长英质岩浆中液态不混溶与成矿作用关系的实验研究

本文的实验研究表明：１２５０℃（１０５Ｐａ）条件下呈均一状态的花岗岩－ＫＢＦ４－Ｎａ２ＭｏＯ４体系,在１０００℃条件下发生液态不混溶形成三种熔体：相对偏酸性的液滴、相对偏基性的熔体和成矿熔体。成矿熔体中富含ＣａＯ、ＭｇＯ和ＭｏＯ２组分。红外光谱研究表明：硅酸盐熔体结构以［ＳｉＯ４］基团为主,而成矿熔体结构中存在Ｃａ—Ｆ、Ｃａ—Ｏ—Ｍｏ、Ｈ—Ｏ—Ｈ以及Ｘ─ＯＨ（Ｘ＝阳离子）基团,说明Ｈ２Ｏ和Ｆ富集在成矿熔体中。液态不混溶作用产生的三种不混溶熔体的结构差异明显,主要表现在结构单元中桥氧数目的变化,相对偏酸性的Ａ类小球体的结构单元中桥氧数最高,基底玻璃熔体中桥氧数则相对较少,而成矿熔体中不存在桥氧。本实验研究结果表明长英质岩浆中的液态不混溶可直接导致成矿熔体的形成。这意味着长英质岩浆中的液态不混溶可直接导致斑岩矿床的形成。     

The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility

The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (La_CN–Yb_CN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate liquid pairs. _Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and _Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has ¹⁸O values of 7·8 to 8·6and ¹³C values of –3·9 to –4·6. ¹⁸Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in ¹⁸O betweencalculated syenitic melts and carbonatites, and the overlapin _Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite     

贵州省梵净山地区玄武质与流纹质岩浆液态不混溶的发现

正液态不混溶(liquid immiscibility)作为最重要的成岩成矿机制之一早就引起了学术界的关注(罗照华等,2007)。但是,自从阿波罗11号采集的月岩中首次发现自然条件下硅酸盐液态不混溶现象(Roedder et al.,1971)以来,地球火成岩中成规模的液态不混溶现象并不多见,涉及液态不混溶过程的论述大多数是基于其最终产物的解释。笔者等于2016年3月在贵州省梵净山地区(图1)进行野外地质工作中发     

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     

The Role of Pressure in Producing Compositional Diversity in Intraplate Basaltic Magmas

Basaltic magmas found in intraplate suites appear to followmore than one differentiation trend. Many ocean island suitesfollow the ocean island tholeiitic trend, with the basalts differentiatingfrom olivine tholeiite through basaltic andesite, andesite,and dacite to sodic rhyolite. Many continental intraplate magmaticregimes, such as those of the Snake River Plain and the plutonicsequences associated with massif anorthosites, follow the potassicsilica-saturated alkalic trend, in which basalt differentiatesfrom olivine tholeiite through ferrobasalt (jotunite or ferrodiorite),Fe-rich intermediate rocks (trachybasalt or monzonite), andtrachyte (syenite) to potassic rhyolites and granites. Crystallizationexperiments on an olivine tholeiite from the Snake River Plainshow that the basaltic portions of the ocean island tholeiitictrend and the potassic silica-saturated alkalic trend (whichleads to strong alkali, P, Ti, and Fe enrichment and silicadepletion) can arise from the same ‘dry’ tholeiiticparental magma. These compositional differences are inducedby changes in phase equilibria as a function of pressure, withthe ocean island tholeiitic series arising from crystal–liquiddifferentiation at low pressure and the potassic silica-saturatedalkalic series arising via differentiation at elevated pressures. KEY WORDS: tholeiite differentiation; experimental petrology; phase equilibria; ferrodiorite; ferrobasalt     

富氟花岗岩浆液态不混溶作用及其成岩成矿效应

除了结晶分异模式外,富氟花岗岩浆液态不混溶作用也是伟晶岩成岩成矿的重要机制。熔体包裹体和实验研究表明,富氟花岗岩浆的液态不混溶会形成富挥发份的贫硅熔体和与其共轭的富硅酸盐熔体。花岗岩中的异离体型伟晶岩和贯入状脉型伟晶岩,挥发份、助熔剂等元素和同位素组成在不混溶相间的突变性是该类不混溶作用的主要标志。富氟花岗岩浆不混溶作用不但可以解释伟晶岩的特征性矿物分带,对传统的伟晶岩成岩理论提出了挑战;还对稀有金属有高度富集作用,这主要归功于F、B、P等挥发份对稀有金属元素的亲和力。然而,目前该类不混溶作用成矿效应的研究还比较薄弱,这主要要归因于没有理想的地质对象。甲基卡矿床是亚洲最大的固体锂矿床,初步研究表明该矿床发生了富氟花岗岩浆液态不混溶作用,是研究氟花岗岩浆液态不混溶过程中稀有金属的分配、迁移、富集规律和机制的理想对象。     

Equilibrium and Fractional Crystallization Experiments at 0{middle dot}7 GPa; the Effect of Pressure on Phase Relations and Liquid Compositions of Tholeiitic Magmas

Two series of anhydrous experiments have been performed in anend-loaded piston cylinder apparatus on a primitive, mantle-derivedtholeiitic basalt at 0·7 GPa pressure and temperaturesin the range 1060–1270°C. The first series are equilibriumcrystallization experiments on a single basaltic bulk composition;the second series are fractionation experiments where near-perfectfractional crystallization was approached in a stepwise mannerusing 30°C temperature increments and starting compositionscorresponding to that of the previous, higher temperature glass.At 0·7 GPa liquidus temperatures are lowered and thestability of olivine and plagioclase is enhanced with respectto clinopyroxene compared with phase equilibria of the samecomposition at 1·0 GPa. The residual solid assemblagesof fractional crystallization experiments at 0·7 GPaevolve from dunites, followed by wehrlites, gabbronorites, andgabbros, to diorites and ilmenite-bearing diorites. In equilibriumcrystallization experiments at 0·7 GPa dunites are followedby plagioclase-bearing websterites and gabbronorites. In contrastto low-pressure fractionation of tholeiitic liquids (1 bar–0·5GPa), where early plagioclase saturation leads to the productionof troctolites followed by (olivine) gabbros at an early stageof differentiation, pyroxene still crystallizes before or withplagioclase at 0·7 GPa. The liquids formed by fractionalcrystallization at 0·7 GPa evolve through limited silicaincrease with rather strong iron enrichment following the typicaltholeiitic differentiation path from basalts to ferro-basalts.Silica enrichment and a decrease in absolute iron and titaniumconcentrations are observed in the last fractionation step afterilmenite starts to crystallize, resulting in the productionof an andesitic liquid. Liquids generated by equilibrium crystallizationexperiments at 0·7 GPa evolve through constant SiO₂ increaseand only limited FeO enrichment as a consequence of spinel crystallizationand closed-system behaviour. Empirical calculations of the (dry)liquid densities along the liquid lines of descent at 0·7and 1·0 GPa reveal that only differentiation at the baseof the crust (1·0 GPa) results in liquids that can ascendthrough the crust and that will ultimately form granitoid plutonicand/or dacitic to rhyodacitic sub-volcanic to volcanic complexes;at 0·7 GPa the liquid density increases with increasingdifferentiation as a result of pronounced Fe enrichment, renderingit rather unlikely that such differentiated melt will reachshallow crustal levels. KEY WORDS: tholeiitic magmas; experimental petrology; equilibrium crystallization; fractional crystallization     

Magma Evolution and Open-System Processes at Shiveluch Volcano: Insights from Phenocryst Zoning

Phenocryst zoning patterns are used to identify open-systemmagmatic processes in the products of the 2001 eruption of ShiveluchVolcano, Kamchatka. The lavas and pumices studied are hornblende–plagioclaseandesites with average pre-eruptive temperatures of 840°Cand fO₂ of 1·5–2·1 log units above nickel–nickeloxide (NNO). Plagioclase zoning includes oscillatory and patchyzonation and sieve textures. Hornblendes are commonly unzoned,but some show simple, multiple or patchy zoning. Apatite microphenocrystsdisplay normal and reverse zoning of sulphur. The textural similarityof patchy hornblende and plagioclase, together with Ba–Srsystematics in patchy plagioclase, indicate that the cores ofthese crystals were derived from cumulate material. Plagioclase–liquidequilibria suggest that the patchy texture develops by resorptionduring H₂O-undersaturated decompression. When H₂O-saturatedcrystallization recommences at lower pressure, reduced pH₂Oresults in lower X_An in plagioclase, causing more Al-rich hornblendeto crystallize. Plagioclase cores with diffuse oscillatory zoning,and unzoned hornblende crystals, probably represent a populationof crystals resident in the magma chamber for long periods oftime. In contrast, oscillatory zoning in the rims of plagioclasephenocrysts may reflect eruption dynamics during decompressioncrystallization. Increasing Fe/Al in oscillatory zoned rimssuggests oxidation as a result of degassing of H₂O during decompression.A general lack of textural overlap between phenocryst typessuggests that different phenocryst populations were spatiallyor temporally isolated during crystallization. We present evidencethat the host andesite has mixed with both more felsic and moremafic magmas. Olivine and orthopyroxene xenocrysts with reactionor overgrowth rims and strong normal zoning indicate mixingwith basalt. Sieve-textured plagioclase resulted from mixingof a more felsic magma with the host andesite. The mineralogyand mineral compositions of a mafic andesite enclave are identicalto those of the host magma, which implies efficient thermalquenching, and thus small volumes of intruding magma. Mixingof this magma with the host andesite results in phenocryst zoningbecause of differences in dissolved volatile contents. We suggestthat small magma pulses differentiated at depth and ascendedintermittently into the growing magma chamber, producing incrementalvariations in whole-rock compositions. KEY WORDS: patchy zoning; magma mixing; Shiveluch     

Immiscible Transition from Carbonate-rich to Silicate-rich Melts in the 3 GPa Melting Interval of Eclogite + CO2 and Genesis of Silica-undersaturated Ocean Island Lavas

We explore the partial melting behavior of a carbonated silica-deficienteclogite (SLEC1; 5 wt % CO₂) from experiments at 3 GPa and comparethe compositions of partial melts with those of alkalic andhighly alkalic oceanic island basalts (OIBs). The solidus islocated at 1050–1075 °C and the liquidus at 1415 °C.The sub-solidus assemblage consists of clinopyroxene, garnet,ilmenite, and calcio-dolomitic solid solution and the near solidusmelt is carbonatitic (<2 wt % SiO₂, <1 wt % Al₂O₃, and<0·1 wt % TiO₂). Beginning at 1225 °C, a stronglysilica-undersaturated silicate melt (34–43 wt % SiO₂)with high TiO₂ (up to 19 wt %) coexists with carbonate-richmelt (<5 wt % SiO₂). The first appearance of carbonated silicatemelt is 100 °C cooler than the expected solidus of CO₂-freeeclogite. In contrast to the continuous transition from carbonateto silicate melts observed experimentally in peridotite + CO₂systems, carbonate and silicate melt coexist over a wide temperatureinterval for partial melting of SLEC1 carbonated eclogite at3 GPa. Silicate melts generated from SLEC1, especially at highmelt fraction (>20 wt %), may be plausible sources or contributingcomponents to melilitites and melilititic nephelinites fromoceanic provinces, as they have strong compositional similaritiesincluding their SiO₂, FeO^*, MgO, CaO, TiO₂ and Na₂O contents,and CaO/Al₂O₃ ratios. Carbonated silicate partial melts fromeclogite may also contribute to less extreme alkalic OIB, asthese lavas have a number of compositional attributes, suchas high TiO₂ and FeO^* and low Al₂O₃, that have not been observedfrom partial melting of peridotite ± CO₂. In upwellingmantle, formation of carbonatite and silicate melts from eclogiteand peridotite source lithologies occurs over a wide range ofdepths, producing significant opportunities for metasomatictransfer and implantation of melts. KEY WORDS: carbonated eclogite; experimental phase equilibria; partial melting; liquid immiscibility; ocean island basalts     

Rates of Thermal and Chemical Evolution of Magmas in a Cooling Magma Chamber: a Chronological and Theoretical Study on Basaltic and Andesitic Lavas from Rishiri Volcano, Japan

Rates of magmatic processes in a cooling magma chamber wereinvestigated for alkali basalt and trachytic andesite lavaserupted sequentially from Rishiri Volcano, northern Japan, bydating of these lavas using ²³⁸U–²³⁰Th radioactive disequilibriumand ¹⁴C dating methods, in combination with theoretical analyses.We obtained the eruption age of the basaltic lavas to be 29·3± 0·6 ka by ¹⁴C dating of charcoals. The eruptionage of the andesitic lavas was estimated to be 20·2 ±3·1 ka, utilizing a whole-rock isochron formed by U–Thfractionation as a result of degassing after lava emplacement.Because these two lavas represent a series of magmas producedby assimilation and fractional crystallization in the same magmachamber, the difference of the ages (i.e. 9 kyr) is a timescaleof magmatic evolution. The thermal and chemical evolution ofthe Rishiri magma chamber was modeled using mass and energybalance constraints, as well as quantitative information obtainedfrom petrological and geochemical observations on the lavas.Using the timescale of 9 kyr, the thickness of the magma chamberis estimated to have been about 1·7 km. The model calculationsshow that, in the early stage of the evolution, the magma cooledat a relatively high rate (>0·1°C/year), and thecooling rate decreased with time. Convective heat flux fromthe main magma body exceeded 2 W/m² when the magma was basaltic,and the intensity diminished exponentially with magmatic evolution.Volume flux of crustal materials to the magma chamber and rateof convective melt exchange (compositional convection) betweenthe main magma and mush melt also decreased with time, from 0·1 m/year to 10^–3 m/year, and from 1 m/yearto 10^–2 m/year, respectively, as the magmas evolved frombasaltic to andesitic compositions. Although the mechanism ofthe cooling (i.e. thermal convection and/or compositional convection)of the main magma could not be constrained uniquely by the model,it is suggested that compositional convection was not effectivein cooling the main magma, and the magma chamber is consideredto have been cooled by thermal convection, in addition to heatconduction. KEY WORDS: convection; magma chamber; heat and mass transport; timescale; U-series disequilibria     

Trace Element Partitioning and Accessory Phase Saturation during H2O-Saturated Melting of Basalt with Implications for Subduction Zone Chemical Fluxes

Experimental phase equilibrium and trace element partitioningdata are reported for H₂O-saturated mid-ocean ridge basalt at2·5 GPa, 750–900°C and oxygen fugacities atthe nickel–nickel oxide buffer. Garnet, omphacite andrutile are present at all temperatures. Amphibole and epidotedisappear as residual phases above 800°C; allanite appearsabove 750°C. The Na–Al-rich silicate glass presentin all run products is likely to have quenched from a supercriticalliquid. Trace element analyses of glasses demonstrate the importantcontrol exerted by residual minerals on liquid chemistry. Inaddition to garnet, which controls heavy rare earth elements(HREE) and Sc, and rutile, which controls Ti, Nb and Ta, allanitebuffers the light REE (LREE; La–Sm) contents of liquidsto relatively low levels and preferentially holds back Th relativeto U. In agreement with previous experimental and metamorphicstudies we propose that residual allanite plays a key role inselectively retaining trace elements in the slab during subduction.Experimental data and analyses of allanite-bearing volcanicrocks are used to derive a model for allanite solubility inliquids as a function of pressure, temperature, anhydrous liquidcomposition and LREE content. The large temperature dependenceof allanite solubility is very similar to that previously determinedfor monazite. Our model, fitted to 48 datapoints, retrievesLREE solubility (in ppm) to within a factor of 1· 40over a pressure range of 0–4 GPa, temperature range of700–1200°C and for liquids with anhydrous SiO₂ contentsof 50–84 wt %. This uncertainty in LREE content is equivalentto a temperature uncertainty of only ± 27°C at 1000K, indicating the potential of allanite as a geothermometer.Silicic liquids from either basaltic or sedimentary protolithswill be saturated in allanite except for Ca-poor protolithsor at very high temperatures. For conventional subduction geothermsthe low solubility of LREE (+ Th) in liquids raises questionsabout the mechanism of LREE + Th transport from slab to wedge.It is suggested either that, locally, temperatures experiencedby the slab are high enough to eliminate allanite in the residueor that substantial volumes of H₂O-rich fluids must pass throughthe mantle wedge prior to melting. The solubility of accessoryphases in fluids derived from subducted rocks can provide importantconstraints on subduction zone thermal structure. KEY WORDS: subduction; experimental petrology; allanite; solubility; supercritical liquid; eclogite     

Magma Evolution of the Sete Cidades Volcano, Sao Miguel, Azores

The Sete Cidades volcano (São Miguel, Azores) is situatedat the eastern end of the ultraslow spreading Terceira riftaxis. The volcano comprises several dominantly basaltic pre-calderaeruptions, a trachytic caldera-forming stage and a post-calderastage consisting of alternating trachytic and basaltic eruptions.The post-caldera flank lavas are more primitive (>5 wt %MgO) than the pre-caldera lavas, implying extended fractionalcrystallization and longer crustal residence times for the pre-caldera,shield-building lavas. Thermobarometric estimates show thatthe ascending alkali basaltic magmas stagnated and crystallizedat the crust–mantle boundary (15 km depth), whereas themore evolved magmas mainly fractionated in the upper crust (3km depth). The caldera-forming eruption was triggered by a basalticinjection into a shallow trachytic magma chamber. Lavas fromall stages follow a single, continuous liquid line of descentfrom alkali basalt to trachyte, although slight differencesin incompatible element (e.g. Ba/Nb, La/Nb) and Sr isotope ratiosimply some heterogeneity of the mantle source. Major and traceelement data suggest similar partial melting processes throughoutthe evolution of the volcano. Slight geochemical differencesbetween post- and pre-caldera stage lavas from the Sete Cidadesvolcanic system indicate a variation in the mantle source compositionwith time. The oxygen fugacity increased from the pre-calderato the post-caldera stage lavas, probably as a result of theassimilation of crustal rocks; this is supported by the presenceof crustal xenoliths in the lavas of the flank vents. The lavasfrom the Sete Cidades volcano generally have low Sr isotoperatios; however, rocks from one post-caldera vent on the westernflank indicate mixing with magmas resembling the lavas fromthe neighbouring Agua de Pau volcano, having higher Sr isotoperatios. The different magma sources at Sete Cidades and theadjacent Agua de Pau volcano imply that, despite their closeproximity, there is only limited interaction between them. KEY WORDS: crystallization depth; fractionation; stratigraphy; Terceira rift; volcanic stages     

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

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

Volcan Popocatepetl, Mexico. Petrology, Magma Mixing, and Immediate Sources of Volatiles for the 1994-Present Eruption

Volcán Popocatépetl has been the site of voluminousdegassing accompanied by minor eruptive activity from late 1994until the time of writing (August 2002). This contribution presentspetrological investigations of magma erupted in 1997 and 1998,including major-element and volatile (S, Cl, F, and H₂O) datafrom glass inclusions and matrix glasses. Magma erupted fromPopocatépetl is a mixture of dacite (65 wt % SiO₂, two-pyroxenes+ plagioclase + Fe–Ti oxides + apatite, 3 wt % H₂O, P= 1·5 kbar, f_O2 = NNO + 0·5 log units) and basalticandesite (53 wt % SiO₂, olivine + two-pyroxenes, 3 wt % H₂O,P = 1–4 kbar). Magma mixed at 4–6 km depth in proportionsbetween 45:55 and 85:15 wt % silicic:mafic magma. The pre-eruptivevolatile content of the basaltic andesite is 1980 ppm S, 1060ppm Cl, 950 ppm F, and 3·3 wt % H₂O. The pre-eruptivevolatile content of the dacite is 130 ± 50 ppm S, 880± 70 ppm Cl, 570 ± 100 ppm F, and 2·9 ±0·2 wt % H₂O. Degassing from 0·031 km³ of eruptedmagma accounts for only 0·7 wt % of the observed SO₂emission. Circulation of magma in the volcanic conduit in thepresence of a modest bubble phase is a possible mechanism toexplain the high rates of degassing and limited magma productionat Popocatépetl. KEY WORDS: glass inclusions; igneous petrology; Mexico; Popocatépetl; volatiles     

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet

The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock ⁸⁷Sr/⁸⁶Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial ⁸⁷Sr/⁸⁶Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An₇₂ plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An₄₃; Mg# 5 clinopyroxene; Fo₁ olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V₂O₅ of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO₂ and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO₂ and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation     

Zircon Inheritance Reveals Exceptionally Fast Crustal Magma Generation Processes in Central Iberia during the Cambro-Ordovician

The Variscan basement of the Central Iberian Zone contains abundantCambro-Ordovician calc-alkaline to peraluminous metagranitesand metavolcanic rocks with two notable features: first, theywere apparently produced with no connection to any major tectonicor metamorphic event; second, they have an unusually high zirconinheritance. U–Pb dating combined with cathodoluminescenceimaging reveals that about 70–80%, in some samples nearer100%, of the zircon grains contain inherited pre-magmatic cores,despite the temperature reached by the magmas (about 900°C,calculated using the Ti-in-zircon thermometer) being high enoughto dissolve all the available zircon (from the rock's zirconsaturation temperature, 770–860°C). The fact thatthe dissolution of zircon was so incomplete can only be attributedto the kinetics of heat transfer to and from the magmas. Three-dimensionalmodeling of zircon dissolution behavior in melts with a compositionsimilar to the Iberian Cambro-Ordovician magmas indicates thatthe survival of zircons from the suggested late Pan-Africanprotolith would be possible only if melt production was rapid,specifically less than 10⁴ years, and probably about 2 x 10³years, from the beginning of melting (700°C) to the thermalpeak (900°C). Melt production was followed by fast magmatransfer to upper crustal levels resulting either in surfaceeruption or in the emplacement of small (< 400 m thick) sillsor laccoliths. We suggest that these elevated rates of crustalmelting could only have been caused by intrusion of mantle-derivedmafic magmas, most probably at the base of the crust. This scenariois consistent with a rifting regime in which crust and mantlewere mechanically decoupled; this would explain the scarcityof contemporaneous crustal deformation. Furthermore, fast meltingrates in the lower crust followed by fast melt transportationto the upper crust could also explain the lack of contemporaneousmetamorphism. The speed of the partial melting process resultedin the production of felsic magmas that inherited the geochemicalcharacteristics of their granitoid crustal protolith. This explainsthe apparent contradiction between the calc-alkaline to peraluminousgeochemical characteristics of the magmas and the inferred extensional(i.e. rift-related) tectonic setting. Our model is compatiblewith the hypothesis of fragmentation and dispersal of terranesfrom the northern margin of Gondwana that led to the openingof the Rheic and Galicia–South Brittany oceans and, ultimately,caused the detachment of the Iberian microplate from Armoricaand Gondwana during the early Paleozoic. KEY WORDS: igneous petrology; migmatite; granite; geochemistry; crustal contamination; ICP-MS; laser ablation     

The Composition of Near-solidus Partial Melts of Fertile Peridotite at 1 and 1{middle dot}5 GPa: Implications for the Petrogenesis of MORB

We have determined the near-solidus melt compositions for peridotiteMM-3, a suitable composition for the production of mid-oceanridge basalt (MORB) by decompression partial melting, at 1 and1·5 GPa. At 1 GPa the MM-3 composition has a subsolidusplagioclase-bearing spinel lherzolite assemblage, and a solidusat 1270°C. At only 5°C above the solidus, 4% meltis present as a result of almost complete melting of plagioclase.This melting behaviour in plagioclase lherzolite is predictedfrom simple systems and previous experimental work. The persistenceof plagioclase to > 0·8 GPa is strongly dependenton bulk-rock CaO/Na₂O and normative plagioclase content in theperidotite. At 1·5 GPa the MM-3 composition has a subsolidusspinel lherzolite assemblage, and a solidus at 1350°C.We have determined a near-solidus melt composition at 2% meltingwithin 10°C of the solidus. Near-solidus melts at both 1and 1·5 GPa are nepheline normative, and have low normativediopside contents; also they have the highest TiO₂, Al₂O₃ andNa₂O, and the lowest FeO and Cr₂O₃ contents compared with higherdegree partial melts. Comparison of these near-solidus meltswith primitive MORB glasses, which lie in the olivine-only fieldof crystallization at low pressure, indicate that petrogeneticmodels involving aggregation of near-fractional melts formedduring melting at pressures of 1·5 GPa or less are unlikelyto be correct. In this study we use an experimental approachthat utilizes sintered oxide mix starting materials and peridotitereaction experiments. We also examine some recent studies usingan alternative approach of melt migration into, and entrapmentwithin ‘melt traps’ (olivine, diamond, vitreouscarbon) and discuss optimal procedures for this method. KEY WORDS: experimental petrology; mantle melting; near-solidus; fertile peridotite; MORB     

Evolution of a Porphyry-Cu Mineralized Magma System at Santa Rita, New Mexico (USA)

The magmatic processes leading to porphyry-Cu mineralizationat Santa Rita are reconstructed on the basis of petrographicstudies, thermobarometry, and laser-ablation inductively-coupled-plasmamass-spectrometry analyses of silicate melt and sulfide inclusionsfrom dikes ranging from basaltic andesite to rhyodacite. Combinedresults suggest that magma evolution at Santa Rita is similarto that of sulfur-rich volcanoes situated above subduction zones,being characterized by repeated injection of hot, mafic magmainto an anhydrite-bearing magma chamber of rhyodacitic composition.The most mafic end-member identified at Santa Rita is a shoshoniticbasaltic andesite that crystallized at 1000–1050°C,1–3 kbar and log f_O2 = NNO + 0·7 to NNO + 1·0,whereas the rhyodacite crystallized at 730–760°C andlog f_O2 = NNO + 1·3 to NNO + 1·9. Mixing betweenthe two magmas caused precipitation of 0·1–0·2wt % magmatic sulfides and an associated decrease in the Cucontent of the silicate melt from 300–500 ppm to lessthan 20 ppm. Quantitative modeling suggests that temporal storageof ore-metals in magmatic sulfides does not significantly enhancethe amount of copper ultimately available to ore-forming hydrothermalfluids. Magmatic sulfides are therefore not vital to the formationof porphyry-Cu deposits, unless a mechanism is required thatholds back ore-forming metals until late in the evolution ofthe volcanic–plutonic system. KEY WORDS: porphyry-Cu; sulfur; sulfides; magma mixing; LA-ICP-MS     

Textural Maturity of Cumulates: a Record of Chamber Filling, Liquidus Assemblage, Cooling Rate and Large-scale Convection in Mafic Layered Intrusions

Textural maturity describes the extent to which a rock has evolvedfrom the initial reaction-controlled texture towards texturalequilibrium controlled by the minimization of interfacial energy.Solidification in a magma chamber results in the formation ofan impingement texture by the random juxtaposition of planar-sidedgrains. Orthocumulates, in which the initial melt-filled poresare pseudomorphed by later-crystallizing phases, have an ophiticor intersertal texture immediately after complete solidification,which then evolves towards solid-state equilibrium by roundingof initially planar grain boundaries and an increase in themedian dihedral angle subtended at the junctions of two primocrysticgrains with the interstitial phase. The bulk of the increasein angle occurs just below the solidus temperature in kilometre-scalemafic plutons. Quantification of textural maturity via measurementof dihedral angle populations in troctolitic and gabbroic cumulatesfrom the Rum Eastern Layered Intrusion and the Skaergaard Intrusiondemonstrates that the rocks preserve a record of thermal eventsrelated to magma chamber replenishment and the onset of chamber-wideconvection. Textural maturity is also a function of the liquidusphase assemblage: for systems in which only olivine and plagioclaseare liquidus (i.e. cumulus) phases in the main magma body abovethe crystal mush, the texture is significantly less mature thanthat in systems in which clinopyroxene is an additional liquidusphase. The difference in textural maturity reflects differencesin the cooling and solidification rate, and demonstrates directlythat the liquidus phase assemblage plays a role in determiningthe thermal history of plutons. KEY WORDS: cumulates; dihedral angles; Rum; Skaergaard; textures     

High-pressure Partial Melting of Mafic Lithologies in the Mantle

We review experimental phase equilibria associated with partialmelting of mafic lithologies (pyroxenites) at high pressuresto reveal systematic relationships between bulk compositionsof pyroxenite and their melting relations. An important aspectof pyroxenite phase equilibria is the existence of the garnet–pyroxenethermal divide, defined by the enstatite–Ca-Tschermakspyroxene–diopside plane in CaO–MgO–Al₂O₃–SiO₂projections. This divide appears at pressures above 2 GPa inthe natural system where garnet and pyroxenes are the principalresidual phases in pyroxenites. Bulk compositions that resideon either side of the divide have distinct phase assemblagesfrom subsolidus to liquidus and produce distinct types of partialmelt ranging from strongly nepheline-normative to quartz-normativecompositions. Solidus and liquidus locations are little affectedby the location of natural pyroxenite compositions relativeto the thermal divide and are instead controlled chiefly bybulk alkali contents and Mg-numbers. Changes in phase volumesof residual minerals also influence partial melt compositions.If olivine is absent during partial melting, expansion of thephase volume of garnet relative to clinopyroxene with increasingpressure produces liquids with high Ca/Al and low MgO comparedwith garnet peridotite-derived partial melts. KEY WORDS: experimental petrology; mantle heterogeneity; partial melting; phase equilibrium; pyroxenite