Fluid evolution of rare-element and muscovite granitic pegmatites from central Galicia, NW Spain

Fluid inclusions have been studied in three pegmatite fields in Galicia, NW Iberian Peninsula. Based on microthermometry and Raman spectroscopy, eight fluid systems have been recognized. The first fluid may be considered to be a pegmatitic fluid which is represented by daughter mineral (silicates)-rich aqueous inclusions. These inclusions are primary and formed above 500 °C (dissolution of daughter minerals). During pegmatite crystallization, this fluid evolved to a low-density, volatile-rich aqueous fluid with low salinity (93% H₂O; 5% CO₂; 0.5% CH₄; 0.2% N₂; 1.3% NaCl) at minimum P–T conditions around 3 ± 0.5 kbar and 420 °C. This fluid is related to rare-metal mineralization. The volatile enrichment may be due to mixing of magmatic fluids and fluids equilibrated with the host rock. A drop in pressure from 3 ± 0.5 to 1 kbar at a temperature above 420 °C, which may be due to the transition from predominantly lithostatic to hydrostatic pressure, is recorded by two-phase, water-rich inclusions with a low-density vapour phase (CO₂, CH₄ and N₂). Another inclusion type is represented by two-phase, vapour-rich inclusions with a low-density vapour phase (CO₂, CH₄ and N₂), indicating a last stage of decreasing temperature (360 °C) and pressure (around 0.5 kbar), probably due to progressive exhumation. Finally, volatile (CO₂)-rich aqueous inclusions, aqueous inclusions (H₂O-NaCl) and mixed-salt aqueous inclusions with low Th, are secondary in charac- ter and represent independent episodes of hydrothermal fluid circulation below 310 °C and 0.5 kbar. Received: 14 October 1999 / Accepted: 5 October 1999     

Retrograde fluids in the Archean Shawmere anorthosite, Kapuskasing Structural Zone, Ontario, Canada

The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T < 600 °C, XCO₂ < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO₂ < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO₂ > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H₂O-rich to CO₂-rich. The calcite in the retrograde veins has δ¹⁸O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ¹³C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO₂-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO₂ ≈ 1.13 to 1.05 g/cm³). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO₂-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO₂-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO₂-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path. Received: 2 April 1996 / Accepted: 15 November 1996     

Isotope geochemistry and fluid inclusion study of skarns from Vesuvius

Summary We present new mineral chemistry, fluid inclusion, stable carbon and oxygen, as well as Pb, Sr, and Nd isotope data of Ca-Mg-silicate-rich ejecta (skarns) and associated cognate and xenolithic nodules from the Mt. Somma-Vesuvius volcanic complex, Italy. The typically zoned skarn ejecta consist mainly of diopsidic and hedenbergitic, sometimes “fassaitic” clinopyroxene, Mg-rich and Ti-poor phlogopite, F-bearing vesuvianite, wollastonite, gehlenite, meionite, forsterite, clinohumite, anorthite and Mg-poor calcite with accessory apatite, spinell, magnetite, perovskite, baddeleyite, and various REE-, U-, Th-, Zr- and Ti-rich minerals. Four major types of fluid inclusions were observed in wollastonite, vesuvianite, gehlenite, clinopyroxene and calcite: a) primary silicate melt inclusions (T_HOM = 1000–1050 °C), b) CO₂ ± H₂S-rich fluid inclusions (T_HOM = 20–31.3 °C into the vapor phase), c) multiphase aqueous brine inclusions (T_HOM = 720–820 °C) with mainly sylvite and halite daughter minerals, and d) complex chloride-carbonate-sulfate-fluoride-silicate-bearing saline-melt inclusions (T_HOM = 870–890 °C). The last inclusion type shows evidence for immiscibility between several fluids (silicate melt – aqueous chloride-rich liquid – carbonate/sulfate melt?) during heating and cooling below 870 °C. There is no evidence for fluid circulation below 700 °C and participation of externally derived meteoric fluids in skarn formation. Skarns have considerably variable ²⁰⁶Pb/²⁰⁴Pb (19.047–19.202), ²⁰⁷Pb/²⁰⁴Pb (15.655–15.670), and ²⁰⁸Pb/²⁰⁴Pb (38.915–39.069) and relatively low ¹⁴³Nd/¹⁴⁴Nd (0.51211–0.51244) ratios. The carbon and oxygen isotope compositions of skarn calcites (δ¹³C_V-PDB = −5.4 to −1.1‰; δ18O_V-SMOW = 11.7 to 16.4‰) indicate formation from a ¹⁸O- and ¹³C-enriched fluid. The isotope composition of skarns and the presence of silicate melt inclusion-bearing wollastonite nodules suggests assimilation of carbonate wall rocks by the alkaline magma at moderate depths (< 5 km) and consequent exsolution of CO₂-rich vapor and complex saline melts from the contaminated magma that reacted with the carbonate rocks to form skarns. Received March 1, 2000; revised version accepted November 2, 2000     

Kornerupine parageneses in whiteschists and other magnesian rocks: is kornerupine + talc a high-pressure assemblage equivalent to tourmaline + orthoamphibole?

Kornerupine, (□,Fe,Mg)(Mg,Fe,Al)₉(Si,Al,B)₅ (O,OH,F)₂₂, has been reported with talc in rocks from six localities worldwide, but only at Chilapila Hill in the Lufilian Arc, Zambia do textural relationships imply that kornerupine (Krn) equilibrated with talc (Tlc) during a prograde metamorphic event at T≈ 640 °C, P≈ 13 kbar; a prograde Krn + Tlc assemblage has also been reported from Mautia Hill, Tanzania (P ≤ 13 kbar). In order to estimate possible constraints on the stability range for the kornerupine + talc paragenesis in nature, we constructed a P-T diagram in the model system MgO-Al₂O₃-SiO₂-H₂O (MASH) for seven phases quartz (Qtz), B-free kornerupine sensu stricto, anthophyllite (Ath), chlorite (Chl), cordierite (Crd), kyanite (Ky), and talc. The minimum pressure for Krn + Tlc + Ky stability in MASH is close to that for Ky + Tlc stability, i.e., 6–8 kbar, at T≤ 780 °C. However, in the natural system, B₂O₃ and Na₂O are major constituents in Krn and orthoamphibole (Oam), respectively, and dravitic tourmaline (Tur) is widespread. The critical assemblage alternative to Krn + Tlc in nature is Tur + Oam. The upper pressure limit of Tur + Ath is determined by the upper pressure for anthophyllite: 7.7–10.5 kbar at 682–794 °C in the MgO-SiO₂-H₂O system (Chernosky et al. 1985, Am Mineral 70:223–236), and is undoubtedly higher in the presence of Na₂O, CaO, and Al₂O₃. At three of the six localities, talc is a retrograde phase; nonetheless, it possibly equilibrated with kornerupine on the retrograde path or during a later metamorphic event at P-T conditions appropriate for Ky + Tlc. At the sixth locality (Mulvoj, southwestern Pamir Mountains, Tajikistan), Krn is found in the same thin section as talc and kyanite and all three minerals formed during a prograde metamorphic event at T≥ 650 °C, P near 7 kbar. However, Krn is restricted to a lens 4 to 6 mm thick of phlogopite + anthophyllite + Tur and it does not touch either talc or kyanite. A reaction relating the Mulvoj and Chilapila Hill (Krn + Tlc + Ky + Qtz + Tur) parageneses is calculated from compositions in the Mulvoj rock to be 0.40Tur + 2.55Ath + 1.33H₂O + 0.27F = Krn + 2.16Tlc + 0.36B₂O₃ + 0.02Rutile + 0.19Na₂O + 0.17CaO. Given the difference in metamorphic pressures estimated for Mulvoj and Chilapila Hill, Krn + Tlc is inferred to be favored by increasing pressure as well as by low Na₂O and CaO contents. Some FeO, F, Fe₂O₃, and BeO are present in measurable amounts in at least one of the phases in the Mulvoj and Chilapila Hill whiteschists (e.g., Krn contains 0.24–0.67 wt% BeO), but the effect of these constituents is subordinate to that of Na₂O, CaO and B₂O₃. The Krn + Tlc could be a more important assemblage in B-bearing whiteschists than has been reported to date, particularly at pressures where orthoamphibole is no longer stable. Received: 21 April 1997 / Accepted: 13 October 1997     

Stability of phlogopite in granitic melts, an experimental investigation

Water-saturated and water-undersaturated experiments (a _{H2 O} = 1.0 and 0.5) were performed in the temperature range 780–1040°C at 2 and 5 kbar in order to determine the upper thermal stability of phlogopite in granitic melts. Starting compositions were: (A) subaluminous mixtures of 20 wt % synthetic phlogopite and 80 wt % synthetic anhydrous haplogranitic glass; (B) peraluminous mixtures (normative corundum  = 4 %) of 20 wt % synthetic phlogopite and 80 wt % synthetic anhydrous peraluminous haplogranitic glass. The molar quartz: albite: orthoclase ratio of the glasses of the 2␣kbar runs was 35:39:26 and that of the 5 kbar runs 30:42:28. In the subaluminous system, phlogopite is stable up to 820°C at a _{H2 O} = 1.0 and up to 780°C at a _{H2 O} = 0.5. At higher temperatures, it is replaced by enstatite. In the peraluminous system phlogopite has a remarkably higher thermal stability (up to 1000°C at 5 kbar and a _{H2 O} = 1.0) and there is a temperature interval of 80°C at a _{H2 O} = 1.0, and 90–100°C at a _{H2 O} = 0.5 between the first appearance of enstatite and the disappearance of phlogopite. In the peraluminous system, phlogopite is a solid solution (ss) of phlogopite, muscovite, talc and eastonite components. The crystalline product of the phlogopite_ss breakdown reaction is an aluminous enstatite. The MgO-content of the melt depends on the normative corundum content of the starting material and the run temperature. It is independent of pressure. In the subaluminous system, the MgO-content ranges between 0.05 and 0.3 wt % in the temperature interval 780–880°C at both investigated water activities. The MgO-content of the peraluminous melts at a _{H2 O} = 1.0 ranges between 0.4 and 1.7 wt % and at a _{H2 O} = 0.5 between 0.2 and 1.4 wt % in the temperature range 780–980°C. Received: 28 August 1995 / Accepted: 6 August 1996     

Fluid influence on mineral reactions in ultrahigh-pressure granulites: a case study in the Śnieżnik Mts. (West Sudetes, Poland)

Small tectonic slices of undeformed eclogites and ultrahigh-pressure granulites occur in three tectonic units of the Śnieżnik Mts. (SW Poland). Ultrahigh-pressure granulite/eclogite transitions with peak metamorphic conditions between 21 and 28 kbar at 800 to 1000 °C occur only in the Złote unit. Conventional U-Pb multigrain analyses of zircons from a mafic granulite provided ²⁰⁷Pb/²⁰⁶Pb ages between 360 to 369 Ma which are interpreted to approximate timing of original crystallisation from a melt. Diffusion kinetics and the restricted availability of a fluid phase mainly controlled the conversion from granulite to eclogite, although some bulk-chemical differences were also recognised. The ultrahigh-pressure granulites from the Złote unit exclusively contain H₂O-rich inclusions with variable salinities which distinguishes them from high-temperature (HT)-granulites world-wide. This is also in contrast to the fluid regime (H₂O-N₂-CO₂) recognised in the lower-temperature eclogites (600–800 °C) from the closely associated Międzygórze and Śnieżnik units. The variation in fluid composition between the lower-temperature eclogites and ultrahigh-pressure granulites on the one hand and ultrahigh-pressure granulites and HT-granulites on the other hand probably indicates contrasting P-T-t paths as a result of different tectonic environments. Received: 15 June 1998 / Accepted: 2 March 1999     

Origin of the Maoduan Pb–Zn–Mo deposit, eastern Cathaysia Block, China: geological, geochronological, geochemical, and Sr–Nd–Pb–S isotopic constraints

The Maoduan Pb–Zn–Mo deposit is in hydrothermal veins with a pyrrhotite stage followed by a molybdenite and base metal stage. The Re–Os model ages of five molybdenite samples range from 138.6 ± 2.0 to 140.0 ± 1.9 Ma. Their isochron age is 137.7 ± 2.7 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating of the nearby exposed Linggen granite porphyry gave a ²⁰⁶Pb/²³⁸U age of 152.2 ± 2.2 Ma and the hidden Maoduan monzogranite yielded a mean of 140.0 ± 1.6 Ma. These results suggest that the intrusion of the Maoduan monzogranite and Pb–Zn–Mo mineralization are contemporaneous. δ ³⁴S values of sulfide minerals range from 3.4‰ to 4.8‰, similar to magmatic sulfur. Four sulfide samples have ²⁰⁶Pb/²⁰⁴Pb = 18.252–18.432, ²⁰⁷Pb/²⁰⁴Pb = 15.609–15.779, and ²⁰⁸Pb/²⁰⁴Pb = 38.640–39.431, similar to the age-corrected data of the Maoduan monzogranite. These isotope data support a genetic relationship between the Pb–Zn–Mo mineralization and the Maoduan monzogranite and probably indicate a common deep source. The Maoduan monzogranite has geochemical features similar to highly fractionated I-type granites, such as high SiO₂ (73.7–75.2 wt.%) and alkalis (K₂O + Na₂O = 7.8–8.9 wt.%) and low FeO_t (0.8–1.3 wt.%), MgO (~0.3 wt.%), P₂O₅ (~0.03 wt.%), and TiO₂ (~0.2 wt.%). The granitic rocks are enriched in Rb, Th, and U but depleted in Ba, Sr, Nb, Ta, P, and Ti. REE patterns are characterized by marked negative Eu anomalies (Eu/Eu^* = 0.2–0.4). The Maoduan monzogranite, having (⁸⁷Sr/⁸⁶Sr) _t  = 0.7169 to 0.7170 and ε_Nd(t) = −13.8 to −13.7, was probably derived from mixing of partial melts from enriched mantle and the Paleoproterozoic Badu group in an extensional tectonic setting.     

Melting of albite and dehydration of brucite in H2O–NaCl fluids to 9 kbars and 700–900°C: implications for partial melting and water activities during high pressure metamorphism

 The melting reaction: albite_(solid)+ H₂O_(fluid) =albite-H₂O_(melt) has been determined in the presence of H₂O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H₂O–CO₂ fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X _(H2O) ) melting relations in the two systems are almost coincident. By contrast, H₂O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 ^°C higher in H₂O–NaCl brines than in H₂O–CO₂ fluids for X _(H2O) =0.8 and ∼100 ^°C higher for X _(H2O) =0.5. The melting temperature of albite in H₂O–NaCl fluids of X _(H2O)=0.8 is ∼100 ^°C higher than in pure water. The P–T curves for albite melting at constant H₂O–NaCl show a temperature minimum at about 5 kbar. Water activities in H₂O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H₂O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a _(H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996     

Phase relations in the MgO–P2O5–H2O system and the stability of phosphoellenbergerite: petrological implications

The polymorphic relations for Mg₃(PO₄)₂ and Mg₂PO₄OH have been determined by reversed experiments in the temperature-pressure (T-P) range 500–1100 °C, 2–30 kbar. The phase transition between the low-pressure phase farringtonite and Mg₃(PO₄)₂-II, the Mg analogue of sarcopside, is very pressure dependent and was tightly bracketed between 625 °C, 7 kbar and 850 °C, 9 kbar. The high-temperature, high-pressure polymorph, Mg₃(PO₄)₂-III, is stable above 1050 °C at 10 kbar and above 900 °C at 30 kbar. The low-pressure stability of farringtonite is in keeping with its occurrence in meteorites. The presence of iron stabilizes the sarcopside-type phase towards lower P. From the five Mg₂PO₄OH polymorphs only althausite, holtedahlite, β-Mg₂PO₄OH (the hydroxyl analogue of wagnerite) and ɛ-Mg₂PO₄OH were encountered. Relatively speaking, holtedahlite is the low-temperature phase (<600 °C), ɛ-Mg₂PO₄OH the high-temperature, low-pressure phase and β-Mg₂PO₄OH the high-temperature, high-pressure phase, with an intervening stability field for althausite which extends from about 3 kbar at 500 °C to about 12 kbar at 800 °C. Althausite and holtedahlite are to be expected in F-free natural systems under most geological conditions; however, wagnerite is the most common Mg-phosphate mineral, implying that fluorine has a major effect in stabilizing the wagnerite structure. Coexisting althausite and holtedahlite from Modum, S. Norway, show that minor fluorine is strongly partitioned into althausite (K_D ^F/OH≈ 4) and that holtedahlite may incorporate up to 4 wt% SiO₂. Synthetic phosphoellenbergerite has a composition close to (Mg_0.9□_0.1)₂Mg₁₂P₈O₃₈H_8.4. It is a high-pressure phase, which breaks down to Mg₂PO₄OH + Mg₃(PO₄)₂ + H₂O below 8.5 kbar at 650 °C, 22.5 kbar at 900 °C and 30 kbar at 975 °C. The stability field of the phosphate end-member of the ellenbergerite series extends therefore to much lower P and higher T than that of the silicate end-members (stable above 27 kbar and below ca. 725 °C). Thus the Si/P ratio of intermediate members of the series has a great barometric potential, especially in the Si-buffering assemblage with clinochlore + talc + kyanite + rutile + H₂O. Application to zoned ellenbergerite crystals included in the Dora-Maira pyrope megablasts, western Alps, reveals that growth zoning is preserved at T as high as 700–725 °C. However, the record of attainment of the highest T and/or of decreasing P through P-rich rims (1 to 2 Si pfu) is only possible in the presence of an additional phosphate phase (OH-bearing or even OH-dominant wagnerite in these rocks), otherwise the trace amounts of P in the system remain sequestered in the core of Si-rich crystals (5 to 8 Si pfu) and can no longer react. Received: 7 April 1995 / Accepted: 12 November 1997     

Interaction of metamict zircon with fluids of various composition

Interaction of metamict zircon with solutions of various composition (H₂O, 1 m NaCl + 0.5 m HCl, 1 m NaCl, 1 m NaHCO₃, 0.1 m Na₂CO₃, 1 m Na₂CO₃, 2 m Na₂CO₃, and 1 m NaOH) was examined at temperatures of 25–800°C, pressures of 1 and 5 kbar, and exposure times of 3, 7, and 14 days. The choice of fluid compositions was determined by the fact that aqueous, chloride, and carbonate solutions are the principal components of postmagmatic, metamorphic, and metasomatic fluids that can affect zircon in nature. Occurring in contact with these solutions, metamict zircon congruently and incongruently dissolves competing with the sorption of elements, crystallization of newly formed phases, and the restoration of the crystal structure. The intensities of these processes are controlled by the temperature, pressure, exposure time, and the pH and composition of the solution.     

Thermobarometry of mafic igneous rocks based on clinopyroxene-liquid equilibria, 0–30 kbar

 Models for estimating the pressure and temperature of igneous rocks from co-existing clino-pyroxene and liquid compositions are calibrated from existing data and from new data obtained from experiments performed on several mafic bulk compositions (from 8–30 kbar and 1100–1475° C). The resulting geothermobarometers involve thermodynamic expressions that relate temperature and pressure to equilibrium constants. Specifically, the jadeite (Jd; NaAlSi₂O₆)–diopside/hedenbergite (DiHd; Ca(Mg, Fe) Si₂O₆) exchange equilibrium between clinopyroxene and liquid is temperature sensitive. When compositional corrections are made to the calibrated equilibrium constant the resulting geothermometer is (i) 10⁴ T=6.73−0.26^* ln [Jd^px*Ca^liq*Fm^liqDiHd^px*Na^liq*Al^liq] −0.86^* ln [Mg^liqMg^liq+Fe^liq]+0.52^*ln [Ca^liq] an expression which estimates temperature to ±27 K. Compared to (i), the equilibrium constant for jadeite formation is more sensitive to pressure resulting in a thermobarometer (ii) P=−54.3+299^* T10⁴+36.4^* T10⁴ ln [Jd^px[Si^liq]^2*Na^liq*Al^liq] +367^*[Na^liq*Al^liq] which estimates pressure to ± 1.4 kbar. Pressure is in kbar, T is in Kelvin. Quantities such as Na^liq represent the cation fraction of the given oxide (NaO_0.5) in the liquid and Fm=MgO+FeO. The mole fractions of Jd and diopside+hedenbergite (DiHd) components are calculated from a normative scheme which assigns the lesser of Na or octahedral Al to form Jd; any excess Al^VI forms Calcium Tschermak’s component (CaTs; CaAlAlSiO₆); Ca remaining after forming CaTs and CaTiAl₂O₆ is taken as DiHd. Experimental data not included in the regressions were used to test models (i) and (ii). Error on predictions of T using model (i) is ±40 K. A pressure-dependent form of (i) reduces this error to ±30 K. Using model (ii) to predict pressures, the error on mean values of 10 isobaric data sets (0–25 kbar, 118 data) is ±0.3 kbar. Calculating thermodynamic properties from regression coefficients in (ii) gives V^Jd _f of 23.4 ±1.3 cm³/mol, close to the value anticipated from bar molar volume data (23.5 cm³/mol). Applied to clinopyroxene phenocrysts from Mauna Kea, Hawaii lavas, the expressions estimate equilibration depths as great as 40 km. This result indicates that transport was sufficiently rapid that at least some phenocrysts had insufficient time to re-equilibrate at lower pressures. Received: 16 May 1994/Accepted: 15 June 1995     

Metamorphism, isotopic ages and composition of lower crustal granulite xenoliths from the Cretaceous Salta Rift, Argentina

Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get information about the age and the chemical composition of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ± K-feldspar as major minerals. The exceedingly rare mafic xenoliths consist of plagioclase-clinopyroxene-garnet ± hornblende. All xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The Sm-Nd mineral isochron ages are 95.1 ± 10.4 Ma, 91.5 ± 13.0 Ma, 89.0 ± 4.2 Ma (granitoid xenoliths), and 110.7 ± 23.6 Ma (mafic xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130–100 and 80–75 Ma) and are interpreted as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600–700 °C). The Sm-Nd and Rb-Sr isotopic signatures (¹⁴⁷Sm/¹⁴⁴Nd = 0.1225–0.1608; ¹⁴³Nd/¹⁴⁴Nd_{t 0} = 0.512000–0.512324; ⁸⁷Rb/⁸⁶Sr = 0.099–0.172; ⁸⁷Sr/⁸⁶Sr_{t 0} = 0.708188–0.7143161) and common lead isotopic signatures (²⁰⁶Pb/²⁰⁴Pb = 18.43–18.48; ²⁰⁷Pb/²⁰⁴Pb = 15.62–15.70; ²⁰⁸Pb/²⁰⁴Pb = 38.22 –38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower ²⁰⁸Pb/²⁰⁴Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a similar Proterozoic protolith. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths. Received: 21 July 1998 / Accepted: 27 October 1998     

Phase relations of calc-silicate assemblagesin the Auerbach marble, OdenwaldCrystalline Complex, Germany

Summary In the Odenwald Crystalline Complex, calc-silicate rocks are concentrated at the margins of the marble layer of Auerbach. They were presumably formed by metasomatic exchange between the calcite marble and the neighbouring granodioritic and quartz-dioritic intrusives. The investigated samples contain the characteristic mineral assemblages: garnet + clinopyroxene + epidote/clinozoisite + calcite + quartz ± titanite (1) and wollastonite + clinopyroxene + garnet + calcite ± quartz ± epidote/clinozoisite ± titanite (2). Microprobe analyses revealed the following compositional ranges: garnet grs_40–98adr_2–55alm_<5.5sps_<5.5pyp_<1; clinopyroxene di_46–88hed_9–47joh_0–5cats_0–6; epidote/clinozoisite ps_20–80. Different phase diagrams were calculated in the system CaO-MgO-Al₂O₃-TiO₂-SiO₂-CO₂-H₂O (CMATSCH) to decipher the P-T-X_CO2 parameters of metamorphism: isobaric T-X_CO2 sections and a P-T projection with mixed volatiles. The phase diagrams illustrate that the observed assemblages can only form in the presence of an H₂O-rich volatile phase. The assemblages are stable over a large range of temperatures, from 580 °C to < 400 °C (at 4 kbar) and at X_CO2 values of less than 0.055 (at 4 kbar). Higher temperatures can be inferred from reaction textures which indicate that garnet + plagioclase (T > 580 °C, at 4 kbar) and wollastonite + plagioclase (T > 660 °C, at 4 kbar) coexisted during an early metamorphic stage. A minimum pressure of 3.5 kbar can be inferred for the early high-temperature stage. Furthermore, on the basis of the calculated phase diagrams, combined with modal abundances in thin sections, it is possible to evaluate fluid behaviour; in the investigated specimens, infiltration of fluids from an external reservoir occurred. A minimum fluid:rock ratio of 3.6:1 can be estimated.

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Zusammenfassung Phasenbeziehungen in Kalksilikat-Paragenesen des Marmorzuges von Auerbach, Odenwald-Kristallin-Komplex, Deutschland Im kristallinen Odenwald konzentrieren sich kalksilikatische Gesteine in den Randbereichen des Marmorzuges von Auerbach. Die kalksilikatischen Partien wurden vermutlich duch metasomatischen Austausch zwischen dem Calcit-Marmor und benachbarten Granodioriten und Quarzdioriten gebildet. Die untersuchten Proben enthalten die charakteristischen Mineralparagenesen: Granat + Klinopyroxen + Epidot/Klinozoisit + Calcit + Quarz + Titanit (1) und Wollastonit + Klinopyroxen + Granat + Calcit ± Quarz ± Epidot/Klinozoisit ± Titanit (2). Mikrosondenanalysen ergaben folgendes Zusammensetzungsspektrum: Granat grs_40–98adr_2–55alm_<5.5sps_<5.5pyp_<1; Klinopyroxen di_46–88hed_9–47joh_0–5cats_0–6s; Epidot/Klinozoisit ps_20–80. Verschiedene Phasendiagramme wurden für das Modellsystem CaO-MgO-Al₂O₃-TiO₂-SiO₂-CO₂-H₂O (CMATSCH) berechnet, um die P-T-X_CO2-Parameter der Metamorphose einzugrenzen: Isobare T-X_CO2-Schnitte und eine P-T-Projektion mit einer Fluid-Mischphase. Die Phasendiagramme verdeutlichen, da? die beobachteten Paragenesen nur in Anwesenheit eines H₂O-reichen Fluids gebildet werden k?nnen. Die Paragenesen sind über einen gro?en Temperaturbereich hinweg stabil, von 580 °C bis < 400 °C (bei 4 kbar) und bei X_CO2-Gehalten von < 0.055 (bei 4 kbar). Ursprünglich h?here Temperaturen k?nnen anhand von Reaktions-Texturen rekonstruiert werden, die zeigen, da? Granat + Plagioklas (T > 580 °C, bei 4 kbar) und Wollastonit + Plagioklas (T > 660 °C, bei 4 kbar) w?hrend eines früheren Metamorphosestadiums koexistierten. Ein Minimaldruck von 3.5 kbar kann für dieses frühe Hochtemperatur-Stadium abgeleitet werden. Mit Hilfe der berechneten Phasendiagramme, in Kombination mit beobachteten Modalgehalten, ist es m?glich, das Verhalten der fluiden Phase abzusch?tzen. Die untersuchten Gesteine implizieren Fluidinfiltration, wobei ein minimales Verh?ltnis Fluid:Gestein von 3.6:1 abgesch?tzt werden kann.

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Received July 29, 1999; accepted March 28, 2000     

Geochemistry and age of ultrahigh pressure metamorphic rocks from the Kokchetav massif (Northern Kazakhstan)

Isotopic and geochemical data of the Zerenda series metamorphic rocks from the Kokchetav massif are reported. Some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism (P > 40 kbar, T = 900–1000 °C). The diamond-bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks. The REE patterns range from LREE depleted to slightly LREE enriched [chondrite normalized (La/Yb)_N– 0.1–5.4] with a negative Eu anomaly. They are depleted in incompatible elements (e.g. Sr, Ba, U, Th) with respect to the upper crust. In contrast non-diamondiferous rocks of the Zerenda series exhibit normal crustal geochemistry. All rocks of the Zerenda series have very radiogenic lead isotopes. The measured μ values (²³⁸U/²⁰⁴Pb) compared with those calculated for the interval between crust formation and ultrahigh pressure (UHP) metamorphism suggest a decrease by factors of up to 200 during the UHP metamorphism. The Sm-Nd mineral isochrons from the diamond-bearing rocks and other rock types of the Zerenda series give a Middle Cambrian (524–535 Ma) age of metamorphism. The Nd model ages show that crust formation occurred about 2.3 Ga ago. Significant fractionation of Sm and Nd and loss of incompatible elements may be due to partial melting of the protoliths. The Ar-Ar age determinations of secondary biotite and muscovite from the diamond-bearing rocks yield an age of 517 ± 5 Ma. This cooling age requires a short time interval between UHP metamorphism and uplift to a crustal level. Ultrahigh pressure metamorphism might be a significant source of Pb for the mantle. We propose that the radiogenic Pb of the oceanic array is the contamination traces of numerous UHP events. Beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using Nd isotopes. We compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the Sm-Nd system during very high grade metamorphic events. Received: 14 August 1998 / Accepted: 1 June 1999     

Liquidus temperatures and phase compositions in the system Qz-Ab-Or at 5 kbar and very low water activities

Liquidus phase relations have been experimentally determined in the systems Qz-Ab-Or-(H₂O), Qz-Ab-(H₂O) and Qz-Or-(H₂O) at H₂O-undersaturated conditions (a _H2O = 0.07) and P = 5 kbar. Starting materials were homogeneous synthetic glasses containing 1 wt% H₂O. The liquidus temperatures were bracketed by crystallization and dissolution experiments. The results of kinetic studies showed that crushed glasses are the best starting materials to overcome undercooling and to minimize the temperature difference between the lowest temperature of complete dissolution (melting) and the highest temperature at which crystallization can be observed. At P = 5 kbar and a _H2O = 0.07, the Qz-Ab eutectic composition is Qz₃₂Ab₆₈ at 1095 °C (±10 °C) and the Qz-Or eutectic is Qz₃₈Or₆₂ at 1030 °C (±10 °C). The minimum temperature of the ternary system Qz-Ab-Or is 990 °C (±10 °C) and the minimum composition is Qz₃₂Ab₃₅‐ Or₃₃. The Qz content of the minimum composition in the system Qz-Ab-Or-H₂O remains constant with changing a _H2O. The normative Or content, however, increases by approximately 10 wt% with decreasing a _H2O from 1 to 0.07. Such an increase has already been observed in the system Qz-Ab-Or-H₂O-CO₂ at high a _H2O and it is concluded that the use of CO₂ to reduce water activities does not influence the composition of the minima in quartz-feldspar systems. The determined liquidus temperature in melts with 1 wt% H₂O is very similar to that obtained in previous nominally “dry” experiments. This discrepancy is interpreted to be due to problems in obtaining absolutely dry conditions. Thus, the hitherto published solidus and liquidus temperatures for “dry” conditions are probably underestimated. Received: 27 March 1997 / Accepted: 1 October 1997     

An experimental study of phase equilibria in the system H2O-CO2-NaCl at 800 °C and 9 kbar

Phase equilibria in the ternary system H₂O-CO₂-NaCl were studied at 800 °C and 9 kbar in internally heated gas pressure vessels using a modified synthetic fluid inclusion technique. The low rate of quartz overgrowth along the `b' and `a' axes of quartz crystals was used to avoid fluid inclusion formation during heating, prior to attainment of equilibrium run conditions. The density of CO₂ in the synthetic fluid inclusions was calibrated using inclusions in the binary H₂O-CO₂ system synthesised by the same method and measured on the same heating-freezing stage. In the two-phase field, two types of fluid inclusions with different densities of CO₂ were observed. Using mass balance calculations, these inclusions are used to constrain the miscibility gap and the orientation of two-phase tie-lines in the H₂O-CO₂-NaCl system at 800 °C and 9 kbar. The equation of state of Duan et al. (1995) approximately describes the P-T section of the ternary system up to about 40 wt% of NaCl. At higher NaCl concentrations the measured solubility of CO₂ in the brine is much smaller than predicted by the EOS. A “salting out” effect must be added to the equation of state to include coulomb interaction in the model of Anderko and Pitzer (1993) and Pitzer and Jiang (1996). The new experimental data together with published data up to 5 kbar (Shmulovich et al. 1995) encompass practically all subsolidus crustal P-T conditions. A feature of the new experimental results is the large compositional range in the H₂O-CO₂-NaCl system occupied by the stability fields of halite + CO₂-rich fluid ± H₂O-NaCl brine. The prediction of halite stability in equilibrium with CO₂-rich fluid in deep-crustal rocks is supported by recent petrological and fluid inclusion studies of granulites. Received: 29 June 1998 / Accepted: 17 March 1999     

High-temperature metamorphic imprint on calc-silicate granulites of Rayagada,Eastern Ghats,India: implication for the isobaric cooling path

 Calc-silicate granulites from Rayagada, north-central sector of Eastern Ghats granulite belt show a wide range of mineral assemblages and chemical compositions, which can be grouped as Gr. I (grossular- rich garnet-wollastonite-scapolite-calcite-clinopyroxene), Gr. II (andradite-rich garnet-scapolite-calcite-clinopyr- oxene), and Gr. III (scapolite-calcite-clinopyroxene-plagioclase) assemblages. Petrographic features suggest the following several reactions in the CaO–Al₂O₃–SiO₂-vapor system: Mei+4Wo+Cal=3Grs+Qtz +2CO₂, Mei+3Wo+2Cal=3Grs+CO₂, Mei= 3An+Cal, Wo+CO₂=Cal+Qtz, Mei+5Wo =3Grs+2Qtz+CO₂, An+Wo=Grs+Qtz, Mei+ 5Cal+3Qtz=3Grs+6CO₂, and the following reactions in the CaO–FeO–MgO–Al₂O₃–SiO₂-vapor system: Cpx_ss+Scp+Wo=Grt_ss+Qtz+CO₂, 4Hd+ 2Cal+O₂=2Adr+2Qtz+2CO₂, Cpx_ss+Scp= Grt_ss+Cal+Qtz. These reactions have been used to estimate peak T-X _CO2 condition for these granulites. A maximum temperature of ∼920 °C has been calculated at an estimated pressure of 9 kbar. A T-X _CO2 diagram shows an isobaric cooling from ∼920 °C to ∼815 °C. A range of X _CO2 (0.50 at 920 °C to 0.25 at 815 °C) has been observed for Gr. I calc-silicate granulites based on the reaction sequences including coronal garnet-forming reactions. This sequence is suggestive of internal fluid buffering rather than external fluid influx and the differences in X _CO2 conditions has been thought to be due to local buffering of fluid phases. Group II and Gr. III calc-silicate granulites, on the other hand, exhibit relatively lower temperature conditions. Received: 11 September 1995/Accepted: 20 June 1996     

Clinopyroxene geobarometry of magmatic rocks. Part 2. Structural geobarometers for basic to acid, tholeiitic and mildly alkaline magmatic systems

The crystal structures of 212 experimentally synthesized, igneous clinopyroxenes were modeled from electronprobe chemical data. The coexisting melts span a wide range of petrologically relevant, dry and hydrous compositions, characterized by variable enrichment in silica and alkalis. Experimental conditions pertain to Earth's crust and uppermost mantle (P = 0–24 kbar; garnet absent) and a variety of f _O2 values (from CCO-buffered to air-buffered) and mineral assemblages (Cpx ± Opx ± Pig ± Ol ± Plag ± Spl ± Mt ± Amp ± Ilm). Unit-cell volume (V_cell) versus M1-polyhedron volume (V_M1) relations were investigated over a range of pressures and temperatures using data derived from structure modeling and corrected for thermal expansivity and compressibility. The relationships between pressure and clinopyroxene structural parameters were found to be dependent on the nature of the coexisting melt. To reduce compositional effects, only clinopyroxenes belonging to mildly alkaline (MA) and tholeiitic (TH) series were considered. Pressure was modeled as a linear function of V_cell, V_M1, and Mg/(Mg + Fe²⁺)^Cpx ratio. A calibration based on the whole data set (MA + TH) reproduced the experimental pressures within 1.4 kbar at the 1-σ level. The maximum residuals were 3.5 kbar and 3.9 kbar for MA- and TH-clinopyroxenes, respectively. Better statistics were obtained by considering MA- and TH-clinopyroxenes separately. A calibration based on the 69 MA-clinopyroxenes reproduced the experimental pressures within 1.1 kbar (1σ) and with a maximum residual of 2.7 kbar. A calibration based on the 143 TH-clinopyroxenes reproduced the experimental pressures within 1.0 kbar (1σ) and with a maximum residual of 3.4 kbar. When these geobarometers are applied to natural samples for which P is unknown, the correction for compressibility is necessarily made through a trial-and-error procedure. This expedient propagates an additional error that increases the above uncertainties and residuals by a factor of about 2. Applications to natural, igneous rocks for which the pressures of crystallization could be constrained based on experimental, petrological or geological evidence yielded pressure estimates that reproduced the expected values to within ca. 2 kbar. Compared to the MA-formulation, the TH-formulation appears to be less robust to variations in magma composition. When applied to high-pressure (>10 kbar) clinopyroxenes synthesized from very low Na (Na₂O < 1.5%) melts, the latter geobarometer can underestimate P by as much as 6 kbar. Calculation of P through the present geobarometers requires clinopyroxene major-element composition and an independent, accurate estimate of crystallization T. Underestimating T by 20 °C propagates into a 1-kbar increase in calculated P. The proposed geobarometers are incorporated in the CpxBar software program, which is designed to retrieve the pressure of crystallization from a clinopyroxene chemical analysis. Received: 11 June 1998 / Accepted: 12 November 1998     

Fluid inclusions in the Niquelândia and Barro Alto Complexes, Goias State, Brazil

Summary ?Fluid inclusions from two Mesoproterozoic, metamorphosed layered intrusive complexes, Niquelandia and Barro Alto, Goiás State, Brazil record multiple fluid influx events from the magmatic to granulitic and retrograde metamorphic stages. 1. The oldest inclusions contain high density CO₂ ± N₂ ± CH₄ and are found as primaries in plagioclase and orthopyroxene in mafic granulite with homogenization temperatures between − 48 and − 28 °C. These inclusions may correspond to the early, magmatic stage. This type was found in samples from both the Niquelandia and the Barro Alto complexes. 2. Intragranular, relatively high density CO₂ + N₂ inclusions (T_h between − 33 and − 26 °C) together with decrepitated and reequilibrated N₂ inclusions (T_h between − 160 and − 151 °C) in the rock-forming minerals can be associated with the granulite facies metamorphism. Such inclusions were found only in the Barro Alto complex. 3. Transgranular, high density, CO₂–N₂ inclusions (93% CO₂ and 7% N₂, according to Raman analysis, with T_h between − 66.6 and − 50.4 °C) as well as the low density, secondary CO₂ ± N₂ ± CH₄ inclusions (T_h between − 13.0 and + 18.7 °C) and the H₂O–NaCl–CaCl₂ hypersaline inclusions (with halite dissolution temperature between 132 and 354 °C, and T_h between 212 and 490 °C) are attributed to different fluid influx events during the retrograde metamorphism. This inclusion type can be found both in the Niquelandia and in the Barro Alto complexes. The fluid inclusion textures and compositions show several stages of fluid evolution. The fluid inclusion measurements and the geothermobarometric data indicate an anticlockwise P-T path for both the Barro Alto and the Niquelandia complexes. Received October 16, 2000; revised version accepted November 20, 2001     

Mineral inclusions in diamonds from the River Ranch kimberlite, Zimbabwe

More than 99% of mineral inclusions in diamonds from the River Ranch pipe in the Late Archean Limpopo Mobile Belt (Zimbabwe), are phases of harzburgitic paragenesis, namely olivine (Fo_92–93), orthopyroxene (Mg# = 93), G10 garnets and chromites. The diamond inclusion (DI) chemistry demonstrates a limited overlap with River Ranch kimberlite macrocrysts: the DI garnets are more Ca-undersaturated, and DI spinel and garnet are more Mg-rich. Most River Ranch diamond inclusions were equilibrated at T = 1080–1320 °C, P = 47–61 kbar, and f _O2 between IW and WM buffers. The P/T profile beneath the Limpopo Mobile Belt (LMB) is consistent with a paleo-heat flow of 41–42 mW/m², similar to calculations for Roberts Victor, but hotter than for the Finsch, Kimberley, Koffiefontein and Premier Mines. This is ascribed to the younger tectonothermal age of the LMB and its proximity to Late Archean oceans. Like diamond inclusions from all other kimberlites studied, the River Ranch DI have a lithospheric affinity and therefore indicate that an ancient, chemically depleted, thick (at least 200 km) mantle root existed beneath the Limpopo Mobile Belt 530–540 Ma ago. The mantle root might have developed beneath the continental Central Zone of the LMB as early as the Archean, and could be alien to the overthrust allochthonous sheet of the Limpopo Belt. Oxygen fugacity estimates for diamond inclusions at River Ranch are similar to other diamondiferous harzburgites beneath the Kaapvaal craton, indicating that the Kaapvaal mantle as a whole was well buffered and homogeneous with respect to f _O2 at the time of peridotitic diamond crystallization. Received: 11 January 1995 / Accepted: 10 June 1997