Biotite Equilibria and Fluid Circulation in the Klokken Intrusion

Chemical variation in biotite from the KJokken gabbro-syeniteintrusion in the Proterozoic Gardar province in South Greenlandhas been investigated by electron probe and, for F and Li, ionmicroprobe. Most mica occurs in small amounts as fringes onilmenomagnetite or fayalite, rarely as an intercumulus or poikiliticphase. The micas range continuously from Phlog₇₀Ann₃₀ in a gabbro,to Phlog₄Ann₉₆ in the most evolved (slightly persilicic andperalkaline) syenite. In the syenites Fe-Mg partitioning betweenbiotite and olivine can be described by a single distributioncoefficient, K_d = XF X^Biot_Ms/X^Biot_Fe 3, suggesting that thesereactant phases mix ideally at the reaction T. Experimentaldata for Fe-Mg exchange via aqueous chloride solutions (Schulien,1980) imply low T (32Q?C). F was absent in the experiments andmay significantly affect the exchange equilibrium. K_d in thegabbros is 1, consistent with equilibrium via a fluid depletedin F because of crystallization of large amounts of amphibole. Al, Mn, and Ti vary regularly throughout the series and canbe used as markers of cryptic variation in the layered syenites.(Al + Si): 22 O is always in the range 7.7–7.85. A1/(A1+ Si) decreases from 0.31 in gabbros to 0.25 in the most Fe-richmicas. Li is always < 260 ppm w. In the syenite series, Fshows a near-linear inverse relationship with Fe/(Fe + Mg) whichpasses close to OF at Ann₁₀₀ with l.4 wt% F(0–7 F to 44positive charges) at Ann₄₄. Biotites in the gabbro unit (whichforms an outer sheath to the intrusion) have relatively lessF, probably because it was consumed by coexisting amphibole.^I8O is similar for both gabbros and syenites, and it is unlikelythat an envelope fluid was involved in the reactions. G reachesa maximum of 0.3 wt. % in biotite except for that in one syenitesample with 0–7 wt. %. Calculation of relative F-OH fugacitiesfrom the reaction OH-phlogopite + F-annite = F-phlogopite +OH-annite, as calibrated by Munoz (1984), appears to suggestthat each horizon in the layered series was in equilibrium witha slightly different fluid. In view of the intimate interleavingof these lithologies, this is improbable. The equilibrium constantof the exchange reaction, obtained from the experimental data,seems not to be appropriate to the Klokken assemblage, or toother examples of regular F-Fe avoidance. Explanations may includeshort-range Fe-Mg ordering in the natural examples or the effectof additional components in the fluid. F contents are high incomparison with biotites from calc-alkaline complexes; highmagmatic F may account for the igneous layering common in theGardar. Temperatures calculated from reactions involving fayaliteand magnetite show that most biotites grew subsolidus. The F-poorannites grew > 300 ?C subsolidus even when texturally intercumulus.Stable isotope data are consistent with the separation and retentionof a deuteric fluid during the final stage of magmatic crystallization.Klokken was not generally subject to the pervasive, long-range(in both distance and time) dydrothermal interactions demonstratedin calcalkaline and theleiitic intrusions, although more extensivefluid flow is indicated for the more permeable laminated syenites.The biotites preserve chemical variation indicating local equilibriumwith other mafic phases, and halogens provide a useful markerof subsolidus fluid flow.     

Metasomatic overprinting by juvenile igneous fluids,Igdlerfigsalik, South Greenland

The late Igdlerfigsalik centre, part of the Igaliko nepheline syenite complex in the Gardar Province, S Greenland, is bounded to the N and E by compositionally similar, earlier syenites forming the early Igdlerfigsalik and South Qôroq centres. In a circa 1 km wide zone parallel to its contact with late Igdlerfigsalik, the South Qôroq centre shows recrystallisation and alteration of mafic phases. South Qôroq therefore comprises two zones, termed the “unaltered” and the “recrystallised” zones. A study of the biotites from the rocks of the area of the present study has demonstrated variations in biotite chemistry, notably variations in elements inferred to reside on the octahedral and hydroxyl sites. Samples were chosen to provide a representative selection from each centre, avoiding pegmatites and late-stage veins. Electron probe microanalyses demonstrate little Cl in these biotites. Fluorine contents of some biotites can be demonstrated to vary with Fe/(Fe+Mg) and ∑(octahedral Al and Ti). Statistical analysis of the data sets as a whole, however, does not demonstrate simple relationships between fluorine content and these parameters and more complex crystallographic controls over fluorine uptake are inferred. Despite these variations, the relative fluorine contents characteristic of each centre can be determined, which, it is believed, reflect the characteristic F activity of fluids associated with each period of magmatism. In South Qôroq, the unaltered zone has a distinctive F-content in biotite, reflecting the original F-content of fluids associated with South Qôroq magmatism. However in the recrystallised zone, the F-contents are distinct, more closely resembling F-contents characteristic of late Igdlerfigsalik rocks. Reaction between a fluorine-rich, younger fluid and older fluorine-poor rocks is inferred. Despite evidence in the micas for fluid-rock interaction, whole-rock chemical changes are irregular in nature and appear not to conform to simple trends. Mass-balance equations are therefore unable to predict the levels of fluid-rock interaction. Similar exchange between late-stage fluids from younger centres and pre-existing rocks may be commonplace in igneous plutonic environments, and fluorine in micas may provide a relatively sensitive guide to fluid-rock interactions.     

Metamorphism of Calcic Pelitic Schists, Strafford Dome, Vermont: Compositional Zoning and Reaction History

Four assemblages from calcic pelitic schists from South Strafford,Vermont, have been studied in detail to determine the relationshipbetween reaction history and compositional zoning of minerals.The lowest-grade assemblage is garnet + biotite + chlorite +plagioclase + epidote + quartz + muscovite + graphite + fluid.Along a path of isobaric heating, the net reaction is Chl +Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasingFe/(Fe + Mg) and X_Spa, (from 0•2 to 0•05), X_Gra staysnearly constant between 0•20 and 0•25, and plagioclasegrows with X_An increasing from peristerite to 0•2–0•5. The subsequent evolution depends on whether chlorite or epidotereacts out first. If chlorite is removed from the assemblagefirst, the net reaction along an isobaric heating path becomesGrt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. X_An of plagioclaseincreases to 0•20–0•70, depending on the bulk-rockcomposition and changes in pressure and temperature. If epidoteis removed first, the assemblage becomes a simple pelite andthe net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H₂O.Plagioclase is consumed to provide Ca for growing garnet, andX_An, Fe/(Fe + Mg) of garnet, X_Gra, and X_Spa all decrease. Afterboth chlorite and epidote are removed, continued heating upto the metamorphic peak of {small tilde}600C produces littleprogress of the reaction Grt + Ms = Bt + Pl; and X_An increases. The four assemblages have been numerically modeled using theGibbs method starting with measured compositions. The modelssuccessfully predict the observed compositional zoning and trendsof mineral growth and consumption along the computed P–Tpaths. The models also predict the compositional mineral zoningthat would have resulted from other P–T paths. ^* Present address: Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487     

Experimental Petrology of the 1991-1995 Unzen Dacite, Japan. Part II: Cl/OH Partitioning between Hornblende and Melt and its Implications for the Origin of Oscillatory Zoning of Hornblende Phenocrysts

High-temperature–pressure experiments were carried outto determine the chlorine–hydroxyl exchange partitioncoefficient between hornblende and melt in the 1992 Unzen dacite.Cl in hornblende and melt was analyzed by electron microprobe,whereas OH in hornblende and melt was calculated assuming anionstoichiometry of hornblende and utilizing the dissociation reactionconstant for H₂O + O = 2(OH) in water-saturated melt, respectively.The partition coefficient strongly depends on the Mg/(Mg + Fe)ratio of hornblende, and is expressed as ln K₁ = (Cl/OH)_hb/(Cl/OH)_melt= 2·37 – 4·6[Mg/(Mg + Fe)]_hb at 2–3kbar and 800–850°C. The twofold variation in Cl contentin the oscillatory zoned cores of hornblende phenocrysts inthe 1991–1995 dacite cannot be explained by the dependenceof the Cl/OH partition coefficient on the Mg/(Mg + Fe)_hb ratio,and requires c. 80% variation of the Cl/OH ratio of the coexistingmelt. Available experimental data at 200 MPa on Cl/OH fractionationbetween fluid and melt suggest that c. 1·2–1·8wt % degassing of water from the magma can explain the required80% variation in the Cl/OH ratio of the melt. The negative correlationbetween Al content and Mg/(Mg + Fe) ratio in the oscillatoryzoned cores of the hornblende phenocrysts is consistent withrepeated influx and convective degassing of the fluid phasein the magma chamber. KEY WORDS: chlorine; element partitioning; hornblende; oscillatory zoning; Unzen volcano     

An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part I: Calibration and Solubility Models

Experiments were conducted to determine the solubilities ofH₂O and CO₂ and the nature of their mixing behavior in basalticliquid at pressures and temperature relevant to seqfloor eruption.Mid-ocean ridge basaltic (MORB) liquid was equilibrated at 1200°Cwith pure H₂O at pressures of 176–717 bar and H₂O—CO₂vapor at pressures up to 980 bar. Concentrations and speciationof H₂O and CO₂ dissolved in the quenched glasses were measuredusing IR spectroscopy. Molar absorptivities for the 4500 cm^–1band of hydroxyl groups and the 5200 and 1630 cm^–1 bandsof molecular water are 0•67±0•03, 0•62±0•07,and 25±3 l/mol-cm, respectively. These and previouslydetermined molar absorptivities for a range of silicate meltcompositions correlate positively and linearly with the concentrationof tetrahedral cations (Si+Al). The speciation of water in glass quenched from vapor-saturatedbasaltic melt is similar to that determined by Silver &Stolper (Journal of Petrology 30, 667–709, 1989) in albiticglass and can be fitted by their regular ternary solution modelusing the coefficients for albitic glasses. Concentrations ofmolecular water measured in the quenched basaltic glasses areproportional to f H₂O in all samples regardless of the compositionof the vapor, demonstrating that the activity of molecular waterin basaltic melts follows Henry's law at these pressures. Abest fit to our data and existing higher-pressure water solubilitydata (Khitarov et al., Geochemistry 5, 479–492, 1959;Hamilton et al., Journal of Petrology 5, 21–39, 1964),assuming Henrian behavior for molecular water and that the dependenceof molecular water content on total water content can be describedby the regular solution model, gives estimates for the V^{o, m}_H2Oof 12±1 cm³/mol and for the 1-bar water solubility of0•11 wt%. Concentrations of CO² dissolved as carbonate in the melt forpure CO₂-saturated and mixed H₂O-CO₂-saturated experiments area simple function of f_CO2 These results suggest Henrian behaviorfor the activity of carbonate in basaltic melt and do not supportthe widely held view that water significantly enhances the solutionof carbon dioxide in basaltic melts. Using a V^{o, m}_r of 23 cm³/mol(Pan et al., Geochimica et Cosmochimica Acta 55, 1587–1595,1991), the solubility of carbonate in the melt at 1 bar and1200°C is 0•5 p.p.m. Our revised determination of CO₂solubility is 20% higher than that reported by Stolper &Holloway (Earth and Planetary Science Letters 87, 397–408,1988). KEY WORDS: mid-ocean ridge basalts; water and carbon dioxide solubility; experimental petrology     

Quantification of Magmatic and Hydrothermal Processes in a Peralkaline Syenite-Alkali Granite Complex Based on Textures, Phase Equilibria, and Stable and Radiogenic Isotopes

The Puklen complex of the Mid-Proterozoic Gardar Province, SouthGreenland, consists of various silica-saturated to quartz-bearingsyenites, which are intruded by a peralkaline granite. The primarymafic minerals in the syenites are augite ± olivine +Fe–Ti oxide + amphibole. Ternary feldspar thermometryand phase equilibria among mafic silicates yield T = 950–750°C,a_SiO2 = 0·7–1 and an f_O2 of 1–3 log unitsbelow the fayalite–magnetite–quartz (FMQ) bufferat 1 kbar. In the granites, the primary mafic minerals are ilmeniteand Li-bearing arfvedsonite, which crystallized at temperaturesbelow 750°C and at f_O2 values around the FMQ buffer. Inboth rock types, a secondary post-magmatic assemblage overprintsthe primary magmatic phases. In syenites, primary Ca-bearingminerals are replaced by Na-rich minerals such as aegirine–augiteand albite, resulting in the release of Ca. Accordingly, secondaryminerals include ferro-actinolite, (calcite–siderite)_ss,titanite and andradite in equilibrium with the Na-rich minerals.Phase equilibria indicate that formation of these minerals tookplace over a long temperature interval from near-magmatic temperaturesdown to     

Low-Pressure Experimental Constraints on the Evolution of Komatiites

Melting experiments were performed on a komatiitic basalt with17 wt% MgO from Munro Township, Ontario, at I-atm pressure andan oxygen fugacity controlled approximately to the fayalite-magnetite-quartzbuffer. The experiments showed that olivine appears at 1344±5°C,spinel at 1334±6°C plagioclase at 1185±5°C,augite at 1176±5°C and pigeonite at 1154±6°C.Compositionally, olivine varies from Fo₉₀ to Fo₇₄ and displaysan average K^Fe/Mg_D (ol/liq) of 0•32. The spinels are chromitesand chromian spinels with Mg/(Mg + Fe²⁺) ratios between 0•66and 0•;32, which show a marked correlation with meltingtemperature. The pyroxenes show an average K^Fe/Mg_D (px/liq)of 0•26, identical for augite and pigeonite. Plagiodaseranges compositionally between An₈₂ and An₇₂ Plotted in thepseudo-quaternary basalt phase diagram, the liquid line of descentis similar to that observed for quartz tholeiitic magmas. Therefore,the low-pressure, late-stage evolution products of komatiiteand basaltic komatiite parental magmas will chemically and mineralogicallybe ferrobasaltic quartz tholeiites. High-temperature and high-pressuremodeling suggests that the main observed compositional variationof Munro komatiites can be explained by low-pressure crystalfractionation and accumulation of olivine into komatiite liquidswith below 21•5–23•5 wt% MgO and eruptive temperaturesbelow 1435–1465°C for oxygen fugacities between thefayalite-magnetite quartz (FMQ) and iron-wiistite (IW) buffers.The maximum magnesium content of liquid komatiites, assumingequilibrium Fo₉₄ olivine, is 27–29 wt% MgO and eruptivetemperatures are between 1515 and 1540°C. KEY WORDS: komatiites; experimental petrology; Munro Township; Ontario     

Chemistry of Kornerupine and Associated Minerals, a Wet Chemical, Ion Microprobe, and X-Ray Study Emphasizing Li, Be, B and F Contents

Kornerupine and associated minerals in 31 samples of high-graderocks relatively rich in Al and Mg were analysed by wet chemistry,ion microprobe mass analyser, electron microprobe and X-raypowder diffraction. For 11 samples of kornerupine and threesamples of biotite (F only) analysed by both wet chemical andion microprobe methods, the best agreement was obtained forB₂O₃, whereas the ion microprobe Li₂O values were systematicallysomewhat higher than the wet chemical values. The wet chemicalmethods give Li₂O=0–0?19 wt.%; BeO=0–0?032 wt.%;B₂O₃=0–4?01 wt.%; and F=0?07–0?77 wt.% in kornerupine,whereas ion microprobe analyses on other kornerupines give valuesup to 0?35 wt.% Li₂O, O066 wt.% BeO, and 4?72 wt.% B₂O₃. Thesum B+Al+Fe³⁺+Cr is close to 6?9 atoms per 22 (O, OH, F) or21?5 (O) in kornerupine. In general, Li/Fe ratios decrease as follows: kornerupine ?sapphirinebiotite> Crd (Na<0?03 per 18 oxygens)>tourmaline, garnet,orthopyroxene. However, for cordierite with Na>004, Li/Fedecreases as follows: cordierite>kornerupine. Sapphirineand sillimanite are the only associated minerals to incorporatesignificant boron (0?1–0?85 wt.% B₂O₃) and then only whenthe single site for B in kornerupine is approaching capacity.Sillimanite B₂O₃ contents increase regularly with kornerupineF. Fractionation of fluorine increases as follows: kornerupine<biotite<tourmaline,and K^krn-BtD=(F/OH)^Krn/(F/(OH)^Bt (assuming ideal anion composition)increases with biotite Ti. Kornerupine B₂O₃ content is a measureof B₂O₃ activity in associated metamorphic fluid, whereas sillimaniteB₂O₃ content increases with temperature, exceeding 0?4 wt.%whenT=900?C at very low water activities. New data on 11 kornerupines and literature data indicate thatthe unit cell parameters a, c, and V decrease with increasingB content and b, c, and V increase with increasing Fe³⁺ content.In Fe³⁺-poor kornerupines, b increases with Mg and with (Mg+ Fe²⁺) but the effect of Mg on b via the substitution ^VIMg+^IVSi=^VIAl+^IVAloverwhelms the effect of Fe²⁺=Mg substitution.     

Immiscibility in Ca-amphiboles

The literature data of nine different occurrences of coexistingmineral pairs of Ca-amphibole have been studied and the bulkvectors, spanning the miscibility gap, derived. The additivecomponent is always impure Mg-tremolite accompanied by someglaucophane and cummingtonite component. The four major exchangecomponents required to describe the compositional variationin coexisting mineral pairs are the edenite (ED), tschermak's(TS), FeMg_–1 and Fe³⁺-tschermak's (FeTs) vector. Trivalentiron is postulated on the basis of excess charges in the bulkvector the size of which coincides with residuals in Al^tet,–Si, Fe and –Mg. The four cations have equal sizes,forming the vector Fe³⁺ Al^tet Mg_–1Si_–1. This distributionscheme is consistent for all the different occurrences and setsthe basis for a comparison. Deviations from the scheme wouldradically complicate the proposed exchange pattern. The ratioTS:ED in most mineral samples fluctuates between one and two.Projection of the data points in the vector space TS–EDonto the line 1ED: 2TS (Tr–Hbl) or 1ED:1TS (Tr–Prg)provides the projected tremolite content (= 1–X_Hbl or = 1–X_prg). This parameter,applied to coexisting pairs, and plotted against the ratio Mg/(Mg+ Fe) shows some characteristic features about the miscibilitygap. In the Mg-pure system the solvus is almost symmetric andlocated in the temperature range between 800 and 870C. Smallamounts (0.10 pfu) of Fe²⁺ in the M(4) -sites and replacingCa have a dramatic effect, forcing the solvus to much lowertemperatures of 650C. An increase in the ratio Fe/(Fe + Mg)causes a shift of the solvus towards more tremolitic compositionswith temperatures 500–650C. The maximum asymmetry ofthe solvus is reached where the Al-poor member (tremolite) hasa composition of =1.0 and Mg/ (Mg + Fe) 0.6. The corresponding Al-rich member has =0.5 and Mg/ (Mg + Fe) 0.4. An anomalyof the solous is observed at Mg/ (Mg + Fe)=0.8. It manifestsas a kind of highly asymmetric ‘sub-gap’ in thetremolite-rich composition range. This is explained by the partitioningof Fe²⁺ into the single M(3) -site and is characterized by athermal hump to 650–700C. KEY WORDS: tremolite; hornblende; pargasite; immiscibility; solous     

The Klokken Gabbro-Syenite Complex, South Greenland: Quantitative Interpretation of Mineral Chemistry

The layered syenite series in the Klokken stock formed by continuousin situ fractionation of a trachytic magma in a chamber linedby gabbro with 3000 m of cover rocks. The following mineralsand reactions are assessed as geothermometers and barometers:two feldspars; hypersolvus ternary feldspars; ferrohedenbergite-ß-wollastonite;clinopyroxene-olivine Fe-Mg exchange; Fe-Ti oxides; sanidine-magnetite-annite;ferroedenite stability. Estimates of silica activity are obtainedfrom the silica-magnetite-fayalite assemblage. The gabbros ended magmatic crystallization at > 1000–1050°C.The less fractionated members of the syenite range probablycrystallized with P_H2O < P_total, at T > 870°C and,P_H2O 800 bars. In the more fractionated syenites P_H2O = P_totalduring intercumulus feldspar growth, and all magmatic phasescrystallized within the interval 940–830°C and P_H2O< 1100 bars. Magmatic f_O2 (bars) was 1 log unit below theQFM buffer. a_SIO2 in gabbros was slightly above the albite-nephelinebuffer, but rose suddenly to just <1 in the syenites, a jumpmirrored by minor elements in pyroxenes and opaque oxides. Biotitegrew subsolidus in most rocks, at f_O2 < QFM, except in intermediaterocks when f_O2 > QFM and was defined by the sanidine-magnetite-biotiteassemblage. In these rocks P_H2O of 450 bars at 760°C isobtained using existing experimental data, but application ofthis data to Fe-rich biotites in the layered series (where biotiteis an intercumulus phase) requires P > 10 kb at magmatictemperatures. High TiO₂ or F: OH probably accounts for increasedT stability of natural annites at low P. The syenitic liquid fractionated down a low temperature zonein a multicomponent system precipitating alk fsp + ol + cpx+ mt and the more fractionated members of the layered serieshad a negligible crystallization interval, a prerequisite forthe development of the unique Klokken-type inversely gradedmineral layering.     

The Role of Advective Fluid Flow and Diffusion during Localized, Solid-State Dehydration: Sondrum Stenhuggeriet, Halmstad, SW Sweden

A localized dehydration zone, Söndrum stone quarry, Halmstad,SW Sweden, consists of a central, 1 m wide granitic pegmatoiddyke, on either side of which extends a 2·5–3 mwide dehydration zone (650–700°C; 800 MPa; orthopyroxene–clinopyroxene–biotite–amphibole–garnet)overprinting a local migmatized granitic gneiss (amphibole–biotite–garnet).Whole-rock chemistry indicates that dehydration of the graniticgneiss was predominantly isochemical. Exceptions include [Y+ heavy rare earth elements (HREE)], Ba, Sr, and F, which aremarkedly depleted throughout the dehydration zone. Systematictrends in the silicate and fluorapatite mineral chemistry acrossthe dehydration zone include depletion in Fe, (Y + HREE), Na,K, F, and Cl, and enrichment in Mg, Mn, Ca, and Ti. Fluid inclusionchemistry is similar in all three zones and indicates the presenceof a fluid containing CO₂, NaCl, and H₂O components. Water activitiesin the dehydration zone average 0·36, or X_H2O = 0·25.All lines of evidence suggest that the formation of the dehydrationzone was due to advective transport of a CO₂-rich fluid witha minor NaCl brine component originating from a tectonic fracture.Fluid infiltration resulted in the localized partial breakdownof biotite and amphiboles to pyroxenes releasing Ti and Ca,which were partitioned into the remaining biotite and amphibole,as well as uniform depletion in (Y + HREE), Ba, Sr, Cl, andF. At some later stage, H₂O-rich fluids (H₂O activity >0·8)gave rise to localized partial melting and the probable injectionof a granitic melt into the tectonic fracture, which resultedin the biotite and amphibole recording a diffusion profile forF across the dehydration zone into the granitic gneiss as wellas a diffusion profile in Fe, Mn, and Mg for all Fe–Mgsilicate minerals within 100 cm of the pegmatoid dyke. KEY WORDS: charnockite; fluids; CO₂; brines; localized dehydration; Söndrum     

The Effect of Mn on the Phase Relations of Medium-Grade Pelites: Constraints from Natural Assemblages on Petrogenetic Grid Topology

The phase relations of muscovite-quartz-bearing pelitic schistscontaining combinations of garnet (Grt), staurolite (St), chloritoid(Cld), biotitt (Bt) and chlorite (Chl) are examined (1) to assessthe influence of manganese on natural assemblages, and (2) toconstrain the topologies of petrogenetic grids, particularlywith respect to the controversial assemblage Cld +Bt. Two fieldareas were studied: Stonehaven, NE Scotland (p 4•5 kbar)and the SE Tauern Window, Austria (P 7 kbar), both characterizedby the up-grade progression from typical ‘garnet-zone’Grt+Chl assemblages to ‘staurolite-zone’ St+Bt±Grtassemblages via a narrow, complex zone containing Cld+Bt assemblages.In both areas, the following commonly observed chemographicrelations hold: Mg/(Mg+Fe): Grt<St<Cld<<Bt<Chl;Mn/(Mn+Fe+Mg): Chl Bt<<St<Cld<<Grt. These compositionsyield the MnAFM-discontinuous reaction (Ms+Qtz+H₂O in excess):Cld+Chl = Grt+St+Bt. The distributions of mineral assemblages in both areas are moreconsistent with the operation of MnAFM reactions than of traditionalAFM reactions. Clear correlations exist between Mn content andassemblage in rocks that crystallized at the same P and T. In the SE Tauern, low-grade Grt+Chl assemblages show a widerange of Mn contents. The crystallization of low-Mn Grt+Chlassemblages down-grade of, but at similar pressures to, low-MnGrt+Cld+Bt+Chl assemblages implies that the right-hand sideof the reaction Grt+Chl = Cld+Bt (Fe, Mg) is stabilized by increasingT. The distributions of assemblages in the areas studied alsoshow differences that are ascribed to P effects. The assemblageGrt+St+Cld+Chl is common in the SE Tauern but absent from Stonehaven.Mn contents of respective minerals in the assemblage Grt+St+Cld+Bt+Chlare higher at Stonehaven than in the SE Tauern, implying thatthe Cld+Chl = Grt+St+Bt (Mn, Fe, Mg) reaction boundary extendsto the low-P side of the [AIs, Crd] invariant point in the Mn-freesystem. Schreincmakcrs' rules are used to construct two KFMnMASH grids,in which the Cld+Bt assemblage has markedly different stabilitylimits; one is based on the KFMASH grid of Harte & Hudson(Geological Society Special Publication 8, 323–337, 1979),in which Cld+Bt is stable over a narrow T interval at relativelylow P, and the other on the KFMASH grids of Spear & Cheney(Contributions to Mineralogy and Petrology 101, 149–164,1989) and Wang & Spear (Contributions to Mineralogy andPetrology 106, 217–235, 1991), in which Cld+Bt is stableover wide ranges of P and T. It is argued that available natural-rockdata are more compatible with the former. KEY WORDS: pelites; KFMnMASH petrogenetic grid; chloritoid + biotite; Stonehaven; Tauern Window     

The Solubility of Alumina in Orthopyroxene Coexisting with Garnet in FeO-MgO--Al2O3--SiO2 and CaO--FeO--MgO--Al2O3--SiO2

The pressure-temperature-compositional (P-T-X) dependence ofthe solubility of Al₂O₃ in orthopyroxene coexisting with garnethas been experimentally determined in the P-T range 5–30kilobars and 800–1200 ?C in the system FeO—MgO—Al₂O₃—SiO₂(FMAS). These results have been extended into the CaO—FeO—MgO—Al₂O₃—SiO₂(CFMAS) system in a further set of experiments designed to determinethe effect of the calcium content of garnet on the Al₂O₃ contentsof coexisting orthopyroxene at near-constant Mg/(Mg + Fe). Startingmaterials were mainly glasses of differing Mg/(Mg + Fe) or Ca/(Ca+ Mg + Fe) values, seeded with garnet and orthopyroxene of knowncomposition, but mineral mixes were also used to demonstratereversible equilibrium. Experiments were performed in a piston-cylinderapparatus using a talc/pyrex medium. Measured orthopyroxene and corrected garnet compositions werefitted by multiple and stepwise regression techniques to anequilibrium relation in the FMAS system, yielding best-fit,model-dependent parameters G^o_y= –5436 + 2.45T cal mol^–1,and W^M1_FeA1= –920 cal mol^–1. The volume change ofreaction, V^o, the entropy change, S^o₉₇₀ and the enthalpy changeH^o_1,970, were calculated from the MAS system data of Perkinset al. (1981) and available heat capacity data for the phases.Data from CFMAS experiments were fitted to an expanded equilibriumrelation to give an estimate of the term W^ga_CaMg = 1900 ? 400cal/mole cation, using the other parametric values already obtainedin FMAS. The experimental data allow the development of a arnet-orthopyroxenegeobarometer applicable in FMAS and CFMAS: where This geobarometer is applicable to both pelitic and metabasicgranulites containing garnet orthopyroxene, and to garnet peridoditeand garnet pyroxenite assemblages found as xenoliths in diatremesor in peridotite massifs. It is limited, however, by the necessityof an independent temperature estimate, by errors associatedwith analysis of low Al₂O₃ contents in orthopyroxenes in high-pressureor low-temperature parageneses, and by uncertainties in thecomposition of garnet in equilibrium with orthopyroxene. Ananalysis of errors associated with this formulation of the geobarometersuggests that it is subject to great uncertainty at low pressuresand for Fe-rich compositions. The results of application ofthis geobarometer to natural assemblages are presented in acompanion paper.     

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.     

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 model forthe phase relations of mantle peridotite involving more thanone volatile component. Run products obtained in a study ofphase relations of four mantle peridotites in the presence ofH₂O- 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. DecreasingfH₂from 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 fH₂. Consequently,the peridotite mineralogy and/or chemistry is not a good indicatorfor the fH₂ 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.     

The Klokken Gabbro--Syenite Complex, South Greenland: Cryptic Variation and Origin of Inversely Graded Layering

The Klokken stock has an outer sheath of vertically banded alkaligabbro which passes through syenogabbros into unlaminated syenitessurrounding a central layered syenite core. The layered seriescomprises sheets of granular syenite interleaved with drusylaminated syenite which shows repeated inversely graded minerallayering in which normal leucosyenite becomes progressivelymore hedenbergite-rich upwards. Tops to layers are nearly feldspar-free,fayalite—magnetite horizons. Model cumulate textures areexhibited, and cross-bedding and normally graded channel structuresare encountered. Granular layers often rest on ultramafic layersand load structures occur at the junctions. The series is cutby a sheet of biotite-syenodiorite and finally by quartzsyeniteaplites. Plagioclase (An₅₇ in gabbros) is progressively mantled by ternaryalkali feldspar and is absent only in the layered series inwhich alkali feldspars cluster around Ab₆₅ Or₃₅. Pyroxene showsunbroken evolution with respect to Fe/Fe + Mg, from augite toferrohedenbergite and sodie hedenbergite, with acmite in aplites,but Al and Ti decrease suddenly between syenogabbro and unlaminatedsyenite stages. Olivines show parallel evolution but there isa jump in Fe: Mg between gabbros and unlaminated syenites fromFo₅₂ to Fo₃₄, with subsequent steady evolution to Fo₃Te₅Fa₉₂in laminated syenites. Biotites show steady increase in Fe/Fe+ Mg throughout the syenites, approaching pure annite, but thegabbro-syenogabbro series show a reversal, interpreted as indicatingcrystallization under conditions of increasing f_o2. Ilmenomagnetitesshow systematically varying minor element ratios, with a suddendrop in MgAl₂O₄ content between gabbros and syenites. Amphiboles(ferroedenites and sodic hastingsites) appear only in the mostfractionated syenites in the layered series. The Klokken chamber was lined by gabbro with only limited insitu fractionation to syenogabbro. The termination of gabbrocrystallization was marked by ingress of water into the conduit,perhaps causing the liquid to embark on its fractionation trendtowards oversaturated residua. The mineralogical breaks areconsistent with a drop in temperature, and increase in a_SIO2at this point. The chamber was subsequently filled by a drytrachytic liquid which fractionated in situ, initially by congelationagainst the walls as unlaminated syenite. The layered seriesdeveloped within this chamber. The granular syenites show invertedcryptic and phase-layering (downward appearance of amphibole),and downward increase in grain size. They are sheets stopedoff the roof of the intrusion, representing chilled equivsalentsof the fractionating trachytic liquid of which the laminatedsyenites are the bottom accumulated products. In the inverselygraded layers the cumulus phases have slightly more advancedcompositions than the same phases in either normal rock or whenpresent as an intercumulus phase in adjacent parts of layers.The layering is attributed to crystallization at varying degreesof undercooling in a magma in which all phases exhibit a narrowcrystallization interval and which was subject to rhythmic P-build-upand sudden release. Crystal accumulation took place under nearstagnant to rhythmie P-build-up and sudden release. Crystalaccumulation took place under near stagnant conditions in athin chamber immediately beneath the roof of the intrusion.     

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.     

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.     

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.     

Melting and Thermal Reconstitution of Pelitic Xenoliths, Wehr Volcano, East Eifel, West Germany

Pelitic xenoliths derived from amphibolite grade basement rocksoccur within a Pleistocene, trachytic, pyroclastic unit of theWehr volcano, East Eifel, West Germany: With increasing temperatureand/or prolonged heating at high temperature, quartz-plagioclaseand micaceous layers of the xenoliths have undergone meltingto form buchites and thermal reconstitution by dehydration reactions,melting and crystallization to form restites respectively. Thexenoliths provide detailed evidence of melting, high temperaturedecomposition of minerals, nucleation and growth of new phasesand P-T-f_o2 conditions of contact metamorphism of basement rocksby the Wehr magma. Melting begins at quartz-oligoclase (An_17·3Ab_82·3Or_0·4-An_20·0Ab_78·1Or_1·9)grain boundaries in quartz-plagioclase rich layers and the amountof melting is controlled by H₂O and alkalis released duringdehydroxylation/oxidation of associated micas. Initially, glasscompositions are heterogeneous, but with increasing degreesof melting they become more homogeneous and are similar to S-typegranitic minimum melts with SiO₂ between 71 and 77 wt. per cent;A/(CNK) ratios of 1·2–1·4; Na₂O < 2·95and normative corundum contents of 1·9–4·0per cent. Near micas plagioclase melts by preferential dissolutionof the NaAlSi₃O₈ component accompanied by a simultaneous increasein CaAl²Si²O⁸ (up to 20 mol. per cent An higher than the bulkplagioclase composition) at the melting edge. With increasingtemperature the end product of fractional melting is the formationand persistence of refractory bytownite (An_78–80) in thosexenoliths where extensive melting has taken place. Initial stage decomposition of muscovite involves dehydroxylation(H₂O and alkali loss). At higher temperatures muscovite breaksdown to mullite, sillimanite, corundum, sanidine and a peraluminousmelt. Mullite (40–43 mol. per cent SiO₂) and sillimanite(49 mol. per cent SiO₂) are Fe₂O₃ and TiO₂ rich (up to 6·1–0·84and 3·6–0·24 wt. per cent respectively).Al-rich mullite (up to 77 wt. per cent Al₂O₃) occurs with corundumwhich has high Fe₂O₃ and TiO₂ (up to 6·9 and 2·1wt. per cent respectively). Annealing at high temperatures andreducing conditions results in the exsolution of mullite fromsillimanite and ilmenite from corundum. Glass resulting fromthe melting of muscovite in the presence of quartz is peraluminous(A/(CNK) = 1·3) with SiO² contents of 66–69 percent and normative corundum of 4 per cent. Sanidine (An_1·9Ab_26·0Or_72·1-An_1·3Ab_15·9Or_82·9)crystallized from the melt. Dehydroxylation and oxidation of biotite results in a decreaseof K₂O from 8·6 to less than 1 wt. per cent and oxidetotals (less H₂O + contents) from 96·5 to 88·6,exsolution of Al-magnetite, and a decrease in the Fe/(Fe + Mg)ratio from 0·41 to 0·17. Partial melting of biotitein the presence of quartz/plagioclase to pleonaste, Al-Ti magnetite,sanidine(An_2·0Ab_34·9Or63·1) and melt takesplace at higher temperatures. Glass in the vicinity of meltedbiotite is pale brown and highly peraluminous (A/CNK = 2·1)with up to 6 wt. per cent MgO+FeO_{(total iroq)} and up to 10 percent normative corundum. Near liquidus biotite with higher Al₂O₃and TiO₂ than partially melted biotite crystallized from themelt. Ti-rich biotites (up to 6 wt. per cent TiO₂) occur withinthe restite layers of thermally reconstituted xenoliths. Meltingof Ti-rich biotite and sillimanite in contact with the siliceousmelt of the buchite parts of xenoliths resulted in the formationof cordierite (100 Mg/(Mg+Fe+Mn) = 76·5–69·4),Al-Ti magnetite and sanidine, and development of cordierite/quartzintergrowths into the buchite melt. Growth of sanidine enclosedrelic Ca-plagioclase to form patchy intergrowths in the restitelayers. Cordierite (100 Mg/(Mg+Fe+Mn) = 64–69), quartz,sillimanite, mullite, magnetite and ilmenite, crystallized fromthe peraluminous buchite melt. Green-brown spinels of the pleonaste-magnetite series have awide compositional variation of (mol. per cent) FeAl₂O₄—66·6–45·0;MgAl₂O₄—53·0–18·7; Fe₃O₄—6·9–28·1;MnAl₂O₄—1·2–1·5; Fe₂TiO₄—0·6–6·2.Rims are generally enriched in the Fe₃O₄ component as a resultof oxidation. Compositions of ilmenite and magnetite (single,homogeneous and composite grains) are highly variable and resultfrom varying degrees of high temperature oxidation that is associatedwith dehydroxylation of micas and melting. Oxidation mainlyresults in increasing Fe³⁺, Al and decreasing Ti⁴⁺, Fe²⁺ inilmenite, and increasing Fe²⁺, Ti⁴⁺ and decreasing Fe³⁺ in associatedmagnetite. A higher degree of oxidation is reached with exsolutionof rutile from ilmenite and formation of titanhematite and withexsolution of pleonaste from magnetite. Ti-Al rich magnetite(5·1–7·5 and 8·5–13·5wt. per cent respectively) and ilmenite crystallized from meltsin buchitic parts of the xenoliths. Chemical and mineralogic evidence indicates that even with extensivemelting the primary compositions of individual layers in thexenoliths remained unmodified. Apparently the xenoliths didnot remain long enough at high temperatures for desilicationand enrichment in Al₂O₃, TiO₂, FeO, Fe₂O₃, and MgO that resultsby removal of a ‘granitic’ melt, and/or by interactionwith the magma, to occur. T °C-f_o2 values calculated from unoxidized magnetite/ilmenitegive temperatures ranging from 615–710°C for contactmetamorphism and the beginning of melting, and between 873 and1054°C for the crystallization of oxides and mullite/sillimanitefrom high temperature peraluminous melts. f_o2 values of metamorphismand melting were between the Ni-NiO and Fe₂O₃-Fe₃O₄ buffer curves.The relative abundance of xenolith types, geophysical evidenceand contact metamorphic mineralogy indicates that the xenolithswere derived from depths corresponding to between 2–3kb P_load = P_fluid. The xenoliths were erupted during the latestphreatomagmatic eruption from the Wehr volcano which resultedin vesiculation of melts in partially molten xenoliths causingfragmentation and disorientation of solid restite layers.