes. The experiments were conducted in-situ with the melt and fluid at high temperature (up to 800 °C) and pressure (up to 1435 MPa). Redox conditions in the HDAC were controlled with the reaction, Mo + H₂O = MoO₂ + H₂, which is slightly more reducing than the Fe + H₂O = FeO + H₂ buffer at 800 °C and less.The dominant species in the fluid are CH₄ + H₂O together with minor amounts of molecular H₂ and an undersaturated hydrocarbon species. In coexisting melt, CH₃ - groups linked to the silicate melt structure via Si-O-CH₃ bonding may dominate and possibly coexists with molecular CH₄. The abundance ratio of CH₃ - groups in melts relative to CH₄ in fluids increases from 0.01 to 0.07 between 500 and 800 °C. Carbon-bearing species in melts were not detected at temperatures and pressures below 400 °C and 730 MPa, respectively. A schematic solution mechanism is, Si-O-Si + CH₄?Si-O-CH₃+H-O-Si. This mechanism causes depolymerization of silicate melts. Solution of reduced (C-O-H) components will, therefore, affect melt properties in a manner resembling dissolved H₂O.     

Arsenic removal from aqueous solutions by mixed magnetite–maghemite nanoparticles

In this study, magnetite–maghemite nanoparticles were used to treat arsenic-contaminated water. X-ray photoelectron spectroscopy (XPS) studies showed the presence of arsenic on the surface of magnetite–maghemite nanoparticles. Theoretical multiplet analysis of the magnetite–maghemite mixture (Fe₃O₄-γFe₂O₃) reported 30.8% of maghemite and 69.2% of magnetite. The results show that redox reaction occurred on magnetite–maghemite mixture surface when arsenic was introduced. The study showed that, apart from pH, the removal of arsenic from contaminated water also depends on contact time and initial concentration of arsenic. Equilibrium was achieved in 3 h in the case of 2 mg/L of As(V) and As(III) concentrations at pH 6.5. The results further suggest that arsenic adsorption involved the formation of weak arsenic-iron oxide complexes at the magnetite–maghemite surface. In groundwater, arsenic adsorption capacity of magnetite–maghemite nanoparticles at room temperature, calculated from the Langmuir isotherm, was 80 μmol/g and Gibbs free energy (∆G⁰, kJ/mol) for arsenic removal was −35 kJ/mol, indicating the spontaneous nature of adsorption on magnetite–maghemite nanoparticles.     

Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilátero Ferrífero, Brazil

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D₁, D₂ and D₃). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D₃. The orebodies constitute sulfide-bearing D₁-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H₂O-rich containing CO₂ + Na⁺ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H₂O is indicated. The CO₂ is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H₂O to CO₂-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.     

Thermodynamic analysis of the system Fe2O3-FeTiO3

The cluster variation method, in the single prism approximation, is used to model phase relations in the system, Fe₂O₃-FeTiO₃. Ordering in FeTiO₃ is analyzed, and it is shown that the stabilization of FeTiO₃ (relative to mechanical mixing of Fe₂O₃ and Ti₂O₃) includes: (1) a contribution from the redox reaction, Fe³⁺+Ti³⁺→Fe²⁺+Ti⁴⁺ (ΔH _redox～?70kJ mole^?1); and (2) a contribution from ordering (ΔH _OD～?8kJ mole^?1). A theoretical phase diagram is presented and compared with available experimental data. Semiquantitative agreement between theory and experiment (on the location of phase boundaries) is achieved; but, owing to the paucity of experimental data on coexisting phases, these results may be fortuitous.     

Characterization of leached layers on olivine and pyroxenes using high-resolution XPS and density functional calculations

High-resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of olivine [(Mg_0.87Fe_0.13)₂SiO₄], bronzite [(Mg_0.8Fe_0.2)₂Si₂O₆] and diopside [Ca(Mg_0.8Fe_0.2)Si₂O₆] were collected before and after leaching in pH ∼2 solutions with the Kratos magnetic confinement charge compensation system which minimizes differential charge broadening. The leached samples yield Si 2p, Mg 2p, Ca 2p and O 1s XPS spectral linewidths and lineshapes similar to those collected from the respective pristine samples prior to leaching. As with previous XPS studies on crushed samples, our broadscan XPS spectra show evidence for initial, preferential leaching of cations (i.e., Ca²⁺ and Mg²⁺) from the near-surface of these minerals. The O 1s spectra of leached olivine and pyroxenes show an additional peak due to OH⁻, which arises from H⁺ exchange with near-surface cations (Ca²⁺ and Mg²⁺) via electrophilic attack of H⁺ on the M-O-Si moiety to produce the H₂Mg(M1)SiO_4(surf) complex at olivine surfaces, and two complexes, H₂Mg(M1)Si₂O_6(surf) and H₄Si₂O_6(surf) at diopside and enstatite surfaces. The olivine and pyroxene surface complexes H₂Mg(M1)SiO_4(surf) and H₂Mg(M1)Si₂O_6(surf) have been proposed previously, but the second pyroxene surface complex H₄Si₂O_6(surf) has not. Two electrophilic reactions occur in both olivine and pyroxene. For olivine, the more rapid attacks the M2-O-Si moiety producing H₂Mg(M1)SiO_4(surf); while the second attacks the M1-O-Si moiety ultimately producing H₄SiO₄ which is released to solution. For pyroxenes, the first electrophilic reaction produces H₂Mg(M1)Si₂O_6(surf), while the second produces.H₄Si₂O_6(surf). These two reactions are followed by a nucleophilic attack of H₂O (or H₃O⁺) on Si of H₄Si₂O_6(surf). This reaction is responsible for rupture of the brigding oxygen bond of the Si-O-Si moiety and release of H₄SiO₄ to solution. The intensity of the OH⁻ peak for the leached pyroxenes is about double the OH⁻ intensity for the leached olivine, consistent with the equivalent of about a monolayer of the above surface complexes being formed in all three minerals.Valence band XPS spectra and density functional calculations demonstrate the remarkable insensitivity of the valence band to leaching of Ca²⁺ and Mg²⁺ from the surface layers. This insensitivity is due to a dearth of Ca and Mg valence electron density in the valence band: the Ca-O and Mg-O bonds are highly ionic, with metal-derived s orbital electrons taking on strong O 2p character. The valence band spectrum of leached olivine shows an additional very weak peak at about 13.5 eV, which is assigned to Si 3s valence orbitals in the surface complex H₂Mg(M1)SiO₄, as indicated by high quality density functional calculations on an olivine where Mg²⁺ in M2 is replaced by 2H⁺. The intensity of this new peak is consistent with formation of the equivalent of a monolayer of the surface complex.     

Solution mechanisms of silicate in aqueous fluid and H2O in coexisting silicate melts determined in-situ at high pressure and high temperature

The structure of H₂O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na₂O·4SiO₂–H₂O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H₂O at nominal densities, ρ_fluid, (from EOS of pure H₂O) of 0.90 and 0.78 g/cm³. With ρ_fluid = 0.78 g/cm³, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 °C) and pressure (677 MPa), whereas with ρ_fluid = 0.90 g/cm³, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 °C, 0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 °C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na₂O·4SiO₂ starting material.Molecular H₂O (H₂O°) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H₂O)-ratio is positively correlated with temperature. The OH/(H₂O)° in melts is greater than in coexisting fluids. Structural units of Q³, Q², Q¹, and Q⁰ type are observed in all phases under all conditions. An expression of the form, 12Q³ + 13H₂O2Q² + 6Q¹ + 4Q⁰, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a ΔH of the reaction near 425 kJ/mol.     

Porphyry to epithermal transition in the Agua Rica polymetallic deposit, Catamarca, Argentina: An integrated petrologic analysis of ore and alteration parageneses

Agua Rica (27°26′S–66°16′O) is a world class Cu–Au–Mo deposit located in Catamarca, Argentina. In the E–W 6969400 section examined, the Seca Norte and the Trampeadero porphyries that have intruded the metasedimentary rock are cut by interfingered igneous and hydrothermal heterolithic and monolithic breccias, and sandy dikes. Relic biotite and K-feldspar of the early potassic alteration (370° to > 550 °C) with Cu (Mo–Au) mineralization are locally preserved and encapsulated in a widespread, white mica + quartz + rutile or anatase halo (phyllic alteration) with pyrite + covellite that suggests fluids with temperatures ≤ 360 °C and high f(S₂). The Trampeadero porphyry and the surrounding metasedimentary rock with phyllic alteration have molybdenite in stringers and B-type quartz veinlets and the highest Mo grades (> 1000 ppm).Multistage advanced argillic alteration overprinted the earlier stages. Early andalusite ± pyrite ± quartz is preserved in the roots of the argillic halo rimmed by an alumina–silica material and white micas. This alteration assemblage is considered to have been formed at temperatures ≥ 375 °C from condensed magmatic vapor. At higher levels, pyrophyllite replaces muscovite and illite in clasts of hydrothermal breccias in the center and east sector of the study section, suggesting temperatures of 280 to 360 °C. Clasts of vuggy silica in the uppermost levels of the central breccia, indicates that at lower temperatures (< 250 °C), fluids reached very low pH (pH < 2). In this early stage of the advanced argillic alteration, hydrothermal fluids seem to have not precipitated sulfides or sulfosalts.Hydrothermal brecciation was concurrent with fluid exsolution (↑? V), which precipitated intermediate-temperature advanced argillic alunite (svanbergite + woodhouseite) ± diaspore ± zunyite as breccia cement along with abundant covellite + pyrite + enargite ± native sulfur ± kuramite at intermediate depths and in lateral transitional zones to unbrecciated rocks. This mineral assemblage indicates temperatures near 300 °C, oxidized and silica-undersaturated hydrothermal fluids with high sulfur fugacity to prevent gold precipitation. Multiple generations of pyrite, emplectite, colusite, Pb- and Bi-bearing sulfosalts, and native sulfur with Au and Ag, accompanied by alunite introduction in the upper level breccias, probably occurred at lower temperatures, but still high sulfur and oxygen activity. An independent Zn and Pb (as galena) mineralization stage locally coincides with Au–Ag and sulfosalts, and advanced at depth, controlled by fractures and overprinting much of the previous mineralization. A later paragenesis of veinlets of alunite + woodhouseite + svanvergite + pyrite ± enargite that cut the phyllic halo suggests temperatures ~ 250 °C and without woodhouseite + svanvergite, temperatures ~ 200 °C. Kaolinite occurs in the phyllic halo as a late mineral in clots and in veinlets thus, in this zone, the fluid had cooled enough for its formation.     

Thermodynamics of multicomponent pyroxenes: III. Calibration of Fe2+(Mg)-1, TiAl2(MgSi2)-1, TiFe 2 3+ (MgSi2)-1, AlFe3+(MgSi)-1, NaAl(CaMg)-1, Al2(MgSi)-1 and Ca(Mg)-1 exchange reactions between pyroxenes and silicate melts

A thermodynamic model for the Gibbs free energy of igneous pyroxenes with the general formula [Na, Ca, Fe²⁺, Mg]^M2[Fe²⁺, Mg, Ti, Al, Fe³⁺]^M1[Al, Fe³⁺, Si]^TetSiO₆ is calibrated from experimentally determined compositions of coexisting pyroxene and silicate melt. The model is based upon the general formulation, and relies upon the calibration of the “quadrilateral” subsystem, previously published by the present authors. The calibration database of pyroxene-liquid equilibria spans a broad spectrum of temperature, pressure and oxygen fugacity conditions, ranging from 1000°–1600°C, 0.001–30 kbar and iron-wüstite to air. Chemical potentials of endmember pyroxene components as well as exchange potentials between pyroxenes and coexisting liquids are defined utilizing the present authors' thermodynamic melt model. Model parameters are extracted from these relations by regression analysis. The resulting model and derivative endmember properties are internally consistent with an existing standard state thermodynamic database. The success of the model and its applicability to igneous petrogenesis are demonstrated by comparing calculated and experimentally determined liquidus compositions, temperatures and symmetry states for pyroxenes crystallizing from a variety of silicate melts, ranging in composition from tholeiites and angrites through rhyolites to potash ankaratrites.     

Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures

A revised regular solution-type thermodynamic model for twelve-component silicate liquids in the system SiO₂-TiO₂-Al₂O₃-Fe₂O₃-Cr₂O₃-FeO-MgO-CaO-Na₂O-K₂O-P₂O₅-H₂O is calibrated. The model is referenced to previously published standard state thermodynamic properties and is derived from a set of internally consistent thermodynamic models for solid solutions of the igneous rock forming minerals, including: (Mg,Fe²⁺,Ca)-olivines, (Na,Mg,Fe²⁺,Ca)^M2 (Mg,Fe²⁺, Ti, Fe³⁺, Al)^M1 (Fe³⁺, Al,Si)₂ ^TETO₆-pyroxenes, (Na,Ca,K)-feldspars, (Mg,Fe²⁺) (Fe³⁺, Al, Cr)₂O₄-(Mg,Fe²⁺)₂ TiO₄ spinels and (Fe²⁺, Mg, Mn²⁺)TiO₃-Fe₂O₃ rhombohedral oxides. The calibration utilizes over 2,500 experimentally determined compositions of silicate liquids coexisting at known temperatures, pressures and oxygen fugacities with apatite ±feldspar ±leucite ±olivine ±pyroxene ±quartz ±rhombohedral oxides ±spinel ±whitlockite ±water. The model is applicable to natural magmatic compositions (both hydrous and anhydrous), ranging from potash ankaratrites to rhyolites, over the temperature (T) range 900°–1700°C and pressures (P) up to 4 GPa. The model is implemented as a software package (MELTS) which may be used to simulate igneous processes such as (1) equilibrium or fractional crystallization, (2) isothermal, isenthalpic or isochoric assimilation, and (3) degassing of volatiles. Phase equilibria are predicted using the MELTS package by specifying bulk composition of the system and either (1) T and P, (2) enthalpy (H) and P, (3) entropy (S) and P, or (4) T and volume (V). Phase relations in systems open to oxygen are determined by directly specifying the f _{o 2} or the T-P-f _{o 2} (or equivalently H-P-f _{o 2}, S-P-f _{o 2}, T-V-f _{o 2}) evolution path. Calculations are performed by constrained minimization of the appropriate thermodynamic potential. Compositions and proportions of solids and liquids in the equilibrium assemblage are computed.     

Crystallization trends of pyroxenes from alkaline subvolcanites of Punta delle Pietre Nere (Gargano,Southern Italy)

Summary The results of microprobe analyses of clinopyroxenes from alkaline melasyenites and layered melagabbros, produced by intra-plate magmatism of Paleocene age at Punta delle Pietre Nere, are here given and discussed.The analysed pyroxenes range from diopsidic to acmite-rich compositions.The first crystallized pyroxenes (diopside) show Al^VI contents suggesting shallow depths of crystallization. In addition pyroxenes from melasyenite and those from melagabbro display different Cr contents, Al/Ti and Mg/(Mg+Fe²⁺+Fe³⁺) ratios confirming their crystallization from melts produced by different parental liquids.Diopsides and salites show an overall trend towards high Al, Ti and Fe³⁺, suggesting that the crystallization occurred under decreasing SiO₂/Al₂O₃ ratios and under relatively highpH₂O–pO₂ conditions.Pyroxenes from the Pietre Nere melasyenite show a progressive variation towards acmite rich compositions at Mg/(Mg+Fe²⁺+Fe³⁺) lower than 0.5; those from the layered melagabbro, instead, show a continuous enrichment in Ca Fe³⁺ AlSiO₆. This different behaviour is due to the co-crystallization, with the latest pyroxenes, of phases with different K/Na and Si/Al ratios.

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Kristallisations-Tendenzen der Pyroxene aus Alkali-Subvulkaniten auf Punta delle Pietre Nere (Gargano, Süditalien)

Zusammenfassung Es werden die Ergebnisse der Mikrosonden-Untersuchungen von Klinopyroxenen aus Alkali-Melasyeniten und schichtigen Alkali-Melagabbros, die durch Intra-plate-Magmatismus paläozenen Alters auf Punta delle Pietre Nere entstanden sind, beschrieben und erörtert.Die untersuchten Pyroxene reichen von diopsidischen bis zu Akmit-reichen Zusammensetzungen.Die zuerst kristallisierten Pyroxene (Diopsid) zeigen Al^VI-Gehalte, die auf geringe Tiefe des Kristallisationsvorganges hinweisen. Dazu zeigen die Pyroxene aus dem Melasyenit und aus dem Melagabbro unterschiedliche Cr-Gehalte; die Al/Ti- und Mg/(Mg+Fe²⁺+Fe³⁺)-Verhältnisse bestätigen deren Kristallisation aus Schmelzen, die aus unterschiedlichen Ursprungsmagmen stammen.Die Diopside und Salite zeigen eine einheitliche Tendenz zu hohem Al-, Ti- und Fe³⁺-Gehalt; dies deutet darauf hin, daß die Kristallisation unter abnehmenden SiO₂/Al₂O₃-Verhältnissen und unter relativ hohenpH₂O–pO₂-Bedingungen stattfand.Die Pyroxene aus dem Punta delle Pietre Nere-Melasyenit zeigen eine zunehmende Änderung zu Akmit-reichen Zusammensetzungen bei weniger als 0,5 Mg/(Mg+Fe²⁺+Fe³⁺); die Pyroxene aus dem schichtig differenzierten Melagabbro zeigen dagegen eine allmähliche Zunahme von CaFe³⁺AlSiO₆. Dieses unterschiedliche Verhalten rührt daher, daß Mineralphasen mit unterschiedlichen K/Na- und Si/Al-Verhältnissen zugleich mit den zuletzt gebildeten Pyroxenen kristallisierten.

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With 6 Figures     

Metasomatic oxidation of upper mantle periodotite

Examination of Fe³⁺ in metasomatized spinel peridotite xenoliths reveals new information about metasomatic redox processes. Composite xenoliths from Dish Hill, California possess remnants of magmatic dikes which were the sources of the silicate fluids responsible for metasomatism of the peridotite part of the same xenoliths. Mössbauer spectra of mineral separates taken at several distances from the dike remnants provide data on Fe³⁺ contents of minerals in the metasomatized peridotite. Clinopyroxenes contain 33% of total iron (Fe_T) as Fe³⁺ (Fe³⁺/Fe_T=0.33); orthopyroxenes contain 0.06–0.09 Fe³⁺/Fe_T; spinels contain 0.30–0.40 Fe³⁺/Fe_T; olivines contain 0.01–0.06 Fe³⁺/Fe_T; and metasomatic amphibole in the peridotite contains 0.85–0.90 Fe³⁺/Fe_T. In each mineral, Fe³⁺ and Fe²⁺ cations per formula unit (p.f.u.) decrease with distance from the dike, but the Fe³⁺/Fe_T ratios of each mineral do not vary. Clinopyroxene, spinel, and olivine Fe³⁺/Fe_T ratios are significantly higher than in unmetasomatized spinel peridotites. Metasomatic changes in Fe³⁺/Fe_T ratios in each mineral are controlled by the oxygen fugacity of the system, but the mechanism by which each phase accommodates this ratio is affected by crystal chemistry, kinetics, rock mode, fluid composition, fluid/rock ratio, and fluid-mineral partition coefficients. Ratio increases in pyroxene and spinel occur by exchange reactions involving diffusion of Fe³⁺ into existing mineral grains rather than by oxidation of existing Fe²⁺ in peridotite mineral grains. The very high Fe³⁺/Fe_T ratio in the metasomatic amphibole may be a function of the high Fe³⁺/Fe_T of the metasomatic fluid, crystal chemical limitations on the amount of Fe³⁺ that could be accommodated by the pyroxene, spinel, and olivine of the peridotite, and the ability of the amphibole structure to accommodate large amounts of 3 + valence cations. In the samples studied, metasomatic amphibole accounts for half of the bulk-rock Fe₂O₃. This suggests that patent metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe²⁺ and Fe³⁺ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe³⁺. With increasing distance from the dike, the ratio of added Fe³⁺ to added Fe²⁺ increases. The high Fe³⁺/Fe_T of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe³⁺/added Fe²⁺ in pyroxenes and spinel suggest that the Fe³⁺/Fe_T ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe³⁺ and C–O–H volatile species were concentrated in the fluid, increasing the fluid Fe³⁺/Fe_T.