Open-System Magma Chamber Evolution: an Energy-constrained Geochemical Model Incorporating the Effects of Concurrent Eruption, Recharge, Variable Assimilation and Fractional Crystallization (EC-E'RA{chi}FC)

Significant petrogenetic processes governing the geochemicalevolution of magma bodies include magma Recharge (includingformation of ‘quenched inclusions’ or enclaves),heating and concomitant partial melting of country rock withpossible ‘contamination’ of the evolving magma body(Assimilation), and formation and separation of cumulates byFractional Crystallization (RAFC). Although the importance ofmodeling such open-system magma chambers subject to energy conservationhas been demonstrated, the effects of concurrent removal ofmagma by eruption and/or variable assimilation (involving imperfectextraction of anatectic melt from wall rock) have not been considered.In this study, we extend the EC-RAFC model to include the effectsof Eruption and variable amounts of assimilation, A. This model,called EC-E'RAFC, tracks the compositions (trace elements andisotopes), temperatures, and masses of magma body liquid (melt),eruptive magma, cumulates and enclaves within a composite magmaticsystem undergoing simultaneous eruption, recharge, assimilationand fractional crystallization. The model is formulated as aset of 4 + t + i + s coupled nonlinear differential equations,where the number of trace elements, radiogenic and stable isotoperatios modeled are t, i and s, respectively. Solution of theEC-E'RAFC equations provides values for the average temperatureof wall rock (T_a), mass of melt within the magma body (M_m),masses of cumulates (M_ct), enclaves (M_en) and wall rock () and the masses of anatectic melt generated () and assimilated (). In addition, t trace element concentrations and i + s isotopic ratios inmelt and eruptive magma (C_m, _m, _m), cumulates (C_ct, _m, _m), enclaves(C_en, , ) and anatectic melt (C_a, , ) as a function of magma temperature (T_m) are also computed. Input parametersinclude the (user-defined) equilibration temperature (T_eq),a factor describing the efficiency of addition of anatecticmelt () from country rock to host magma, the initial temperatureand composition of pristine host melt (, , , ), recharge melt (, , , ) and wall rock (, , , ), distribution coefficients (D_m, D_r, D_a) and their temperaturedependences (H_m, H_r, H_a), latent heats of transition (meltingor crystallization) for wall rock (h_a), pristine magma (h_m)and recharge magma (h_r) as well as the isobaric specific heatcapacity of assimilant (C_p,a), pristine (C_p,m) and recharge(C_p,r) melts. The magma recharge mass and eruptive magma massfunctions, M_r(T_m) and M_e(T_m), respectively, are specified apriori. M_r(T_m) and M_e(T_m) are modeled as either continuous orepisodic (step-like) processes. Melt productivity functions,which prescribe the relationship between melt mass fractionand temperature, are defined for end-member bulk compositionscharacterizing the local geologic site. EC-E'RAFC has potentialfor addressing fundamental questions in igneous petrology suchas: What are intrusive to extrusive ratios (I/E) for particularmagmatic systems, and how does this factor relate to rates ofcrustal growth? How does I/E vary temporally at single, long-livedmagmatic centers? What system characteristics are most profoundlyinfluenced by eruption? What is the quantitative relationshipbetween recharge and assimilation? In cases where the extractionefficiency can be shown to be less than unity, what geologiccriteria are important and can these criteria be linked to fieldobservations? A critical aspect of the energy-constrained approachis that it requires integration of field, geochronological,petrologic, and geochemical data, and, thus, the EC-ERAFC ‘systems’approach provides a means for answering broad questions whileunifying observations from a number of disciplines relevantto the study of igneous rocks. KEY WORDS: assimilation; energy conservation; eruption; open system; recharge     

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.     

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.     

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.     

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.     

An Experimental Study of Fe-Al Solubility in the System Corundum-Hematite up to 40 kbar and 1300{degrees}C

The mutual solubility in the system corundum–hematite[-(Al, Fe³⁺)₂O₃] was investigated experimentally using bothsynthetic and natural materials. Mixtures of -Al₂O₃ and -Fe₂O₃(weight ratios of 8:2 and 10:1) were used as starting materialsfor synthesis experiments in air at 800–1300°C withrun times of 7–34 days. Experiments at 8–40 kbarand 490–1100°C were performed in a piston-cylinderapparatus (run times of 0·8–7·4 days) usinga natural diasporite consisting of 60–70 vol. % diasporeand 20–30 vol. % Ti-hematite. During the diasporite–corunditetransformation, the FeTiO₃ component (12–18 mol %) ofTi-hematite only slightly increased, implying that oxygen fugacitywas maintained at high values. Run products were studied byelectron microprobe and X-ray diffraction (Rietveld) techniques.An essentially linear volume of mixing exists in the solid solutionwith a slight positive deviation at the hematite side. Up to1000°C, corundum contains <4 mol % Fe₂O₃ and hematite<10 mol % Al₂O₃; at 1200°C these amounts increase to9·3 and 17·0 mol %, respectively. At 1300°Chematite was no longer stable and coexists with the orthorhombic phase . The present results agree with corundum (solvus) compositions obtained inprevious studies but indicate a larger solubility of Al in hematite.The miscibility gap in the solution can be modelled with anasymmetric Margules equation with interaction parameters (2uncertainties): ; ; ; . Application of the corundum–hematite solution as a solvus geothermometer is limited because of thescarcity of suitable rock compositions. KEY WORDS: corundum; hematite; corundum–hematite miscibility gap; experimental study; Margules model; metabauxite     

Mineral Chemistry and Crystallization Conditions of the Mboutou Layered Gabbro-Syenite-Granite Complex, North Cameroon

The Mboutou complex is one of a line of early Tertiary ringcomplexes which runs from Lake Chad to the Gulf of Guinea, noneof which has hitherto been described in detail. The main rocktypes are layered gabbros and gabbronorites, with minor bodiesof quartz-syenodiorite, quartz-syenite and hypersolvus granite.Feldspars form a continuum with exceptional compositional range,from An₈₅Ab₁₃Or₂ to around An₁Ab₄₆Or₅₅, and form an entirelyhypersolvus sequence with very strong zoning in the syenodiorites.Ca-rich clinopyroxenes (salite and calcic augite) and olivines(Fo_78–62) have restricted range. Orthopyroxene-bearingleucogabbros and syenodiorites contain minor orthopyroxene (En₆₂Fs₃₅Wo₃)and quartz; olivine and orthopyroxene never coexist. In moreevolved rocks amphibole (magnesio-hornblende to ferroedenite)and minor biotite, showing progressive Fe-enrichment, are theonly mafic silicates.Major-element rock chemistry, minor elementsin clinopyroxenes and biotite chemistry show that, notwithstandingits thoroughly anorogenic setting, Mboutou was, at the outset,only very mildly alkaline. Its more evolved members embarkedon a line of evolution with some calc-alkaline characteristics,probably because of ingress of water into residual batches ofmagma, a possibility supported by stable isotope data. Thischange in behaviour corresponded with the sudden appearanceof quartz and orthopyroxene, which was not in equilibrium withclinopyroxene on the two-pyroxene surface. Amphibole then becamethe main mafic silicate with further increase in . The more evolved rocks are relatively highly altered,but Fe-Ti oxide pairs suggest that was maintained near to and above the QFM buffer and the rangeof biotite compositions further suggests crystallization undera regime of decreasing . Biotites maintain alkaline characteristics throughout the sequence. Zoningpatterns in the ternary feldspars in the syenodiorites, andthe hypersolvus character of the final granite, limit maximumvalues of to < 1 kb, and suggest minimum temperatures for the end of crystallizationin the syenodiorites of{small tilde} 850 ?C.     

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.     

Experimental Petrology of Melilite Nephelinites

Experimental study of natural melilite nephelinite lavas ofintermediate K/Na ratio at low pressure (fo₂ reveals the presenceof a peritectic ‘point’ of distributary type (1090?C)for liquids saturated with leucite, nepheline, and spinel. Withdecreasing temperature on the olivine + melilite cotectic, botholivine and melilite react with such liquids to produce high-calciumpyroxene at the peritectic. Both the olivine + high-calciumpyroxene and melilite + high-calcium pyroxene cotectics arestable at temperatures below the peritectic. Olivines coexistingwith such liquids are much more magnesian than those in comparabletholeiitic liquids. The olivine-liquid Fe-Mg distribution coefficient is a monotonically increasing function of silica activity over the composition range spannedby melilite nephelinite, ugandite, alkali basalt, and tholeiitebasalt liquids. The analogous Fe-Mg distribution coefficientfor melilite and liquid is effectively constant , while that for high-calcium pyroxene and liquidis highly dependent on the chemistry of high-calcium pyroxene(cf., Sack & Carmichael, 1984). Pseudoternary liquidus projectionsof multiply saturated liquids coexisting with nepheline, leucite,and spinel (?olivine?high-calcium pyroxene?melilite) have beenprepared to facilitate graphical analysis of the evolution oflava compositions during hypabyssal cooling. Major element chemicalanalyses and petrographic features of lavas from Mt. Nyiragongo,East Africa and Oahu, Hawaii (e.g., Denaeyer et al., 1965; Wilkinson& Stolz, 1983) confirm the validity of these diagrams andthe systematics established from the experimental data. ^*Reprint requests to R.O. Sack     

Subsolidus Phase Relations in the System MgO--SiO2--Gr--O in Equilibrium with Metallic Cr, and their Significance for the Petrochemistry of Chromium

Subsolidus phase relations have been determined in the systemsSiO₂-Cr-0 and MgO-SiO₂-Cr-O in equilibrium with metallic Cr,at 1100–1500C and 0–288 kbar. There are no ternaryphases in the SiO₂-Cr-O system at these conditions, ie. onlythe assemblage eskolaite + Cr-metal + SiO₂ is found. However,in the MgO-containing system, extensive substitution of Cr²⁺for Mg is observed in (Mg,Cr²⁺)₂SiO₄ olivine, (Mg,Cr²⁺)₂Si₂O₆pyroxene, and (Mg,Cr²⁺)Cr³⁺₂0₄ spinel. Cr ³⁺ levels in olivineand pyroxene are below detection limits. The pyroxene is orthorhombicat monoclinic at higher . The structure of the spinels becomestetragonally distorted at is limited by the breakdown of olivine to pyroxene + spinel+ metal. This maximum amount increases strongly with increasingtemperature, reaching >0.2 at 1500C and 48 kbar. Increasing pressure reduces the maximum. Increasing temperature also increases the maximum amounts of Cr²⁺ which substitute intopyroxene and spinel, indicating that end-member Cr₂Si₂0₆ andCr₃O₄ may become stable above 1650C if melting does not intervene.Powder X-ray diffraction analysis of selected runs has beenused to extract molar volumes of the Mg-Cr²⁺ solid solutionsas a function of composition, which may be extrapolated to predictmolar volumes for Cr₂SiO₄ (olivine), Cr₂Si₂O₆ (ortho- and clino-pyroxene)and Cr₂O₄ (cubic spinel) of 477, 680 and 449 cm³, respectively.The experimental data have been fitted to a thermodynamic model,including free energies of formation for end member Cr₂SiO₄,Cr₂Si₂O₆ and Cr₃O₄. This model is then used to predict the amountsof Cr²⁺ which can be expected in olivine in equilibrium withCr-bearing spinel as a function of T, P and fo₂. This amountincreases strongly with temperature along standard T-fo₂ buffercurves, and is sufficient to explain the observed high Cr contentsof olivine from komatiites and diamond inclusions at reasonableterrestrial fo, values. The lower fo₂ of the lunar environmentresults in significant Cr²⁺ in olivine being stable to muchlower temperatures. The tendency for the oxidation state ofCr, and hence its geochemical properties, to change with temperaturerelative to other redox reactions makes it a potentially usefulmonitor of the temperatures of uppermantle processes, and isa significant factor in the differing styles of igneous differentiationin the Earth and Moon. Corresponding author     

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     

Metamorphism, Fluid Flow and Partial Melting in Pelitic Rocks from the Onawa Contact Aureole, Central Maine, USA

Field, petrologic and geochemical data were used to characterizefluid infiltration and partial melting during metamorphism ofpelitic rocks in the contact aureole of the Onawa pluton, centralMaine, USA. Mineral assemblages delineate five metamorphic zoneswithin the contact aureole: chlorite zone, andalusite–cordierite(a–c) zone, alkali feldspar zone, sillimanite zone andleucocratic-vein (l–v) zone. The sequence of observedmineral assemblages and mineral–fluid reactions calculatedby mass balance is similar to those observed in other contactaureoles. Pressure of contact metamorphism is 3 kbar, on thebasis of optimum geothermobarometry calculations. Metamorphictemperatures vary from 500C in the andalusite–cordieritezone to 65OC in the leucocratic-vein zone. Data from fieldobservations, mineral textures, observed reaction stoichiometry,geothermometry and major-element geochemistry suggest that theleucocratic veins of the l-v zone represent crystallized, partialmelts. Two overall calculated mineral reactions are responsiblefor vein formation: which can be modeled as combinations of two NKFMTASH meltingreactions: Progress of (M1) and (M2) was measured in eight samples, andreaction (M1) is the dominant melt-forming reaction in all samples.Partial melting (and vein formation) was therefore driven byinfiltration of the l-v zone by H²O-rich fluids. Calculatedtime-integrated fluid fluxes for l-v zone samples range from09 10⁴ to 31 10⁴ mol/cm², and flow was in the directionof increasing temperature. KEY WORDS: pelites; contact metamorphism; fluid infiltration; partial melting; Onawa Pluton; Maine; USA ^*Corresponding author. Telephone:(516) 632–8192. Fax (516)632–8240 e-mail: gsymmes{at}ccmail.sunysb.edu     

The Ni-S System and Related Minerals

The system Ni-S has been studied systematically from 200^? to1, 030^? C by means of evacuated, sealed silica-glass tube experimentsand differential thermal analyses. Compounds in the system areNi₃S₂ (and a high temperature, non-quenchable Ni_3?S₂ phase),Ni₇S₆, Ni₁–S₄ Ni₃S₄, and NiS₂. The geologic occurrenceof the minerals heazlewoodite (Ni₂S₂), millerite (ßSNi₁-₂S),polydymite (Ni₃S₄), and vaesite (NiS₂) can now be describedin terms of the stability ranges of their synthetic equivalents. Hexagonal heazlewoodite, which is stoichiometric within thelimit of error of the experiments, inverts on heating to a tetragonalor pseudotetragonal phase at 556^? C. This high-temperature phase(Ni₃ has a wide field of stability, from 23.5 to 30.5 wt percent sulfur at 600^? C, and melts incongruently at 806^??3^? C.The ßNi₇S₆ phase inverts to Ni₇₈ at 397^? C6 when inequilibrium with Ni₃S₂, and at 400^? C when in equilibrium withNiS. Crystals of Ni₇S₆ break down to Ni₃-S₂+NiS at 573^??3^?C.The low-temperature form of Ni₁-S₁ corresponding to the mineralmillerite, is rhombohedral, and the high-temperature form hasthe hexagonal NiAs structure. Stoichiometric NiS inverts at379^??3^?C, whereas Ni₁-S with the maximum nickel deficiency invertsat 282^??5OC. The Ni₁-alphS-NiS₂ solvus was determined to 985^??3^?C,the eutectic temperature of these phases. Stoichiometric NiSis stable at 600^?C but breaks down to Ni₂-S₂ and Ni₁-S below797^?C, whereas Ni₁-S with 38.2 wt per cent sulfur melts congruentlyat 992^??3^?C. Vaesite does not vary measurably from stoichiometricNiS₂ composition, and melts congruently at 1.007?5^?C. Polydymitebreaks down to aNi-S? vaesite at 356^??3^?C. Differential thermalanalyses showed the existence of a two-liquid field in the sulfur-richportion of the system above 991^?C and over a wide compositionalrange.     

Calibration and Applications of Spinel Equilibria in the System FeO-Al2O3-SiO2

A new thermobarometer, based on the equilibrium: has been calibrated with experiments carried out in the piston-cylinderapparatus. Reversed equilibria were obtained using well-calibrated2.54 cm NaCl furnace assemblies and Ag₈₀Pd₂₀capsules with f_O2bufferedat or near iron-wustite. The equilibrium is located between5.2–5.4, 6.6–6.8, and 8.6–8.8 kb at 880, 940,and 1020?C, respectively, and at 5.2 and 8.8 kb between 865–880and 1020–1030?C, respectively. X-ray refinement data indicate that the hercynite (a = 8.15546?) has approximately 18 per cent inverse character. M?ssbauerspectra reveal that 4 mol per cent of the Fe is ferric (2 percent magnetite component). Broad Mossbauer lines and a Fe²+energy level splitting of 3.7 kJ mol^–1 calculated fromthe Mossbauer spectra are consistent with the X-ray determineddegree of inversion, although no separate octahedral Fe²⁺ spectraldoublet is resolved. Calibration of this equation allows calculation of the equilibrium: Thermobarometers based on the above equilibria are widely applicablein granulite fades rocks and yield pressure/temperature datathat are consistent with other well-calibrated barometers andthermometers.     

Caledonian Magma Genesis and Crustal Recycling

The isotopic compositions of Sr, Nd, Pb and O together withabundance data for Rb, Sr, Sm, Nd, U and Pb are reported forsamples from the component parts of the c. 400 Ma old EtiveComplex, temporally and spatially related Lorne and Glencoelavas, and the Dalradian country rocks into which the Complexhas been emplaced. These and published data available for otherCaledonian granites are used to evaluate the petrogenesis ofthe Etive Complex in particular, and the role of crustal recyclingin the generation and evolution of the Caledonian granites ingeneral. Nd-isotope compositions of Etive samples at 400 Ma range from – 9.9 to – 4.7 compared with–8.4 < – 3.2 for the associated volcanics investigated here, and an estimatedvalue for depleted mantle 400 Ma ago at approximately ? 7. Dalradiancountry rocks have – 23.4 < – 7.5 and two partially digested metasedimentaryxenoliths within the granite have values of –9. 3 and -4.0. Initial ⁸⁷Sr/ ⁸⁶Sr ratios forthe Etive Complex range from 0–7043 to 0–7079, whereasDalradian metasediment in the immediate vicinity of the granitehas an initial ratio of 0–726. Oxygen isotopes in theComplex have 7. 6 per mil <¹⁸O < 10.0 per mil, all inexcess of typical values of mantle oxygen and reflect a crustalcomponent. An upper limit of 25 per cent Dalradian assimilationis set by the Nd-Sr isotopic variations with the granites andxenoliths. The Etive complex parent magma prior to Dalradian xenolith assimilationis estimated to have values between – 10 and – 5. In order to satisfy the Srand Pb isotope composition, additional components from a deepersource within the lithosphere (lower crust or continental lithosphericmantle) with relatively unradiogenic Sr, Nd and Pb are required. The crustal residence ages of the Etive Complex average about1.5 Ga, similar to those of many other late and post-tectonicCaledonian granites. The generation of the Etive Complex andCaledonian granites in general has been dominated by recyclingof the continental lithosphere, rather than the addition ofnew material from asthenospheric sources.     

Properties of Synthetic Triclinic KFeSi3O8, Iron-microcline, with some Observations on the Iron-microcline{rightleftharpoons}Iron-sanidine Transition

Triclinic KFeSi₃O₈, iron-microcline, has been synthesized fromoxide mixes and by complete conversion of monoclinic KFeSi₃O₈,iron-sanidine. Iron-microcline is triclinic, C, a=8?68?0?01?, b=13?10?0?01, c=7?34?0?01, =90? 45'?10', ß=116?03'?10', =86?14'?10'. The optical properties (Na light) are:=1?585?0?002, ß=1?596?0?002, =1?605?0?002, 2V=85?(calc.), Xb, Z c=20??5?. A reversible phase transition betweentriclinic and monoclinic KFeSi₃O₈ occurs at 704??6? C at 2000bars total pressure. Iron-microcline is the low-temperaturepolymorph; no intermediate polymorphs were observed in eitherhydrothermal or dry heating experiments.     

A Method for Thermodynamical Calculation of Basicity Indicators of Rocks and Minerals

A new method has been suggested for evaluating the overall basicityof minerals and rocks by using ionization reactions involvingone proton: (sum of cations) + H₂O = mineral + H⁺, (sum of cations) + H₂O = (sum of normative minerals of a rock)+ H⁺. The basicity indicators are expressed as standard free energychanges of these reactions (). At standard water pressure (logP_H2O = 0) and chemical activity of the metal ions ( log _Mⁿ⁺= 0), the relationship between and alkalinity of solutions(pH) becomes: = –2.303 RTlog _H⁺ = 2.303 RT pH. The overall basicities of rock-forming oxides, minerals andmajor rocks were calculated from the thermodynamic data on ionsin water solutions and solid compounds.     

Fluid--Rock Interaction during Low-Pressure Polymetamorphism of the Reynolds Range Group, Central Australia

Marbles and metapelites from the Reynolds Range Group (centralAustralia) were regionally metamorphosed at low pressure duringM₂ at 1.6 Ga, M₂ ranged in grade from greenschist to granulitefacies along the length of the Reynolds Range, and overprinted1.78 Ga granites and their contact aureoles in the ReynoldsRange Group metasediments. At all M₂ grades the marbles andmetapelites have highly variable oxygen isotope ratios [marbles:¹⁸O(carb) 14–20%; metapelites: ¹⁸O 6–14%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(¹⁸O 5–13%), with the lowest values occurring at thegranite margins. In all rock types, the lowest oxygen isotopevalues are consistent with the infiltration of channelled magmaticand/or meteoric fluids. The variable lowering of oxygen isotopevalues resulted from pre-M₂ contact metamorphism and fluid—rockinteraction around the 1.78 Ga granites. In contrast, mineralassemblages in the marbles define a trend of increasing X_CO2with increasing grade from <0.05 (greenschist facies) to0.7–1.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluid—rock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology ^*Corresponding author Fax: 61–3–94791272. e-mail: geoisb{at}lure.latrobe.edu.au     

Sapphirine+Forsterite and Sapphirine+Humite-Group Minerals in an Ultra-Magnesian Lens from Kuhi-lal, SW Pamirs, Tajikistan: Are these Assemblages Forbidden?

Sapphirine occurs with humite-group minerals and forsteritein Precambrian amphibole-facies rocks at Kuhi-lal, SW PamirMountains, Tajikistan, a locality also for talc+kyanite magnesiohornblendewhiteschist. Most of these sapphirine-bearing rocks are graphiticand sulfidic (pyrite and pyrrhotite) and contain enstatite,clinohumite or chondrodite, spinel, rutile, gedrite, and phlogopite.A phlogopite schist has the assemblage with X_Fe = Fe/(Fe+Mg)increasing as follows: chlorite (0-003)<phlogopite (0.004–0.005)sapphirine (0.004–0.006) enstatite (0-006)forsterite (0-006–0-007)<spinel (0-014). This assemblage includes the incompatiblepair sapphirine+forsterite, but there is no textural evidencefor reaction. In one rock with clinohumite, X_Fe increases asfollows: clinohumite (0-002) <sapphirine (0-003) <enstatite(0-004–0-006) <spinel (0-010). Ion microprobe and wet-chemicalanalyses give 0-57–0-73 wt.% F in phlogopite and 0-27wt.% F in chlorite in the phlogopite schist; 0-04, 1.5–1.9,and 4.4 wt.% F in forsterite, clinohumite, and chondrodite,respectively; and 0-0-09 wt.% BeO and 0-05–0-21 wt.% B₂O₃in sapphirine. Stabilization of sapphirine+clinohumite or sapphirine+chondroditeinstead of sapphirine+phlogopite is possible at high F contentsin K-poor rocks, but minor element contents appear to be toolow to stabilize sapphirine as an additional phase with forsterite+enstatite+spinel.Although sapphirine+forsterite is metastable relative to spinel+enstatitein experiments conducted at a_H2O=1 in the MgO-Al₂O₃-SiO₂-H₂Osystem, it might be stabilized at a_H2O0.5, P4 kbar, T650–700C.Textures in the Kuhi-lal whiteschists suggest a polymetamorphicevolution in which the rocks were originally metamorphosed atT650C, P 7 kbar, conditions under which sapphirine+clinohumiteand sapphirine+chondrodite are inferred to have formed, andsubsequently affected by a later event at lower P, similar T,and lower a_H2O. The latter conditions were favorable for sapphirine+forsteriteto form in a rock originally containing chlorite+forsterite+spinel+enstatite.     

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.