Activity-composition relationship in Tschermak's substituted Fe biotites at 700°C, 2 kbar

The reaction-displacement technique was applied to the end-member reaction annite = sanidine + magnetite + H₂ in order to determine the activity of the annite component (a _Ann) in iron biotites with variable degrees of the Tschermak's substitution (^[6]Fe + ^[4]Si = ^[6]Al + ^[4]Al). Based on the simplified relation a _Ann = f _{H 2}/foH₂ (foH₂ = hydrogen fugacity of the end-member reaction at P, T), two types of experiments were performed at 700°C / 2 kbar: Type I used Fe-Al biotites of known starting composition together with sanidine + magnetite + H₂O. This assemblage was exposed to various f _{H 2} conditions (f _{H 2} < foH₂) produced in the pressure vessel either by using different ratios of water/oil as pressure medium (f _{H 2} in this case was measured by the hydrogen sensor technique), or by the Ni′NiO buffer. The composition of the Fe-Al biotites changed through incorporation or release of the annite component in response to the externally imposed f _{H 2}. By using opposite biotite starting compositions, the equilibrium composition as a function of f _H2 was bracketed. For type II, f _{H 2} in equilibrium with a specific combination of fine-grained Fe-Al biotite (+ sanidine + magnetite + H₂O) was measured internally by application of the hydrogen sensor technique. Both type I and type II experiments yield consistent results demonstrating that a fine-grained assemblage of Fe-Al biotite (+ sanidine + magnetite + H₂O) is able to act as a sliding-scale buffer. The final chemical composition of the Fe-Al biotite after the experiments was determined by electron microprobe and Mössbauer spectroscopy. The ^[4]Al and ^[6]Al in the biotites are coupled according to the Tschermak's substitution. In the tetrahedral sheet 0.1 Al-atoms per formula unit are present in excess to the amount required to balance ^[6]Al, and all Fe-Al biotites contain 8–10% Fe³⁺. Therefore, they are not members of the pure annite - siderophyllite join, but have an almost constant amount (15 Mol%) of two additional Fe³⁺-bearing components (ferri-siderophyllite and a vacancy end-member). The volume - composition relationship obtained does not indicate excess molar volumes of mixing for the annite (Ann) - siderophyllite (Sid) binary. The data are consistent with a molar volume of annite of 15.46 ± 0.02 Jbar^–1 and of 15.06 ± 0.02 Jbar^–1 for siderophyllite. The experimentally determined activity - composition relation shows that biotites on the join annite - siderophyllite deviate negatively from ideality. A symmetric interaction parameter W_AnnSid is sufficient to represent the data within error. This was constrained as: W _AnnSid = –29 ± 4 kJmol^–1. This is in contradiction to empirical interaction parameters derived from natural assemblages for this binary that predict positive deviation from ideality. Reasons for this discrepancy are discussed.     

Staurolite Stability and Related Parageneses: Theory, Experiments, and Applications

The results of recent investigations on the stability limitsof staurolite have been combined together with those of thepresent study to develop a semi-quantitative model of the P–T–f_o2–Xrelations of staurolite±quartz±magnetite. Theproblem with respect to the hydroxyl content of staurolite hasbeen analysed; it is concluded that no evidence has yet beenmustered to discount the idealised stoichiometry proposed byNaray-Szabó & Sasvari (1958), at least as a limitingcomposition. The stability limits of staurolite±magnetitehave been calculated from the experimental data for the equilibriainvolving quartz. Also the conditions over which the assemblagecordierite+magnetite+quartz could be stable, as well as a quantitativemodel for the fo₂-P stability of almandine ± quartz havebeen deduced theoretically. An analysis is presented of the paragenetic relations of staurolitein common pelitic schists. It is suggested that the formationof staurolite at the expense of either chloritoid or chlorite,rather than the unqualified first appearance of staurolite asproposed by Winkler (1970), should define a ‘staurolite-in’isograd in the range of 500–575 °C. In regional metamorphism,chloritoid, staurolite, and aluminum silicates should, underequilibrium conditions, be unstable relative to almandine ingraphitic pelitic schists involving magnetite (chloritoid/staurolite/Al₂SiO₆+magnetite+quartzalmandine+O₂+H₂O).The limits of P-T conditions over which staurolite and cordieritemay coexist in natural assemblages have been deduced; it isrestricted, almost entirely within the field of andalusite,between 500–700 °C, and 2–6 kbars, thus definingthe range of P-T conditions for the ‘low-pressure intermediate’—or ‘Buchan’–type amphibolite facies discussedby Miyashiro (1961). In assemblages involving staurolite andandalusite, cordierite rather than almandine should usuallybe stable; the reverse holds for assemblages involving stauroliteand sillimanite.     

The stability of annite+quartz: reversed experimental data for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite +2 H2O

Reversals for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite+2 H₂O have been experimentally determined in cold-seal pressure vessels at pressures of 2, 3, 4 and 5?kbar, limiting annite +quartz stability towards higher temperatures. The equilibrium passes through the temperature intervals 500–540°?C (2?kbar), 550–570°?C (3?kbar), 570–590°?C (4?kbar) and 590–610°?C (5?kbar). Starting materials for most experiments were mixtures of synthetic annite +fayalite+sanidine+quartz and in some runs annite+quartz alone. Microprobe analyses of the reacted mixtures showed that the annites deviate slightly from their ideal Si/Al ratio (Si per formula unit ranges between 2.85 and 2.92, Al^VI between 0.06 and 0.15). As determined by Mössbauer spectroscopy, the Fe³⁺ content of annite in the assemblage annite+fayalite +sanidine+quartz is around 5–7%. The experimental data were used to extract the thermodynamic standard state enthalpy and entropy of annite as follows: H ⁰ _f,?Ann =?5125.896±8.319 [kJ/mol] and S ⁰ _Ann=432.62±8.89 [J/mol/K] (consistent with the Holland and Powell 1990 data set), and H ⁰ _f,Ann =?5130.971±7.939 [kJ/mol] and S ⁰ _Ann=424.02±8.39 [J/mol/K] (consistent with the TWEEQ data base, Berman 1991). The preceeding values are close to the standard state properties derived from hydrogen sensor data of the redox reaction annite=sanidine+magnetite+H ₂ (Dachs 1994). The experimental half-reversal of Eugster and Wones (1962) on the annite +quartz breakdown reaction could not be reproduced experimentally (formation of annite from sanidine+fayalite+quartz at 540°?C/1.035?kbar/magnetite-iron buffer) and probable reasons for this discrepancy remain unclear. The extracted thermodynamic standard state properties of annite were used to calculate annite and annite+quartz stabilities for pressures between 2 and 5?kbar.     

An Experimentally Constrained Petrogenetic Grid in the Silica-Saturated Portion of the System KFMASH at High Temperatures and Pressures

Experiments in the quartz-saturated part of the system KFMASHunder fO₂ conditions of the haematite–magnetite bufferand using bulk compositions with X_Mg of 0·81, 0·72,0·53 define the stability limits of several mineral assemblageswithin the P–T field 9–12 kbar, 850–1100°C.The stability limits of the mineral assemblages orthopyroxene+ spinel + cordierite ± sapphirine, orthopyroxene + garnet+ sapphirine, sapphirine + cordierite + orthopyroxene and garnet+ orthopyroxene + spinel have been delineated on the basis ofP–T and T–X pseudosections. Sapphirine did not appearin the bulk composition of X_Mg = 0·53. A partial petrogeneticgrid applicable to high Mg–Al granulites metamorphosedat high fO₂, developed in our earlier work, was extended tohigher pressures. The experimental results were successfullyapplied to several high-grade terranes to estimate P–Tconditions and retrograde P–T trajectories. KEY WORDS: KFMASH equilibria; experimental petrogenetic grid at high fO₂     

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.     

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

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

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     

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.     

An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa

Crystallization experiments were performed at 200 MPa in thetemperature range 1150–950°C at oxygen fugacitiescorresponding to the quartz–fayalite–magnetite (QFM)and MnO–Mn₃O₄ buffers to assess the role of water andf_O2 on phase relations and differentiation trends in mid-oceanridge basalt (MORB) systems. Starting from a primitive (MgO9·8 wt %) and an evolved MORB (MgO 6·49 wt %),crystallization paths with four different water contents (0·35–4·7wt % H₂O) have been investigated. In primitive MORB, olivineis the liquidus phase followed by plagioclase + clinopyroxene.Amphibole is present only at water-saturated conditions below1000°C, but not all fluid-saturated runs contain amphibole.Magnetite and orthopyroxene are not stable at low f_O2 (QFM buffer).Residual liquids obtained at low f_O2 show a tholeiitic differentiationtrend. The crystallization of magnetite at high f_O2 (MnO–Mn₃O₄buffer) results in a decrease of melt FeO^*/MgO ratio, causinga calc-alkaline differentiation trend. Because the magnetitecrystallization temperature is nearly independent of the H₂Ocontent, in contrast to silicate minerals, the calc-alkalinedifferentiation trend is more pronounced at high water contents.Residual melts at 950°C in a primitive MORB system havecompositions approaching those of oceanic plagiogranites interms of SiO₂ and K₂O, but have Ca/Na ratios and FeO^* contentsthat are too high compared with the natural rocks, implyingthat fractionation processes are necessary to reach typicalcompositions of natural oceanic plagiogranites. KEY WORDS: differentiation; MORB; oxygen fugacity; water activity; oceanic plagiogranite     

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

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

Biotites as Indicators of Fluorine Fugacities in Late-Stage Magmatic Fluids: the Gardar Province of South Greenland

The major element chemistry and fluorine contents of biotitesfrom the Gardar alkaline province of South Greenland providean insight into the F contents of late-stage fluids associatedwith the magmatism. Biotites were taken from composite intrusionsranging from alkali gabbro to syenites, nepheline syenites andalkali granites. In each complex they show a large range ofFe/(Fe+Mg) (from 0•2 to 1) and exhibit strong Fe-F avoidance.There is considerable variation in F for any value of Fe/(Fe+Mg)but for each centre maximum F values plotted against Fe/(Fe+Mg)define a nearly straight line of characteristic slope towardslow or zero F at pure annite. Micas in the SiO₂ undersaturatedcentres have higher F contents than those from oversaturatedcentres. Cl is low (0•69 wt%) except in the Kûngnâtintrusion, where it reaches 1•4 wt%. Phase equilibriumand textural considerations suggest that most or all biotitesgrew subsolidus in a pervasive deuteric fluid. ¹⁸O values suggestthat these fluids were largely magmatic in character and thatextensive reactions with envelope fluids did not occur. Metasomaticresetting of F in biotites in early intrusive units in the aureoleof later units can be demonstrated. Experimental data of Munoz (Mineralogical Society of America,Reviews in Mineralogy 13, 469–494, 1984) were used tocalculate families of curves showing variation in F with Fe/(Fe+Mg)for biotites in equilibrium with fluids of fixed fugacity ratio,f(HF)/f(H₂O), at fixed P and T. The resulting curves cut sharplyacross the maximum fluorine lines observed in the natural examples.As it seems highly unlikely that changes in fluid compositionand T, acting together, could produce the observed linear relationship,we conclude that the partitioning of F between fluid and micain the plutonic environment is not well modelled by the experiments.Possible explanations are short-range order (SRO) of Fe andMg on octahedral sites in biotite (Mason, Canadian Mineralogist30, 343–354, 1992) or effects resulting from differentF speciation in alkaline fluids. If perfect Fe-F avoidance isassumed, Fe-Mg SRO can increase maximum F content. The F levelsseen in the silica-saturated centres are broadly consistentwith a cooling-rate related control. It is possible that thehigher F in biotites in the undersaturated centres reflectsdifferent speciation in the fluid rather than higher F contents,with strongly bound SiF₄° complexes more common. KEY WORDS: biotites; Gardar; fluorine; alkaline rocks; fluids     

Annite stability revised. 1. Hydrogen-sensor data for the reaction annite = sanidine + magnetite + H2

In P - T - logfO₂ space, the stability of annite (ideally KFe ₃ ²⁺ (OH)₂AlSi₃O₁₀) at high fO₂ (low fH₂) is limited by the reaction: annite = sanidine + magnetite + H₂. Using the hydrogen-sensor technique, the equilibrium fH₂ of this reaction was measured between 500 and 800° C at 2.8 kbar in 50° C intervals. Microbrobe analyses of the reacted annite+sanidine+magnetite mixtures show that tetrahedral positions of annite have a lower Si/Al ratio than the ideal value of 3/1. Silicon decreases from 2.9 per formula unit at low temperatures to 2.76 at high temperatures. As determined by Mössbauer spectroscopy in three experimental runs, the Fe³⁺ content of annite in the equilibrium assemblage is 11%±3. A least squares fit to the hydrogensensor data gives H _R ⁰ = 50.269 ± 3.987 kJ and S _R ⁰ = 83.01 ± 4.35 J/K for equilibrium (1). The hydrogene-sensor data are consistent with temperature half brackets determined in the classical way along the nickel-nickel oxide (NNO) and quartz-fayalite-magnetite (QFM) buffers with a mixture of annite+sanidine+magnetite for control. Compared to published oxygen buffer reversals, agreement is only found at high temperature and possible reasons for that discrepancy are discussed. The resulting slope of equilibrium (1) in logfO₂ – T dimensions is considerably steeper than previously determined and between 400 and 800°C only intersects with the QFM buffer curve. Based on the hydrogen-sensor data and on the thermodynamic dataset of Berman (1988, and TWEEQ data base) for sanidine, magnetite and H₂, the deduced standard-state properties of annite are: H _f ⁰ =-5127.376±5.279 kJ and S ⁰=422.84±5.29 J/(mol K). From the recently published unit cell refinements of annites and their Fe³⁺ contents, determined by Mössbauer spectroscopy (Redhammer et al. 1993), the molar volume of pure annite was constrained as 15.568±0.030 J/bar. A revised stability field for annite is presented, calculated between 400 and 800°C.     

The Solubility of Sulphur in Hydrous Rhyolitic Melts

Experiments performed at 2 kbar, in the temperature range 800–1000°C,with fO₂ between NNO–2·3 and NNO+2·9 (whereNNO is the nickel–nickel oxide buffer), and varying amountsof sulphur added to hydrous metaluminous rhyolite bulk compositions,were used to constrain the solubility of sulphur in rhyolitemelts. The results show that fS₂ exerts a dominant control onthe sulphur solubility in hydrous silicate melts and that, dependingon fO₂, a rhyolitic melt can reach sulphur contents close to1000 ppm at high fS₂. At fO₂ below NNO+1, the addition of ironto a sulphur-bearing rhyolite magma produces massive crystallizationof pyrrhotite and does not enhance the sulphur solubility ofthe melt. For a given fO₂, the melt-sulphur-content increaseswith fS₂. For fixed fO₂ and fS₂, temperature exerts a positivecontrol on sulphur solubilities, at least for fO₂ below NNO+1.The mole fraction of dissolved sulphur exhibits essentiallylinear dependence on fH₂S at low fO₂ and, although the experimentalevidence is less clear, on fSO₂ at high fO₂. The minimum insulphur solubility corresponds to the redox range where bothfH₂S and fSO₂ are approximately equal. A thermodynamic modelof sulphur solubility in hydrous rhyolite melts is derived assumingthat total dissolved sulphur results from the additive effectsof H₂S and SO₂ dissolution reactions. The model reproduces wellthe minimum of sulphur solubility at around NNO+1, in additionto the variation of the sulphide to sulphate ratio with fO₂.A simple empirical model of sulphur solubility in rhyoliticmelts is derived, and shows good correspondence between modeland observations for high-silica rhyolites. KEY WORDS: sulphur; solubility; rhyolite; thermodynamics; fO₂; fS₂     

A Clinopyroxene Paragenesis of Albite--Epidote--Amphibolite Facies in Meta-Syenites from the South-East Tauern Window, Austria

Mineral assemblages and textures are described from clinopyroxene-bearingmeta-syenites and related rocks from a small area in the PenninicBasement Complex of the south-east Tauern Window. Evidence from mineral textures, mineral compositions and geobarometryindicate that the clinopyroxene, a sodic salite, crystallizedas part of an equilibrium albite-epidote-amphibolite faciesparagenesis in the 35–40 Ma meso-Alpine metamorphic event.Phase relations in co-facial quartz + albite + K-feldspar +sphene-bearing meta-syenites and meta-granites are examinedusing a projection from these minerals onto the plane (A1₂O₃+ Fe₂O₃)-CaO-(MgO + FeO + MnO). The projection demonstratesthat salitic clinopyroxene can only be a stable phase in suchrocks if the bulk-rock Al/Na + K ratios are low. This is confirmedby comparing the whole-rock analyses of clinopyroxene-bearingmeta-syenites with those of clinopyroxene-free meta-syenitesand meta-granites. Mineral assemblages in a variety of lithologies from the south-eastTauern Window are used to construct a generalized AKM diagramfor magnesian albite + epidote + quartz-bearing rocks of thealbite-epidote-amphibolite facies. Thermochemical calculations indicate that the meta-syeniteswere metamorphosed at temperatures close to 500 C and at a pressureof 6+2 –4 kb. Fluids in equilibrium with meta-syeniteand meta-granite mineral assemblages had X_H2O values of 0–95,assuming X_H2O + X_CO2O= 1.0.     

Melting Experiments on a Sanidine Phlogopite Lamproite at 4-7 GPa and their Bearing on the Sources of Lamproitic Magmas

Suprasolidus phase relations at pressures from 4 to 7 GPa andtemperatures from 1000 to 1700C have been determined experimentallyfor a sanidine phlogopite lamproite from North Table Mountain,Leucite Hills, Wyoming. The lamproite is silica rich and hasbeen postulated to be representative of the magmas which wereparental to the Leucite Hills volcanic field. Near-liquidusphases above 5 GPa are pyrope-rich garnet and jadeite-rich pyroxene.Below 5 GPa, jadeite-poor pyroxene is the only near-liquidusphase. Near-solidus assemblages consist of clinopyroxene, titanianpotassium richterite and titanian phlogopite with either potassiumtitanian silicate above 5 GPa or potassium feldspar below 5GPa. The potassium titanian silicate is a newly recognized high-pressurephase ranging in composition from K₄Ti₂Si₇O₂₀ to K₄TiSi₈O₂₀.It coexists with coesite at pressures above 6 GPa at 1100–1400C.A previously unrecognized K-Ba-phosphate is a common near-solidusphase. The phase relationships are interpreted to suggest thatlamproites cannot be derived by the partial melting of simplelherzolitic sources. However, it is proposed that sanidine phlogopitelamproites an derived by high degrees of partial melting ofancient metasomatic veins within a harzburgitic–lherzoliticlithospheric substrate mantle. The veins are considered to consistof phlogopite, K–Ti-richterite, K–Ba-phosphate andK–Ti-silicates. KEY WORDS: lamproilte; experimental petrology; upper mantle ^*Corresponding author     

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.     

Formation of Carbon and Hydrogen Species in Magmas at Low Oxygen Fugacity

Studies of iron-bearing silicate melt (ferrobasalt) + iron metallicphase + graphite + hydrogen equilibria show that carbon andhydrogen solubilities in melts are important for the evolutionof the upper mantle. In a series of experiments conducted at3·7 GPa and 1520–1600°C, we have characterizedthe nature (oxidized vs reduced) and quantified the abundancesof C- and H-compounds dissolved in iron-bearing silicate melts.Experiments were carried out in an anvil-with-hole apparatuspermitting the achievement of equal chemical potentials of H₂in the inner Pt capsule and outer furnace assembly. The fO₂for silicate melt–iron equilibrium was 2·32 ±0·04 log units below iron–wüstite (IW). Theferrobasalt used as starting material experienced a reductionof its iron oxides and silicate network. The counterpart wasa liberation of oxygen reacting with the hydrogen entering thecapsule. The amount of H₂O dissolved in the glasses was measuredby ion microprobe and by step-heating and was found to be between1 and 2 wt %. The dissolved carbon content was found to be 1600ppm C by step-heating. The speciation of C and H componentswas determined by IR and Raman spectroscopy. It was establishedthat the main part of the liberated oxygen was used to formOH^– and to a much lesser extent H₂O, and only traces ofH₂, CO₂ and     

High fO2 During Sillimanite Zone Metamorphism of Part of the Barrovian Type Locality, Glen Clova, Scotland

The redox state of sillimanite zone (650–700°C, 5–6kbar) metasediments of the Barrovian type area, Scotland, wasinvestigated using estimates of metamorphic oxygen fugacity(f_O2), sulfur fugacity (f_S2), and fluid chemistry based on newdeterminations of mineral and rock compositions from 33 samples.A total of 94% of the samples lack graphite, contain both ilmenite–hematitesolid solutions (RHOMOX) and magnetite, and had metamorphicf_O2 about 2 log₁₀ units above the quartz–fayalite–magnetite(QFM) buffer. The regional variation in metamorphic f_O2 forthese rocks was minimal, about ±0·3 log₁₀ units,reflecting either a protolith that was homogeneous with respectto redox state, or an initially variable protolith whose redoxstate was homogenized by metamorphic fluid–rock interaction.RHOMOX inclusions in garnet porphyroblasts that become richerin ilmenite from the interior to the edge of the host porphyroblastsuggest that at least some syn-metamorphic reduction of rockoccurred. Significant variations in bulk-rock oxidation ratio(OR) that are probably inherited from sedimentary protolithsare found from one layer to the next; OR ranges mostly between     

Phase Relations of Peralkaline Silicic Magmas and Petrogenetic Implications

The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO₂relative to the Ni–NiO solid buffer), between 800 and660°C and for melt H₂O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckite–arfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H₂O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H₂O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 50–60°C,and that of amphibole by 30°C. Riebeckite–arfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40–100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO₂ lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H₂O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H₂O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO₂ at or below the fayalite–quartz–magnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H₂O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria     

Metamorphic History of Sapphirine-bearing and Related Magnesian Gneisses from Namaqualand, South Africa

Sapphirine occurs with cordierite, phlogopite, spinel, sillimanite,corundum, orthopyroxene, and gedrite in granulite facies Mg-and Al-rich paragneisses within the low P, high T NamaqualandMetamorphic Complex. The gneisses reveal a three-stage texturalhistory. Sapphirine appeared during a second stage of progrademineral growth which produced nodular structures and intergrowthsinvolving spinel, corundum, and sillimanite, pseudomorphingan earlier generation of coarse, amphibolite facies minerals.A third generation of coarse, cross-cutting, mainly hydrousminerals (gedrite, kornerupine, phlogopite) is sporadicallydeveloped. The wide variety of cofacial mineral assemblages allows thedelineation of the stable mineral associations of sapphirinein the system K₂O-MgO-FeO-Al₂O₃-SiO₂-H₂O under P-T conditionsindependently estimated at about 5 kb, 750–800 °C.The natural assemblages provide constraints which, taken togetherwith existing thermodynamic and experimental data, allow theestimation of P-T slopes of sapphirine equilibria. The mineraltextures thus indicate sapphirine growth under increasing T,decreasing a(H₂O), and constant or slightly increasing P. The preservation of prograde reaction textures during fine-grainedmineral growth probably results from the reduced importanceand/or more CO₂-rich composition of the metamorphic fluid undergranulite facies conditions in these refractory rocks. Aqueousfluids were locally reintroduced after the metamorphic peak.