Subduction zones are one of the most characteristic features of planet Earth. Convergent plate junctions exert enormous influence on the formation and recycling of continental crust, and they are also responsible for major mineral resources and earthquakes, which are of crucial importance for society. A subduction-related geologic unit containing high-pressure rocks occurs in the Barragan area (Valle del Cauca Department) on the western flank of the Central Cordillera of the Colombian Andes. Blueschists and amphibolites, serpentinized meta-ultramafic rocks, graphite-chlorite-muscovite-quartz schists, protocataclasites, and graphite-chlorite-andalusite-andesine-garnet-muscovite +/- titanite schists are exposed in this region. In spite of the petrotectonic importance of blueschists, the high-pressure metamorphism of the Central Cordillera of Colombia has been rarely studied. New geochemical data indicate that protoliths of the blueschist- and amphibolite-facies rocks possessed normal mid-ocean ridge basalt bulk compositions. Ar-40/Ar-39 geochronology for a metapelite rock associated with the blueschists shows a plateau age of similar to 120 million years. We suggest that high-P/T conditions were present from similar to 150 to 125 Ma, depending on the model of generation and exhumation considered.

Martian meteorite Allan Hills (ALH) 84001 contains sub-micron magnetite grains, suggested to be of biogenic origin, in its globules of Fe–Mg carbonate mineral. There is disagreement on whether the low Mg content of the magnetite could only arise from biological metabolism (Treiman, 2003, Thomas-Keprta et al., 2009). However, constraints on the magnetite’s biogenicity are far less certain than had been inferred. The thermochemical bases for the equilibrium calculations are reviewed in detail; there are inconsistencies and gaps in fundamental data for siderite, macromolecular carbons, and magnesioferrite. The calculations of Treiman (2003), assuming formation of magnetite via “siderite=magnetite+CO2+CO”, are incorrect because of a flaw in the computer code used. The corrected location of this equilibrium (Thomas-Keprta et al., 2009) is no longer crucial, because of recent finds that the magnetite grains are associated with macromolecular carbon; this implies that the dominant magnetite-forming reaction was “siderite=magnetite+CO2+C”. From the location of this equilibrium, using the corrected computer code and best available thermochemical data, the Mg-poor magnetite grains (and macromolecular carbon) in carbonates in ALH 84001 could have formed by decomposition of the carbonates at geologically reasonable pressures and temperatures. The low-Mg compositions of the magnetite grains remain consistent with an abiotic origin within the known geological history of ALH 84001.

Quantitative thermobarometers require selection of a univariant reaction, consideration of dilutions in all phases and application to fully buffered systems. Empirical thermobarometers include Al-IV in chlorite, Al in hornblende, Ti in biotite, phengite barometry and various clay and organic transformations. For the most part they were not evaluated with univariant reactions, buffering phases or corrections for solid or fluid solutions and as a result they are not adequately calibrated.Recently proposed thermometers in the system CaO-ZrO2-TiO2-SiO2 (CazrtiQ) include Ti in zircon, Ti in quartz, Zr in rutile and Zr in sphene. Most of the original calibrations assumed that pressure (P) had no influence on the thermometer despite a significant volume change in buffering reactions. The Zr in sphene was calibrated as a thermobarometer, the P dependence of Ti in zircon and Zr in rutile has now been evaluated and Ti in quartz probably has a P dependence as well. Recent work indicates that zircon thermometer has a -80 degrees to -90 degrees C temperature (T) correction when applied to high level granitic intrusions crystallised at 1-2 kbar and that these are minima in the absence of rutile. Calculations of a(TiO2) from ilmenite-magnetite in the Bishop tuff gives 0.4 +/- 0.02, suggesting that granitoids may have a lower a(TiO2) than previously thought, which will shift T back by +30 to +80 degrees C if the zircon can be shown to have equilibrated with the oxides. Combination of thermobarometers in CazrtiQ offers a new set of phase equilibria potentially of wide use in metamorphic and igneous rocks.

The heat capacity (C,,) of two synthetic spessartine samples (Sps) was measured oil 20-30 mg-size samples in the temperature range 2-864 K by relaxation calorimetry (RC) and differential scanning calorinictry (DSC). The polycrystalline spessartine samples were synthesized in two different laboratories at high pressures and temperatures from glass and oxide-mixture starting materials and characterized by X-ray powder diffraction and electron-microprobe analysis. The low-temperature heat capacity data show a prominent lambda transition with a peak at 6.2 K, which is interpreted to be the result of a paramagnetic-antiferromagnetic phase transition. The DSC data around ambient T agree excellently with the RC data and can be represented by the C-p polynomial for T> 250 K:C-p(Sps) = 610 - 3060. T-0.5 - 1.45.10(7).T-2 + 1.82.10(9).T-3.Integration of the low temperature C,, data yields a calorimetric standard entropy for the two different samples of S-o = 334.6 +/- 2.7 J/mol K and 336.0 +/- 2.7 J/mol.K. The preferred standard third-law entropy for spessartine is S-o = 335.3 +/- 3.8 J/mol.K, which is the mean value from the two separate determinations. The lattice (vibrational) heat capacity of spessartine was calculated using the single-parameter phonon dispersion model of Komada and Westrum. The lattice entropy at 298.15 K is S-ulb(298.15) = 297.7 J/mol.K, which represents 89% of the calorimetric entropy. The magnetic heat capacity and entropy of spessartme, S-mag at 298.15 K were also calculated. The S-mag of the two samples is 38.7 and 37.4 J/mol.K, which is 87% and 83% of the maximum possible magnetic entropy given by 3Rln6 = 44.7 J/mol . K. Published model-dependent lattice-dynamic calculations S-ulb(298.15) are analyzed and compared to the experimental data.Using the calorimetrically determined S-o and the C-p polynomial for spessartine, together with high P-T experimental phase-equilibrium data oil Mn2+-Mg partitioning between garnet and olivine, allows calculation of the standard enthalpy of formation of spessartine. This gives Delta H-f,Sps(o) = -5693.6 +/- 1.4 kJ/mol, a value nearly 50 kJ more negative than some published values. The Gibbs free energy of spessartine was also calculated and gives Delta G(f,Sps)(o) = -5364.3 kJ/mol at 298.15 K. The new standard entropy and enthalpy of formation values for spessartine lead to revised estimates for the enthalpies of formation of other Mn2+-silicates. Resulting Delta H-f(o) values for Mn-biotite, Mn-chlotire, Mn-cordierite, Mn-staurolite and Mn-chloritoid are 7-34 kJ more negative than their values listed in the thermodynamic database "THERMOCALC".As an example, the new standard entropy and enthalpy of formation for spessartine have been applied to Mn-Fe partitioning between garnet and orthopyroxene from manganiferous iron formations. Excellent agreement between the predicted and observed distribution coefficient was obtained. (C) 2009 Elsevier Ltd. All rights reserved.

Understanding how Ti partitions between the Si and Zr sites in zircon is crucial for developing the Ti-in-zircon geothermometer. Energies calculated using quantum-mechanical methods (VASP, CASTEP, Dmol(3), Crystal) were used to compare the relative favorability of substitution into each site at pressures ranging from 0 to 10 GPa. The results of these quantum-mechanical calculations were used in Monte-Carlo calculations to derive the excess enthalpy of mixing (Delta H-excess), entropy of mixing (Delta S-excess), and free energy of mixing (Delta G(excess)) for the binaries ZrSiO4-ZrTiO4 and ZrSiO4-TiSiO4 (assuming that all compositions have zircon structure) at temperatures ranging from 333 K to 3000 K, and estimates are made of the maximum amount of Ti that may be incorporated into each site as a function of temperature and pressure. The results are considered in thermodynamic reference to other oxides, such as SiO2, ZrO2, and TiO2, that are involved in substitution reactions. At pressures below about 3.5 GPa, substitution into the Si site is more thermodynamically favorable and thus dominates, whereas at higher pressures, substitution into the Zr site becomes more important in zircon. The latter result suggests that the reaction TiO2(rutile) + ZrSiO4(zircon) <-> TiSiO4(zircon) + ZrO2(baddeleyite) becomes predominant for the substitution of Ti-in-zircon for ultra-high pressure assemblages. The molar volume of the theoretical zircon-structured compound ZrTiO4 was calculated using quantum Mechanics (VASP, CASTEP, Dmol(3)) and determined to be 44.21 +/- 0.45 cm(3)/mol. The resulting AV for the reaction ZrSiO4(zircon) + TiO2(rutile) = ZrTiO4(zircon) + SiO2(quartz) is doubled. The Clapeyron slope (dP/dT) of the reaction is halved, and the pressure correction to the Ti-in-zircon thermometer is twice as large as a previous estimate.

Quantitative thermobarometry of inclusions in zoned garnet from a Franciscan eclogite block record a counter-clockwise P–T path from blueschist to eclogite and back. Garnet retains prograde zoning from inclusion-rich Alm52Grs30Pyp6Sps12 cores to inclusion-poor Alm62Grs25Pyp12Sps1 mantles, with overgrowths of highly variable composition. Barometry using the Waters–Martin version of the garnet–phengite–omphacite thermobarometer yields conditions of 7–15 kbar, 400–500°C (garnet cores), 18–22 kbar, ∼550°C (mantles), and 10–14 kbar, 350–450°C (overgrowths), in agreement with clinozoisite–sphene–rutile–garnet–quartz barometry. These pressures are ∼10–15 kbar less than those obtained using more recent, fully thermodynamic calibrations of the phengite–omphacite–garnet thermobarometer. Low early temperatures suggest that the block was subducted in a thermally mature subduction zone and not at the inception of subduction when prograde temperature is expected to be higher. Franciscan high-grade blocks likely represent crust subducted throughout the history of this convergent margin, rather than only at the inception of the subduction zone.

The new mineral species coskenite-(Ce) [(Ce,Nd,La)z(SO+)z(CzO+).8HzO] occurs as transparent, pinkto cream-colored clusters of tabular crystals 0.1 to 1.0 mm across embedded within, or in cavities in, epsomite and "hair salts" (principally apjohnite) at AIum Cave Bluff, Great Smoky Mountains, Tennessee. A complex suite of sulfates occurs in the soil or on the ceiling of a rock shelter, as a result of weathering of pyritic phyllite and evaporation of the resulting solutions. Coskrenite-(Ce) is closely assocrated wi th two other new minerals, lev insoni te-(Y) [ (Y,Nd,Ce)Al(SO4)2(C2O4).12H2O] and zugshunst i te-(Ce) [(Ce,Nd,La)A(SO+)z(CzO+).72H2O]. They are thefirst natural examples of rare-earth oxalate salts. Coskrenite-(Ce) is triclinic, space group Pl, a 6.007(1), , 8.368 (2), c 9.189Q) A, a99.90(2), B 105.55 (2), t l0'7 .71(2)", V 4O'7 .4Q) 43, and Z = I. The basic structural unit is a four-membered ring of two REE(O,H2O)e polyhedra and two SOa tetrahedra, each sulfate tetrahedron sharing two of its vertices with two REE polyhedra These rings are linked into chains, and adjacent chains are linked by oxalate groups and hydrogen bonds into sheets oriented parallel to {001 }. Adjacent sheets are weakly bonded together through hydrogen bonds. The REE site is nine-coordinated by a distorted monocapped square antiprism of five O and four H2O, withan average REE-O distance of 2.518 A. The strongestlines in the powder-diffraciion pauein Id in L(D(hkl)l arc:5.4S(100X110), 8.52(70)(001), 6.72(60)(011),3.84(60)(020,110), 4.26(s0X002,012),3.3s(40)(022), and2.744(40)022,212,1?,0,220). Coskrenite-(Ce) is biaxi a l nega t i ve ,a=1 .544 (4 ) , 9=1 .578 (a ) , 1=1 .602 (4 ) , 2V (obs . )=65 (10 ) ' , 2V (ca l c . )=69 (3 ) "d i spe rs i onmed ium( r>v ) , and Z Ac = 21o (in plane ofcleavage). The mineral is named after its discoverer, T. Dennis Coskren.

Published electron microprobe analyses of mattheddleite, a lead sulpho-silicate apatite from Leadhills, Scotland, have 9-13% IV site deficiencies. However, galena was used as a standard for S, which suggested that low S resulted from a shift in the S-K alpha peak. Wavelength scans with a PET crystal show that the S-K alpha peak is shifted down by 0.0026 angstrom for sulphates relative to sulphides. Quantitative analyses show a similar to 30% increase of S in mattheddleite using a celestite standard, which fills the IV site, but with Si > S, on average Pb5S1.2Si1.8O11.7Cl0.6(OH)(0.4). Direct analysis of oxygen with the electron microprobe implies that the charge imbalance engendered from the inequality of Si and S is compensated with substitution of a vacancy (square), as in Pb5S1.2Si1.8[O-11.7 square(0.3)][Cl-0.6(OH)(0.4)] or Pb5S1.2Si1.8[O-11.7(Cl,OH)(0.3)][Cl,OH)(0.7)square(0.3)]. Calculation of OH as l-Cl suggests the presence of both OH- and Cl-dominant mattheddleite at Leadhills, but direct analysis of H is needed to confirm the dominance of OH in the channel site. Wavelength-dispersive analyses of S in apatite and other sulphates must be undertaken with sulphate standards: use of sulphide standards yields a negative error on the order of 10-20% in the resultant S concentration. Reactions of mattheddleite with other Pb minerals at Leadhills show that their stability depends on fluid composition as well as pressure and temperature. An X-ray map of Cl shows complex zoning between Cl-poor and Cl-rich mattheddleite, recording rapid changes in the fluid chemistry during late-stage hydrothermal processes at Leadhills.

Precise U–Pb monazite and 40Ar/39Ar hornblende ages have been obtained from three locations in the high-grade Archean core of the Wind River Range, Wyoming. Monazites from metapelites in the Paradise Basin, Medina Mountain, and Crescent Lake have U–Pb ages of 2718 ± 1, 2633 ± 5, and 2657 ± 2 Ma, respectively. Hornblendes from amphibolites and granulites from the same locations yield plateau 40Ar/39Ar isotope ages of 2652 ± 11, 2572 ± 9, and 2527 ± 8 Ma, respectively, and are interpreted as cooling ages from the last thermal event. The three localities experienced similar peak pressure–temperature conditions. The timing of high-grade metamorphism in the Paradise Basin is older than the emplacement of large subjacent batholiths at 2.63–2.67 Ga. Calculated cooling rates based on monazite–hornblende pairs of 3.4 ± 1.0 °C/Ma for Paradise Basin, 3.8 ± 1.2 °C/Ma for Medina Mountain, and 1.7 ± 0.4 °C/Ma for Crescent Lake cannot be used to rule out reheating during subsequent pluton emplacement. The markedly slowe...

Metapelites with unusually high levels of several transition elements, including V, Cr and Zn, have been found in Archean rocks from the Paradise Basin area of the Wind River Range, Wyoming. These rocks contain sillimanite with up to 0.56 wt% V2O3 and 0.32% Cr2O3. Coexisting minerals are also enriched in V and Cr, including garnet, hercynite and biotite. The K-D values for V and Cr versus Al have been calculated among garnet, sillimanite and hercynite. The partitioning of V/Al is nearly equal (K-D = 0.98) between garnet and sillimanite, whereas Cr/Al is strongly favored by garnet (K-D = 6.9). Hercynite strongly partitions V/Al over both garnet and sillimanite (K-D approximate to 15) and very strongly partitions Cr/Al over garnet and sillimanite (K-D of 27 and 189, respectively). The hercynite also contains most of the Zn in the rock, as 10% of the gahnite component in solid solution. The P-T conditions recorded in the cores of matrix minerals indicate peak metamorphism in the granulite facies at > 9.5 +/- 0.1 kbar and 780 +/- 50 degrees C. Rim compositions in the matrix indicate lower-grade conditions that constrain a portion of the retrograde P-T path. There is no evidence of introduction of elements along veins or fractures, and the source of the V, Cr, and Zn is presumably provided by weathering of older source materials that were deposited as clastic grains.

The occurrences of natural coexisting feldspars including hyalophane and also celsian delineate two-, three- and possibly four-phase fields in the system BaAl2Si2O8-CaAl2Si2O8-NaAlSi3O8-KAlSi3O8. Hyalophane occurs with albite and microcline in a very low grade (anchizonal to epizonal) metasedimentary association from the Uppony Mountains, Hungary, and in Grenville marbles from Ontario. Analyses show very little Ba in albite and only limited Na in hyalophane. One marble from the garnet zone has albite (Ab(95-98)), oligoclase (An(17)Sl(3)Ab(80)), hyalophane (Cn(65)Sl(3)An(2)Ab(9)Or(24)) and celsian (Cn(92)Sl(3)An(1)Ab(2)Or(2)). The albite and oligoclase are complexly intergrown and may indicate unmixing during cooling. A marble in the sillimanite zone contains albite (Ab(95-98)Or(2-3)), oligoclase (An(17)An(2)Sl(3)Ab(33)Or(24)), hyalophane (Cn(65)Sl(3)An(2)Ab(9)Or(24)), and an inclusion of celsian (Cn(67)An(1)Sl(2)Ab(2)Or(91)) in an albite. Sanidine from the Peshtigo monzonite in Wisconsin unmixed to a symplectic perthite with barian microcline (Cn(8-11)An(2)Ab(8-13)Or(75-80)) and oligoclase (CnlAn(18)Ab(79)Or(2)). The former compositions of the ternary igneous feldspars (Cn(3)An(9)Ab(46)Or(42), Cn(1)An(18)Ab(69)Or(12)) were obtained by reintegration.The Na content of hyalophane equilibrated with albite is correlated with metamorphic grade. Hyalophane has 5 +/- 2 mol % Ab in very low-grade associations, 10 +/- 3 mol % Ab in the greenschist facies, 16 +/- 2 mol % Ab in the low to middle amphibolite facies, and 30 mol % Ab in the upper amphibolite to granulite facies even when not buffered with albite. The limited Na content of celsian equilibrated with albite in the greenschist facies is in striking disagreement with the narrow solvi obtained from unreversed experiments on the join BaAl2Si2O86-NaAlSi3O8.Up to 8 four-feldspar and 24 three-feldspar assemblages may be stable in the system BaAl2Si2O8-CaAl2Si2O8-NaAlSi3O8-KAlSi3O8. In contrast, the repeatedly observed and variably located discontinuities within zoned hyalophane grains may represent changes in the environment during mineral growth rather than internal miscibility gaps. Given its miscibility gaps with microcline and celsian, the name hyalopbane is justified for intermediate feldspars near the Cn-Or join.

The Nova Brasilândia metasedimentary belt (NBMB) of western Brazil marks a fundamental crustal boundary in the Amazon craton. The metasedimentary rocks of the NBMB (calc‐silicates, metapelites, quartzites, metabasites) contrast strongly with the older, polycyclic granitoid rocks of the adjacent Amazon craton. Aeromagnetic anomalies indicate that the belt is continuous for at least 1000 km in an E‐W direction, although the easternmost extent of the NBMB is covered by the Cretaceous sediments of the Parecis Formation. Additional geologic evidence suggests that the belt extends along an E‐W trend for ∼2000 km. The northern portion of the NBMB preserves vestiges of an early high pressure‐temperature (P‐T) assemblage (kyanite + staurolite) overprinted by sillimanite during prograde metamorphism. A higher metamorphic grade is observed in the southern portion of the belt, with peak conditions calculated to be 800 MPa and 800°C for granulitic assemblages. The combined P‐T path demonstrates that the competing processes of imbrication (northern domain) and magma generation (southern domain) are responsible for regional metamorphism and crustal thickening. Cooling from peak metamorphic conditions is recorded by U‐Pb monazite ages of 1090 Ma and titanite ages of ∼1060 Ma. Integrated cooling rates of 2°–3°C/Myr from regional metamorphism are calculated from these U/Pb ages combined with 40Ar/39Ar ages of hornblende (∼970 Ma) and biotite (∼910 Ma). The NBMB marks the Mesoproterozoic limit of the SW Amazon craton. The discordance of the NBMB to the NNW structural trend of the younger Aguapeí belt (200 km SE of NBMB), together with marked differences between the two belts in sedimentary environment, metamorphic grade, and timing of deformation, signify that these two belts are not geologically continuous. The “Grenvillian” deformation recorded by the NBMB belt marks the final docking of the Amazon craton and Paragua craton within the Rodinia framework. The Aguapeí belt, in contrast, seems to record only limited deformation internal to the Paragua craton.

INTRODUCTION Experimental studies were carried out to evaluate phase relations involving titanite–F–Al-titanite solid solution in the system CaSiO3– Geological background Al2SiO5–TiO2–CaF2. The experiments were conducted at Titanite is a common accessory mineral in mafic, pelitic 900–1000°C and 1·1–4·0 GPa. The average F/Al ratio in and granitic rocks from many geological environments titanite solid solution in the experimental run products is 1·01 ± (Higgins & Ribbe, 1976; Ribbe, 1982; Enami et al., 0·06, and XAl ranges from 0·33 ± 0·02 to 0·91 ± 0·05, 1993). Titanite may deviate significantly from its ideal consistent with the substitution [TiO]–1[AlF ]1. Analysis of composition by the substitution Al and F for Ti and O the phase relations indicates that titanite solid solutions coexisting (Hollabaugh, 1980; Franz & Spear, 1985; Bernau et al., with rutile are always low in XAl , whereas the maximum XAl of 1986; Fehr, 1991; Oberti et al., 1991; Carswell et al., titanite solid solution occurs with fluorite and either anorthite or 1996). The Al + OH ⇔ Ti + O substitution leads Al2SiO5. Reaction displacement experiments were performed by to the Al–OH end-member vuagnatite CaAlSiO4(OH), adding fluorite to the assemblage anorthite + rutile = titanite + which has a different structure from titanite (McNear et kyanite. The reaction shifts from 1·60 GPa to 1·15 ± 0·05 al., 1976) and is typical of low-temperature geological GPa at 900°C, from 1·79 GPa to 1·375 ± 0·025 GPa at environments (Enami et al., 1993). By contrast, the F–Al 1000°C, and from 1·98 GPa to 1·575 ± 0·025 GPa at substitution is isostructural and is common at high meta1100°C. The data show that the activity of CaTiSiO4O is very morphic temperatures (>500–600°C) and pressures. For close to the ideal molecular activity model (XTi ) at 1100°C, but example, up to 55 mol % F–Al substitution has been shows a negative deviation at 1000°C and 900°C. The results reported from highand ultra-high-pressure metaconstrain G°f,298·15 of CaAlSiO4F to be −2595 ± 3 kJ/mol morphic rocks (Franz & Spear, 1985; Sobolev & Shatsky, and S°298·15 to be in the range of 105·2–109·6 J/mol K, which 1991; Carswell et al., 1996). The common association of in turn can be used to calculate petrogenetic grids involving titanite F–Al-rich titanites with high-pressure environments has solid solutions in the system CaTiSiO4O–CaAlSiO4F. led to the suggestion that equilibria involving titanite solid solution may be useful for constraining pressure, temperature and/or F2 fugacity during metamorphism (Smith, 1977; Franz & Spear, 1985; Gibert et al., 1990;

Eclogites of the Ianca Valley in northern Italy underwent Eo-Alpine eclogite-facies metamorphism and subsequent retrogression under blueschist- and greenschist-facies conditions, which led to the formation of a sequence of mineral assemblages. A first eclogite-facies assemblage (high-pressure stage I: paragonite + clinozoisite + zoisite + garnet + kyanite + quartz) is preserved in the cores of some garnets. Inclusion assemblages in garnet rims represent two further stages in the eclogite-facies: (1) a first stage has the assemblage omphacite + clinozoisite + garnet + kyanite + rutile + quartz zoisite (high-pressure stage II) and (2) a second stage with the assemblage omphacite + barroisite + rutile + quartz + zircon (high-pressure stage III) is occasionally found in the outermost rims. The latter assemblage represents hydration in the eclogite-facies, which also led to the formation of the matrix assemblage barroisite + omphacite + garnet + clinozoisite + paragonite + muscovite + dolomite + rutile + quartz zoisite, without kyanite. Hydration continued, forming veins crosscutting the main foliation with the assemblage paragonite + clinozoisite + zoisite + barroisite + quartz omphacite. During a subsequent blueschist-facies event, glaucophane appeared, and finally, a greenschist-facies assemblage actinolite + chlorite + clinozoisite + muscovite + talc + albite + titanite + calcite + quartz formed. The successive mineral assemblages were used to construct a P-T path, yielding a decompression path from the eclogite-facies from 550-650 degreesC, 17-21 kbar (high-pressure stages I and II) to 520-650 degreesC, 16-21 kbar (high-pressure stage III) and 400-500 degreesC and 8-10 kbar in the blueschist-facies. The conditions of the greenschist-facies are around 250-350 degreesC and 3-6 kbar. In addition, an attempt was made to characterize the evolution of a(H(2)O) during the P-T path. The activity of the metamorphic fluid is characterized by a(H(2)O) of 0.39-0.81 for the mineral assemblage in the inner rims of the garnets (high-pressure stage II) and high a(H(2)O) activities of 0.84-0.98 for the hydration stage of the eclogite-facies (high-pressure assemblage III) and the subsequent stages of decompression (blueschist- and greeschist-facies).

Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na2O–MgO–Al2O3–SiO2–H2O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 20–30 kbar and 700–800 °C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite–talc ± quartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyböite (Nyb), Al–Na-cummingtonite (Al–Na-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite + 4quartz + 4H2O=7talc, nyböite + 3quartz=3jadeite + talc, 3Al–Na-cummingtonite + 11quartz + 2H2O=6jadeite + 5talc, and 3 sodium anthophyllite + 13quartz + 4H2O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of AlIV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [NaAAlIV]<=>[□ASi] (edenite) toward nyböite, and [Na(M4)AlVI]<=>[Mg(M4)MgVI] toward cummingtonite. The extent of nyböite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.

An oxygen barometer has been developed using garnet–epidote equilibria and the latest available thermodynamic data via the reaction:2 Ca2FeAl2Si3O12(OH)=2 Ca2FeAl2Si3O12+H2O+1/2 O2epidote (Ps33Czo67)garnet (Alm33Grs67)fluidwhere epidote (Ps33Czo67) reacts to form (Alm33Grs67), water and oxygen. The garnet–epidote oxybarometer has been applied to a wide variety of metamorphic rocks including low to medium-pressure greenschists and amphibolites, as well as high-pressure eclogites. Results indicate that this equation represents a sensitive oxygen monitor with only a minor dependence on pressure. Andradite solid solution of less than 15 mol% has only minor influence in the locus of the curve in fO2/T space. Garnet–epidote equilibria may also provide a thermometer when combined with an independent oxygen buffer. The garnet–epidote oxythermobarometer is a reliable oxygen sensor that can be applied to many low through medium-grade metamorphic rocks. The extensive solid solution found in both garnet and epidote allows this equation to be applied to mineral pairs with a wide range of compositions. The garnet–epidote oxythermobarometer is potentially applicable to any rocks that contain the equilibrium assemblage garnet+epidote.