The Four-Phase AFM Assemblage Staurolite--Aluminum Silicate--Biotite--Garnet: Extra Components and Implications for Staurolite--Out Isograds

The prograde disappearance of staurolite can be described inthe model system K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (KFMASH) by thereaction: staurolite + muscovite + quartz = biotite + aluminumsilicate + garnet + water. The common occurrence and world—widedistribution of the assemblage staurolite-biotite-aluminum silicate-garnet(SBAG) in quartz-mica-schist suggest that the model reactionmay be over-simplified. Previous workers have suggested thatthe SBAG assemblage (1) is a strictly divariant assemblage thatbuffered water activity, (2) is stabilized by non-KFMASH components,and (3) did not attain equilibrium. We used least-squares regression to show that balanced reactionsdo not exist among the minerals in samples of SBAG assemblagesfrom Califonia and New England. The absence of reaction relationshipscan be explained by imbalances in two or three of the minorelements Zn, Mn, and either Ca or Na. The assemblage is apparentlystabilized by non-KFMASH components. Criteria for mapping staurolite-out isograds that representthe conditions of the KFMASH staurolite-out reaction dependon which of the four phases is the ‘extra’ phase,and require an understanding of the thermodynamic effects ofall the ‘extra’ components. Our results suggestthat transition zones of SBAG assemblages near staurolite-outisograds are the result of ‘extra’ components. However,it is uncertain whether µH²O of fluids in equilibriumwith SBAG assemblages varied across such zones.     

Stability Relations of Mg-Chlorite-Muscovite and Quartz between 5 and 10 kb Water Pressure

Three reactions involving Mg-chlorite-muscovite and quartz werestudied between 5 and 10 kilobars water pressure over a temperaturerange of 600–700 °C using mixtures of synthetic clinochlore,muscovite and quartz as starting materials. Three reactionswere reversed in this system. 3 chlorite+5 muscovite + 8 kyanite+ 5 phlogopite + 1 quartz + 12 H₂O was reversed between 639.5±5.3°C and 531.8±5.1 °C at 7.24 kb and between 645.9±5.7°C and634.1±6.0°C at8.27kb. A second reaction: 1 chlorite+ 1 muscovite+2 quartz = 1 phlogopite+ 1 cordierite + 3.5 H₂O was reversed between 6370±60°C and 622.8±5.2°C at 6.21 kb. A third reaction: 3 cordierite+2 muscovite = 2 phlogopite +8 alumino-silicate + 7 quartz + 1.5 H₂O was reversed between660.l±5.7 °C and 650.l±5.3 °C at 6.21kb. This reaction is terminated at the beginning of melting around725 °C at 5.65 kb. These reactions determine the upper stability limits of Mg-chlorite-muscoviteand quartz assemblages between 5 and 10 kb water pressure. Theresults may be used in delimiting the upper stability of similarassemblages in natural systems.     

An Experimental Determination of Iron Biotite-Alkali Feldspar Equilibria

Experiments in the system KAlSi0₄-NaAlSi0₄-SiOj-Fe-0-H indicatethat approximately 20 mole per cent of sodium annite [NaFe₃AlSiO₁₀(OH)₂]can enter into solid solution with potassium annite. This substitutionreduces the biotite stability field. With excess alkali feldsparand no free quartz, the following reactions limit the biotitestability: Feldspar(s)+biotite feldspar(s)+magnetite+vapor. (1) Feldspar+biotite feldspar+fayalite+leucite+vapor. (2) Feldspar+biotite feldspar+fayalite+liquid+vapor. (3) Biotite feldspar+fayalite+nepheline+vapor. (4) On the QFM buffer (P_Total = 2 Kb), reaction 1 occurs at 685°C in the sodium free system and at 625 °C when twofeldspars are present; on NNO, it occurs at 630 and 590 °C.On the G-CH buffer, reaction 2 is stable at high temperatures(795–830 °Q and potassic compositions (), reaction3 at intermediate conditions (T = 740–95 °C; ) andreaction 4 at lower temperatures (710–40 °C) and sodiccompositions (). In the presence of excess quartz, reaction1 remains stable below the QFM buffer, but the reaction Feldspars)+biotite+quartz feldspars)+fayalite+vapor (5) is stable at higher hydrogenfugacities. On the G-CH buffer, reaction 5 occurs at 610 °Cin the sodium free system and at 595 °C when two alkalifeldspars are present. On the MW buffer, the temperatures are585 and 515 °C (M.I.). The experimental data presented suggest that biotite will notbe stable in the presence of granitic liquids at total pressuresbelow 4 kb, providing the fluorine and titanium content of thebiotite is low. They also suggest that gradients in the a£1S,l0 in a rock could produce variations in the biotite Fe/Fe+Mgratio and in extreme cases could result in the complete breakdown(decreased ) of biotite or its crystallization (increased )where previously there was none.     

The Stability of Sodalite in a Synthetic Syenite plus Aqueous Chloride Fluid System

Natural feldspathoidal syenites may be approximated by assemblagescontaining some or all of the phases sodalite, nepheline, oneor two alkali feldspars, and aqueous chloride fluid in the systemNaAISi₃O₈-KAISi₃O₈-NaAISiO₄-KAISiO₄-NaCI-KCI-H₂O. The stabilityof sodalite in these assemblages was studied in the range 500–700°C and 600–2000 bars fluid pressure. Sodalite appears to be a stable phase on the vapor-saturatedliquidus in this system over a wide range of pressure. At or near the vapor-saturated liquidus minimum in this system,three distinct types of sodalite-bearing syenite can crystallize.Nepheline-sodalite-one alkali feldspar rocks, nepheline-sodalite-twoalkali feldspars rocks and sodalite-analcime-bearing rocks crystallizebelow 1600 bars, between 1600 and 2750 bars and above 2750 barsfluid pressure, respectively. The effects of closed-system cooling on the assemblage sodalite-nepheline-twoalkali feldspars-aqueous fluid are different and distinguishablefrom the effects of metasomatism. Closed-system cooling resultsin replacement of K-feldspar by albite, feldspathoids remainingnearly unchanged, while metasomatism generally results in sismultaneousenrichment or impoverishment in sodalite plus K-feldspar.     

Low-temperature Compatibility Relations of Cordierite in Haplopelites of the System K2O--MgO--Al2O3--SiO2--H2O

The equilibrium temperatures of the reaction muscovite+chlorite+quartz= cordierite+phlogopite+H₂O (1) in the pure system K₂O—MgO—Al₂O₂—SiO₂—H₂Owere found to be 495±10°C at 1 kb P_H2O; 525±10°Cat 2 kb; 610±15°C at 5 kb; 635±10°C at6 kb. From intersection of this curve with the lower temperaturestability limit of cordierite close to 645°C, 6.5 kb P_H2O,a reaction cordieritc+muscovite = phlogopite+aluminum silicate+quartz+H₂O(2) is generated which has a negative slope and passes throughthe points 645°C, 6.5 kb PH₂O and 700°C, 5 kb PH₂O.On the high-pressure side of this reaction curve cordieriteis restricted to K₂O—poor bulk compositions. Application of the experimentally determined phase relationsto more complex natural pelitic rocks suggests that reaction(1) represents maximum temperatures for the disappearance ofchlorite from pelitic assemblages containing muscovite and quartz,whereas reaction (2) gives maximum water pressures for the disappearanceof cordierite from these rocks.     

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     

Low-Pressure Phase Relationships in the System Na2O--CaO--FeO--MgO--Al2O3--SiO2 at 1100C, with Implications for the Differentiation of Basaltic Magmas

Experiments were performed on the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂at 1100C, with the interest focused on the assemblage Liq+Aug+Pl+Oland its boundaries. Glass synthesized in a very reducing atmospherewas used as starting material. To avoid sodium loss during theexperiment, the starting material was loaded into iron capsules,and the experiments were carried out in evacuated silica glasstubes. All phases in the products were identified and analysedwith an electron microprobe. The probe analyses indicate thatthe assemblage Liq+Aug+Pl+Ol is stable over a wide range ofcompositions, and is bounded by the appearance of pigeonitein the silica-rich compositions. In the silica-poor compositions,the assemblage is successively bounded by the appearance ofkirschsteinite, melilite, and nepheline with increasing sodiumcontent. Owing to the isothermal and ‘isobaric’divariant nature of the assemblage Liq+Aug+Pl+Ol in the studiedsystem, a numerical method has been used to describe the phasecompositions with Si and Na contents in the liquid as two arbitrarilychosen independent variables. This procedure results in quantitativecharacterization of the assemblage Liq+Aug+Pl+Ol over a rangeof compositions. ^*Present address: Geochemistry Group, Geology Dept., Beijing University, Beijing, 100871, P.R. China.     

The Stability of Andradite, Hedenbergite, and Related Minerals in the System Ca--Fe--Si--O--H

Phase relations for the bulk compositions 3CaO·2FeO_x·3SiO₂+excessH₂O and CaO·FeO_x·2SiO₂+excess H₂O were determinedusing conventional hydrothermal techniques with solid phaseoxygen buffers to control fO₂. Andradite, Ca₃Fe³⁺₂Si₃O₁₂, synthesized above 550 °C hasan average unit cell edge, a_o, of 12.055±0.001 Å,and an index of refraction, n, of 1.887±0.003. Belowthis temperature, a_o increases whereas n decreases, indicatingthe formation of a member of the andradite-hydroandradite solidsolution. At 2000 bars P_fluid andradite is stable above an f_O2of 10¹⁵ bar at 800 °C and 10-³² bar at 400 °C. At lowerf_O2 andradite+fluid gives way at successively lower temperaturesto the condensed assemblages magnetite+wollastonite, kirschsteinite(CaFe²⁺SiO₄)+ wollastonite and kirschsteinite+xonotlite (Ca₆Si₆O₁₇(OH)₂). Synthetic hedenbergite, CaFe²⁺Si₂O₆, has average unit cell dimensionsof a_o = 9.857± 0.004 Å, b_o = 9.033±0.002Å, c_o = 5.254±0.002 Å and ß = 104.82°±0.03°,and refractive indices of n = 1.731±0.003 and n = 1.755±0.005.At 2000 bars P_fiuid, hedenbergite is stable below an f_O2 of10-¹³ bar at 800 °C and 10-²⁸ bar at 400 °C. Above thesef_O2 values, hedenbergite+O₂ breaks down to andradite+magnetite+quartz. The mineral pair andradite +hedenbergite thus limit the f_O2range possible for their joint formation under equilibrium conditions. The hydration of wollastonite to xonotlite occurs at much lowertemperatures than previous experimental work indicated. A tentativehigh temperature limit for this reaction is set at 185°±15°C and 5000±25 bars and 210°±15 °Cand 2000±20 bars. Inasmuch as the growth of xonotlitefrom wollastonite + H₂O was never accomplished, this high temperaturelimit does not represent an equilibrium univariant curve. Nine phases were encountered in the study of andradite and hedenbergite.They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite,xonotlite, quartz, ilvaite, and vapor (fluid). An invariantpoint analysis using the method of Schreinemakers shows thetopologic relations of the reactions involved. The resultinggrid can be used to interpret natural occurrences.     

Transition from the Zeolite to Prehnite-Pumpellyite Facies in the Karmutsen Metabasites, Vancouver Island, British Columbia

The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A1₂O₃-Fe₂O₃-SiO₂-H₂O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the X_Fe³⁺ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe³⁺ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-X_Fe³⁺ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H₂O, where the X_Fe³⁺ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-X_Fe³⁺ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant.     

Fractionation and Assimilation Processes in the Alkaline Augite Syenite Unit of the Ilimaussaq Intrusion, South Greenland, as Deduced from Phase Equilibria

The early augite syenite unit in the 1·13-Ga-old Ilímaussaqintrusive complex, South Greenland, consists of a magmatic assemblageof ternary alkali feldspar + fayalitic olivine + augite + titanomagnetite+ apatite + baddeleyite ± nepheline ± quartz ±ilmenite ± zircon. Feldspar, nepheline and QUILF thermometryyield T = 1000–700°C, at P = 1 kbar, which is derivedfrom fluid inclusion data from other parts of the complex. Ternaryfeldspar was the first major liquidus phase. It crystallizedat temperatures between 950 and 1000°C from a homogeneousmagma with a_SiO2 = 0·8 and f_O2 about 1·5–2log units below the fayalite–magnetite–quartz (FMQ)buffer. Later, closed system fractionation produced nepheline-bearingassemblages with a_SiO2 = 0·4 and log f_O2 = FMQ –3 to FMQ – 5. Assimilation of wall rocks produced localvariations of melt composition. Four traverses through the unitwere sampled parallel to the assumed direction of crystallization.They exhibit significant differences in their mineral assemblagesand compositions. The chemical zoning and calculated intensiveparameters of four sample suites reflect both closed systemfractional crystallization and local assimilation of wall rocks. KEY WORDS: alkaline magmatism; assimilation; fractionation; redox equilibria; QUILF     

The System Na2O-Al2O3-Fc2O3-SiO2 at 1 Atmosphere, and the Petrogenesis of Alkaline Rocks

Equilibria involving acmite, albite, nepheline, quartz, anda liquid phase constitute the petrologically important partof the system Na₂O–Al₂O₃–Fe₂O₂–SiO₂, and theunivariant and invariant relations provide useful analogiesfor a wide variety of alkaline igneous rocks. These relationsare dominated by the incongruent melting behaviour of acmite,which does not appear on the liquidus of the join acmite-nepheline-silica;instead, a broad field of hematite is present and acmite crystallizesonly from liquids containing potential sodium silicate. Consequently,the oversaturated and undersaturated eutectics, correspondingto granitic and nepheline syenitic liquids, are rich in sodiumsilicate and distinct from those found in Petrogeny's Residuasystem: the temperatures of the eutectics are 7285C and 7155C, respectively. Survival of peralkaline granite in the aluminouscontinental crust can be explained by the strongly peralkalinecomposition of the oversaturated eutectic. Magma of this typemay be the primitive granite of the non-orogenic zones. Theubiquitous alkali metasomatism around alkaline complexes canalso be interpreted in terms of residual liquids enriched inalkali silicates. Transition from undersaturated to oversaturatedliquids is possible by fractionation of hematite and a new processfor achieving the reverse transition has been found. This dependson the substitution of Fe³ for Al³ in feldspar and suggestsa more important role for syenite in any scheme of petrogenesis. Each of the two eutectics is linked to a corresponding peritecticat which hematite reacts to give acmite. The liquid at the undersaturated,quaternary reaction point is of ijolitic type, providing thefirst intimation that ijolite may represent a low-melting fractionin nature. The system Na₂O–Al₂O₃–Fe₂O₃–SiO₂thus constitutes the peralkaline residua system and on thisbasis a coherent picture of stable continental magmatism canbe constructed. Ijolite is seen as the low-melting fractionfrom a range of peralkaline compositions and from rocks suchas melilite basalt, while the frequently associated carbonatiteis considered to be the volatile-rich, fugitive material fromthe mantle. Such a relationship is consistent with the dualassociation of carbonatite with either ijolite or kimberliteunder different tectonic conditions. The more common syenite,nepheline syenite, and alkaline granite of the non-orogenicregions are regarded as low-melting fractions from basalticmaterials in the deep crust. Most of this activity, involvingmagmas of residual type, could thus be explained in terms ofpartial melting in the deep crust and upper mantle. A possiblemechanism for this would be arching of the rigid continentalcrust, the consequent relief of lithostatic load giving riseto melting, and the concentration of fugitive constituents,in the underlying zones.     

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.     

Nepheline--Alkali Feldspar Equilibria: I. Experimental Data and Thermodynamic Calculations

Nepheline-alkali feldspar equilibria with alkali chloride aqueoussolutions have been determined for the temperature range 400to 700 °C at 1000 bars pressure. Nepheline-alkali feldsparequilibria with alkali chloride melts have been determined forthe temperature range 800 to 1100 °C at approximately 6bars pressure. (1) NaAlSiO₄ + KCl(aq) = NaCl(aq) + KAlSiO₄ (2) NaAlSiO₄ + KCl(melt) = NaCl(melt) + KAlSiO₄ (3) NaAlSi₃O₈(high) + KCl(aq) = NaCl(aq) + KAlSi₃O₈(San) (4) NaAlSi₃O₈(low) + KCl(aq) = NaCl(aq) + KAlSi₃O₈(Mic) (5) NaAlSi₃O₈(high) + KCl(melt) = NaCl(melt) + KAlSi₃O₄(San) (6) NaAlSi₃O₈(low) + KCl(melt) = NaCl(melt) + KAlSi₃O₈(Mic) From these, two diagrams of phase relationships were derivedfor the following exchange equilibria: (7) NaAlSiO₄ + KAlSi₃O₈(San) = NaAlSi₃O₈(high) + KAlSiO₄; (8) NaAlSiO₄ + KAlSi₃O₈(Mic) = NaAlSi₃O₈(low) + KAlSiO₄. The effect of pressure on these equilibria has been determinedby comparing the experimental data for 1000 and 5000 bars (t= 500 °C) and thermodynamic calculations. It has also beenshown that the effect of excess silica in nepheline solid solutionon the K—Na distribution between nepheline and alkalifeldspar is substantial and opposite to that of temperature.In the high temperature region an increase in silica contentin nepheline of 2 wt. per cent eliminates the effect on theredistribution of a temperature increase of 100 °C. Thesecation exchange data and unit cell data for the crystal phasesare used to calculate thermodynamic mixing properties of nephelinesolid solution and alkali feldspar solid solution for a widerange of temperature and pressure.     

Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2 {+/-} K2O at 1{middle dot}1 GPa

The compositions of multiply saturated partial melts are valuablefor the thermodynamic information that they contain, but aredifficult to determine experimentally because they exist onlyover a narrow temperature range at a given pressure. Here wetry a new approach for determining the composition of the partialmelt in equilibrium with olivine, orthopyroxene, clinopyroxeneand spinel (Ol + Opx + Cpx + Sp + Melt) in the system CaO–MgO–Al₂O₃–SiO₂(CMAS) at 1·1 GPa: various amounts of K₂O are added tothe system, and the resulting melt compositions and temperatureare extrapolated to zero K₂O. The ‘sandwich’ experimentalmethod was used to minimize problems caused by quench modification,and Opx and Cpx were previously synthesized at conditions nearthose of the melting experiments to ensure they had appropriatecompositions. Results were then checked by reversal crystallizationexperiments. The results are in good agreement with previouswork, and establish the anhydrous solidus in CMAS to be at 1320± 10°C at 1·1 GPa. The effect of K₂O is todepress the solidus by 5·8°C/wt %, while the meltcomposition becomes increasingly enriched in SiO₂, being quartz-normativeabove 4 wt % K₂O. Compared with Na₂O, K₂O has a stronger effectin depressing the solidus and modifying melt compositions. Theisobaric invariant point in the system CMAS–K₂O at whichOl + Opx + Cpx + Sp + Melt is joined by sanidine (San) is at1240 ± 10°C. During the course of the study severalother isobaric invariant points were identified and their crystaland melt compositions determined in unreversed experiments:Opx + Cpx + Sp + An + Melt in the system CMAS at 1315 ±10°C; in CMAS–K₂O, Opx + Cpx + Sp + An + San + Meltat 1230 ± 10°C and Opx + Sp + An + San + Sapph +Melt at 1230 ± 10°C, where An is anorthite and Sapphis sapphirine. Coexisting San plus An in three experiments helpdefine the An–San solvus at 1230–1250°C. KEY WORDS: feldspar solvus; igneous sapphirine; mantle solidus; partial melting; systems CMAS and CMAS–K₂O     

The Mineralogy and Petrology of the Transition from the Lower to Upper Sillimanite Zone in the Oquossoc Area, Maine

Systematic changes in the assemblages and compositions of mineralssuggest that the transition from lower to upper sillimanitezone in the Oquossoc area, Maine, is marked by the reaction Staurolite+Sodic Muscovite+QuartzSillimanite+Biotite+K-richerMuscovite+Albite+Garnet+H₂O. Moreover, the mineralogic data for the silicates suggest thatH₂ O is buffered by some of the assemblages present but PH₂O<P_total. A consideration of the equilibria among the opaqueminerals enables a calculation of the composition of the fluidphase assuming specific PT conditions and that P_fluid = P_total.At several reasonable PT conditions the calculated fluid phaseis reasonable in terms of the model for P_H2O based upon thesilicate equilibria. Probable PT conditions for the metamorphism considered in thisstudy range from 575 to 630°C and 3 to 5 kb.     

The Effect of Reaction Overstep on Garnet Microtextures in Metapelitic Rocks of the Ilesha Schist Belt, SW Nigeria

Garnet-bearing assemblages of K-rich and K-poor metapelitesfrom the Ilesha Schist belt, SW Nigeria, are investigated. K-richsamples contain the assemblages (A) garnet–staurolite–muscovite–chlorite–magnetite,(B) andalusite–garnet–staurolite–muscovite–chlorite–magnetiteand (C) sillimanite–andalusite–garnet–muscovite–chlorite–magnetite.K-poor samples contain the assemblages (D) garnet–staurolite–cordierite–chloriteand (E) garnet–cordierite–chlorite ± staurolite.All assemblages contain quartz, plagioclase, biotite and ilmenite.P–T pseudosections calculated in the system CaO–Na₂O–K₂O–TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂ –H₂O ± O₂ suggest peak metamorphismat 590 ± 20°C at 5 ± 0·5 kbar, followedby retrogression to 550°C at 3·0 kbar, in agreementwith field evidence, domain assemblages, mineral compositions,modes and geothermobarometry. The absence of compositional zonationshows that garnet in all investigated rocks nucleated and grewat constant P–T–X in equilibrium with associatedminerals on the thin-section scale. However, the garnet-in reactiondid not begin until the establishment of a significant temperatureoverstep of     

Metamorphism of Calcareous Rocks in Three Roof Pendants in the Sierra Nevada, California

Two roof pendants in the Hope Valley area, Alpine County, containabundant calc-silicate assemblages which can be related to univariantor invariant equilibria in the CaO-Al₃O₃-SiO₂-H₂O-CO₂ system.Such assemblages are considered to represent components of reactionsthat buffered the chemistry of the pore fluid. Through dataobtained from microprobe analysis it is concluded that solidsolution in plagioclase, garnet, and clinozoisite are importantvariables such that on a TXco₂ projection each sample had aunique path during metamorphism. Differences in the plagioclasecomposition of nearby samples with assemblages related by thereaction: grossularite_(s.s)+quartz = anorthite_(s.s.)+wollastonite, suggest unique equilibration temperatures for assemblages inlocal domains. In the Twin Lakes pendant in Fresno County, thereaction: clinohumite+calcite+CO₂= 4forsterite+dolomite+H₂O, is importantin magnesian marbles. Contrasting parageneses, which are relatedby this equilibrium, are considered to reflect variations influid composition. Constrasting assemblages in calc-silicaterocks, which are linked by the reactions: calcite+quartz= wollastonite+CO₂, tremolite+calcite= dolomite+diopside+CO₂+H₂O, exist down to the scale of a thin section. Variation in Ti contentof idocrase may be an important factor in assemblages linkedby reactions involving this phase. This study suggests that during contact metamorphism of calcareousrocks in the Sierra Nevada, H₂O and CO₂ behaved as ‘initialvalue components’ (Zen, 1963) whose activities were controlledby reactions withion local systems.     

Lawsonite-Omphacite-Bearing Metabasites of the Pam Peninsula, NE New Caledonia: Evidence for Disrupted Blueschist- to Eclogite-Facies Conditions

The Diahot terrane of NE New Caledonia contains an interbeddedsequence of Cretaceous to Eocene metasediments, felsic and maficmetavolcanics that experienced c. 40 Ma high-P/T metamorphism.Metabasaltic assemblages define two prograde events (M₁ andM₂) and a tectonically disrupted crustal profile that extendsfrom lawsonite–blueschist conditions in the SW to paragonite–eclogiteconditions in the NE. Weakly deformed metabasalts from lowest-gradeparts of the Diahot terrane contain M₁ omphacite, chlorite,lawsonite and glaucophane-bearing assemblages that partiallypseudomorph igneous plagioclase and augite, and reflect P =0·7–1·0 GPa and T = 350–400°C.M₁ assemblages are enveloped by a steeply SW-dipping S₂ foliationthat becomes progressively more intense towards the NE overa distance of c. 15 km. S₂ assemblages are divided into fourzones: (1) lawsonite–omphacite; (2) lawsonite–clinozoisite–spessartine;(3) clinozoisite–hornblende–almandine; (4) almandine–omphacite.S₂ assemblages reflect a P–T gradient that spans the exposed15 km of the Diahot terrane from P = 0·8–1·0GPa and T = 350–400°C (Zone 1) to P = 1·6–1·7GPa and T = 550–600°C (Zone 4). The systematic mineralogicalchanges reflect parts of a P–T array between 1·0and 1·7 GPa that was extensively disrupted by tectonicthinning during exhumation. KEY WORDS: blueschist; eclogite; New Caledonia; CNFMASH; pseudosection     

Sediment Melts at Sub-arc Depths: an Experimental Study

The phase and melting relations in subducted pelites have beeninvestigated experimentally at conditions relevant for slabsat sub-arc depths (T = 600–1050°C, P = 2·5–4·5GPa). The fluid-present experiments produced a dominant paragenesisconsisting of garnet–phengite–clinopyroxene–coesite–kyanitethat coexists with a fluid phase at run conditions. Garnet containsdetectable amounts of Na₂O (up to 0·5 wt%), P₂O₅ (upto 0·56 wt%), and TiO₂ (up to 0·9 wt%) in allexperiments. Phengite is stable up to 1000°C at 4·5GPa and is characterized by high TiO₂ contents of up to 2 wt%.The solidus has been determined at 700°C, 2·5 GPaand is situated between 700 and 750°C at 3·5 GPa.At 800°C, 4·5 GPa glass was present in the experiments,indicating that at such conditions a hydrous melt is stable.In contrast, at 700°C, 3·5 and 4·5 GPa, asolute-rich, non-quenchable aqueous fluid was present. Thisindicates that the solidus is steeply sloping in P–T space.Fluid-present (vapour undersaturated) partial melting of thepelites occurs according to a generalized reaction phengite+ omphacite + coesite + fluid = melt + garnet. The H₂O contentof the produced melt decreases with increasing temperature.The K₂O content of the melt is buffered by phengite and increaseswith increasing temperature from 2·5 to 10 wt%, whereasNa₂O decreases from 7 to 2·3 wt%. Hence, the melt compositionschange from trondhjemitic to granitic with increasing temperature.The K₂O/H₂O increases strongly as a function of temperatureand nature of the fluid phase. It is 0·0004–0·002in the aqueous fluid, and then increases gradually from about0·1 at 750–800°C to about 1 at 1000°C inthe hydrous melt. This provides evidence that hydrous meltsare needed for efficient extraction of K and other large ionlithophile elements from subducted sediments. Primitive subduction-relatedmagmas typically have K₂O/H₂O of 0·1–0·4,indicating that hydrous melts rather than aqueous fluids areresponsible for large ion lithophile element transfer in subductionzones and that top-slab temperatures at sub-arc depths are likelyto be 700–900°C. KEY WORDS: experimental petrology; pelite; subduction; UHP metamorphism; fluid; LILE     

Phase Relations and Crystallization Sequence in a Contact-Metamorphosed Rock from the Gunflint Iron Formation, Minnesota

Portions of the Gunflint Iron Formation, originally a ferruginoussediment, were metamorphosed by the intrusion of the DuluthComplex to assemblages containing: pigeonite (Wo₁₀En₂₄Fs₆₆)+olivine(Fo₁₃Fa₃₇)+Fe-Ti oxide (Mt₆₂Usp₃₄Hc₄)+plagioclase (An₉₄Ab₆)+vapor+augite (Wo₄₀En₂₀Fs₄₀) or cummingtonite Fe/(Fe+Mg) {smalltilde} 0.69; quartz was present but probably was not in equilibriumwith olivine. Comparison with synthetic phase-equilibrium studiesindicate conditions of initial recrystallization of T 800 °C,P_total 2kb, fo₂ slightly below that of the pure fayalite-magnetite-quartzassemblage, and P_H2O < P_total. During the slow cooling process following initial recrystallization,the phases present underwent a complex series of exsolution,inversion, oxidation, and hydration reactions. Pigeonite initiallyexsolved augite along (001), then inverted to orthopyroxene,which then exsolved augite along (100). The augite exsolvedonly pigeonite on (001) during its cooling history. The Fe-Tioxide for the most part oxidized to an intergrowth of magnetiteand ilmenite, although unoxidized portions later exsolved ulvöspinel.Cummingtonite exsolved actinolite, forming irregular patchesof the latter. Olivine, orthopyroxene, and augite reacted withplagioclase to form retrograde amphiboles. Orthopyroxene had difficulty nucleating during this slow coolingprocess, forming only at widely spaced points in mosaics ofprimary pigeonite grains, and never nucleating within primaryaugite grains. The resulting orthopyroxene grains are much largerthan the original pigeonite grains.