Experimental Petrology of the 1991-1995 Unzen Dacite, Japan. Part I: Phase Relations, Phase Composition and Pre-eruptive Conditions

Crystallization experiments were conducted on dry glasses fromthe Unzen 1992 dacite at 100–300 MPa, 775–875°C,various water activities, and fO₂ buffered by the Ni–NiObuffer. The compositions of the experimental products and naturalphases are used to constrain the temperature and water contentsof the low-temperature and high-temperature magmas prior tothe magma mixing event leading to the 1991–1995 eruption.A temperature of 1050 ± 75°C is determined for thehigh-temperature magma based on two-pyroxene thermometry. Theinvestigation of glass inclusions suggests that the water contentof the rhyolitic low-temperature magma could be as high as 8wt % H₂O. The phase relations at 300 MPa and in the temperaturerange 870–900°C, which are conditions assumed to berepresentative of the main magma chamber after mixing, showthat the main phenocrysts (orthopyroxene, plagioclase, hornblende)coexist only at reduced water activity; the water content ofthe post-mixing dacitic melt is estimated to be 6 ± 1wt % H₂O. Quartz and biotite, also present as phenocrysts inthe dacite, are observed only at low temperature (below 800–775°C).It is concluded that the erupted dacitic magma resulted fromthe mixing of c. 35 wt % of an almost aphyric pyroxene-bearingandesitic magma (1050 ± 75°C; 4 ± 1 wt % H₂Oin the melt) with 65 wt % of a phenocryst-rich low-temperaturemagma (760–780°C) in which the melt phase was rhyolitic,containing up to 8 ± 1 wt % H₂O. The proportions of rhyoliticmelt and phenocrysts in the low-temperature magma are estimatedto be 65% and 35%, respectively. It is emphasized that the strongvariations of phenocryst compositions, especially plagioclase,can be explained only if there were variations of temperatureand/or water activity (in time and/or space) in the low-temperaturemagma. KEY WORDS: Unzen volcano; magma mixing; experimental study     

Granulite-Facies Metamorphism at Molodezhnaya Station, East Antarctica

Granulite-facies quartzofeldpathic gneisses metamorphosed 1000m.y. ago are exposed around Molodezhnaya Station (67°40'S,46°E) in East Antarctica. In addition to quartz, K-feldspar,and plagioclase, the fourteen samples studied in detail consistof the assemblages biotite-orthopyroxene-magnetite, biotite-garnet-orthopyroxene-ilmenite±magnetite, biotite-garnet ± ilmenite ± magnetite,biotite-garnet-sillimanite-ilmenite ± rutile, and biotite-garnet-cordierite-ilmenite-(sillimanite-rutile).Garnets are pyrope-almandine (13 to 34 mol per cent pyrope).Biotite (X_Fe = 0.33 to 0.57) is rich in TiO₂ (4 to 6.3 wt percent) and its Al₂O₃ content depends on the mineral assemblage.Orthopyroxene (X_Fe = 0.45 to 0.60) contains 1.5 to 3.0 weightper cent Al₂O₃. By and large, the minerals are chemically homogeneousand compositional variations are systematic, which indicatecrystallization under equilibrium conditions. On the basis ofthe compositions of coexisting garnet-biotite, garnet-cordierite,garnet-plagioclase (with sillimanite), and garnet-plagioclase-orthopyroxene,temperatures and pressures during the granulite-facies metamorphismare estimated to be 700°C ± 30°C and 5.5 ±1 kb. Water pressure apparently was significantly less thantotal pressure. Alteration during events following the granulite-facies metamorphismhas resulted in chemical zoning in garnet, in which grain edgesare more iron-rich than cores, heterogeneous biotite compositions,and anomalous trends involving MnO. Temperatures based on biotiteand garnet-edge compositions range from 410 to 580°C. Differences in the chemical potential (µ) of water andoxygen in the fluid phase can explain compositional variationsamong the three sillimanite-bearing samples and the relativelyiron-rich compositions of garnet and biotite associated withcordierite. Apparently, the water released by the formationof cordierite remained in the rock, forcing µH₂O to increaseas cordierite formed. Buffering of fluid phase composition bythe mineral assemblage suggests that water was not removed fromthe Molodezhnaya rocks by flushing with CO₂-rich fluids duringmetamorphism, a hypothesis evoked to explain ‘dry’mineral assemblages in other granulite-facies terrains.     

The effects of metamorphism on O and Fe isotope compositions in the Biwabik Iron Formation, northern Minnesota

The Biwabik Iron Formation of Minnesota (1.9 Ga) underwent contact metamorphism by intrusion of the Duluth Complex (1.1 Ga). Apparent quartz–magnetite oxygen isotope temperatures decrease from ∼700°C at the contact to ∼375°C at 2.6 km distance (normal to the contact in 3D). Metamorphic pigeonite at the contact, however, indicates that peak temperatures were greater than 825°C. The apparent O isotope temperatures, therefore, reflect cooling, and not peak metamorphic conditions. Magnetite was reset in δ¹⁸O as a function of grain size, indicating that isotopic exchange was controlled by diffusion of oxygen in magnetite for samples from above the grunerite isograd. Apparent quartz–magnetite O isotope temperatures are similar to calculated closure temperatures for oxygen diffusion in magnetite at a cooling rate of ∼5.6°C/kyr, which suggests that the Biwabik Iron Formation cooled from ∼825 to 400°C in ∼75 kyr at the contact with the Duluth Complex. Isotopic exchange during metamorphism also occurred for Fe, where magnetite–Fe silicate fractionations decrease with increasing metamorphic grade. Correlations between quartz–magnetite O isotope fractionations and magnetite–iron silicate Fe isotope fractionations suggest that both reflect cooling, where the closure temperature for Fe was higher than for O. The net effect of metamorphism on δ¹⁸O–δ⁵⁶Fe variations in magnetite is a strong increase in δ¹⁸O_Mt and a mild decrease in δ⁵⁶Fe with increasing metamorphic grade, relative to the isotopic compositions that are expected at the low temperatures of initial magnetite formation. If metamorphism of Iron Formations occurs in a closed system, bulk O and Fe isotope compositions may be preserved, although re-equilibration among the minerals may occur for both O and Fe isotopes. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C-O-H-S) on Phase Equilibria and Mineral Composition

Crystallization experiments at 400 MPa, oxidized condition (logfO₂= NNO + 1, where NNO is nickel–nickel oxide buffer) andover a range of temperatures (850–950°C) and fluidcomposition (XH₂O_in = 0·3–1) have been carriedout to constrain the storage conditions of the sulphur-richmagma of the Huerto Andesite (an anhydrite, pyrrhotite, andS-rich apatite-bearing, post-Fish Canyon Tuff mafic lava). Theresults are used to evaluate the role of fluids released fromthe crystallization of magmas such as the Huerto Andesite onthe remobilization of the largely crystallized dacitic FishCanyon magma body. Experiments were performed using the naturalandesitic bulk composition with and without added sulphur. Thepresence of sulphur slightly affects the phase equilibria bychanging the phase proportions, stability fields of plagioclase,pyroxenes and ilmenite, and also affects the plagioclase composition.Phase equilibria and mineral composition data indicate thatthe magma may have contained 4·5 wt % water in the meltand that the pre-eruptive temperature was 875 ± 25°C.Assuming that the magma was in equilibrium with a fluid phase,the CO₂ concentration of the melt is estimated to be in therange 2000–4000 ppm (at 400 MPa). Before eruption, theandesite had an oxidation state very close to, or slightly within,the co-stability field of anhydrite–pyrrhotite at NNO+ 1·1. At these conditions, the sulphur content in themelt is 500 ppm. Assuming open-system degassing resulting fromcontinuing crystallization at depth, most of the CO₂ dissolvedin the andesitic melt should be released after the crystallizationof <10 vol. % of the magma, corresponding to a cooling from875 to 825–850°C. Thus, the fluids released owingto crystallization processes should be mainly composed of waterat temperatures below 825°C. KEY WORDS: experimental study; andesite; volatile; Fish Canyon Tuff; Huerto Andesite     

Thermal magnetic susceptibility data on natural iron sulfides of northeastern Russia

Dependences of magnetic susceptibility (MS) on the temperature of natural iron sulfide samples (pyrite, marcasite, greigite, chalcopyrite, arsenopyrite, pyrrhotite) from the deposits of northeastern Russia were studied. The thermal MS curves for pyrite and marcasite are the same: On heating, MS increases at 420–450 °C, and unstable magnetite (maghemite) and monoclinic pyrrhotite with a well-defined Hopkinson peak are produced. In oxygen-free media with carbon or nitrogen, magnetite formation is weak, whereas pyrrhotite generation is more significant. The heating curves for chalcopyrite are similar to those for pyrite. They show an increase in MS at the same temperatures (420–450 °C). However, stable magnetite is produced, whereas monoclinic pyrrhotite is absent. In contrast to that in pyrite, marcasite, and chalcopyrite, magnetite formation in arsenopyrite begins at > 500 °C. Arsenopyrite cooling is accompanied by the formation of magnetite (S-rich arsenopyrite) or maghemite (As-rich arsenopyrite) with a dramatic increase in MS. Arsenopyrite with an increased S content is characterized by insignificant pyrrhotite formation. Greigite is marked by a decrease in MS on the heating curves at 360–420 °C with the formation of unstable cation-deficient magnetite.Monoclinic pyrrhotite is characterized by a decrease in MS at ～ 320 °C, and hexagonal pyrrhotite, by a transition to a ferrimagnetic state at 210–260 °C. The addition of organic matter to monoclinic pyrrhotite stimulates the formation of hexagonal pyrrhotite, which transforms back into monoclinic pyrrhotite on repeated heating. The oxidation products of sulfides (greigite, chalcopyrite) show an increase in MS at 240–250 °C owing to lepidocrocite.     

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.     

Petrology of Rhyolitic and Mixed Magma Ejecta from the 1875 Eruption of Askja, Iceland

Volcanic activity in Askja central volcano and its fissure swarmin 1875 occurred in response to a crustal rifting episode inIceland, resulting in up to 70 km lateral flow of magma withinthe crust, caldera collapse and a plinian eruption of acid magma(0·2 km³ dense-rock equivalent). Petrologic studies ofthe predominantly rhyolitic and crystal-poor ejecta reveal thata complex array of other liquid compositions was also present,including icelandite (0.75 per cent) and basalt (1.9 per cent),as well as leucocratic xenoliths of trondhjemite type. Mineralgeothermometers indicate that the rhyolite evolved at 990 to1010 °C and 0·5 Kb P_H2O, the icelandite at 1005 to1020 °C and at f_O2 10^–10 atm. and the basalt at 1140to 1170 °C. A petrologic model of Askja in 1875 consists of a density-stratifiedmagma chamber with a rhyolitic upper part and a lower part offerrobasalt, with an intervening layer of icelandite. The modelcalculations show that the icelandite can be derived from ferrobasaltby 50 per cent fractional crystallization, but one-stage fractionalcrystallization models cannot account for generation of theacid magma. Simple partial or complete fusion of the field-associatedtrondhjemite xenoliths cannot produce the acid magma. Instead,a more complex fusion, hybridization and fractional crystallizationmodel is presented, which is consistent with the available petrologicevidence. This model involves large-scale fusion of pre-existingtrondhjemite intrusions or reactivation of previously consolidatedroof-rock in the magma chamber followed by hybridization ofthe acid magma with 7 to 14 per cent basaltic magma. Finally,10 to 11 per cent fractional crystallization of the dacite hybridis required to produce the observed compositional range withinthe rhyolite ejecta. The 1875 explosive eruption was causedby the ascent of tholeiitic basalt magma from depth during crustalrifting. Influx of new basalt magma in 1874–75 triggeredconvective mixing and hybridization in the compositionally zonedmagma chamber.     

Experimental Study of the Composition Join NaAlSiO4-CaCO3-H2O and the Genesis of Alkalic Rock--Carbonatite Complexes

Phase equilibrium studies have been carried out on the compositionjoin NaAlSiO₄-CaCO₃-H₂O with 25 wt per cent H₂O at 1 kb pressurein the temperature range 600–960 °C. Liquid, in equilibriumwith crystalline phases and a sodic, aqueous vapor phase persistsacross the join down to temperatures of about 600 °C. Fractionalcrystallization of a carbonated nepheline-rich liquid in equilibriumwith vapor is capable of generating successively the crystallineassemblages (1) nepheline, (2) melilite+nepheline, (3) hydroxyhaüyne+melilite,(4) cancrinite+melilite, and (5) calcite+cancrinite+melilite.Late-stage liquid fractions are rich in CaCO₃, whereas the vaporphase is enriched in Na. The experimental assemblages are strikinglysimilar to rocks in alkalic rock-carbonatite complexes in generaland in the Oka, Quebec, complex in particular. The successionof assemblages at Oka and at other melilite rock-bearing complexesmay be interpreted as the products of fractionation of a carbonatednephelinite magma by analogy with the experimental results.The sodium-bearing vapor phase of the experiments may be analogousto the fenitizing agent of some carbonatite complexes.     

Mineralogical and Chemical Zonation of the Laacher See Tephra Sequence (East Eifel, W. Germany)

The late Quaternary Laacher See Tephra (LLST, MLST, ULST: Lower,Middle, Upper Laacher See Tephra) of the East Eifel volcanicfield (W. Germany) is zoned from highly evolved, volatile-richand crystal-poor phonolite at its base towards a mafic, crystal-richphonolite at the top of the deposit. This is shown by phenocrystabundances, major and trace element chemistry of whole rocks,matrix-glass and some mineral compositions. MgO content in wholerocks ranges from 0.07 wt. per cent in lower LLST to 0.85 wt.per cent in ULST phonolite. Late mafic hybrid pyroclasts containup to 7.0 wt. per cent MgO. Na₂O shows a reverse trend from11.7 wt. per cent in LLST to 5.1 per cent in ULST. Trace elementsare divided into depleted (compatible) elements (e.g. Sr, Sc,Co), and enriched (incompatible) elements (e.g. Zn, Zr, Nb).‘Semi-compatible’ elements (Ta, Y) show minimumconcentrations at an intermediate stratigraphic level (MLST).All compositional gradients are smooth showing a major compositionalinterface between LLST/MLST and ULST. Twelve phenocryst phasesoccur: sanidine, plagioclase, hauyne, amphibole, clinopyroxene,sphene, apatite, Ti-magnetite, biotite, nepheline, cancriniteand zircon. The latter three are restricted to LLST phonolite.There is a steep gradient in mineral abundances from the LLSTand MLST to the mafic ULST phonolite. Microprobe analyses definetwo phenocryst populations: (a) Fe, Na and Mn-rich evolved phenocrysts(hastingsite, aegirine-augite, Ab-rich plagioclase) which predominatewithin highly differentiated (LLST) phonolite; (b) Mg and Ti-richand Fe, Na, Mn-poor phenocrysts, which are most abundant inULST. This compositional zonation is interpreted as the resultof continuous eruption from a zoned magma column; highly evolvedearly magma representing the upper part and mafic late phonoliticmagma representing the lower part of a stratified magma chamberemplaced at shallow, crustal levels.     

Alkali Feldspars with Water at 5 kb Pressure

Alkali feldspars with water at 5 kb begin to melt at an isobariceutectic: 703±2 °C, Or ₂₈.₅ (wt. per cent) recalculatedanhydrous. The liquidus of albite-H₂O at 5 kb is 758±3°C. The K feldspar-rich, water-saturated liquidus is essentiallythat of Yoder, Stewart, & Smith (1957), who furthermorefound orthoclase-H₂O to melt at 876 °C. The alkali feldspar solvus, determined by synthesis from glassand a few reversals using crystalline material, has a calculatedcritical temperature of 730 °C and a critical compositionof Or₃₁ (wt. per cent) using parametric equations (Thompson& Waldbaum, 1969). The Margules equation yields T_c = 738°C. The feldspars produced are structural equivalents oflow sanidine on the basis of their 060 and 04 powder X-ray diffractionpeaks. The critical line intersects the beginning of meltingcurve for the system Ab-Or-H₂O at 4.2 kb, 715 °C, the minimumconditions for the coexistence of two feldspars and liquid inthis system. The slope of the critical line, using Orville's (1963) 2 kbresults, is 18.3 °C/kb, leading to an intersection withthe kyanite-sillimanite curve of Richardson, Bell, & Gilbert(1968) at about 10.6 kb, 834 °C. The intersection with theandalusite-sillimanite curve of these authors (1969) falls atabout 3.5 kb, 703 °C. Such intersections may be used toestimate limits of P and T for a variety of crustal rocks. Because isobaric crystallization of feldspars in the water-deficientregion must enrich liquids in H₂O to the saturation point, haplosyeniticliquids with a finite initial water content must, at 5 kb totalpressure, eventually crystallize two feldspars.     

The origin and evolution of the parental magmas of frontal volcanoes in Kamchatka: Evidence from magmatic inclusions in olivine from Zhupanovsky volcano

The paper presents data on naturally quenched melt inclusions in olivine (Fo 69–84) from Late Pleistocene pyroclastic rocks of Zhupanovsky volcano in the frontal zone of the Eastern Volcanic Belt of Kamchatka. The composition of the melt inclusions provides insight into the latest crystallization stages (∼70% crystallization) of the parental melt (∼46.4 wt % SiO₂, ∼2.5 wt % H₂O, ∼0.3 wt % S), which proceeded at decompression and started at a depth of approximately 10 km from the surface. The crystallization temperature was estimated at 1100 ± 20°C at an oxygen fugacity of ΔFMQ = 0.9–1.7. The melts evolved due to the simultaneous crystallization of olivine, plagioclase, pyroxene, chromite, and magnetite (Ol: Pl: Cpx: (Crt-Mt) ∼ 13: 54: 24: 4) along the tholeiite evolutionary trend and became progressively enriched in FeO, SiO₂, Na₂O, and K₂O and depleted in MgO, CaO, and Al₂O₃. Melt crystallization was associated with the segregation of fluid rich in S-bearing compounds and, to a lesser extent, in H₂O and Cl. The primary melt of Zhupanovsky volcano (whose composition was estimated from data on the most primitive melt inclusions) had a composition of low-Si (∼45 wt % SiO₂) picrobasalt (∼14 wt % MgO), as is typical of parental melts in Kamchatka and other island arcs, and was different from MORB. This primary melt could be derived by ∼8% melting of mantle peridotite of composition close to the MORB source, under pressures of 1.5 ± 0.2 GPa and temperatures 20–30°C lower than the solidus temperature of “dry” peridotite (1230–1240°C). Melting was induced by the interaction of the hot peridotite with a hydrous component that was brought to the mantle from the subducted slab and was also responsible for the enrichment of the Zhupanovsky magmas in LREE, LILE, B, Cl, Th, U, and Pb. The hydrous component in the magma source of Zhupanovsky volcano was produced by the partial slab melting under water-saturated conditions at temperatures of 760–810°C and pressures of ∼3.5 GPa. As the depth of the subducted slab beneath Kamchatkan volcanoes varies from 100 to 125 km, the composition of the hydrous component drastically changes from relatively low-temperature H₂O-rich fluid to higher temperature H₂O-bearing melt. The geothermal gradient at the surface of the slab within the depth range of 100–125 km beneath Kamchatka was estimated at 4°C/km.     

Magnetite‐out and pyrrhotite‐in temperatures in shales and slates

Clay‐rich basins have undergone varying degrees of magnetic transformation during burial, affecting their ability to retain accurate records of Earth's dynamic magnetic field. We propose to bracket the magnetite‐out and pyrrhotite‐in temperatures in shales and slates from Taiwan and the Pyrenees by using a combination of low‐temperature magnetic transitions and geothermometers. For T_burial < 340°C, the magnetic assemblage is dominated by magnetite. Gradually with increasing burial temperature, the concentration of magnetite decreases to a few ppmv. We observe the magnetite‐out isograd at T_burial ~350°C. At T_burial >60°C and T_burial >340°C respectively, fine‐grained and coarse‐grained pyrrhotite develop. In the course of burial, a clay‐rich basin gradually loses its capability to retain a record of Earth's magnetic field. It is only during basin uplift, that coarse pyrrhotite might acquire a thermo remanent magnetization. Our results point out therefore highly contrasted magnetic properties and palaeomagnetic records between deeply buried basins and exhumed ones.     

Mineralogy,fluid inclusions,and isotopes of the Cihai iron deposit,eastern Tianshan,NW China: Implication for hydrothermal evolution and genesis of subvolcanic rocks-hosted skarn-type deposits

Most skarn deposits are closely related to granitoids that intruded into carbonate rocks. The Cihai (>100 Mt at 45% Fe) is a deposit with mineral assemblages and hydrothermal features similar to many other typical skarn deposits of the world. However, the iron orebodies of Cihai are mainly hosted within the diabase and not in contact with carbonate rocks. In addition, some magnetite grains exhibit unusual relatively high TiO₂ content. These features are not consistent with the typical skarn iron deposit. Different hydrothermal and/or magmatic processes are being actively investigated for its origin. Because of a lack of systematic studies of geology, mineral compositions, fluid inclusions, and isotopes, the genetic type, ore genesis, and hydrothermal evolution of this deposit are still poorly understood and remain controversial.The skarn mineral assemblages are the alteration products of diabase. Three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observations, which show a prograde skarn stage (garnet-clinopyroxene-disseminated magnetite), a retrograde skarn stage (main iron ore stage, massive magnetite-amphibole-epidote ± ilvaite), and a quartz-sulfide stage (quartz-calcite-pyrite-pyrrhotite-cobaltite).Overall, the compositions of garnet, clinpyroxene, and amphibole are consistent with those of typical skarn Fe deposits worldwide. In the disseminated ores, some magnetite grains exhibit relatively high TiO₂ content (>1 wt.%), which may be inherited from the diabase protoliths. Some distinct chemical zoning in magnetite grains were observed in this study, wherein cores are enriched in Ti, and magnetite rims show a pronounced depletion in Ti. The textural and compositional data of magnetite confirm that the Cihai Fe deposit is of hydrothermal origin, rather than associated with iron rich melts as previously suggested.Fluid inclusions study reveal that, the prograde skarn (garnet and pyroxene) formed from high temperature (520–600 °C), moderate- to high-salinity (8.1–23.1 wt.% NaCl equiv, and >46 wt.% NaCl equiv) fluids. Massive iron ore and retrograde skarn assemblages (amphibole-epidote ± ilvaite) formed under hydrostatic condition after the fracturing of early skarn. Fluids in this stage had lower temperature (220°–456 °C) and salinity (8.4–16.3 wt.% NaCl equiv). Fluid inclusions in quartz-sulfide stage quartz and calcite also record similar conditions, with temperature range from 128° to 367 °C and salinity range from 0.2 to 22.9 wt.% NaCl equiv. Oxygen and hydrogen isotopic data of garnet and quartz suggest that mixing and dilution of early magmatic fluids with external fluids (e.g., meteoric waters) caused a decrease in fluid temperature and salinity in the later stages of the skarn formation and massive iron precipitation. The δ¹⁸O values of magnetite from iron ores vary between 4.1 and 8.5‰, which are similar to values reported in other skarn Fe deposits. Such values are distinct from those of other iron ore deposits such as Kiruna-type and magmatic Fe-Ti-V deposits worldwide. Taken together, these geologic, geochemical, and isotopic data confirm that Cihai is a diabase-hosted skarn deposit related to the granitoids at depth.     

Petrologic and experimental evidence for the movement and heating of the pre-eruptive Minoan rhyodacite (Santorini, Greece)

Hydrothermal experiments combined with petrologic observations form the basis for a new two-stage model for the evolution of the pre-eruption Minoan magma chamber at Santorini, Greece. Ninety-nine percent of the erupted volume is two-pyroxene, rhyodacitic magma that had been stored at a temperature of ∼885 °C, based on magnetite-ilmenite and QUILF geothermometry. The rest of the volume is basaltic to andesitic magma, which occurs as <10 cm scoria clasts and as small inclusions in rhyodacite pumices. Petrologic observations show that these magmas mixed at different scales and at different times (i.e., multiple batches of mafic magma). Hydrothermal experiments were carried out on samples of rhyodacite and a mafic scoria in order to determine magma storage conditions and the mixing history of the two magmas. At 885 °C, the rhyodacite must have been stored at water-saturated pressures of ∼50 MPa, based on its phase assemblage, matrix-glass composition, and crystal content. However, glass inclusions inside rhyodacitic plagioclase phenocrysts contain more than 6 wt% H₂O, indicating they formed at pressures >200 MPa. In addition, the composition of the plagioclase hosts (An_{56 ± 6}) of the inclusions require temperatures of 825 ± 25 °C at pressures >200 MPa. This demonstrates that the Minoan rhyodacitic magma underwent a two-stage evolution, first crystallizing at ∼825^∘C and >200 MPa, and then rinsing to a shallow ∼50 MPa storage region with a concomitant rise in temperature to ∼885 °C. We suggest that the episodic intrusion of mafic magmas provided the necessary heat and perhaps contributed to the ascent of the magma to shallow crustal depths where it reequilibrated before the cataclysmic eruption. Phase equilibria suggest that much of the heating of the rhyodacite occurred in the shallow storage region. Thermal budget calculations suggest that the rhyodacite magma could have been heated by intrusions of basalt rising at reasonable upwelling rates and injected into the storage zone over several hundred years. Preservation of amphibole in the mafic scoria indicate that injection of mafic magma continued up until days before the cataclysmic eruption, perhaps triggering the event. Received: 30 September 1997 / Accepted: 5 October 1998     

Spinels,Fe–Ti oxide minerals,apatites, and carbonates hosted in the ophiolites of Eastern Desert of Egypt: mineralogy and chemical aspects

Spinels, Fe–Ti oxide minerals, apatites, and carbonates hosted in ophiolitic serpentinites and metagabbros of Gabal Garf (southern ED) and Wadi Hammariya (central ED) of Egypt are discussed. Microscopic and electron probe studies on these minerals are made to evaluate their textural and compositional variations. Alteration of chromites led to form ferritchromite and magnetite; rutile–magnetite intergrowths and martite are common in serpentinites. Fine trillis exsolution of ilmenite–magnetite and ilmenite–hematite and intergrowth of rutile–magnetite and ilmenite–sphene are recorded. Composite intergrowth grains of titanomagnetite–ilmenite trellis lamellae are common in metagabbros. The formation of ilmenite trellis and lamellae in magnetite and titanomagnetite indicate an oxidation process due to excess of oxygen contained in titanomagnetite; trapped and external oxidizing agents. This indicates the high P _H2O and oxygen fugacity of the parental magma. The sulfides minerals include pyrrhotite, pyrite and chalcopyrite. Based on the chemical characteristics, the Fe–Ti oxide from the ophiolitic metagabbros in both areas corresponds to ilmenite. The patites from the metagabbros are identified as fluor-apatite. Carbonates are represented by dolomites in serpentinites and calcite in metagabbros. Spinel crystals in serpentinites are homogenous or zoned with unaltered cores of Al-spinel to ferritchromit and Cr-magnetite toward the altered rims. Compared to cores, the metamorphic rims are enriched in Cr# (0.87–1.00 vs. 0.83–0.86 for rims and cores, respectively) and impoverished in Mg# (0.26–0.48 vs. 0.56–0.67) due to Mg–Fe and Al (Cr)–Fe³⁺ exchange with the surrounding silicates during regional metamorphism rather than serpentinization process. The Fe–Ti oxides have been formed under temperature of ~800 °C for ilmenite. Al-spinels equilibrated below 500–550 °C, while the altered spinel rims correspond to metamorphism around 500–600 °C. Geochemical evidence of the podiform Al-spinels suggest a greenschist up to lower amphibolite facies metamorphism (at 500–600 °C), which is isofacial with the host rocks. Al-spinel cores do not appear to have re-equilibrated completely with the metamorphic spinel rims and surrounding silicates, suggesting relic magmatic composition unaffected by metamorphism. The composition of Al-spinel grains suggest an ophiolitic origin and derivation by crystallization of boninitic magma that belonging to a supra-subduction setting could form either in forearcs during an incipient stage of subduction initiation or in back-arc basins.     

Towards pyrrhotite/magnetite geothermometry in low-grade metamorphic carbonates of the Tethyan Himalayas (Shiar Khola,Central Nepal)

In Mesozoic metacarbonates of the Tethyan Himalayas (Shiar Khola area, Central Nepal) two characteristic remanent magnetisations (ChRM₁ and ChRM₂) were identified by their unblocking temperature spectra. The ChRM₁ is carried by pyrrhotite (unblocking temperature: 270–360°C) and the ChRM₂ by magnetite (unblocking temperature spectra: 430–580°C). The temperature-related formation of pyrrhotite at the expense of primary magnetite during low-grade metamorphism in marly carbonates allows the determination of thermal gradients by the pyrrhotite/magnetite ratio. This new method can be used as a geothermometer for T≤300°C in low-grade metamorphic carbonates, where other methods are not available. This method is applied for the first time in the Tethyan Himalayas of Central Nepal.In the Shiar Khola valley, systematic variations in the ferrimagnetic content of the metacarbonates along an E–W profile were detected by the ratio of remanence intensity of pyrrhotite to magnetite, derived from natural remanent magnetisation (R_PYR/MAG) and saturation magnetisation (S_PYR/MAG). Over a stretch of 10 km the R_PYR/MAG and S_PYR/MAG increase from W to E from ~0.42 to ~0.91 and ~0.48 to ~1.0, respectively. Based on temperature estimates, the eastern part experienced upper anchizone–epizone (~250–300°C) conditions, while the western part underwent only diagenesis (~200°C). The temperature gradient and the temperature ranges suggested are consistent with the findings of the calcite twin lamellae geothermometry which is a non-magnetic method.     

A Study of the Strathcona Mine and Its Bearing on the Origin of the Nickel-Copper Ores of the Sudbury District, Ontario

The Strathcona iron-nickel-copper sulfide ore deposit lies atthe base of the Sudbury Nickel Irruptive along the north rimof the Sudbury basin. In the vicinity of the deposit the mainbody of the Nickel Irruptive consists of an upper unit of 3700ft (1200 m) of granophyre (the ‘micropegmatite’)and a lower unit of 1500 ft (500 m) of augite norite (the ‘felsicnorite’) separated by 300 ft (100 m) of transitional rock(the ‘transition zone’). Two augite norite intrusions(the ‘mafic norite’ and the ‘xenolithic norite’)that are younger than the felsic norite occur along its lowercontact. The xenolithic norite is relatively rich in xenolithsand grades downwards into a unit known as the ‘hanging-wallbreccia’. The breccia resembles the xenolithic noritebut contains a higher proportion of xenoliths. A quartz-plagioclase-augite gneiss (the ‘footwall gneiss’)underlies all units of the Nickel Irruptive. A cataclastic breccia(the ‘footwall breccia’) which formed as a resultof comminution of both gneiss and overlying Irruptive rocksis present in most areas between the gneiss and the Nickel Irruptive.The ore body occurs partly as a dissemination of sulfides inthe matrix of the hanging-wall breccia (‘hanging-wallore’), partly as a fine dissemination and massive stringersof sulfide in the footwall breccia matrix (‘main-zoneore’), and partly as massive stringers of sulfide in thefootwall gneiss (‘deep-zone ore’). Xenoliths in the xenolithic norite and hanging-wall brecciarange from dunite to olivine gabbro. Olivine in the xenoliths(composition estimated by an X-ray method) varies from Fo73to Fo85, and hypersthene and augite (composition estimated byelectron microprobe analysis) vary from Fs25 to Fsi3, and Fsi3to Fs5, respectively. The iron content of the mafic mineralsshows a positive correlation with the proportion of felsic mineralsin the xenoliths, suggesting that the xenoliths have been derivedfrom a cryptically layered body of mafic and ultramafic rock.The wide distribution of xenoliths around the margin of theNickel Irruptive coupled with the absence of any obvious externalsource is strong evidence that the xenoliths are cognate, supportingWilson's (1956) proposal that the Irruptive is a funnel-shapedintrusion with a zone of ultramafic rocks towards its base. Hypersthene ranges from Fs33 to Fs28 in the felsic norite, fromFs28 to Fs22 in the mafic norite, and from Fs28 to Fs20 in thexenolithic norite. Augite ranges from Fsl6 to Fs14 in the felsicnorite and from Fs14 to Fsn in both the mafic and xenolithicnorites. The distribution coefficient for iron and magnesiumbetween coexisting augite and hypersthene ranges from 1-0 insome of the xenoliths to 1-5 in some samples of felsic norite,indicating that the two pyroxenes equilibrated at, or near,magmatic temperature. The composition of plagioclase in thefelsic norite, mafic norite, and xenolithic norite is aroundAn65-70 but decreases to An44 in those Irruptive rocks closestto the footwall breccia. The composition of plagioclase withinthe breccia varies between An32 and An43. Sodium metasomatismappears to have affected the breccia and to have spread outto affect adjacent rocks. The concentration of nickel and copper in the sulfides variessystematically across the ore deposit. The nickel content ofiron-nickel sulfides varies between 2-5 and 3 per cent in thehanging-wall ore but increases regularly from 3 per cent to5 or 5-5 per cent from hanging wall to footwall across the main-zoneore. Copper concentration shows a similar but more erratic variation.The variation is attributed to thermal diffusion of nickel andcopper within the main-zone ore along a gradient induced bythe overlying, hot, Nickel Irruptive. The principal opaque minerals in the ore body are, in the orderof their abundance, pyrrho-tite of at least two types, magnetite,pentlandite, chalcopyrite, and pyrite. All of the sulfides inthe hanging-wall ore are the result of exsolution from a high-temperature,pyrrhotite solid solution. Pyrite started to exsolve below 700C, chalcopyrite below 450 C, and pentlandite below 300 C.Monoclinic pyrrhotite formed from the host hexagonal pyrrhotiteprobably between 300 and 250 C. The temperature of formationof the sulfides in the main-zone ore has been obscured by reworkingof the ore after its first emplacement. The principal ore sulfides, pyrrhotite and pentlandite, arecommon throughout the mafic norite, xenolithic norite, and hanging-wallbreccia, occurring in amounts around 5 per cent in most samples.Pyrrhotite and pentlandite are extremely rare in the overlyingfelsic norite where pyrite is the most common sulfide. It occursin amounts between 01 and 0-5 per cent, commonly together withsecondary amphibole after pyroxene. The sulfides in the maficand xenolithic norites and in the hanging-wall breccia occupyspaces interstitial to the silicates, and little or no replacementof silicates by sulfides has occurred. In the main-zone ore,evidence of small-scale replacement of silicates by sulfidesis common. The high percentage of pyrrhotite and pentlandite in the maficand xenolithic norites in contrast to the felsic norite, texturalrelations between sulfides and silicates, and the high temperaturesindicated by the pyroxene distribution coefficients lead tothe conclusion that the hanging-wall sulfides (including thehanging-wall ore) at Strathcona were introduced with these youngernoritic intrusions. Data on the solubility of sulfides in silicatemagmas rule out the possibility that the bulk of the sulfideswere in solution in the noritic magmas; the data support thehypothesis that during intrusion the sulfides were held in suspensionin the in the magmas as droplets of immiscible sulfide-oxideliquid. Calculations on the rate of settling to be expectedfor such sulfide droplets are consistent with this hypothesis.The manner of emplacement of the main-zone ore is less certain;our explanation is that this ore consists of sulfides that originallysettled out of, or collected along, the base of the hanging-wallbreccia zone and were subsequently incorporated in the brecciationthat gave rise to the footwall breccia. The origin of the sulfides at Strathcona is clearly connectedclosely with the origin of the younger noritic intrusions. Asimilar connexion exists between sulfides and young marginalintrusions at many other Sudbury deposits. Jt is possible thatboth sulfides and intrusions are portions of the Nickel Irruptivemagma that lagged behind the main body of magma and were intrudedat a later stage. Alternatively, the young intrusions may haveassimilated sulfides from a sulfide-rich zone within or at themargin of the deeper layers of the Irruptive.     

Direct evolution of W-rich brines from crystallizing melt within the Mariktikan granite pluton, west Transbaikalia

Uneconomic tungsten mineralization associated with the Mariktikan granite pluton of the Transbaikalian igneous province, eastern Siberia, is confined to a marginal part of the pluton referred to as the Andreyevsky body. This is composed of rocks similar to those of the main pluton, but is likely to be an autonomous, although kindred, body. On the basis of cross-cutting relationships between rock varieties, and their textural features, the crystallization history is subdivided into three stages of unequal duration. Melt and fluid inclusions related to each of them have been studied using microthermometric and micro-analytical procedures, including a technique for atomic emission spectroscopy of individual fluid inclusions opened by a laser microprobe. During the main crystallization stage (1045–1012 °C) more than 70% of the parental magma is believed to have crystallized (at the level studied), resulting in the formation of a crystalline framework rigid enough for the appearance of fractures within which some portion of the intercrystalline melt accumulated, giving rise to thin aplite veins. Almost complete crystallization of intercrystalline and fracture-hosted melt occurred during the late stage (1012–990 °C), whereas during the final stage only small bunches of schlieric, sometimes miarolitic pegmatite were formed from a few pockets of residual melt (990–917 °C). In spite of the low water content of the melt (about 1 wt.%), fluid separation took place from the onset of crystallization. During the main crystallization stage, the exsolved fluid divided into two immiscible phases, CO₂-rich gas and salt-rich liquid (brine). However, it was homogeneous thereafter. Major ore components of the brine (Mn, Fe, W) displayed different behaviour as crystallization progressed. W concentration was below 0.1 wt.% at the main stage, attained 1 wt.% during the late stage and increased to 1.8 wt.% at the final stage. Comparison of these data with calculations carried out using estimated parameters for the parental magma enables us to infer that obtained concentration values are reasonable for the special case studied here. However, they are unlikely to be attainable if the magma is H₂O enriched and lacks an anomalously high W content. In the Andreyevsky body of the Mariktikan pluton, the following features of the parental magma made possible the generation of W-rich solutions: (1) high liquidus temperature (1045 °C) and elevated Cl content (c.0.15 wt.%) that resulted in enlargement of the fluid/melt distribution coefficient (c.10), (2) low water content (c.1%) and elevated W content (c.0.001 wt.%) that provided a relatively high W/H₂O ratio within the system. At the same time, because of the low water content of the magma, the total mass of W-bearing solutions has proved to be insufficient for the production of large-scale mineralization (a reduced W content of the great bulk of the exsolved brine may have had an unfavourable effect as well). Received: 2 May 1996 / Accepted: 29 January 1997     

Petrology of the Younger Andesites and Dacites of Iztacc?huatl Volcano, Mexico: II. Chemical Stratigraphy, Magma Mixing, and the Composition of Basaltic Magma Influx

The Younger Andesites and Dacites of Iztacc?huatl volcano, Mexico,constitute a medium-K calcalkaline rock suite (58–66 wt.per cent SiO₂) characterized by high Mg-numbers (100Mg/(Mg+0?85Fe²⁺=55–66) and relatively high abundances of MgO (2?5–6?6wt. per cent), Ni(17–158 p.p.m.), and Cr (42–224p.p.m.). Chemical stratigraphy plots of eruptive sequences indicatethe existence of a plexus of long-lived dacite magma chambersperiodically replenished by influxes of basaltic magma ascendingfrom depth. Short-term geochemical evolution after batch influxwas dictated by magma mixing and eventual dilution of the basalticcomponent by ‘quasi-steady state’ hornblende dacitemagma. The chemical data support textural and mineralogicalevidence for rapid homogenization of originally diverse magmasby convective blending of residual liquids accompanied by dynamicfractional crystallization (Nixon, 1988). Internally-consistent mixing calculations used to derive thecomposition of basaltic magma influx incorporate analyticaluncertainties and the observed range of salic end-member compositions.Mafic end-members are basalts to basaltic andesites (52–54wt. per cent SiO₂) with Mg-numbers (73–76), MgO (9–11wt. per cent), Ni (250 p.p.m.), and Cr (340–510 p.p.m.)concentrations, and liquidus olivine compositions (Fo_90–88),appropriate for unfractionated partial melts of mantle peridotite.The majority of model compositions are Ol-Hy-normative, similarto those of primitive basaltic lavas on the flanks of Iztacc?huatland in the Valley of Mexico. However, calculated magma batchesrange from weakly Qz-normative to strongly Ne-normative. Bothcalculated and analyzed basaltic compositions are distinguishedby highly variable abundances of alkalies and incompatible traceelements, notably Rb, Ba, Sr, P, Zr, and Y. Initial ⁸⁷Sr/⁸⁶Sr ratios for Iztacc?huatl lavas (0?7040–0?7046;n=24) are comparable to those for primitive basaltic rocks (0?7037–0?7045;?=4) and indicate that (1) mantle source regions are isotopicallyheterogeneous; and (2) contamination of iztacc?huatl magma chambersby radiogenic crustal rocks was not a significant factor inthe evolution of calc-alkaline andesites and dacites. The replenishment of Iztacc?huatl dacite reservoirs by Ne-normativemagmas late in the history of cone growth precludes exhaustionof mantle source regions by progressive partial melting. Thewaning stages of volcanic activity at Iztacc?huatl appear toreflect the inability of dense basaltic influxes to successfullypenetrate a large high-level chamber of low density hornblendedacite magma.     

Oxide and sulphide minerals in highly oxidized, Rb-depleted, Archaean granulites of the Shevaroy Hills Massif, South India: Oxidation states and the role of metamorphic fluids

The Shevaroy Hills of northern Tamil Nadu, southern India, expose the highest-grade granulites of a prograde amphibolite facies to granulite facies deep-crustal section of Late Archaean age. These highly oxidized quartzofeldspathic garnet charnockites generally show minor high-TiO₂ biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements Rb and Th. Peak metamorphic temperatures (garnet–orthopyroxene) and pressures (garnet–orthopyroxene–plagioclase–quartz) are near 750 °C and 8 kbar, respectively. Pervasive veinlets of K-feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene. They are suggested to have been produced by a low H₂O activity, migrating fluid phase under granulite facies conditions, most likely a concentrated chloride/carbonate brine with high alkali mobility accompanied by an immiscible CO₂-rich fluid. Silicate, oxide and sulphide mineral assemblages record high oxygen fugacity. Pyroxenes in the felsic rocks have high Mg/(Mg+Fe) (0.5–0.7). The major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite. Utilizing three independent oxygen barometers (ferrosilite–magnetite–quartz, ferrosilite–hematite–quartz and magnetite–hematite) in conjunction with garnet–orthopyroxene exchange temperatures, samples with XIlmHm>0.1 yield a consistent oxygen fugacity about two log units above fayalite stability. Less oxidized samples (XIlmHm<0.1) show some scatter with indications of having equilibrated under more reducing conditions. Temperature-f (O₂ ) arrays result in self consistent conditions ranging from 660 °C and 10^?16 bar to 820 °C and 10^?11.5 bar. These trends are confirmed by calculations based on the assemblage clinopyroxene–orthopyroxene–magnetite–ilmenite using the QUIlF program. In the most oxidized granulite samples (XIlmHm>0.4) pyrite is the dominant sulphide and pyrrhotite is absent. Pyrite grains in these samples have marginal alteration to magnetite along the rims, signifying a high-temperature oxidation event. Moderately oxidized samples (0.1no coexisting magnetite. Chalcopyrite is a common accessory mineral of pyrite and pyrrhotite in all the samples. Textures in some samples suggest that it formed as an exsolution product from pyrrhotite. Extensive vein networks of magnetite and pyrite, associated principally with the pyroxene and amphibole, give evidence for a pervasive, highly oxidizing fluid phase. Thermodynamic analysis of the assemblage pyrrhotite, pyrite and magnetite yields consistent high oxidation states at 700–800 °C and 8 kbar. The oxygen fugacity in our most oxidized pyrrhotite-bearing sample is 10^?12.65 bar at 770 °C. There are strong indications that the Shevaroy Hills granulites recrystallized in the presence of an alkali-rich, low H₂O-activity fluid, probably a concentrated brine. It cannot be demonstrated at present whether the high oxidation states were set by initially oxidized protoliths or effected by the postulated fluids. The high correspondence of maximally Rb-depleted samples with the highest recorded oxidation states suggests that the Rb depletion event coincided with the oxidation event, probably during breakdown of biotite to orthopyroxene+K-feldspar. We speculate that these alterations were effected by exhalations from deep-seated alkali basalts, which provided both heat and high oxygen fugacity, low a_H2O fluids. It will be of interest to determine whether greatly Rb-depleted granulites in other Precambrian terranes show similar highly-oxidizing signatures.