Problems of pressure estimation in high temperature experiments using solid media apparatus with particular reference to piston-cylinder and anvil-with-hole apparatus (Part I)

The high albite (Ab)⇄jadeite (Jd)+quartz(Q) reaction (1) and the quartz(Q)⇄coesite (Cs) transformation (2) have been determined within the temperature range of 1000–1100°C and 1000–1400°C respectively under variable pressures using an anvil-with-hole apparatus. The equilibrium curves for the two reactions as a function ofP andT are as follows: P=−1·33+0·0296T (reaction 1);P=18·949+0·0111T(reaction 2). These two lines intersect at 31·1±0·5kb and 1096°C. The possibility of using an anvil-with-hole apparatus for conducting current investigations is discussed in this paper.     

Geothermobarometry of a stilpnomelane–garnet-bearing metapegmatite: P–T constraints on the Eo-Alpine metamorphic overprint of the Austroalpine nappes north of the Tauern Window

The Ordovician Kellerjochgneiss (Schwaz Augengneiss) is a polymetamorphic orthogneiss-bearing unit and is part of the Austroalpine basement nappes north of the Tauern Window. Within the Kellerjochgneiss a small, strongly deformed metapegmatite dike occurs. The pegmatite crosscuts the gneiss discordantly and contains the mineral assemblage muscovite 1,2+plagioclase+K-feldspar+chlorite+quartz+garnet 1 (Alm_67–76Andr_0.9–2Sps_17–28Prp_0.4–5)+garnet 2 (Grs_36–46Alm_24–32Andr_8–21Sps_15–17Prp_0–1)±stilpnomelane±biotite±clinozoisite. The magmatic protolith assemblage is comprised of relict K-feldspar, quartz and garnet 1. Textural observations indicate that biotite and muscovite cores (muscovite 1) are either part of the magmatic- or an earlier (Variscan?) metamorphic assemblage. Geothermobarometry of the metapegmatite was done on the latest-stage (Eo-Alpine) mineral assemblage garnet 2+muscovite 2+chlorite+stilpnomelane+plagioclase+quartz. Calculations of H₂O-absent intersections in the system [KCNFMAS] with the multi-equilibrium program THERMOCALC v.3.1 yielded P–T estimates of 4.4 to 6.7 kbar and 321°C to 376°C. Calculations of the P–T conditions by using the assemblage muscovite 2+chlorite+stilpnomelane+quartz yielded slightly higher pressures of 6.4 to 7.2 kbar at temperatures of 310–325°C. Correlating these P–T data with geochronological data from the neighbouring lithologies (Kellerjochgneiss, Innsbruck Quartzphyllite, Wildschönau Schists) and with structural investigations from these units indicate that the P–T estimates obtained in this investigation represent the Eo-Alpine metamorphic overprint. Hence, these unusual rocks provide important information on the Eo-Alpine P–T conditions since most samples studied from the investigated Austroalpine basement nappes north of the Tauern Window rarely contain mineral assemblages suitable for geothermobarometry.     

Experimental study of subsolidus phase relations and mixing properties of pyroxene in the system CaO-Al2O3-SiO2

Subsolidus phase relations in the system CaO-Al₂O₃-SiO₂ (CAS) were experimentally determined with tight reversals of several univariant curves and with 14 equilibration experiments containing the assemblage pyroxene + anorthite, where pyroxene is a binary solid solution of Ca-Tschermak (CaTs-CaAl₂SiO₆) and Ca-Eskola (CaEs-Ca_0.5AlSi₂O₆) endmembers.Reversals were obtained on the following reactions (bar, °C): 3An = Gr + 2Ky + Q (P = 22T ? 700), 3An + Cor = Gr + 3Ky (P = 21.8T ? 950), 3CaTs= Gr + 2Cor(P = 55T ? 53900), and 6CaTs_{(1 ? x)}CaEs_x = 2(1 ? 2x)Gr + 4(1 ? 2x)Cor + 9xAn. Observed slopes indicate 9.8 J/mol · K of Al-Si disorder in Ca-Tschermak pyroxene and 5.3 J/mol·K of Al-Si disorder in anorthite, at 1300°C. It is suggested that Al-Si disorder in anorthite increases by 1.9 J/mol · K from 700°C to 1300°C.Compositions of CaTs-CaEs pyroxene in equilibrium with anorthite and PbO-rich liquid were experimentally determined at 1400–1430°C and 22.7–30.8 kbar. Microprobe measurements gave compositions which are consistent with an ideal pyroxene solution and the following parameters for the reaction 3An = 2CaTs + 2CaEs (J, bar, K): 2RTln(X_CaTs · X_CaEs) + 60200 + 86.4T ? (5.06 + 13 × 10^?7P)P = 0, resulting in ΔH⁰_j = ?39.8 kJ/mol and S⁰ = 461.8 J/mol · K for the Ca-Eskola endmember at 1300°C. The obtained properties of the Ca-Eskola component are necessary for thermobarometry based on pyroxene bearing assemblages containing plagioclase, quartz, or kyanite.     

Internally consistent recalibrations of mineral equilibria for geothermobarometry involving garnet–orthopyroxene–plagioclase–quartz assemblages and their application to the South Indian granulites

Abstract Three reactions are calibrated as geothermobarometers for garnet–orthopyroxene–plagioclase–quartz assemblages, namely: 1/2 ferrosilite + 1/3 pyrope ± 1/2 enstatite + 1/3 almandine (A): ferrosilite + anorthite ± 2/3 almandine + 1/3 grossularite + quartz (B); and enstatite + anorthite ± 2/3 pyrope + 1/3 grossularite + quartz (C). The internally consistent geothermobarometers based on reactions (A), (B) and (C) are calibrated from experimental data only. The thermodynamic parameters of reaction (A) are derived from published experimental data in the FMAS system (n= 104) in the range 700–1400°C and 5–50 kbar, while those for reaction (B) are derived by summation of the existing reversed experimental data of the mineral equilibria: ferrosilite ± fayalite + quartz (D) and anorthite + fayalite ± 2/3 almandine + 1/3 grossularite (E). The retrieved thermodynamic parameters for reactions (A), (B) and (C) are, respectively: (ΔH⁰, cal) -3367 ± 209, -2749 ± 350 and +3985 ± 545; (ΔS⁰, cal K^?1) -1.634 ± 0.163, -8.644 ± 0.298 and -5.376 ± 0.391; and (ΔV⁰_1,298, cal bar^?1) -0.024, -0.60946 and -0.5614. On a one-cation basis, the derived Margules parameters of the ternary Ca–Fe–Mg in garnet are: W_Fe–Mg= -1256 + 1.0 (～0.23) T(K), W_Mg–Fe= 2880 -1.7 (～0.13) T(K), W_Ca–Mg= 4047 (～77) -1.5 T(K), W_Mg–Ca= 1000 (～77) -1.5 T(K), W_Ca–Fe= -723 + 0.332 (～0.02) T(K), W_Fe–Ca= 1090, (cal) and the ternary constant C₁₂₃= -4498 + 1.516 (～0.265) T(K) cal (subregular solution model of non-ideal mixing); and Fe–Mg–Al in orthopyroxene: W_Fe–Mg= 948 (～200) -0.34 (～0.10) T(K), W_Fe–Al= -1950 (～500) and W_Mg–Al= 0 (cal) (regular solution model of non-ideal mixing). The anorthite activity in plagioclase is calculated by the ‘Al-avoidance’model of subregular Ca–Na mixing commonly used for geobarometry based on reactions (B) and (C). When the geothermobarometers are applied to garnet–orthopyroxene–plagioclase–quartz assemblages (n= 45) of wide compositional range from the Precambrian South Indian granulites, temperature ranges of 690–860°C (X= 760 ± 45°C) and pressure ranges of 5–10 kbar were obtained. The P–T values were estimated simultaneously and there is no difference in the pressure calculated from P_Mg (reaction C) and P_Fe (reaction B). In the existing calibrations this difference is 1 kbar or more. Furthermore, there is no compositional dependence of the ln K of the experimental data in the FMAS (n= 104) and the CFMAS (n= 78) systems at different temperatures and the estimated temperatures of the South Indian granulites.     

Évolution pression–température des amphibolites de la zone axiale au cours du métamorphisme hercynien des Pyrénées orientales

Thin levels of amphibolites from the Canigou, Albères and Cap de Creus massifs have been studied in order to investigate their pressure and temperature evolution during time. P and T values have been calculated using the amphibole–plagioclase–quartz thermo-barometer. Si, Al, Mg and Fe of zoned amphiboles have been analysed from core to rim by microprobe. By combining the results obtained from several (or different) crystals, P–T–t paths have been determined using the amphibole-plagioclase-quartz equilibriums. In the Canigou Massif, the amphibolites have recorded anti-clockwise P–T–t paths around a peak of metamorphism located at about 650?°C–6.1 kbar, whereas in the Albères Massif, the calculated P–T–t paths of amphibolites near the paragneisses are retrograde only, from 600?°C–5 kbar to 450?°C–2.5 kbar, but one cummingtonite-bearing amphibolite has also recorded an anti-clockwise evolution around 650?°C–4.5 kbar. The retrograde P–T–t paths recorded for amphibolites from the ‘Cap de Creus’ Massif are retrograde only, from 650?°C–6 kbar and 400?°C–1 kbar. To cite this article: C. Triboulet et al., C. R. Geoscience 337 (2005).     

Chloritoid–glaucophane schist in the north Qilian orogen, NW China: phase equilibria and P–T path from garnet zonation

Chloritoid–glaucophane‐bearing rocks are widespread in the high‐pressure belt of the north Qilian orogen, NW China. They are interbedded and cofacial with felsic schists originated from greywackes, mafic garnet blueschists and low‐T eclogites. Two representative chloritoid–glaucophane‐bearing assemblages are chloritoid + glaucophane + garnet + talc + quartz (sample Q5‐49) and chloritoid + glaucophane + garnet + phengite + epidote + quartz (sample Q5‐12). Garnet in sample Q5‐49 is coarse‐, medium‐ and fine‐grained and shows two types of zonation patterns. In pattern I, X_grs is constant as X_py rises, and in pattern II X_grs decreases as X_py rises. Phase equilibrium modelling in the NC(K)MnFMASH system with Thermocalc 3.25 indicates that pattern I can be formed during progressive metamorphism in lawsonite‐stable assemblages, while pattern II zonation can be formed with further heating after lawsonite has been consumed. Garnet growth in Q5‐49 is consistent with a continuous progressive metamorphic process from ～14.5 kbar at 470 °C to ～22.5 kbar at 560 °C. Garnet in sample Q5‐12 develops with pattern I zonation, which is consistent with a progressive metamorphic process from ～21 kbar at 540 °C to ～23.5 kbar at 580 °C with lawsonite present in the whole garnet growth. The latter sample shows the highest P–T conditions of the reported chloritoid–glaucophane‐bearing assemblages. Phase equilibrium calculation in the NCKFMASH system with a recent mixing model of amphibole indicates that chloritoid + glaucophane paragenesis does not have a low‐pressure limit of 18–19 kbar as previously suggested, but has a much larger pressure range from 7–8 to 27–30 kbar, with the low‐pressure part being within the stability field of albite.     

The thermodynamic properties and phase relations of some minerals in the system CaO-AI2O3-SiO2-H2O

The heat capacities of lawsonite, margante, prehnite and zoisite have been measured from 5 to 350 K with an adiabatic-shield calorimeter and from 320 to 999.9 K with a differential-scanning calorimeter. At 298.15 K, their heat capacities, corrected to end-member compositions, are 66.35, 77.30, 79.13 and 83.84 cal K^?1 mol^?1; their entropies are 54.98, 63.01, 69.97 and 70.71 cal K^?1 mol^?1, respectively. Their high-temperature heat capacities are described by the following equations (in calories, K, mol): Lawsonite (298–600 K): Cp° = 66.28 + 55.95 × 10^?3T ? 15.27 × 10⁵T^?2 Margarite (298–1000 K): Cp° = 101.83 + 24.17 × 10^?3T ? 30.24 × 10⁵T^?2 Prehnite (298–800 K): Cp° = 97.04 + 29.99 × 10^?3T ? 25.02 × 10⁵T^?2 Zoisite (298–730 K): Cp° = 98.92 + 36.36 × 10^?3T ? 24.08 × 10⁵T^?2 Calculated Clapeyron slopes for univariant equilibria in the CaO-Al₂O₃-SiO₂-H₂O system compare well with experimental results in most cases. However, the reaction zoisite + quartz = anorthite + grossular + H₂O and some reactions involving prehnite or margarite show disagreements between the experimentally determined and the calculated slopes which may possibly be due to disorder in experimental run products. A phase diagram, calculated from the measured thermodynamic values in conjunction with selected experimental results places strict limits on the stabilities of prehnite and assemblages such as prehnite + aragonite, grossular + lawsonite, grossular + quartz, zoisite + quartz, and zoisite + kyanite + quartz. The presence of this last assemblage in eclogites indicates that they were formed at moderate to high water pressure.     

Petrology and P–T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex,China

The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphosed ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, along with lesser amounts of siliciclastic and carbonate rocks and banded iron formations. Petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M₁ assemblage is composed of plagioclase+quartz+actinolite+chlorite+epidote+biotite+rutile, preserved as mineral inclusions in garnet porphyroblasts. The metamorphic conditions for this assemblage cannot be quantitatively estimated. The M₂ stage is represented by garnet porphyroblasts in a matrix of quartz, plagioclase, amphibole, biotite, rutile and ilmenite. P–T conditions for this assemblage have been estimated using the program Tweequ at 10–12 kbar and 600–650°C. The M₃ assemblage is shown by amphibole+plagioclase±ilmenite symplectic coronas around embayed garnets and yields P–T conditions of 6.0–7.0 kbar and 600–650°C. M₄ is represented by chlorite and epidote rimming garnet, chlorite rimming amphibole and epidote replacing plagioclase under greenschist-facies conditions of 400–500°C and relatively lower pressures. Taken together, the qualitative P–T estimates from M₁ and M₄ and the quantitative P–T estimates from M₂ and M₃ define a clockwise P–T path for the Wutai amphibolites.The estimated P–T path from the four stages suggests that the Wutai Complex underwent initial burial and crustal thickening (M₁+M₂), subsequent isothermal exhumation (M₃), and finally cooling and retrogression (M₄). This tectonothermal path, along with those of the Fuping and Hengshan complexes, which bound the southeast and northwest margins, respectively, of the Wutai Complex, is considered to record the early Paleoproterozoic collision between the eastern and western segments of the North China craton.     

Convergent phase equilibria at the massif anorthosite—granulite interface near Bolangir,Orissa, India and thermal evolution of a part of the Indian shield

This investigation is based on detailed geological mapping of the western part of the Bolangir anorthosite massif of Orissa, India and its granulite borders, detailed petrography, whole-rock chemistry, mineralogy and an equilibrium thermodynamic analysis of the mineral phase relations. Structural analysis of the foliations of the granulites and the primary flow layers and the joint system of the anorthositic rocks strongly indicates that the pluton was forcefully intruded into the granulitic cover with considerable stretching and extension and approached the form and structure of a schlieren dome. The anorthositic suite of rocks includes anorthositic norites (median plagioclase composition An₇₅), noritic anorthosites (median plagioclase composition An₇₀) and anorthosites (median plagioclase composition An₅₂), while the bordering granulites include leptynites (K-feldspar + plagioclase + quartz + orthopyroxene + biotite + garnet + ilmenite), khondalites (K-feldspar + quartz + sillimanite + garnet + graphite + ilmenite ± biotite), basic granulites (plagioclase + diopsidic clinopyroxene + orthopyroxene + garnet + hornblende + ilmenite ± K-feldspar ± quartz ± magnetite) and calc-granulites (diopside + scapolite + calcite + garnet + microline + quartz + sphene ± magnetite ± apatite). The anorthositic rocks have a relatively high K₂O/SiO₂ ratio with the MgO/FeO ratio mainly between 1 and 2. The MgO/FeO vs. plagioclase/mafics relations of the anorthositic suite indicate the fractionation trend: anorthositic norite → noritic anorthosite → anorthosite.The calculated P−T curves for nine different mineral equilibria in the anorthosites and the granulites converge to a broad cluster within the region, 3–7 kbar and 600–740°C. The orthopyroxene—garnet thermometer (Harley) and orthopyroxene—plagioclase—garnet—quartz barometer (Newton and Perkins) restrict the convergence to a slightly narrower P−T region: 4.7–7 kbar and 620–740°C. The two-pyroxene equilibria were possibly quenched at a somewhat higher temperature region and the temperature spread of the order of 200°C at pressures between 4.7 and 7 kbar may represent the P−T path over which the pluton cooled in the final stage of its evolution. The parent magma of the anorthosites, believed to be coeval with the 1312 Ma old Chilka Lake massif anorthosites of Orissa, may have formed under a Proterozoic continental crust of well over 20 km thickness.     

The stability of annite+quartz: reversed experimental data for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite +2 H2O

Reversals for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite+2 H₂O have been experimentally determined in cold-seal pressure vessels at pressures of 2, 3, 4 and 5?kbar, limiting annite +quartz stability towards higher temperatures. The equilibrium passes through the temperature intervals 500–540°?C (2?kbar), 550–570°?C (3?kbar), 570–590°?C (4?kbar) and 590–610°?C (5?kbar). Starting materials for most experiments were mixtures of synthetic annite +fayalite+sanidine+quartz and in some runs annite+quartz alone. Microprobe analyses of the reacted mixtures showed that the annites deviate slightly from their ideal Si/Al ratio (Si per formula unit ranges between 2.85 and 2.92, Al^VI between 0.06 and 0.15). As determined by Mössbauer spectroscopy, the Fe³⁺ content of annite in the assemblage annite+fayalite +sanidine+quartz is around 5–7%. The experimental data were used to extract the thermodynamic standard state enthalpy and entropy of annite as follows: H ⁰ _f,?Ann =?5125.896±8.319 [kJ/mol] and S ⁰ _Ann=432.62±8.89 [J/mol/K] (consistent with the Holland and Powell 1990 data set), and H ⁰ _f,Ann =?5130.971±7.939 [kJ/mol] and S ⁰ _Ann=424.02±8.39 [J/mol/K] (consistent with the TWEEQ data base, Berman 1991). The preceeding values are close to the standard state properties derived from hydrogen sensor data of the redox reaction annite=sanidine+magnetite+H ₂ (Dachs 1994). The experimental half-reversal of Eugster and Wones (1962) on the annite +quartz breakdown reaction could not be reproduced experimentally (formation of annite from sanidine+fayalite+quartz at 540°?C/1.035?kbar/magnetite-iron buffer) and probable reasons for this discrepancy remain unclear. The extracted thermodynamic standard state properties of annite were used to calculate annite and annite+quartz stabilities for pressures between 2 and 5?kbar.     

Metamorphic evolution of low-T eclogite from the North Qilian orogen, NW China: evidence from petrology and calculated phase equilibria in the system NCKFMASHO

Low‐T eclogites in the North Qilian orogen, NW China share a common assemblage of garnet, omphacite, glaucophane, epidote, phengite, quartz and rutile with or without paragonite. Phase relations for the low‐T eclogites can be modelled well in the system NCKFMASHO with the updated solid‐solution models for amphibole and clinopyroxene. Garnet in the eclogite typically exhibits growth zonations in which pyrope increases while grossular somewhat decreases from core to rim, which is modelled as having formed mainly in the P–T conditions of lawsonite‐eclogite facies at the pre‐peak stage. Omphacite shows an increase in jadeite component as aegirine and also total FeO decrease in going from the inclusions in garnet to grains in the matrix, and from core to rim of zoned crystals, reflecting an increase in metamorphic P–T conditions. Glaucophane exhibits a compositional variation in X(gl) (= Fe²⁺/(Fe²⁺ + Mg)) and F(gl) (= Fe³⁺/(Fe³⁺ + Al) in M2 site), which decrease from the inclusions in garnet to crystals in the matrix, consistent with an increase in P–T conditions. However, for zoned matrix crystals, the X(gl) and F(gl) increase from core to rim, is interpreted to reflect a late‐stage decompression. Using composition isopleths for garnet rim and phengite in P–T pseudosections, peak P–T conditions for three samples Q5–45, Q5–01 and Q7–28 were estimated as 530–540 °C at 2.10–2.25 GPa, 580–590 °C at 2.30–2.45 GPa and 575–590 °C at 2.50–2.65 GPa, respectively, for the same assemblage garnet + omphacite + glaucophane + lawsonite (+ phengite + quartz + rutile) at the peak stage. The eclogites suggest similar P–T ranges to their surrounding felsic–pelitic schists. During post‐peak decompression of the eclogites, the most distinctive change involves the transformation of lawsonite to epidote, releasing large amount of water in the rock. The released fluid promoted further growth of glaucophane at the expense of omphacite and, in appropriate bulk‐rock compositions, paragonite formed. The decompression of eclogite did not lead to pronounced changes in garnet and phengite compositions. Peak P–T conditions of the North Qilian eclogite are well constrained using both the average P–T and pseudosection approaches in Thermocalc. Generally, the conventional garnet–clinopyroxene geothermometer is too sensitive to be used for constraining the temperature of low‐T eclogite because of the uncertainty in Fe³⁺ determination in omphacite and slight variations in mineral compositions because of incomplete equilibration.     

Petrology and phase equilibrium modeling of sapphirine ＋ quartz assemblage from the Napier Complex,East Antarctica： Diagnostic evidence for Neoarchean ultrahigh-temperature metamorphism

A synthesis of the petrological characters of granulite facies rocks that contain equilibrium sapphirine ＋ quartz assemblage from two localities （Tonagh Island （TI） and Priestley Peak （PP）） in the Napier Complex,East Antarctica,provides unequivocal evidence for extreme crustal metamorphism possibly associated with the collisional orogeny during Neoarchean.The reaction microstructures associated with sapphirine ＋ quartz vary among the samples,probably suggesting different tectonic conditions during the metamorphic evolution.Sapphirine and quartz in TI sample were probably in equilibrium at the peak stage,but now separated by corona of Grt ＋ Sil ＋ Opx suggesting near isobaric cooling after the peak metamorphism,whereas the Spr ＋ Qtz ＋ Sil ＋ Crd ＋ Spl assemblage replaces garnet in PP sample suggesting post-peak decompression.The application of mineral equilibrium modeling in NCKFMASHTO system demonstrated that Spr ＋ Qtz stability is lowered down to 930 ℃ due to small Fe3＋ contents in the rocks （mole Fe2O3/（FeO ＋ Fe2O3） =0.02）.The TI sample yields a peak p-T range of 950-1100 ℃ and 7.5-11 kbar,followed by cooling toward a retrograde stage of 800-950 ℃ and 8-10 kbar,possibly along a counterclockwise p-T path.In contrast,the peak condition of the PP sample shows 1000-1050 ℃ and ＞12 kbar,which was followed by the formation ofSpr ＋ Qtz corona around garnet at 930-970 ℃ and 6.7-7.7 kbar,suggesting decompression possibly along a clockwise p-T trajectory.Such contrasting p-T paths are consistent with a recent model on the structural framework of the Napier Complex that correlates the two areas to different crustal blocks.The different p-T paths obtained from the two localities might reflect the difference in the tectonic framework of these rocks within a complex Neoarchean subduction/collision belt.     

Das P{\text{ - }}T{\text{ - }}X_{{\text{CO}}_{\text{2}} } Stabilitätsfeld von Lawsonit

At pressures which are expected in the earth's crust, the high temperature border of the lawsonite stability field is marked by reaction lawsonite = zoisite + kyanite/andalusite + pyrophyllite + H₂O. (1a) The equilibrium data of reaction (1a) have been experimentally determined, and the equilibrium curve is characterized by the following P, T-data: 4 kb; 360±20° C; 5 kb; 375 ±20° C; 7kb;410±20° C. In the P, T diagram the equilibrium curve of reaction lawsonite + quartz = zoisite + pyrophyllite + H₂O (6) is very close to the curve of reaction (1a); the distance is smaller than the error stated for curve (1a), i.e. below ±20° C. The stability fields of lawsonite and anorthite + H₂O are not adjacent fields in the P, T diagram. This means that no stable reaction of lawsonite to anorthite + H₂O can exist. Thus, the CaAl-silicate formed by the decomposition of lawsonite is always zoisite. Further, as shown by experimental determination of reaction calcite + pyrophyllite + H₂O = lawsonite + quartz + CO₂, (7) lawsonite can coexist with a gas phase only if the CO₂ content of the gas phase does not exceed 3±2 Mol-%. This means, for metamorphism of lawsonite glaucophane rocks, that the fluid phase that was present during metamorphism has been quite rich in H₂O. Ernst (1971, in press) who applied a different, indirect investigation method when studying the composition of the fluid-attending Franciscan and Sanbagawa metamorphism has come to the result that during metamorphism of lawsonite-glaucophane rocks the fluid phase did not contain more than 1–3 Mol-% of CO₂.     

TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz

Quartz and rutile were synthesized from silica-saturated aqueous fluids between 5 and 20 kbar and from 700 to 940°C in a piston-cylinder apparatus to explore the potential pressure effect on Ti solubility in quartz. A systematic decrease in Ti-in-quartz solubility occurs between 5 and 20 kbar. Titanium K-edge X-ray absorption near-edge structure (XANES) measurements demonstrate that Ti⁴⁺ substitutes for Si⁴⁺ on fourfold tetrahedral sites in quartz at all conditions studied. Molecular dynamic simulations support XANES measurements and demonstrate that Ti incorporation onto fourfold sites is favored over interstitial solubility mechanisms. To account for the P–T dependence of Ti-in-quartz solubility, a least-squares method was used to fit Ti concentrations in quartz from all experiments to the simple expression

Metamorphic evolution of the coesite-bearing ultrahigh-pressure terrane in the North Qaidam, Northern Tibet, NW China

Widespread evidence for ultrahigh‐pressure (UHP) metamorphism is reported in the Dulan eclogite‐bearing terrane, the North Qaidam–Altun HP–UHP belt, northern Tibet. This includes: (1) coesite and associated UHP mineral inclusions in zircon separates from paragneiss and eclogite (identified by laser Raman spectroscopy); (2) inclusions of quartz pseudomorphs after coesite and polycrystalline K‐feldspar + quartz in eclogitic garnet and omphacite; and (3) densely oriented SiO₂ lamellae in omphacitic clinopyroxene. These lines of evidence demonstrate that the Dulan region is a UHP metamorphic terrane. In the North Dulan Belt (NDB), eclogites are characterized by the peak assemblage Grt + Omp + Rt + Phn + Coe (pseudomorph) and retrograde symplectites of Cpx + Ab and Hbl + Pl. The peak conditions of the NDB eclogites are P = 2.9–3.2 GPa, and T = 631–687 °C; the eclogite shows a near‐isothermal decompression P–T path suggesting a fast exhumation. In the South Dulan Belt (SDB), three metamorphic stages are recognized in eclogites: (1) a peak eclogite facies stage with the assemblage Grt + Omp + Ky + Rt + Phn at P = 2.9–3.3 GPa and T = 729–746 °C; (2) a high‐pressure granulite facies stage with Grt + Cpx (Jd < 30) + Pl (An24–29) + Scp at P = 1.9–2.0 GPa, T = 873–948 °C; and (3) an amphibolite facies stage with the assemblage Hbl + Pl + Ep/Czo at P = 0.7–0.9 GPa and T = 660–695 °C. The clockwise P–T path of the SDB eclogites is different from the near‐isothermal decompression P–T path from the NDB eclogites, which suggests that the SDB was exhumed to a stable crustal depth at a slower rate. In essence these two sub‐belts formed in different tectonic settings; they both subducted to mantle depths of around 100 km, but were exhumed to the Earth's surface separately along different paths. This UHP terrane plays an important role in understanding continental collision in north‐western China.     

A Fluid Inclusion Study of Vein-type Copper Mineralization in the East Wulasigou Pb–Zn–Cu Deposit, Altay, Northwestern China

The Wulasigou Cu-Pb-Zn deposit,located 15 km northwest of Altay city in Xinjiang,is one of many Cu-Pb-Zn polymetallic deposits in the Devonian Kelan volcanic-sedimentary basin in southern Altaids.Two mineralizing periods can be distinguished:the marine volcanic sedimentary PbZn mineralization period,and the metamorphic hydrothermal Cu mineralization period,which is further divided into an early bedded foliated quartz vein stage(Q1) and a late sulfide-quartz vein stage(Q2) crosscutting the foliation.Four types of fluid inclusions were recognized in the Q1 and Q2 quartz from the east orebodies of the Wulasigou deposit:H_2O-CO_2 inclusions,carbonic fluid inclusions,aqueous fluid inclusions,and daughter mineral-bearing fluid inclusions.Microthermometric studies show that solid CO_2 melting temperatures(T_(m,CO2)) of H_2O-CO_2 inclusions in Ql are from-62.3℃ to-58.5C,clathrate melting temperatures(T_(m,clath)l) are from 0.5 C to 7.5 C,partial homogenization temperatures(T_(h,CO2)) vary from 3.3℃ to 25.9℃(to liquid),and the total homogenization temperatures(T_(h,tot)) vary from 285℃ to 378℃,with the salinities being 4.9%-15.1%NaCl eqv.and the CO_2-phase densities being 0.50-0.86 g/cm~3.H_2O-CO_2 inclusions in Q2 have T_(m,CO_2) from-61.9℃ to-56.9℃,T_(m,clath)from 1.3℃ to 9.5℃,T_(h,CO2) from 3.4℃ to 28.7℃(to liquid),and T_(h,tot) from 242℃ to 388℃,with the salinities being 1.0%-15.5%NaCl eqv.and the CO_2-phase densities being 0.48-0.89 g/cm~3.The minimum trapping pressures of fluid inclusions in Q1 and Q2 are estimated to be 260-360 MPa and180-370 MPa,respectively.The δ~(34)S values of pyrite from the volcanic sedimentary period vary from2.3‰ to 2.8‰(CDT),and those from the sulfide-quartz veins fall in a narrow range of-1.9‰ to 2.6‰(CDT).The δD values of fluid inclusions in Q2 range from-121.0‰ to-100.8‰(SMOW),and theδ~(18)O_(H2O) values calculated from δ~(18)O of quartz range from-0.2‰ to 8.3‰(SMOW).The δD-δ~(18)O_(H2O)data are close to the magmatic and metamorphic fields.The fluid inclusion and stable isotope data documented in this study indicate that the vein-type copper mineralization in the Wulasigou Pb-Zn-Cu deposit took place in an orogenic-metamorphic enviroment.     

Phase relations to 3 kbar in the systems edenite + H2O and edenite + excess quartz + H2O

Amphiboles approached edenite (NaCa₂Mg₅Si₇AlO₂₂(OH)₂), richterite (Na₂CaMg₅Si₈O₂₂(OH)₂), tremolite (□Ca₂Mg₅Si₈O₂₂(OH)₂) solid solutions were studied by conventional hydrothermal techniques employing the bulk compositions edenite, and edenite + additional quartz, all with excess H₂O. For the stoichiometric edenite bulk composition + excess H₂O, the equilibrium phase assemblage is diopside + Na-phlogopite + forsterite + fluid at, and just above the amphibole high-temperature limit at 850 ± 5°C, 500 bar, and 880 ± 5°C, 1000 bar. The breakdown temperature of sodic phlogopite is 855 ± 3°C at 500 bar, and 890 ± 5°C at 700 bar, producing nepheline + plagioclase (or melt), additional forsterite and fluid. Diopside and Na-phlogopite solid solution coexist over a broad P_fluid-T region, even within the amphibole field, where they are associated with an edenite-richterite (-tremolite) solid solution of approximate composition Ed₃₅Rc₅₀Tr₁₅.In the system edenite + 4 quartz + excess H₂O, nearly pure tremolite and albite coexist stably between 670° and 830°C at 1000 bar and give way to the possibly metastable assemblage diopside + talc + albite below 670°C. In the presence of albite, tremolite reacts to produce diopside + quartz + enstatite + fluid above 830°C at 1000 bar. For the investigated silica-rich bulk composition, amphibole P_fluid-T stability is divided by the albite melting curve into a tremolite + albite field, and a tremolite + aqueous melt field. Substantial equilibrium solid solution of tremolite towards edenite or richterite was not observed for silica-excess bulk compositions. Metastable edenite-rich amphiboles initially synthesized change to tremolite with increasing run length in the presence of free SiO₂.Edenitic amphibole is stable only over a very limited temperature range in silica-undersaturated environments, thus accounting for its rarity in nature. Na-phlogopite solid solutions are also disfavored by high a_SiO2; even for nepheline-normative lithologies, a hypothesized rapid low-temperature conversion to vermiculite or smectite could partly explain the scarcity of sodic phlogopite in rocks.     

The electrical conductivity of calcite between 300 and 1200° C at a CO2 pressure of 40 bars

The electrical conductivity of calcite cleavage fragments was measured using direct current. Five temperature intervals, characterized by different activation energies of conduction, were established — T ₁: (300°)–500° C, T ₂: 500–725° C, T ₃: 725°–800° C, T ₄: 800°–985° C and T ₅: 985°–(1,200)° C. The data above 600° C indicate various intrinsic conduction mechanisms which are likely to be related to disorder in the CO₃-sublattice of calcite. The transitions between the temperature intervals T ₂-T ₃-T ₄ and T ₄-T ₅ are probably correlated with high-temperature transitions of calcite at 800° and 985° C. Measurement of the thermoelectric voltage indicates a conductivity dominated by negatively charged carriers.     

Eclogites from the south Tianshan, NW China: petrological characteristic and calculated mineral equilibria in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O system

Eclogites from the south Tianshan, NW China are grouped into two types: glaucophane and hornblende eclogites, composed, respectively, of garnet + omphacite + glaucophane + paragonite + epidote + quartz and garnet + omphacite + hornblende (sensu lato) + paragonite + epidote + quartz, plus accessory rutile and ilmenite. These eclogites are diverse both in mineral composition and texture not only between the two types but also among the different selected samples within the glaucophane eclogite. Using thermocalc 3.1 and recent models of activity–composition relation for minerals, a P–T projection and a series of P–T pseudosections for specific samples of eclogite have been calculated in the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCFMASH) with quartz and water taken to be in excess. On the basis of these phase diagrams, the phase relations and P–T conditions are well delineated. The three selected samples of glaucophane eclogite AK05, AK11 and AK17 are estimated to have peak P–T conditions, respectively, of 540–550 °C at c. 16 kbar, c. 560 °C at 15–17 kbar and c. 580 °C at 15–19 kbar, and two samples of hornblende eclogite AK10 and AK30 of 610–630 °C and 17–18 kbar. Together with H₂O‐content contours in the related P–T pseudosections and textural relations, both types of eclogite are inferred to show clockwise P–T paths, with the hornblende eclogite being transformed from the glaucophane eclogite assemblage dominantly through increasing temperature.     

High-pressure granulites in the Sanggan area, North China craton: metamorphic evolution, P–T paths and geotectonic significance

High‐pressure basic granulites are widely distributed as enclaves and sheet‐like blocks in the Huaian TTG gneiss terrane in the Sanggan area of the Central Zone of the North China craton. Four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles. The P–T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. The low‐Ca core of growth‐zoned garnet, along with inclusion minerals, defines a prograde assemblage (M1) of garnet + clinopyroxene + plagioclase + quartz, yielding 700 °C and 10 kbar. The peak of metamorphism at about 750–870 °C and 11–14.5 kbar (M2) is defined by high‐Ca domains in garnet interiors and inclusion minerals of clinopyroxene, plagioclase and quartz. Kelyphites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions (M3) at conditions around 770–830 °C and 8.5–10.5 kbar. Garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 500–650 °C and 5.5–8 kbar (M4). These results help define a sequential P–T path containing prograde, near‐isothermal decompression (ITD) and near‐isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P–T paths of the HP granulites in the Sanggan area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high‐pressures (6–14.5 kbar) of the four metamorphic stages and the geometry of the P–T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened during collision. The corresponding upper‐plate might be the tectonically overlying Khondalite series, which was subjected to medium‐ to low‐pressure (MP/LP: 7–4 kbar) granulite facies metamorphism with a clockwise P–T path including an ITD segment. Both the HP and the MP/LP granulite facies events occurred contemporaneously at c. 1.90–1.85 Ga in a collisional environment created by the assembly process of the North China craton.