摘要:

AbstractThe reaction of phlogopite plus quartz to enstatite, potassium feldspar, and aqueous fluid in the system KMASH-KCl was reversed at 2-12 kbar and 750-875 °C and at low H2O activities by reversal of the H2O content of concentrated KCl solutions equilibrated with product and reactant assemblages. Synthetic 1M phlogopite [KMg3AlSi3O10(OH)2] and enstatite (MgSiO3) maintained end-member stoichiometry throughout, and the potassium feldspar (KAlSi3O8) was a high sanidine, based on unit-cell refinements. The broad P-T-XH₂Oand narrow reversal ranges of this investigation were possible because of the low and well-defined H2O activity, yet powerful fluxing action, of concentrated KCl solutions. Solubility experiments on quartz and potassium feldspar in our P-T-XH₂Orange showed that fluid-phase solution of silicate constituents was too small to have affected the H2O activity in the experiments. The new determinations are more definitive than previous work done at very low pressures with pure H2O or in CO2-H2O mixtures. They establish the standard free energy of the reaction in the experimental range with an uncertainty of about 1 kJ and indicate that the synthetic phlogopite has maximal (Al-Si) disorder under our experimental conditions. The standard enthalpy of reaction at 298 K is 106.54 ± 2.0 kJ (2σ) based on our reversals, a value 6 kJ less positive than that currently used by many workers in calculations of biotite stability and H2O activity in the petrogenesis of high-grade metamorphic rocks. The lower thermal stability that we find for phlogopite requires revision in estimates of H2O activity of granulite facies metamorphism: typical values for the natural assemblage orthopyroxene-biotite-garnet-potassium feldspar-plagioclase-quartz at deepcrustal metamorphic conditions (750-850 ³C, 5-10 kbar) are aH₂O= 0.4-0.6 compared with values of 0.15-0.30 which would have been estimated with previously available data on phlogopite stability. An important consequence of the expanded H2O activity range of granulites is that alkali chloride solutions of only moderate concentration [XH₂O= 0.5-0.7], H2O which are the values observed in actual fluid inclusions in many kinds of igneous and metamorphic rocks, are a feasible alternative to the vapor-absent conditions considered necessary by many workers based on previous low estimates of aH₂O. Participation of concentrated brines in deep-crust/upper mantle metamorphic processes enables alkali metasomatism and other kinds of chemical transport in an aqueous fluid without large-scale melting of the crust.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-401

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter

摘要:

AbstractThe dehydroxylation of brucite was determined in the presence of NaCl-H2O fluids containing 0, 2, 6, 11, 14, 18, 30, and 38 mol% NaCl (0.0, 5.5, 17.0, 28.8, 34.5, 41.7, 58.2, and 66.6 wt% NaCl) to ∼2500 bars and ∼700 °C by high-pressure differential thermal analysis (HP-DTA). At pressures below 150 bars, brucite dehydrates in two separate reactions involving at least one intermediate phase. The dehydroxylation reaction of brucite at pressure to ∼2 kbars follows lnP(bars) = 22.23 2 13620/T(K). In the presence of NaCl- H2O fluids, the dehydroxylation temperature is lowered because of the reduction of the activity of H2O in these fluids. At 1 kbar pressure, mixing in the NaCl-H2O fluids is virtually ideal, assuming a nearly complete association of NaCl, but at 2 kbars, the dissociation of NaCl is substantial. The phase relations in the system MgO-NaCl-H2O show five univariant reactions emanating from an invariant assemblage, periclase = brucite = halite = liquid = vapor, located at 565 ± 5 °C and 440 ± 30 bars.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-402

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter

摘要:

AbstractThe P-T equilibrium curve of the reaction of magnesite + enstatite = forsterite and CO2was determined in the ranges 6-25 kbar and 700-1100 °C by 53 reversed experiments carried out in 1.91 cm diameter piston-cylinder apparatus with NaCl pressure medium. Silver oxalate was used as a CO2source and charges were buffered at hematite-magnetite oxygen fugacity. Reaction progress was monitored by weight changes upon puncturing of CO2-inflated quenched charge capsules and was confirmed by comparing X-ray diffractograms of charges made before and after the experiments. The data satisfy the relation: PEQ= -3.215 - 4.784 × 10-3T + 2.542 × 10-5T2with P in kbar and T in °C. A P-T curve calculated with the 1994 version 2.3 THERMOCALC program agrees well with our results except in the highest pressure range. Our data, together with three existing tight experimental brackets on the decarbonation reaction of magnesite to periclase and CO2, give ΔH0f(298 K) (forsterite) = -61.20 ± 0.87 kJ/mol from the oxides. This value is in 0f agreement with the value adopted by Berman (1988). An analysis of various proposed equations of state for CO2shows that only those of Mäder and Berman (1991) and Frost and Wood (1997) meet the constraints imposed by this study near 1000 °C and 10-20 kbar. The modified Redlich-Kwong (MRK)-based equations of state overestimate CO2fugacity in this pressure range.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-403

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter

摘要:

AbstractThe stability field of MgMgAl-pumpellyite. Mg5Al5Si6O21(OH)7,was determined in the system MgO-Al2O3-SiO2-H2O in reversal experiments at pressures between 34 and 100 kbar and temperatures in the range of 597 to 1050 °C. Brackets were obtained on five breakdown reactions (in order of increasing pressure):This phase becomes stable only at pressures of more than 34 kbar and temperatures up to 820 °C. Thus, MgMgAl-pumpellyite may be an H2O-containing phase at depths greater than 100 km in the coldest parts of subduction zones.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-404

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter

摘要:

AbstractDouble-thermocouple experiments were carried out at 10 kbar in a 1.27 cm (1/2 in.) piston-cylinder apparatus to map the thermal gradient and locate the peak temperature of a CaF2-graphite furnace assembly over the temperature interval 900-1500 °C. An unexpected but reproducible result was that the thermal peak is displaced upward significantly (toward the steel base plug) from the vertical center of the graphite heater tubes.The hot spot of our assembly, defined as the region no more than 10 °C cooler than the peak temperature, is displaced upward from the center of the furnace and moves slightly toward the center with increasing temperature. At a maximum temperature of 914 °C, the center of the hot spot was 2.6 mm above the mid-point of the furnace, while at 1510 °C, the highest temperature investigated, the center of the hot spot was 2.2 mm above the center of the furnace. The hot spot varies in width from 3.2 mm at lower temperatures to 2.2 mm at 1510 °C. We attribute the upward displacement of the hot spot to the greater thermal conductivity of the lower tungsten-carbide piston (k = 100W/mK at 200 °C) relative to the upper stainless steel base plug (k = 25 W/mK at 600 °C). The piston apparently conducts heat more efficiently downward than the steel plug does upward, thus displacing the hot spot upward from the furnace center.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-405

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter

摘要:

AbstractWe performed 27 viscosity determinations on dry and water-bearing peraluminous haplogranitic melts. The dry melt compositions cover the range of normative corundum to be expected in peraluminous granitic melts in nature. The compositions are based on addition of Al2O3to a haplogranitic melt (HPG8) whose composition is near that of the projection of the 2 kbar H2O-saturated minimum melt composition into the system NaAlSi3O8- KAlSi3O8-SiO2. The H2O contents of the hydrous melts were analyzed using Karl Fischer titration ranging from 1 to 3 wt%. The viscosity determinations were performed using a modified micropenetration method in the viscosity range of 1010to 1011Pa·s, at 1 atm pressure, and in the temperature ranges of 880-940 °C and 470-640 °C for the dry and wet melts, respectively. For the dry peraluminous melts in this high viscosity range, addition of the first few percent of normative corundum to a metaluminous granitic melt increases the viscosity, which remains nearly constant despite further addition of Al2O3. Thus a viscosity maximum is inferred for dry slightly peraluminous granitic melts. The hydrous melt viscosity data were compared with the recent calculational model of Hess and Dingwell (1996), which was based on and designed for metaluminous melt viscosities. That model is capable of describing the viscosities of hydrous peraluminous granitic melts within the uncertainties stated for its application in metaluminous melts.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-406

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter

摘要:

AbstractSr-bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the Su-Lu ultra-high pressure (UHP) province, eastern China. The SrO content of prograde zoisite and epidote reaches 3.2 wt% in crystal cores and generally decreases toward crystal margins. Retrograde epidote is poorer in SrO (70% of the whole-rock SrO is contained in these minerals. Apatite and K-white mica are only minor reservoirs for SrO in these rocks. Zoisite and epidote are thus regarded as the most important Sr reservoirs at UHP conditions where calcic plagioclase and titanite are unstable.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-407

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter

摘要:

AbstractAccessory monazite crystals in granites are commonly unstable during amphibolite facies regional metamorphism and typically become mantled by newly formed apatite-allanite- epidote coronas. This distinct textural feature of altered monazite and its growth mechanism were studied in detail using backscattered electron imaging in a sample of metagranite from the Tauern Window in the eastern Alps. It appears that the outer rims of the former monazites were replaced directly by an apatite ring with tiny thorite intergrowths in connection with Ca supply through metamorphic fluid. Around the apatite zone, a proximal allanite ring and a distal epidote ring developed. This concentric corona structure, with the monazite core regularly preserved in the center, shows that the reaction kinetics were diffusion controlled and relatively slow.Quantitative electron microprobe analyses suggest that the elements released from monazite breakdown (P, REE, Y, Th, U), were diluted and redistributed in the newly formed apatite, allanite, and epidote overgrowth rings and were unable to leave the corona. This supports the common hypothesis that these trace elements are highly immobile during metamorphism. Furthermore, microprobe data suggest that the preserved monazite cores lost little, possibly none of their radiogenic lead during metamorphism. Thus, metastable monazite grains from orthogneisses appear to be very useful for constraining U-Th-Pb protolith ages.On the basis of these findings and a review of literature data, it seems that monazite stability in amphibolite facies metamorphic rocks depends strongly on lithologic composition. While breaking down in granitoids, monazite may grow during prograde metamorphism in other rocks such as metapelites.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-408

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter

摘要:

AbstractPeraluminous granites of the Erzgebirge-Fichtelgebirge, Germany, are hosts of various members of the monazite group of minerals that display an unprecedented compositional diversity. The Eibenstock S-type granite constitutes the third reported occurrence worldwide of brabantite and the first occurrence of this mineral in a granite. Many new occurrences of cheralite-(Ce), as well as a monazite-group mineral intermediate between monazite-( Ce) and huttonite for which the term huttonitic monazite is proposed, were discovered. Even ‘‘common’’ monazite-(Ce) may show extreme ranges of actinide and lanthanide element concentrations.The granites that host brabantite and cheralite-(Ce) are highly differentiated, strongly peraluminous, low-temperature residual melts of S-type affinity, which are rich in fluorine and other volatile constituents but depleted in thorium and the light rare-earth elements. Such highly evolved, volatile-rich compositions resemble rare-element pegmatites and appear favorable for the precipitation of cheralite-(Ce) and brabantite, but not of monazite with large amounts of huttonitic substitution. Instead, these minerals occur preferentially in F-poor biotite and F-rich Li-mica granites of A-type affinity. Irrespective of the level of uranium in silicate melts, which may exceed that of thorium, the substitution of uranium in monazite remains limited.The compositional data reported here are consistent with complete miscibility in the monazite-(Ce)-brabantite solid solution series under magmatic conditions. These granites contain monazites that span almost the entire compositional range reported for monazitegroup minerals worldwide, and therefore granites appear to be ideal rocks in which to study the crystal chemistry of this mineral group in general.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-409

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter

摘要:

AbstractThe chemical microstructure of five jadeitic pyroxenes from three Franciscan quartzose metagraywacke samples was investigated by backscattered electron imagery, element mapping, and electron microprobe microanalysis. These clinopyroxenes are neoblastic, subhedral grains or subradial aggregates replacing albite. For each compositionally zoned Cpx grain, a series of up to five parallel, polished sections were cut essentially normal or parallel to the crystallographic c axis. Five investigated grains, single and subradial aggregates, were examined quantitatively and three-dimensional chemical sections of the grains were constructed. Chemical zoning progresses from the Na-Cpx core to rim in four distinct regions (Q, L, Acm, and T). Region Q consists of microcrystalline blebs of quartz and jadeite (xjd = 0.95) and has a bulk composition of nearly pure albite. Very rarely it contains albite. L is jadeite (xjd = 0.80) that contains lawsonite inclusions. Acm is acmiterich Cpx (xjd = 0.65), and T consists of TiO2-bearing jadeite (1-2 wt% TiO2, xjd = 0.95). Isolated Cpx grains and prismatic aggregates display the same chemical architecture regardless of crystallographic orientation. The earliest growth stage, Q, represents a volumefor-volume replacement of pre-existing albite by nearly stoichiometric jadeite plus quartz. As prisms grew, diffusion from the lithic matrix progressively enriched outer Na-Cpx zones in Fe3+, Ca, and Mg. Terminal stages of high-pressure growth are represented by TiO2- bearing jadeitic pyroxenes, possibly reflecting a temperature increase or relatively longterm annealing under the same conditions in the presence of titanite and rutile. The core Q and rim T regions represent a jadeite plus quartz assemblage and confirm our earlier notion that the jadeite zone of Franciscan metamorphic rocks was subducted to depths where the assemblage jadeite and quartz was stable, at pressures of more than 10 kbar.

ISSN:0003-004X    

DOI:10.2138/am-1998-3-410

出版商:Mineralogical Society of America    

年代:1998

数据来源: Degruyter