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     

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

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

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

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

Liquidus Equilibria in the System K2O-Na2O-Al2O3-SiO2-F2O-1-H2O to 100 MPa: II. Differentiation Paths of Fluorosilicic Magmas in Hydrous Systems

We investigated phase equilibria in the six-component systemNa₂O–K₂O–Al₂O₃–SiO₂–F₂O_–1–H₂Oat 100 MPa to characterize differentiation paths of naturalfluorine-bearing granitic and rhyolitic magmas. Topaz and cryoliteare stable saturating solid phases in calcium-poor systems.At 100 MPa the maximum solidus depression and fluorine solubilityin evolving silicic melts are controlled by the eutectics haplogranite–cryolite–H₂Oat 640°C and 4 wt % F, and haplogranite–topaz–H₂Oat 640°C and 2 wt % F. Topaz and cryolite form a binaryperalkaline eutectic at 660°C, 100 MPa and fluid saturation.The low-temperature nature of this invariant point causes displacementof multiphase eutectics with quartz and alkali feldspar towardsthe topaz–cryolite join and enables the silicate liquidusand cotectic surfaces to extend to very high fluorine concentrations(more than 30 wt % F) for weakly peraluminous and subaluminouscompositions. The differentiation of fluorine-bearing magmasfollows two distinct paths of fluorine behavior, depending onwhether additional minerals buffer the alkali/alumina ratioin the melt. In systems with micas or aluminosilicates thatbuffer the activity of alumina, magmatic crystallization willreach either topaz or cryolite saturation and the system solidifiesat low fluorine concentration. In leucogranitic suites precipitatingquartz and feldspar only, the liquid line of descent will reachtopaz or cryolite but fluorine will continue to increase untilthe quaternary eutectic with two fluorine-bearing solid phasesis reached at 540°C, 100 MPa and aqueous-fluid saturation.The maximum water solubility in the haplogranitic melts increaseswith the fluorine content and reaches 12· 5 ±0· 5 wt % H₂O at the quartz–cryolite–topazeutectic composition. A continuous transition between hydrousfluorosilicate melts and solute-rich aqueous fluids is not documentedby this study. Our experimental results are applicable to leucocraticfluorosilicic magmas. In multicomponent systems, however, thepresence of calcium may severely limit enrichment of fluorineby crystallization of fluorite. KEY WORDS: granite; rhyolite; topaz; cryolite; magmatic differentiation     

A Melt Inclusion Record of Volatiles, Trace Elements and Li-B Isotope Variations in a Single Magma System from the Plat Pays Volcanic Complex, Dominica, Lesser Antilles

Glass inclusions in plagioclase and orthopyroxene from daciticpumice of the Cabrits Dome, Plat Pays Volcanic Complex in southernDominica reveal a complexity of element behavior and Li–Bisotope variations in a single volcanic center that would gounnoticed in a whole-rock study. Inclusions and matrix glassesare high-silica rhyolite with compositions consistent with about50% fractional crystallization of the observed phenocrysts.Estimated crystallization conditions are 760–880°C,200 MPa and oxygen fugacity of FMQ + 1 to +2 log units (whereFMQ is the fayalite–magnetite–quartz buffer). Manyinclusion glasses are volatile-rich (up to 6 wt % H₂O and 2900ppm Cl), but contents range down to 1 wt % H₂O and 2000 ppmCl as a result of shallow-level degassing. Sulfur contents arelow throughout, with <350 ppm S. The trace element compositionof inclusion glasses shows enrichment in light rare earth elements(LREE; (La/Sm)_n = 2·5–6·6) and elevatedBa, Th and K contents compared with whole rocks and similaror lower Nb and heavy REE (HREE; (Gd/Yb)_n = 0·5–1·0).Lithium and boron concentrations and isotope ratios in meltinclusions are highly variable (20–60 ppm Li with ⁷Li= +4 to +15 ± 2; 60–100 ppm B with ¹¹B = +6 to+13 ± 2) and imply trapping of isotopically heterogeneous,hybrid melts. Multiple sources and processes are required toexplain these features. The mid-ocean ridge basalt (MORB)-likeHREE, Nb and Y signature reflects the parental magma(s) derivedfrom the mantle wedge. Positive Ba/Nb, B/Nb and Th/Nb correlationsin inclusion glasses indicate coupled enrichment in stronglyfluid-mobile (Ba, B) and less-mobile (Th, Nb) trace elements,which can be explained by fractional crystallization of plagioclase,orthopyroxene and Fe–Ti oxides. The ⁷Li and ¹¹B valuesare at the high end of known ranges for other island arc magmas.We attribute the high values to a ¹¹B and ⁷Li-enriched slabcomponent derived from sea-floor-altered oceanic crust and possiblyfurther enriched in heavy isotopes by dehydration fractionation.The heterogeneity of isotope ratios in the evolved, trappedmelts is attributed to shallow-level assimilation of older volcanicrocks of the Plat Pays Volcanic Complex. KEY WORDS: subduction; volcanic arcs; igneous processes; melt inclusions; SIMS; trace elements; lithium and boron isotopes; diffusion     

An Experimental Study of the Sulfur Content in Basaltic Melts Saturated with Immiscible Sulfide or Sulfate Liquids at 1300{degrees}C and 1{middle dot}0 GPa

The sulfur content in basaltic melts coexisting with eithersulfide or sulfate melts was determined experimentally. Theexperimental conditions were in the range of 1300–1355°Cand 1·0–1·6 GPa, conditions appropriatefor the melting of the upper mantle above subduction zones.Under these conditions, both sulfide and sulfate were presentas immiscible liquids, as inferred from the round geometriesof the quenched sulfide and sulfate phases. The measured S contentin basaltic melts saturated with sulfate liquids ([S] = 1·5± 0·2 wt %) was 10 times higher than the S contentin basaltic melts saturated with sulfide liquids ([S] = 0·14± 0·02 wt %). In our experiments, sulfate liquidswere stable at fO₂ as low as FMQ = +1·85 [FMQ = log (fO₂)_sample– log (fO₂)_FMQ, where FMQ is the fayalite–magnetite–quartzoxygen buffer], and evidence from other sources indicates thatsulfates will be stable at lower fO₂ in melts with lower activitiesof silica. Because chalcophile and highly siderophile elements,such as Cu, Ni, Au, and Pd, are partitioned preferentially intosulfide phases, melting of sufficiently oxidized sources, inwhich sulfides are not stable, would favor incorporation ofthese elements into the silicate melt produced. Such melts wouldhave a higher potential to generate ore deposits. This studyshows that the high sulfur contents of such oxidized basaltsalso means that relatively small amounts of such magmas canprovide significant amounts of sulfur to exsolving volatilephases and account for the bulk of the sulfur expelled in somevolcanic eruptions, such the 1991 eruption of Mount Pinatubo. KEY WORDS: basalt; mantle; oxidation state; sulfate; sulfur     

Petrogenesis and 40Ar/39Ar Geochronology of the Brandberg Complex, Namibia: Evidence for a Major Mantle Contribution in Metaluminous and Peralkaline Granites

Anorogenic granites of the Brandberg igneous complex in NW Namibiaformed during early Cretaceous rifting and continental break-upof Gondwana. A metaluminous series [SiO₂ = 62–77 wt %,molar (Na + K)/Al = 0·8–0·95] includes anearly monzonite body, major biotite–hornblende granite,late biotite granite segregations and peripheral dykes of trachydacite.Volumetrically minor peralkaline granites of the Amis complex[SiO₂ = 72–77 wt %, (Na + K)/Al = 1·0–1·5]intrude the main granite and adjacent country rocks. Comparedwith the metaluminous main granite, these are in part highlyenriched in Zr, Nb, Y, U and Th. Initial Nd and Sr isotope ratiosof the metaluminous suite are     

Geochronology and Petrogenesis of the Cretaceous Antampombato-Ambatovy Complex and Associated Dyke Swarm, Madagascar

The Antampombato–Ambatovy complex is the largest intrusionin the central–eastern part of the Cretaceous flood basaltprovince of Madagascar, with an exposed surface area of about80 km². It has an ⁴⁰Ar/³⁹Ar incremental heating age of 89·9± 0·4 Ma and a U–Pb age of 90 ± 2Ma. The outcropping plutonic rocks range from dunite and wehrlite,through clinopyroxenite and gabbro, to sodic syenite. A dykeswarm cross-cutting some of the above lithologies (and the nearbyPrecambrian basement rocks) is formed of picritic basalts, alkalito transitional basalts, benmoreites and rhyolites; some ofthe latter are peralkaline. A few basaltic dykes have cumulateolivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereasothers have characteristics more akin to those of primitiveliquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200ppm Ni). These basalts have relatively high TiO₂ (2·2wt %) and total iron (14 wt % as Fe₂O₃), and moderate contentsof Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basaltdykes are 0·7030–0·7037 and 0·51290–0·51283,respectively. Syenites and peralkaline rhyolites have Sr- andNd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274,respectively. The data suggest derivation of the parental magmasfrom a time-integrated depleted mantle source, combined withsmall amounts of crustal contamination in the petrogenesis ofthe more evolved magmas. The isotopic compositions of the mafic–ultramaficrocks are most similar to those of the mid-ocean ridge basalt(MORB)-like igneous rocks of eastern Madagascar, and suggestthe existence of an isotopically ‘depleted’ componentin the source of the entire Madagascar province, even thoughthe Antampombato basalts are chemically unlike the lavas anddykes with the same depleted isotopic signature found in westernMadagascar. If this depleted component is plume-related, thissuggests that the plume has a broadly MORB-source mantle composition.The existence of isotopically more enriched magma types in theMadagascan province has several possible petrogenetic explanations,one of which could be the interaction of plume-related meltswith the deep lithospheric mantle beneath the island. KEY WORDS: geochronology; flood basalts; Antampombato–Ambatovy intrusion; Cretaceous; Madagascar     

The 26{middle dot}5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km³ of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO₂ and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO₂. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and ⁸⁷Sr/⁸⁶Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO₂)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO₂) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO₂ content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics     

Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites

La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km³. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (76–77 wt % SiO₂) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (66–70 wt % SiO₂), dense (40–60% vesicularity)and crystal rich (30–40 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770–790°C with melt water contents of 3·1–4·4wt %. The pre-eruptive Toconao magma was cooler (730–750°C)and its melt more water rich (6·3–6·8 wt% H₂O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (⁸⁷Sr/⁸⁶Sr_i 0·7094–0·7131;¹⁴³Nd/¹⁴⁴Nd 0·51222–0·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H₂O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber     

Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism

Calc-alkaline batholiths of the Archaean Minto block, northeasternSuperior Province, Canada, have pyroxene- and hornblende-bearingmineral assemblages inferred to have crystallized from hot,water-undersaturated magmas at 2·729–2·724Ga. A regional amphibolite- to granulite-facies tectonothermalevent at 2·70 Ga resulted in mild to negligible metamorphiceffects on the dominantly granodioritic units. Geochemical,textural and thermobarometric studies define the crystallizationhistory in compositions ranging from cumulate pyroxenite throughquartz diorite, granodiorite, granite, and syn-magmatic gabbroicdykes. Early magmatic assemblages include orthopyroxene, clinopyroxene,plagioclase, biotite, Fe–Ti oxides and ternary feldspar,indicating crystallization from magmas containing <2 wt %H₂O at 1100–900°C. Water enrichment in the residualmelt induced hornblende crystallization at 5 ± 1 kbar,800–600°C. Characterized by a continuum of large ionlithophile element (LILE)-enriched, high field strength element(HFSE)-depleted compositions, the I-type suite resembles moderncontinental arc batholiths in composition and size but not primarymineralogy. Magmatic arcs produced between 2·75 and 1·85Ga commonly have charnockitic components, possibly because slab-derivedfluids interacted with mantle wedges at ambient temperatureshigher by 100°C than at present, producing large volumesof water-deficient magma. KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated magma; charnockite     

Origin of the Charnockites of the Louis Lake Batholith, Wind River Range, Wyoming

The 2·63 Ga Louis Lake batholith, a calc-alkalic plutonexposed in Wind River Range of western Wyoming, consists ofminor diorite, quartz diorite, granodiorite, and granite. Atshallow structural levels the batholith is pyroxene free, butat deeper levels, all units of the batholith contain pyroxenes.On its northern margin the batholith was emplaced at P = 5–6kbar, T = 775–800°C, fO₂ at FMQ (fayalite–magnetite–quartz)+ 1·5 to FMQ + 1·8, and aH₂O     

Volatiles in Submarine Basaltic Glasses from the Northern Kerguelen Plateau (ODP Site 1140): Implications for Source Region Compositions, Magmatic Processes, and Plateau Subsidence

Submarine pillow basalts (34 Ma) recovered from the NorthernKerguelen Plateau at ODP Site 1140 contain abundant unalteredglass, providing the first opportunity to measure the volatilecontents of tholeiitic basaltic magmas related to the Kerguelenmantle plume. The glasses have La/Sm and Nb/Zr ratios that varyfrom values similar to Southeast Indian Ridge (SEIR) MORB (Unit1), to slightly more enriched (Unit 6), to values transitionalbetween SEIR MORB and basaltic magmas formed by melting of theKerguelen plume (Units 2 and 3). Volatile contents for glassesin Units 1 and 6 are similar to depleted mid-ocean ridge basalt(MORB) values (0·25–0·27 wt % H₂O, 1240–1450ppm S, 42–54 ppm Cl). In contrast, H₂O contents are higherfor the enriched glasses (Unit 2, 0·44 wt % H₂O; Unit3, 0·69 wt %), as are S (1500 ppm) and Cl (146–206ppm). Cl/K ratios for all glasses are relatively low (0·03–0·04),indicating that assimilation of hydrothermally altered materialdid not occur during shallow-level crystallization. H₂O/Ce forthe enriched glasses (Units 2 and 3) is significantly lowerthan Pacific and South Atlantic MORB values, suggesting thatlow H₂O/Ce may be an inherent characteristic of the Kerguelenplume source. Vapor saturation pressures calculated using theH₂O and CO₂ contents of the glasses indicate that     

Experimental Crystallization of a High-K Arc Basalt: the Golden Pumice, Stromboli Volcano (Italy)

The near-liquidus crystallization of a high-K basalt (PST-9golden pumice, 49·4 wt % SiO₂, 1·85 wt % K₂O,7·96 wt % MgO) from the present-day activity of Stromboli(Aeolian Islands, Italy) has been experimentally investigatedbetween 1050 and 1175°C, at pressures from 50 to 400 MPa,for melt H₂O concentrations between 1·2 and 5·5wt % and NNO ranging from –0·07 to +2·32.A drop-quench device was systematically used. AuPd alloys wereused as containers in most cases, resulting in an average Feloss of 13% for the 34 charges studied. Major crystallizingphases include clinopyroxene, olivine and plagioclase. Fe–Tioxide was encountered in a few charges. Clinopyroxene is theliquidus phase at 400 MPa down to at least 200 MPa, followedby olivine and plagioclase. The compositions of all major phasesand glass vary systematically with the proportion of crystals.Ca in clinopyroxene sensitively depends on the H₂O concentrationof the coexisting melt, and clinopyroxene Mg-number shows aweak negative correlation with NNO. The experimental data allowthe liquidus surface of PST-9 to be defined. When used in combinationwith melt inclusion data, a consistent set of pre-eruptive pressures(100–270 MPa), temperatures (1140–1160°C) andmelt H₂O concentrations is obtained. Near-liquidus phase equilibriaand clinopyroxene Ca contents require melt H₂O concentrations<2·7–3·6 and 3 ± 1 wt %, respectively,overlapping with the maximum frequency of glass inclusion data(2·5–2·7 wt % H₂O). For olivine to crystallizeclose to the liquidus, pressures close to 200 MPa are needed.Redox conditions around NNO = +0·5 are inferred fromclinopyroxene compositions. The determined pre-eruptive parametersrefer to the storage region of golden pumice melts, which islocated at a depth of around 7·5 km, within the metamorphicarc crust. Golden pumice melts ascending from their storagezone along an adiabat will not experience crystallization ontheir way to the surface. KEY WORDS: basalt; pumice; experiment; phase equilibria; Stromboli     

Liquidus Equilibria in the System K2O-Na2O-Al2O3-SiO2-F2O-1-H2O to 100 MPa: I. Silicate-Fluoride Liquid Immiscibility in Anhydrous Systems

Liquidus relations in the four-component system Na₂O–Al₂O₃–SiO₂–F₂O_–1were studied at 0· 1 and 100 MPa to define the locationof fluoride–silicate liquid immiscibility and outlinedifferentiation paths of fluorine-bearing silicic magmas. Thefluoride–silicate liquid immiscibility spans the silica–albite–cryoliteand silica–topaz–cryolite ternaries and the haplogranite-cryolitebinary at greater than 960°C and 0· 1–100 MPa.With increasing Al₂O₃ in the system and increasing aluminum/alkalication ratio, the two-liquid gap contracts and migrates fromthe silica liquidus to the cryolite liquidus. The gap does notextend to subaluminous and peraluminous melt compositions. Forall alkali feldspar–quartz-bearing systems, the miscibilitygap remains located on the cryolite liquidus and is thus inaccessibleto differentiating granitic and rhyolitic melts. In peralkalinesystems, the magmatic differentiation is terminated at the albite–quartz–cryoliteeutectic at 770°C, 100 MPa, 5 wt % F and cation Al/Na =0· 75. The addition of topaz, however, significantlylowers melting temperatures and allows strong fluorine enrichmentin subaluminous compositions. At 100 MPa, the binary topaz–cryoliteeutectic is located at 770°C, 39 wt % F, cation Al/Na 0·95, and the ternary quartz–topaz–cryolite eutecticis found at 740°C, 32 wt % F, 30 wt % SiO₂ and cation Al/Na 0· 95. Such location of both eutectics enables fractionationpaths of subaluminous quartz-saturated systems to produce fluorine-rich,SiO₂-depleted and nepheline-normative residual liquids. KEY WORDS: silicate melt; granite; rhyolite; fluorine; liquid immiscibility     

Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions

Many basaltic flood provinces are characterized by the existenceof voluminous amounts of silicic magmas, yet the role of thesilicic component in sulphur emissions associated with trapactivity remains poorly known. We have performed experimentsand theoretical calculations to address this issue. The meltsulphur content and fluid/melt partitioning at saturation witheither sulphide or sulphate or both have been experimentallydetermined in three peralkaline rhyolites, which are a majorcomponent of some flood provinces. Experiments were performedat 150 MPa, 800–900°C, fO₂ in the range NNO –2 to NNO + 3 and under water-rich conditions. The sulphur contentis strongly dependent on the peralkalinity of the melt, in additionto fO₂, and reaches 1000 ppm at NNO + 1 in the most stronglyperalkaline composition at 800°C. At all values of fO₂,peralkaline melts can carry 5–20 times more sulphur thantheir metaluminous equivalents. Mildly peralkaline compositionsshow little variation in fluid/melt sulphur partitioning withchanging fO₂ (D_S 270). In the most peralkaline melt, D_S risessharply at fO₂ > NNO + 1 to values of >500. The partitioncoefficient increases steadily for S_bulk between 1 and 6 wt% but remains about constant for S_bulk between 0·5 and1 wt %. At bulk sulphur contents lower than 4 wt %, a temperatureincrease from 800 to 900°C decreases D_S by 10%. These results,along with (1) thermodynamic calculations on the behaviour ofsulphur during the crystallization of basalt and partial meltingof the crust and (2) recent experimental constraints on sulphursolubility in metaluminous rhyolites, show that basalt fractionationcan produce rhyolitic magmas having much more sulphur than rhyolitesderived from crustal anatexis. In particular, hot and dry metaluminoussilicic magmas produced by melting of dehydrated lower crustare virtually devoid of sulphur. In contrast, peralkaline rhyolitesformed by crystal fractionation of alkali basalt can concentrateup to 90% of the original sulphur content of the parental magmas,especially when the basalt is CO₂-rich. On this basis, we estimatethe amounts of sulphur potentially released to the atmosphereby the silicic component of flood eruptive sequences. The peralkalineEthiopian and Deccan rhyolites could have produced 10¹⁷ and10¹⁸ g of S, respectively, which are comparable amounts to publishedestimates for the basaltic activity of each province. In contrast,despite similar erupted volumes, the metaluminous Paraná–Etendekasilicic eruptives could have injected only 4·6 x 10¹⁵g of S in the atmosphere. Peralkaline flood sequences may thushave greater environmental effects than those of metaluminousaffinity, in agreement with evidence available from mass extinctionsand oceanic anoxic events. KEY WORDS: silicic flood eruptions; sulphur; experiment; Ethiopia; Deccan     

Experimental Crystallization of Leucogranite Magmas

Both crystallization and melting experiments have been carriedout on two natural, biotite-muscovite (DK) and tourmaline-muscovite(GB) High Himalayan leucogranites (HHL) at 4 kbar, logfO₂ =FMQ–05, aH₂O = 1–0•03, and at five temperaturesbetween 803 and 663C H₂O contents of the quenched glasses wereanalysed by ion microprobe. Plagioclase and biotite are theliquidus phases for reduced melt H₂O contents and H₂O-rich conditions,respectively. H₂O saturation limits range from 8 to 10 wt%.DK has a wider crystallization interval than GB (150 vs 80Cfor conditions close to H₂O saturation), and a slightly higherH₂O-saturated solidus (645 compared with 630C for GB). Tourmalinenever crystallized spontaneously from the melt. Tourmaline seedsalways reacted out to biotite in the biotite-muscovite sample,whereas they remained stable in the tourmaline-muscovite sample.Biotite is replaced by hercynite as the main ferromagnesianphase at high temperature and reduced aH₂O. Muscovite crystallizationis restricted to near-solidus conditions. The compositions ofplagioclase, alkali feldspar, biotite and muscovite are givenas a function of bulk composition, temperature and aH₂O. Glasscompositions are richer in normative quartz than the 4 kbarH₂O-saturated Qz–Ab–Or eutectic, and become moreperaluminous and less mafic with increasing fractionation. Biotitecrystallization in peraluminous liquids is favoured by elevatedFe, Mg and Ti contents. Muscovite crystallization is not promotedunder H₂O-saturated conditions. Tourmaline stability is stronglydependent on aH₂O. For GB, tourmaline is present at elevatedtemperatures for intermediate values of aH₂O (803 C, 0–7),but not above 650C for H₂O-saturated conditions. Comparisonof the natural crystallization sequence with experiments suggestsinitial water contents between 5 and 75 wt % for the DK magma,and > 7 wt% for the GB magma. Plagioclase core compositionsgive minimum temperatures of 700C for GB and 750C for DK,consistent with an emplacement of these HHL as almost entirelyliquid bodies. The restricted occurrence of biotite in the GBgranite suggests that it reacted out during the magmatic evolution,owing to a marked change in fO₂ toward more oxidizing conditions.Tourmaline leucogranites can be generated from biotite leucogranitesby fractional crystallization under conditions of increasingdegree of oxidation. KEY WORDS: leucogranite; melting experiments; crystlization experiments; Himalayas; phase relations ^*Corresponding author     

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     

Crustal Evolution of Island-Arc Ultramafic Magma: Galmoenan Pyroxenite-Dunite Plutonic Complex, Koryak Highland (Far East Russia)

Alaskan-type platinum-bearing plutons and potassium-enrichedmafic to ultramafic volcanic rocks are temporally and spatiallyassociated within the Late Cretaceous–Paleocene Achaivayam–Valaginskiiintra-oceanic palaeo-arc system, allochthonously present inthe Koryak Highland and Kamchatka Peninsula (Far East Russia).The compositions of the parental magmas to the Alaskan-typecomplexes are estimated using the Galmoenan plutonic complexas an example. This complex, composed of dunites, pyroxenitesand minor gabbros, is the largest (20 km³) in the system andthe best studied owing to associated platinum placer deposits.The compositions of the principal mineral phases in the Galmoenanintrusive rocks [olivine (Fo_79–92), clinopyroxene (1–3·5wt % Al₂O₃, 0·1–0·5 wt % TiO₂), and Cr-spinel(5–15 wt % Al₂O₃ and 0·3–0·7 wt %TiO₂)] are typical of liquidus assemblages in primitive island-arcmagmas in intra-oceanic settings, and closely resemble the mineralcompositions in the Achaivayam–Valaginskii ultramaficvolcanic rocks. The temporal and spatial association of intrusiveand extrusive units, and the similarity of their mineral compositions,suggest that both suites were formed from similar parental magmas.The composition of the parental magma for the Galmoenan plutonicrocks is estimated using previously reported data for the Achaivayam–Valaginskiiultramafic volcanic rocks and phenocryst-hosted melt inclusions.Quantitative simulation of crystallization of the parental magmain the Galmoenan magma chamber shows that the compositions ofthe cumulate units are best modelled by fractional crystallizationwith periodic magma replenishment. The model calculations reproducewell the observed mineral assemblages and the trace elementabundances in clinopyroxene. Based upon the estimated compositionof the parental magmas and their mantle source, we considerthat fluxing of a highly refractory mantle wedge (similar tothe source of boninites) by chlorine-rich aqueous fluids isprimarily responsible for both high degrees of partial meltingand the geochemical characteristics of the magmas, includingtheir enrichment in platinum-group elements. KEY WORDS: subduction; platinum-group elements; clinopyroxene; trace elements; fractional crystallization; Alaskan-type plutons     

Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas

Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (⁸⁷Sr/⁸⁶Sr_initial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (⁸⁷Sr/⁸⁶Sr_initial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (⁸⁷Sr/⁸⁶Sr_initial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy