Low-grade Metamorphism of the Karmutsen Volcanics, Vancouver Island, British Columbia

Low-variance assemblages occurring within amygdules of Karmutsenlavas from the Elk Creek and Upper Campbell Lake areas, VancouverIsland, British Columbia, provide important constraints on thepressure and temperature of metamorphism as well as on the compositionof the attendant fluid. The P-T stability of the assemblagesepidote-muscovite-K-feldspar-prehnite and epidote-prehnite-quartz-wairakitecoupled with mean isochores derived from homogenization temperaturesof H₂O inclusions within amygdaloidal quartz indicate that theUpper Campbell Lake area was subjected to metamorphism at 1?5kb (?0?5 kb), 260 ?C (? 15?C) and the Elk Creek area at somewhatlower P or higher T. Isobaric T-a(CO₂) diagrams show that the occurrence of epidote-oligoclase,prehnite-orthoclase-albite, and prehnite-andesine assemblagescollected from the Elk Creek area is consistent with the P-Tconstraints and that these assemblages formed in water-richfluids containing very low concentrations of CO₂. The presenceof Ca-zeolite-epidote assemblages in the Upper Campbell Lakearea is also compatible with P-T estimates. The consistencyof epidote and prehnite rim compositions in low-variance assemblagesand the lack of incompatible phases in these assemblages demonstratethat equilibrium was obtained on limited domains within amygdules. Because epidote and prehnite compositions in low-variance assemblagesare very sensitive to changes in concentration of CO2, low-varianceassemblages involving these phases can serve as monitors offluid composition. It is postulated that low-variance assemblagesin Karmutsen flows originated by reaction of previously formed,high-variance assemblages with infiltrating CO₂-bearing aqueousfluids during a subsequent hydrothermal event. These fluidspreferentially exploited more permeable amygdaloidal portionsof the Karmutsen flows. The low-variance assemblages not onlyrecord the extremely H₂O-rich composition of the permeatingfluid, but also outline the paths the fluid took. ^* Offprint requests to B. R. Frost     

Water-Saturated and -Undersaturated Melting of Metaluminous and Peraluminous Crustal Compositions at 10 kb: Evidence for the Origin of Silicic Magmas in the Taupo Volcanic Zone, New Zealand, and Other Occurrences

The melting relations of two proposed crustal source compositionsfor rhyolitic magmas of the Taupo Volcanic Zone (TVZ), New Zealand,have been studied in a piston-cylinder apparatus at 10 kb totalpressure and a range of water activities generated by H₂O-CO₂vapour. Starting materials were glasses of intermediate composition(65 wt.% Si0₂ representing a metaluminous ‘I-type’dacite and a peraluminous ‘S-type’ greywacke. Crystallizationexperiments were carried out over the temperature range 675to 975?C, with aH₂O values of approximately 1?0, 0?75, 0?5,and 0?25. Talc-pyrex furnace assemblies imposed oxygen fugacitiesclose to quartz-fayalite-magnetite buffer conditions. Assemblages in both compositions remain saturated with quartzand plagioclase through 675–700?C at high aH₂O, 725–750?Cat aH₂O0?5, and 800–875?C at aH₂O0?25, corresponding to<60–70% melting. Concentrations of refractory mineralcomponents (Fe, Mg, Mn, P, Ti) in liquids increase throughoutthis melting interval with increasing temperature and decreasingaH₂O. Biotite and hornblende are the only mafic phases presentnear the solidus in the dacite, compared with biotite, garnet,gedritic orthoamphibole, and tschermakitic clinoamphibole inthe greywacke. Near-solidus melting reactions are of the type:biotite + quartz + plagioclase = amphibole ? garnet, potentiallyreleasing H₂O for dehydration melting in the greywacke, butproducing larger amounts of hornblende and releasing littleH₂O in the dacite. At aH₂O0?25 and temperatures 825–850?C,amphibole dehydration produces anhydrous mineral phases typicalof granulite fades assemblages (clinopyroxene, orthopyroxene,plagioclase?quartz in the dacite; garnet, orthopyroxene, plagioclase?quartzin the greywacke) coexisting with melt proportions as low as40%. Hornblendce-saturated liquids in the dacite are weaklyperaluminous (0?3–1?6 wt.% normative C—within therange of peraluminous TVZ rhyolites), whereas, at aH₂O0?25 andtemperatures 925?C, metaluminous partial melt compositions (upto 1?8 wt.% normative Di) coexist with plagioclase, orthopyroxene,and clinopyroxene. At all water activities, partial melts ofthe greywacke are uniformly more peraluminous (1?5–2?6wt.% normative C), reflecting their saturation in the componentsof more aluminous mafic minerals, particularly garnet and Al-richorthopyroxene. A metaluminous source for the predominantly Di-normativeTVZ rhyolites is therefore indicated. With decreasing aH₂O the stability fields of plagioclase andquartz expand, whereas that of biotite contracts. These changesare reflected in the proportions of normative salic componentsin partial melts of both the dacite and greywacke. At high aH₂O,partial melts are rich in An and Ab and poor in Or (trondhjemitic-tonalitic);with decreasing aH₂O they become notably poorer in An and richerin Or (granodioritic-granitic). These systematic variationsin salic components observed in experimental metaluminous tostrongly peraluminous melts demonstrate that a wide varietyof granitoid magmas may be produced from similar source rocksdepending upon P-T-aH₂O conditions attending partial melting.Some peraluminous granitoids, notably trondhjemitic leucosomesin migmatites, and sodic granodiorites and granites emplacedat deep crustal levels, have bulk compositions similar to nearsolidus melt compositions in both the dacite and greywacke,indicating possible derivation by anatexis without the involvementof a significant restite component.     

Petrology of an Andalusite-Type Regional Metamorphic Terrane, Panamint Mountains, California

The terrane in the Panamint Mountains, California, was regionallymetamorphosed under low-pressure conditions and subsequentlyunderwent retrograde metamorphism. Prograde metamorphic isogradsthat mark the stability of tremolite + calcite, diopside, andsillimanite indicate a westward increase in grade. The studywas undertaken to determine the effects of the addition of Caon the types of assemblages that may occur in pelitic schists,to contribute to the understanding of the stability limits inP – T – aH₂O – X_Fe of the pelitic assemblagechlorite + muscovite + quartz, and to estimate the change inenvironment from prograde to retrograde metamorphism. Peliticassemblages are characterized by andalusite + biotite + stauroliteand andalusite + biotite + cordierite. Within a small changein grade, chlorite breaks down over nearly the entire rangein Mg/(Mg + Fe) to biotite + aluminous mineral. Chlorite withMg/(Mg + Fe) = 0.55 is stable to the highest grade, and thegeneralized terminal reaction is chlorite + muscovite + quartz= andalusite + biotite + cordierite + H₂O. Calcic schists arecharacterized by the assemblage epidote + muscovite + quartz+ chlorite + actinolite + biotite + calcite + plagioclase atlow grades and by epidote + muscovite + quartz + garnet + hornblende+ biotite + calcite + plagioclase at high grades. Epidote doesnot coexist with any AFM phase that is more aluminous than garnetor chlorite. Lithostatic pressure ranged from 2.3 kb to 3.0kb. During prograde-metamorphism temperatures ranged from lessthan 400° to nearly 700°C, and X_H2O (assuming P_H2O +P_CO3 = P_total) is estimated to be 0.25 in siliceous dolomite,0.8 in pelitic schist, and 1.0 in calcic schist. Temperatureduring retrograde metamorphism was 450° ± 50°C,and all fluid were H₂O-rich. A flux of H₂O-rich fluid duringfolding is believed to have caused retrograde metamorphism.The petrogenetic grid of Albee (1965b) is modified to positionthe (A, Cd) invariant point relative to the aluminosilicatetriple point, which allows the comparison of facies series thatinvolve different chloritoid-reactions.     

Granulite Metamorphism, Fluid Buffering, and Dehydration Melting in the Madras Charnockites and Metapelites

Interlayered and cofolded charnockites and metapelites of thetype charnockite area near Madras were metamorphosed under granulitefades conditions. Fe-Mg partitioning between orthopyroxene,garnet, and biotite indicates that chemical equilibrium wasapproached under similar P-T conditions in the two rock suites.Several geothennometers and geobarometers give P-T values whichconverge at 750–800?C and 6.5–7.5 kb. Computations utilizing data from high pressure phase equilibriumexperiments of Bohlen et al. (1983a) and Wones & Dodge (1977)point to several significant relations regarding the behaviourof H₂O during the granulite metamorphism. a_H2O values, computedfrom Bohlen et al.'s (1983a) reversal data and the a-X modelfor phlogopite after Bohlen et al. (1980), show distinctly lowermagnitudes in metapelites (0.10–0.16) than in charnockites(0.23–0.34). No systematic spatial gradients exist withinthe charnockites or metapelites, and a_H2O has similar valuesin metapelite exposures widely separated in the field. Theseimply an internal, rather than an external (e.g., by CO2 influx),control of the fluids. Applying the algebraic method developed by Rumble (1976), Gibbsanalysis in the system K₂O-MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂-TiO₂-H₂Oshows that the chemical potentials of H₂O and to O₂, as monitoredagainst biotite composition and , exhibit gradients with respect to X_Mg in the two rock suites under isothermal-isobaricconditions. µ_H2O was found to decrease with X_Mg^bt in both,while µ_O2 increases with decreasing X_Mg^bt in metapelitesbut increases sympathetically with X_Mg^bt in charnockites. Thesefindings point out again that µ_H2O and µ_O2 wereinternally buffered. The absence of graphite in the metapelites,at an estimated f_O2 = 10^–14.7 b, also argues against anexternal influx of CO₂ and, inter alia, supports internal buffering.A complementary enquiry into variations of a_TIO2 reveals aninverse relation between a_TIO2 and a_H2O, suggesting a similarcontrol for a_TIO2. The inferences from biotite dehydration equilibria, when combinedwith the P-T data and with several field and chemical featuresof these rocks noted earlier (Sen, 1974), make dehydration meltinga distinct possibility for the Madras rocks. It is argued thatthe low a_H2O and high a_TIO2 ({small tilde} 0.9) observed inthe metapelites have been caused by a greater extent of meltingin the precursors of metapelites, which were more hydrous thanthose of charnockites, coupled with preferential partitioningof Ti into the residual rocks—thus strengthening the casefor dehydration melting.     

Metamorphism in the Adirondacks: II. The Role of Fluids

Quantitative estimates of metamorphic fluid speciation, stableisotopic analyses, and studies of fluid inclusions all documentthe local complexity of fluids in the deep crustal rocks exposedin the Adirondack Mountains, NY. Estimates of the activity ofH₂O in the granulite facies are substantially lower than inthe amphibolite facies gneisses. The onset of low water activitiesin semi-pelitic gneisses generally correlates with migmatitictextures in the uppermost amphibolite facies, suggesting thatpartial melts absorbed H₂O at the peak of metamorphism. In granulitefacies marbles and calc-silicates, conditions varied from extremelyundersaturated in H₂O-CO₂ fluid to fluid saturated, and _H2Oand _CO2 show sharp gradients within single outcrops. Low valuesof f_O2 and f_H2O, or of f_CO2, and f_H2O indicate fluid-absentconditions for some orthogneisses and marbles, which are inferredto have been ‘dry’ rocks before and during granulitefacies recrystallization. Wollastonite is preserved from earlycontact metamorphism and serves as an index mineral for fluid-absentconditions in granulites where _H2O is low. Values off_O2 rangefrom near the hematite + magnetite buffer in metamorphosed ironformation to substantially below the quartz + magnetite + fayalitebuffer in some orthogneisses. The anorthosite suite is moreoxidized than some associated granitic gneisses. Halogens (Fand Cl) substitute extensively for OH in micas and amphiboles,extending their stability, although F₂, Cl₂, HCl, and HF areminor components in any fluid. Oxybiotite-type exchanges involvingO for OH are also important, extending the stability of biotite.Stable isotopic ratios of O and C demonstrate that premetamorphicwhole-rock compositions are commonly preserved whereas mineralcompositions generally reflect equilibration at the peak ofmetamorphism. The Marcy Anorthosite Massif was intruded as ahigh ¹⁸O magma. The combination of mineral equilibria, stable isotope data,and fluid inclusions is used to identify and to distinguishamong pre-orogenic contact metamorphic/hydrothermal events,peak metamorphic events, and retrograde/postmetamorphic events.Polymetamorphism is documented at skarn zones adjacent to anorthosite,where large volumes of hydrothermal fluid were channeled duringearly, shallow contact metamorphism and where conditions werefluid poor during subsequent regional metamorphism. Peak metamorphicevents are inferred to have been caused primarily by magmaticprocesses of intrusion and anatexis. Partial melting has causedlow values of _H2O in many rocks, but in other cases low valuesof _H2O are recorded in orthogneisses derived from H₂O-poor magmas.Isotopic studies show that maximum fluid/rock ratios were <0?land possibly 0?0 for infiltrating fluids at the peak of metamorphismin many localities. No evidence of pervasive, regional infiltrationby a fluid at the peak of metamorphism has been substantiatedin the Adirondacks. Fluid inclusions containing high-densityCO₂ or CO₂ + H₂O represent conditions from after the peak ofmetamorphism and document isobaric cooling, in agreement withestimates from garnet zoning. Fine-scale retrograde veins arecommon and are associated with high-density CO₂-rich fluid inclusions.     

Melting of a Hydrous Mantle: I. Phase Relations of Natural Peridotite at High Pressures and Temperatures with Controlled Activities of Water, Carbon Dioxide, and Hydrogen

Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO₂=43?7–45?7 wt. percent, Al₂O3=1?6O–8?21 wt. per cent, CaO=0?70–8?12wt. per cent,alk=0?10–0?90 wt. per cent and Mg/(Mg+Fe²⁺)=0?94–0?85)have been investigated in the hypersolidus region from 800?to 1250?C with variable activities of H₂O, CO₂, and H₂. Thevapor-saturated peridotite solidi are 50–200?C below thosepreviously published. The temperature of the beginning of meltingof peridotite decreases markedly with decreasing Mg/(Mg+Fe)of the starting material at constant CaO/Al₂O₃. Conversely,lowering CaO/Al₂O₃ reduces the temperature at constant Mg/(Mg+Fe)of the starting material. Temperature differences between thesolidi up to 200?C are observed. All solidi display a temperatureminimum reflecting the appearance of garnet. This minimum shiftsto lower pressure with decreasing Mg/(Mg+Fe) of the startingmaterial. The temperature of the beginning of melting decreasesisobarically as approximately a linear function of the mol fractionof H₂O in the vapor (X_H2O). The data also show that some CO₂may dissolve in silicate melts formed by partial melting ofperidotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H₂O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or coexist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable a_H2O conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane couldform by direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of (). Such activities of H₂Oresult in melting at depths ranging between 125 and 175 km inthe mantle. This range is within the minimum depth generallyaccepted for the formation of kimberlite.     

Petrology of Biotite-Cordierite-Garnet Gneiss of the McCullough Range, Nevada II. P-T-aH2O Path and Growth of Cordierite During Late Stages of Low-P Granulite-Grade Metamorphism

Thermodynamic calculations based on addition of mass balanceequations to the Gibbs Method (Spear, 1986) are used to modelthe cordierite-producing reaction in pelitic gneiss from theMcCullough Range, southern Nevada. Calculations which treatthe model paragenesis as a system open to transfer of H₂O areconsistent with textural relations. Results indicate that cordieritegrew by the continuous net-transfer reaction: 0?76 BIO+1?72 SILL+3? 55 QTZ+0?27 PLG+0?005 GRT +0?06Al₂R²⁺_–1Si_–1[BIO]1?02 KSP+0?76 H₂O +0?30 FeMg_–1[CRD]+0?15FeMg_–1[BIO]+0?0005 FeMg_–1[GRT] +0?005 CaNaAl_–1Si_–1[PLG] with decreasing P, decreasing T, and increasing a_H2O The steepretrograde dP/dT path for these low-pressure granulites contrastswith isobaric cooling paths typical of higher pressure granulites,and suggests uplift and erosion were active during Proterozoicgranulite-grade metamorphism in this area.     

Petrology of Biotite-Cordierite-Garnet Gneiss of the McCullough Range, Nevada I. Evidence for Proterozoic Low-Pressure Fluid-Absent Granulite Grade Metamorphism in the Southern Cordillera

Proterozoic migmatitic paragneisses exposed in the McCulloughRange, southern Nevada, consist of cordierite+almanditic garnet+biotite+sillimanite+plagioclase+K-feldspar+quartz+ilmenite+hercynite.This assemblage is indicative of a low-pressure fades seriesat hornblende-granulite grade. Textures record a single metamorphicevent involving crystallization of cordierite at the expenseof biotite and sillimanite. Thermobarometry utilizing cation exchange between garnet, biotite,cordierite, hercynite, and plagioclase yields a preferred temperaturerange of 590–750?C and a pressure range of 3–4 kb.Equilibrium among biotite, sillimanite, quartz, garnet, andK-feldspar records a_H2O between 0?03 and 0?26. The low a_H2Otogetherwith low f_O2 (QFM) and optical properties of cordierite indicatemetamorphism under fluid-absent conditions. Preserved mineralcompositions are not consistent with equilibrium with a meltphase. Earlier limited partial melting was apparently extensiveenough to cause desiccation of the pelitic assemblage. The relatively low pressures attending high-grade metamorphismof the McCullough Range paragneisses allies this terrane withbiotite-cordierite-garnet granulites in other orogenic belts.aosure pressures and temperatures require a transient apparentthermal gradient ofat least 50?C/km during part of this Proterozoicevent in the southern Cordillera. ^*Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-1567     

Experimentally Determined Conditions in the Fish Canyon Tuff, Colorado, Magma Chamber

The Fish Canyon Tuff, Colorado, forms one of the largest (3000km³ known silicic eruptions in Earth history. The tuff is ahomogeneous quartz latite consisting of 40% phenocrysts (plagioclase,sanidine, biotite, hornblende, quartz, magnetite, apatite, sphene,and ilmenite) in equilibrium with a highly evolved rhyoliticmelt now represented by the matrix glass. Melt inclusions trappedin hornblende and quartz phenocrysts are identical to the newlyanalyzed matrix glass composition indicating that hornblendeand quartz crystallized from a highly evolved magma that subsequentlyexperienced little change. This study presents experimentalphase equilibrium data which are used to deduce the conditions(P, T, f_O2, f_H2O, etc.) in the Fish Canyon magma chamber priorto eruption. These new data indicate that sanidine and quartzare not liquidus phases until 780?C temperatures are achieved,consistent with Fe-Ti oxide geothermometry which implies thatthe magmatic temperature prior to eruption was 760?30?C. NaturalFe-Ti oxide pairs also suggest that log f_O2 was -12.4 (intermediatebetween the Ni-NiO and MnO-Mn₃O₄ oxygen buffers) in the magmachamber. This f_O2.102 is supported by the experimentally determinedvariations in hornblende and melt Mg-numbers as functions off_O2 A new geobarometer based on the aluminum content of hornblendesin equilibrium with the magmatic assemblage hornblende, biotite,plagioclase, quartz, sanidine, sphene, ilmenite or magnetite,and melt is calibrated experimentally, and yields pressuresaccurate to ?0.5 kb. Total pressure in the Fish Canyon magmachamber is inferred to have been 2.4 kb (equivalent to a depthof 7.9 km) based on the Al-content of natural Fish Canyon hornblendesand this new calibration. This depth is much shallower thanhas been proposed previously for the Fish Canyon Tuff. Variationsin experimental glass (melt) composition indicate that the magmawas water-undersaturated prior to eruption. X_H2O in the fluidphase that may have coexisted with the Fish Canyon magma isestimated to have been 0.5 by comparing the An-content of naturalplagioclases to experimental plagioclases synthesized at differentX_H2O and P_totals. This ratio corresponds to about 5 wt.% waterin the melt at depth. The matrix glass chemistry is reproducedexperimentally under these conditions: 760?C, 2.4 kb, X_H2O=0.5,and log f_o2=NNO+2 log units. The fugacity of SO₂ (91 b) is calculatedfrom the coexistence of pyrrhotite and magnetite. Maximum CO₂fugacity (2520 b) is inferred assuming the magma was volatilesaturated at 2.4 kb.     

Melting Relations of Basalt with Equilibrium Water Pressure Less Than Total Pressure

Equilibrium H₂O pressure (P_eH2O) was fixed at values less thantotal pressure (P_T) in melting experiments on mixtures of 1921Kilauea tholeiite, H₂O, and CO₂ (58.5 mole per cent H₂O, 41.5mole per cent CO₂), buffered by Ni+NiO. New determinations ofthe beginning of melting of mixtures of 1921 Kilauea tholeiiteand H₂O buffered by quartz+fayalite+magnetite were made at 2and 3 kb. Microprobe analyses of coexisting glass, clinopyroxene,?olivine, ?amphibole were determined for several runs. Decreasing H₂O fugacity (fH₂O) to about six-tenths the fugacityof pure H₂O (f?II₂O) raises the solidus and the upper stabilitylimit of plagioclase. Plagioclase and clinopyroxene coexistin equilibrium with liquid-a feature not observed in the pureH₂O system. Amphibole is stable to about 970 ?C at 2 kb, 1025?C at 5 kb and 1060 ?C at 8 kb. The Al (VI)+Ti contents of theamphibole increase with P, yielding kaersutite at 1050 ?C and8 kb. Calculated modes for the condensed phases reveal large differencesin the amount of glass (liquid) present and large differencesin liquid composition below and above the breakdown temperatureof amphibole at 5 and 8 kb. Liquids coexisting with amphibole,clinopyroxene, olivine, and magnetite are dacitic near the solidusand silica-rich andesites around 1000 ?C at 5 and 8 kb. Theresults of this study substantiate the model for the generationof the calc-alkaline suite by partial melting of H₂O-rich basalts.     

Melting of a Hydrous Mantle: II. Geochemistry of Crystals and Liquids Formed by Anatexis of Mantle Peridotite at High Pressures and High Temperatures as a Function of Controlled Activities of Water, Hydrogen, and Carbon Dioxide

The system peridotite-H₂O–CO₂ serves as a simplified modelfor the phase relations of mantle peridotite involving morethan one volatile component. Run products obtained in a studyof phase relations of four mantle peridotites in the presenceof H₂O- and (H₂O+CO₂)- bearing vapors and with controlled hydrogenfugacity (f_H2) at high pressures and temperatures have beensubjected to a detailed chemical investigation, principallyby the electron microprobe. Mg/(Mg+Fe) of all phases generally increases with increasingtemperature and with increasing Mg/(Mg+Fe) of the starting material.This ratio appears to decrease with increasing pressure forolivine, and for amphibole coexisting with garnet. Decreasingf_H2 from that of IW buffer to that of MH buffer decreases Mg/(Mg+Fe)of the partial melt from approximately 0-85 to approximately0.50, whereas the Fo content of coexisting olivine increasesslightly less than 3 per cent and the Mg/(Mg+Fe) of clinopyroxeneincreases about 4 per cent. However, the variations in Fo contentof olivines are within those observed in olivines from naturalmantle peridotite. The chemistry of other silicate mineralsdoes not significantly reflect variations of f_H2. Consequently,the peridotite mineralogy and/or chemistry is not a good indicatorfor the f_H2 conditions during crystallization. All crystalline phases, except amphibole, and to some extentgarnet, show increasing Cr content with increasing temperatureand increasing Cr content of the starting material, resultingin a positive correlation with Mg/(Mg+Fe). Partial melts aredepleted in Cr₂O₃ relative to the crystalline phases. High Mg/Mg+Fe)and Cr₂O₃ are thus expected in crystal residues after partialmelting. The absolute values depend on degree of melting andthe composition of the parent peridotite. Liquids formed by anatexis of mantle peridotite are andesiticunder conditions of X_H2O^v > 0.6 to at least 25 kb total pressureand to more than 200?C above the peridotite solidus. This observationsupports numerous suggestions that andesite genesis in islandarcs may result from partial melting of underlying peridotitemantle. In contrast to basaltic rocks, the absence of amphibole(paragasitic hornblende) does not affect the silica-saturatednature of the liquids. Increasing K₂O content of the startingmaterial (up to 1 wt. per cent K₂O) results in increasing potassiumcontent of the amphibole (1 wt. per cent K₂O) as well as theappearance of phlogopite. The liquid under these conditionsis relatively K₂0-poor (less than 1 wt. per cent K₂O). Partial melts are olivine normative with X_H2O 0.5, and initialliquids contain normative ol and ne at X_H2O 0.4. The alkalinityof these liquids increases with decreasing X_H2O below valuesof 0.5. The (ol+opx)-normative liquids resemble oceanic basaltswhereas (ol+ne)-normative liquids resemble olivine nepheliniteand melilite basalt. Low aHlo and high a_Co2 conditions may bethose under which kimberlites and related rocks are formed inthe mantle.     

P--T Stabilities of Laumontite, Wairakite, Lawsonite, and Related Minerals in the System CaAl2Si2O8-SiO2-H2O

Hydrothermal investigation of the bulk composition CaO.Al₂O₃.4SiO₂+excessH₂O has been conducted using conventional techniques over thetemperature ranges 200–450 °C and 500–6000 barsP_fluid. A number of reactions have been studied by employingmineral mixtures consisting of reactants and products in about9: 1 and 1: 9 ratios. The phase relations were deduced fromrelatively long experiments by observing which seeded assemblagedisappeared or decreased markedly in one of the paired run charges. Laumontite was synthesized in the laboratory, probably for thefirst time. Laumontite was grown from seeded wairakite to over99 per cent using a weak NaCl solution. The refractive indicesof the synthetic material are about = 1.504 and = 1.514. Theaverage unit cell dimensions are a₀ = 14.761±0.005 Å;b₀ = 13.077±0.005 Å; c₀ = 7.561±0.003 Å;and ß = 112.02°±0.04°. Within the errorof measurement, the optical properties and cell parameters arein good agreement with those of natural laumontite. The equilibriumdehydration of laumontite involves two reactions: (1) laumontite= wairakite+2H₂O, passing through about 230 °C at 0.5 kb,255±5 °C at 1 kb, 282±5 °C at 2 kb, 297±5°C at 3 kb and 325±5 °C at 6 kb; and (2) laumontite= lawsonite+2 quartz+2H₂O, taking place at about 210 °Cat 3 kb and 275 °C at 3.2 kb. Above 300 °C, the equilibriumcurve for the solid-solid reaction (3) lawsonite+2 quartz =wairakite passes through 305 °C, 3.4 kb and 390 °C,4.4 kb. Equilibrium has been demonstrated unambiguously forthe above three reactions. The hydrothermal decomposition ofnatural laumontite above its own stability limit appears tobe a very slow process. Combined with previously published equilibria determined hydrothermallyfor wairakite, the phase relations are further investigatedby chemographic analysis interrelating the phases, laumontite,wairakite, lawsonite, anorthite, prehnite+kaolinite, and 2 pumpellyite+kaolinitein the system CaAl₂Si₂O₈-SiO₂-H₂O. This synthesis allowed theconstruction of a semiquantitative petrogenetic grid applicableto natural parageneses and the delineation of the physical conditionsfor the various low-grade metamorphic facies in low µCO₂environments. The similar stratigraphic zonations, consistentlyfound in a variety of environments, are recognized to be a functionof burial depth, geothermal gradient, and mineralogical andchemical composition of the parental rocks. Departures fromthe normal sequences are believed to be due to the combinationsof mineralogical variations, availability of H₂O, differencesin the ratio µCO₂/µH₂O, and the rate of reaction.The possible P-T boundaries for diagenesis, the zeolite facies,the lawsonite-albite facies, the prehnite-pumpellyite facies,and the adjacent metamorphic facies are illustrated diagrammatically.     

Infiltration-Driven Metamorphism in Northern New England, USA

Mineral reactions at the biotite isograd were investigated inpelitic schists, micaceous sandstones, micaceous limestones,and metaigneous rocks from three stratigraphic units over anarea of 10000 km² in north-central New England. The biotiteisograd in north-central New England represents a metamorphicdecarbonation front that affected all major rock types in eachstratigraphic unit. Pressure at the isograd was near 3500 bat the northern end of the study area and near 5500 b in thesouth. Temperature was in the range 400–450?C. Equilibriummetamorphic fluids were approximately CO₂-H₂O mixtures withX_CO2=0?04–0?07. Volumetric fluid]-rock ratios were calculatedfor more than 70 samples of all major rock types from each formationusing measured progress of the prograde reactions and the estimatedP-T-X_CO2, conditions of metamorphism. Regardless of stratigraphicunit, limestones record low values of 0–0?2, pelites andmetaigneous rocks generally record high values of 1–3,and standstones record intermediate values of 0?2–1. With exception of the limestones, all samples from the biotitezone record fluid-rock ratios significantly greater than likelyrock porosity during metamorphism. The prograde decarbonationreactions therefore were driven by infiltration of rock by reactiveaqueous fluids. The observed correlations between fluid-rockratio and rock type indicate that significant permeability contrastsoccurred during low-grade metamorphsim with permeability increasingin the order: limestones<sandstones<pelites rocks. Asa corollary, reactive fluid flow must have been channelizedwith enhanced flow in pelites and metaigneous rocks relativeto sandstones and limestones. Results of this study in north-centralNew England taken together with studies of the biotite isogradin south-central Maine (Ferry, 1984, 1986a, 1988) demonstratethat low-grade metamorphism over much of the northern Appalachianorogen was infiltration-driven.     

Melting of a Hydrous Mantle: I. Phase Relations of Natural Peridotite at High Pressures and Temperatures with Controlled Activities of Water, Carbon Dioxide, and Hydrogen

Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO₂ = 43.7–45.7 wt.per cent, A1₂O₃ = 1.6O–8.21 wt. per cent, CaO = 0.70–8.12wt. per cent, alk = 0.10–0.90 wt. per cent and Mg/(Mg+Fe²⁺)= 0.94–0.85) have been investigated in the hypersolidusregion from 800? to 1250?C with variable activities of H₂O,CO₂, and H₂. The vapor-saturated peridotite solidi are 50–200?Cbelow those previously published. The temperature of the beginningof melting of peridotite decreases markedly with decreasingMg/(Mg+SFe) of the starting material at constant CaO/Al₂O₃.Conversely, lowering CaO/Al₂O₃ reduces the temperature at constantMg/(Mg+Fe) of the starting material. Temperature differencesbetween the solidi up to 200?C are observed. All solidi displaya temperature minimum reflecting the appearance of garnet. Thisminimum shifts to lower pressure with decreasing Mg/(Mg + Fe)of the starting material. The temperature of the beginning ofmelting decreases isobarically as approximately a linear functionof the mol fraction of H₂O in the vapor (X_H2O^v). The data alsoshow that some CO₂ may dissolve in silicate melts formed bypartial melting of peridotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H₂O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or co-exist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aHjo conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. Itis suggested that komatiite in Precambrian terrane could formby direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of X_H2O^v = 0.5–0.25 (X_CO2^v= 0.5–0.75). Such activities of H₂O result in meltingat depths ranging between 125 and 175 km in the mantle. Thisrange is within the minimum depth generally accepted for theformation of kimberlite.     

Eclogite Facies Regional Metamorphism of Hydrous Mafic Rocks in the Central Alpine Adula Nappe

The Adula Nappe is a slice of Pre-Mesozoic continental basementaffected by Early Alpine (Mesozoic or Lower Tertiary) high-pressuremetamorphism. Mineral compositions in mafic rocks containingomphacite + garnet + quartz record a continuous regional trendof increasing recrystalliza tion temperatures and pressuresthat can be ascribed to this regional high-pressure metamorphicevent. P-T estimates derived from mineral compositions gradefrom about 12 kb and 500 ?C or less in the north of the nappeto more than 20 kb/800 ?C in the south. The regional P-T trend is associated with a mineralogical transitionfrom assemblages containing additional albite and abundant amphiboles,epidote minerals, and white micas in the north (omphacite-garnetamphibolites) to kyanite eclogites containing smaller amountsof hornblende and zoi.site in the south. Textures and mineralcompositional data show that these hydrous and anhydrous silicatesassociated with omphacite + garnet + quartz arc primary partsof the high-pressure assem blages. Observed phase relationsbetween these primary silicates, theoretical Schreinemakersanalysis, and the thermobarometric results, together indicatethat the regional transition from omphacite amphibolites tokyanite eclogites can be described by two simplified reactions: alb+epi+hbl=omp+kya+qtz+par (H₂O-conserving) (15) par+epi+hbl+qtz=omp+kya+H₂O (dehydration) (12) which have the character of isograd reactions. Local variations of water activity (a_H2O) as indicated by isofacialmineral assemblages, and the H₂O character of the reaction (15),are interpreted to reflect largely H and predominantly fluid-absenthigh-pressure metamorphism within the northern part of the nappe.The omphacite amphibolites and paragonite eclogites in thisarea are thought to have formed by H₂O reactions from Pre-Mesozoichigh-grade amphibolites, i.e. from protoliths of similar bulkH₂O-countent. The second ‘isograd’ (12) is interpreted to markthe regional transition from largely fluid-absent metamorphismin the north to fluid-present metamorphism in the south, wheremetamorphic pressures and temperatures in excess of 12-15kband 500-600?C were sufficient for prograde in-situ dehydrationof similar hydrous protoliths to kyanite eclogites. The observationof abundant veins, filled with quartz+kyanite+omphacite, suggeststhat a free fluid coexisted locally with the kyanite eclogitesof the southern Adula Nappe at some time during progressivedehydration.     

Synthesis and Stability Relations of Mg--Al Pumpellyite, Ca4Al5MgSi6O21(OH)7

Hydrothermal synthesis and investigations of stability relationsof Mg—Al pumpellyite were conducted using high-pressurecold-seal apparatus over the temperature range 250–600°C and 2–8 kb P_fluid. Mg—Al pumpellyite Ca₄Al₅MgSi₆O₂₁(OH)₇was synthesized from partially crystalline gel mixtures of stoichiometriccomposition at 275–410 °C, 6–9 kb P_fluid, andruns of 7–90 days. Pure monomineralic synthetic Mg—Alpumpellyite has refractive index n_ß = 1.624 (2) andcell dimensions = 8.825 (8) Á, b = 5.875 (5) Á,c = 19.10 (1) Á, and ß = 97.39 (7)°. The high temperature assemblage of the equivalent bulk compositionconsists of clinozoisite, hydrogrossular/grossular, aluminousseptechlorite/chlorite, quartz, and H₂O. Hydrogrossular wassynthesized in the presence of quartz at 8 kb from 400–500°C, and hydrogrossular + quartz are unstable with respectto grossular + H₂O at 400 °C and 8 kb P_fluid. At 8 kb P_fluid,aluminous septechlorite forms at temperatures below 500 °Cwhereas aluminous 14 Á chlorite crystallizes at 500–600°C. The equilibrium relations of Mg—Al pumpellyite were determinedusing subequal mixtures of synthetic Mg—Al pumpellyiteand its high temperature assemblage. The reaction 9 Mg—Alpumpellyite = 9 clinozoisite + 6 grossular + 2 chlorite + 4quartz + 19 H₂O occurs at temperatures of 390 °C at 8 kb,368 °C at 5 kb, and near 325 °C at 2 kb P_fluid. Thereversal data yield an approximate value of –3141 joules/mole°K for the standard entropy of formation for the syntheticMg—Al pumpellyite. The Schreinemakers' relations for pumpellyite, prehnite, clinozoisite,tremolite, grossular, and amesite in the presence of excessquartz and fluid were constructed in the pseudo-ternary systemCaO–Al₂O₃–MgO(SiO₂–H₂O). The results, togetherwith reconnaissance experiments on the reaction 4 Mg—Alpumpellyite + 2 quartz = 8 prehnite + aluminous septechlorite+ 2 H₂O, locate the invariant point [TR] at approximately 5.7kb P_fluid and 375 °C. The results of the present study arenot compatible with previous experimental data on the invariantpoint [GR]. The P–T oriented phase relations are used to interpretsome natural parageneses developed in low-grade metabasalticrocks recrystallized under conditions of low _co2. The high-temperaturestability relations of Mg—Al pumpellyite are useful todenote the onset of greenschist facies metamorphism in rocksof basaltic composition.     

Stability and Phase Relations of Trioctahedral Calcium Brittle Micas (Clintonite Group)

On the pseudobinary join CaO:3MgO:Al₂O₃:2SiO₂:_xH2O–CaO:1.25MgO:2.75 Al₂O₃: 0.25SiO₂:_xH₂O clintonite mixed crystals Ca(Mg_{1+ x}Al_{2 – x}) (Al_{4 – x}Si_xO₁₀)(OH)₂ with x rangingfrom 0.6 to 1.4 occur in the temperature range 600–830?C, 2 kb fluid pressure. On the MgSirich side clintonites coexistwith chlorite, forsterite, diopside, and calcite (due to smallamounts of CO₂ in the gas phase) and, at lower temperatures,also with idocrase, hydrogrossularite, and aluminous serpentine.Decomposition of clintonite over a divariant temperature rangeoccurs above 830 ?C, 2 kb; clintonite-free subsolidus assemblagescomprising three or four solid phases are formed in the temperatureranges 890 ?–1120 ?C. The subsolidus assemblages can berepresented in a polyhedron defined by the corners forsterite,diopside, melilite, spinel, anorthite, corundum, and calciumdialuminate. Above 1120 ?C partial melting occurs. The upper thermal stability limits of three selected compositionshave been reversed in the P-T range 0.5–20 kb and 730–1050 ?C, respectively. Below some 4 kb breakdown is dueto the divariant reactions: (1)Ca(Mg_2.25Al_0.75)(Al_2.75)(Si_1.25O₁₀)(OH)₂ spinel+diopside_ss+forsterite+clintonite_ss+vapor, (2)Ca(Mg₂Al)(Al₃SiO₁₀)(OH)₂ spinelx002B;melilite_ss+anorthite+clintonite_ss+vapor, (3)Ca(Mg_1.75Al_1.25)(Al_3.25)(Si_0.75O₁₀)(OH)₂ spinel+melilite_ss+corundum+clintonite_ss+vapor, At the terminations of the divariant temperature ranges (1)melilite_ss, (2) diopside_ss, and (3) anorthite enter those assemblagesand clintonitess disappears completely. The reactions can berepresented by the following equations (1)log,_H2O = 10.2879–8113/T+0.0856(P–1)/T, (2)log = 9.5852–7325/T+0.0794(P–1)/T, (3)log = 7.8358–5250/T+0.077(P–1)/T, with P expressed in bars and Tin ?K. Above 4 kb the upper thermalstability limit of clintonite is defined by incongruent melting,with grossularite participating at pressures above 9 kb. Thesecurves exhibit a very steep, probably even negative slope inthe P-T diagram. There is a close correspondence between natural clintonite-bearingassemblages and thosefound experimentally. The rarity of clintonitein nature is not due to special conditions of pressure and temperaturebut rather due to special bulk compositions of the rocks.     

Aluminous Granulites of the Archean Craton of Kasai (Zaire): Petrology and P-T Conditions

Aluminous granulites of the Archean (2?8 Ga) Kasai craton (Zaire)consist of two main mineral assemblages: Grt-Opx and Sil?Grt?Crdrocks. The high-grade metamorphic conditions as deduced from Grt-Opxand Grt-Opx-Pl-Qtz equilibria are 720?C-6?7 kb. Consideringthe zoning of the same minerals, the slope of the P-T path isestimated at 15 b/?C. Thermobarometry involving Crd is consistentwith those P-T conditions. Three cordierite-forming reactions have been observed petrographically: These equilibria are continuous reactions; end-member reactionshave slopes less than 15 b/?C; they are decompression reactionsoccurring after the metamorphic climax. Using available thermodynamic data, (R3) fixes the oxygen fugacityto a value below the QFM buffer (log₁₀f_O2 = – 17?6 at720?C, 6?7 kb and in the graphite stability field. The absence of graphite in the rocks showsthat the end of the granulite facies metamorphism did not occurunder important CO₂ streaming. The polymetamorphic history of this Archean craton is considered.     

Phase Equilibria for Calcic Scapolite, and Implications of Variable Al-Si Disorder for P-T, T-XCO2, and a-X Relations

Carbonate scapolite is a potentially powerful mineral for calculatingCO₂ activities in non-calcareous rocks, but an analysis of thethermodynamics and phase equilibria of carbonate scapolite isfirst necessary. This includes an evaluation of Al-Si disorderin meionite, as this has the greatest effect on derived phaserelations. Available experimental data on meionite stability,X-ray diffraction refinements and nuclear magnetic resonancespectra for calcic scapolite do not uniquely constrain the Al-Siordering state of synthetic meionite. However, the data aremost consistent with a high degree of Al-Si disorder and inconsistentwith long-range Al-Si order. An internally consistent thermodynamicdata set was derived and used to calculate P-T and T-X_CO2 equilibriainvolving meionite in the CaO-Al₂O₃-SiO₂-CO₂-H₂O (CASCH) system.The effect of Al-Si disorder is illustrated by calculating thephase equilibria using an ordered, an arbitrary intermediatedisordered, and a completely Al-Si disordered standard statefor meionite. The Gibbs free energy of meionite was calculatedfrom reversals (at 790–815?C, 2–15 kb) on the reaction 3 Anorthite +Calcite =Meionite The _fG?_{m, 298} for each of the standard states is –13 146?6,–13128?8, and –130930kJ/mol, respectively. Becauseof the steep slope of reaction (1) and limited temperature rangeover which it breaks down, meionite used in the experimentsto constrain reaction (1) must possess a limited range of Al-Sidisorder. The P-T slope of reaction (1) increases, and the slopeof meionite decarbonation equilibria changes from positive tonegative in T-X_CO2 and P-T space, as a function of increasingAl-Si disorder. Meionite has a wide stability field at highT in T-X space at 5 and 10 kb (P_Total=P_Fluid), being stableto X_CO2=0?06. Meionite alone breaks down to undersaturated gehleniteand/or corundum-bearing assemblages at 5 kb, and to clinozoisiteat 10 kb. The effect of solid solutions on the T-X stabilityof meionite is similar to that of increasing pressure, stabilizingmeionite to lower temperature. Variable Al-Si disorder doesnot significantly affect the upper limit of meionite stabilityin T-X_CO2 space. Activity-composition relations for meionitein carbonate scapolite were calculated relative to reaction(1) from data on natural scapolite-plagioclase-calcite assemblages.The extent of departure from ideality varies as a function ofAl-Si disorder. Negative deviations from ideality are indicatedfor natural scapolite solid solutions at T<750?C, based ona disordered Al-Si standard state for meionite. This is likelyto reflect a more ordered Al-Si distribution in natural scapolitescompared with the synthetic endmember standard state. Present address: Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100     

Thermodynamic Mixing Properties of Muscovite-Paragonite Crystalline Solutions at High Temperatures and Pressures, and their Geological Applications

Reversed Na-K exchange data between mica and a 2 molal aqueous(Na,K)Cl fluid (Flux & Chatterjee, 1986) have been employedto model the thermodynamic mixing behaviour of muscovite-paragonitecrystalline solutions on the basis of the Redlich-Kister equation.For these binary micas, G^ex_m may be expressed as where A=11222+1.389 T+0.2359 P, B=–1134+6.806 T–0.0840 P, and C=–7305+9.043 T, with T in K, P in b, G^ex_m, A, B, and C in joules/mol. G_m^ex is well constrained between 450 and 620?C, and may be extrapolatedbeyond that range with caution. The calculated solvi are skewedtoward the paragonite end member. In the range up to 15 kb,the critical temperature, T_c and the critical composition, X_cmay be expressed as a function of P by the relations: and with P indicated in bars. Calculated phase relations of muscovite-paragonite crystallinesolutions have been depicted in terms of the system KAlSi₃O₈-NaAlSi₃O₈-Al₂O₃-SiO₂-H₂O.These data may be applied to appropriate assemblages involvingmica, alkali feldspar, an Al₂ polymorph, and quartz to estimateP, T and a_H2O conditions of their equilibration. In principle,the muscovite limb of the solvus may be used to obtain geothermometricdata for coexisting muscovite-paragonite pairs, provided theequilibrium pressure is independently known. However, such applicationmust be restricted for the present to micas on the ideal muscovite-paragonitejoin. Mica-alkali feldspar-Al₂SiO₅-quartz or mica-plagioclase-Al₂SiO₅-quartzassemblages may be used to deduce a_H2O in the coexisting fluid,if P, and T of equilibrium are independently known. Examplesof such geological applications are given.