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     

Variations in Melt Productivity and Melting Conditions along SWIR (70{degrees}E-49{degrees}E): Evidence from Olivine-hosted and Plagioclase-hosted Melt Inclusions

Melt inclusion and host glass compositions from the easternend of the Southwest Indian Ridge show a progressive depletionin light rare earth elements (LREE), Na₈ and (La/Sm)_n, but anincrease in Fe₈, from the NE (64°E) towards the SW (49°E).These changes indicate an increase in the degree of mantle meltingtowards the SW and correlate with a shallowing of the ridgeaxial depth and increase in crustal thickness. In addition,LREE enrichment in both melt inclusions and host glasses fromthe NE end of the ridge are compatible with re-fertilizationof a depleted mantle source. The large compositional variations(e.g. P₂O₅ and K₂O) of the melt inclusions from the NE end ofthe ridge (64°E), coupled with low Fe₈ values, suggest thatmelts from the NE correspond to a variety of different batchesof melts generated at shallow levels in the mantle melting column.In contrast, the progressively more depleted compositions andhigher Fe₈ values of the olivine- and plagioclase-hosted meltinclusions at the SW end of the studied region (49°E), suggestthat these melt inclusions represent batches of melt generatedby higher degrees of melting at greater mean depths in the mantlemelting column. Systematic differences in Fe₈ values betweenthe plagioclase- and the olivine-hosted melt inclusions in theSW end (49°E) of the studied ridge area, suggest that theplagioclase-hosted melt inclusions represent final batches ofmelt generated at the top of the mantle melting column, whereasthe olivine-hosted melt inclusions correspond to melts generatedfrom less depleted, more fertile mantle at greater depths. KEY WORDS: basalt; melt inclusions; olivine; plagioclase; Southwest Indian Ridge     

Silica-rich Melts in Quartz Xenoliths from Vulcano Island and their Bearing on Processes of Crustal Anatexis and Crust-Magma Interaction beneath the Aeolian Arc, Southern Italy

Quartz-rich xenoliths in lavas and pyroclastic rocks from VulcanoIsland, part of the Aeolian arc, Italy, contain silicic meltinclusions with high SiO₂ (73–80 wt %) and K₂O (3–6wt %) contents. Two types of inclusions can be distinguishedbased on their time of entrapment and incompatible trace element(ITE) concentrations. One type (late, ITE-enriched inclusions)has trace element characteristics that resemble those of themetamorphic rocks of the Calabro-Peloritano basement of theadjacent mainland. Other inclusions (early, ITE-depleted) havevariable Ba, Rb, Sr and Cs, and low Nb, Zr and rare earth element(REE) contents. Their REE patterns are unfractionated, witha marked positive Eu anomaly. Geochemical modelling suggeststhat the ITE-depleted inclusions cannot be derived from equilibriummelting of Calabro-Peloritano metamorphic rocks. ITE-enrichedinclusions can be modelled by large degrees (>80%) of meltingof basement gneisses and schists, leaving a quartz-rich residuerepresented by the quartz-rich xenoliths. Glass inclusions inquartz-rich xenoliths represent potential contaminants of Aeolianarc magmas. Interaction between calc-alkaline magmas and crustalanatectic melts with a composition similar to the analysed inclusionsmay generate significant enrichment in potassium in the magmas.However, ITE contents of the melt inclusions are comparablewith or lower than those of Vulcano calc-alkaline and potassicrocks. This precludes the possibility that potassic magmas inthe Aeolian arc may originate from calc-alkaline parents throughdifferent degrees of incorporation of crustal melts. KEY WORDS: melt inclusions; crustal anatexis; magma assimilation; xenoliths; Vulcano Island     

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     

Magma Ascent and Crustal Accretion at Ultraslow-Spreading Ridges: Constraints from Plagioclase Ultraphyric Basalts from the Arctic Mid-Ocean Ridge

Plagioclase ultraphyric basalts (PUBs) with up to 54% plagioclasephenocrysts were dredged in the rift valley and adjacent flanksof the ultraslow-spreading Mohns and Knipovich ridges. The PUBsshow large variations in crystal morphologies and zoning. Thelarge variations suggest that single basalt samples containa mixture of plagioclase crystals that aggregated at differentlevels in the magma conduits. Resorbed crystals and repeatedreverse zones suggest that the magma reservoirs were replenishedand heated several times. Thin concentric zones with melt inclusions,and sharp reductions in the anorthite content of 3–7%,are common between the reverse zones. These zones, and skeletalcrystals with distinctly lower anorthite contents than massivecrystals, are interpreted to be the result of rapid crystalliztionduring strong undercooling. The changes between short periodsof cooling and longer periods with reheating are explained bymultiple advances of crystal-rich magma into cool regions followedby longer periods of gradual magma inflow and temperature increase.The porphyritic basalts are characterizd by more depleted andmore fractionated compositions than the aphyric basalts, withlower (La/Sm)_N, K₂O and Mg-numbers. This relationship, and theobservation that PUBs are sampled only close to segment centresalong these ridges, suggests that the PUBs formed by higherdegrees of melting and evolved in more long-lived magma reservoirs.We propose that the zoning patterns of plagioclase crystalsand crystal morphologies of these PUBs reflect the developmentand flow of magma through a stacked sill complex-like conduitsystem, whereas the aphyric equivalents represent later flowof magma through the conduit. The formation of voluminous higher-degreemelts may trigger the development of the magma conduits andexplain the generally depleted compositions of PUB magmas. KEY WORDS: basalt; mineral chemistry; MORB; magma mixing; magma chamber; major element     

Melt Inclusions in Primitive Olivine Phenocrysts: the Role of Localized Reaction Processes in the Origin of Anomalous Compositions

Melt inclusions are small portions of liquid trapped by growingcrystals during magma evolution. Recent studies of melt inclusionshave revealed a large range of unusual major and trace elementcompositions in phenocrysts from primitive mantle-derived magmaticrocks [e.g. in high-Fo olivine (Fo > 85 mol %), spinel, high-Anplagioclase]. Inclusions in phenocrysts crystallized from moreevolved magmas (e.g. olivine Fo < 85 mol %), are usuallycompositionally similar to the host lavas. This paper reviewsthe chemistry of melt inclusions in high-Fo olivine phenocrystsfocusing on those with anomalous major and trace element contentsfrom mid-ocean ridge and subduction-related basalts. We suggestthat a significant portion of the anomalous inclusion compositionsreflects localized, grain-scale dissolution–reaction–mixing(DRM) processes within the magmatic plumbing system. The DRMprocesses occur at the margins of primitive magma bodies, wheremagma is in contact with cooler wall rocks and/or pre-existingsemi-solidified crystal mush zones (depending on the specificenvironment). Injection of hotter, more primitive magma causespartial dissolution (incongruent melting) of the mush-zone phases,which are not in equilibrium with the primitive melt, and mixingof the reaction products with the primitive magma. Localizedrapid crystallization of high-Fo olivines from the primitivemagma may lead to entrapment of numerous large melt inclusions,which record the DRM processes in progress. In some magmaticsuites melt inclusions in primitive phenocrysts may be naturallybiased towards the anomalous compositions. The occurrence ofmelt inclusions with unusual compositions does not necessarilyimply the existence of new geologically significant magma typesand/or melt-generation processes, and caution should be exercisedin their interpretation. KEY WORDS: melt inclusions; olivine; geochemistry; mush zones; MORB; subduction-related magmas     

Remobilization of Highly Crystalline Felsic Magma by Injection of Mafic Magma: Constraints from the Middle Sixth Century Eruption at Haruna Volcano, Honshu, Japan

The latest eruption of Haruna volcano at Futatsudake took placein the middle of the sixth century, starting with a Plinianfall, followed by pyroclastic flows, and ending with lava domeformation. Gray pumices found in the first Plinian phase (lowerfall) and the dome lavas are the products of mixing betweenfelsic (andesitic) magma having 50 vol. % phenocrysts and maficmagma. The mafic magma was aphyric in the initial phase, whereasit was relatively phyric during the final phase. The aphyricmagma is chemically equivalent to the melt part of the phyricmafic magma and probably resulted from the separation of phenocrystsat their storage depth of 15 km. The major part of the felsicmagma erupted as white pumice, without mixing and heating priorto the eruption, after the mixed magma (gray pumice) and heatedfelsic magma (white pumice) of the lower fall deposit. Althoughthe mafic magma was injected into the felsic magma reservoir(at 7 km depth), part of the product (lower fall ejecta) precedederuption of the felsic reservoir magma, as a consequence ofupward dragging by the convecting reservoir of felsic magma.The mafic magma injection made the nearly rigid felsic magmaerupt, letting low-viscosity mixed and heated magmas open theconduit and vent. Indeed the lower fall white pumices preservea record of syneruptive slow ascent of magma to 2 km depth,probably associated with conduit formation. KEY WORDS: high-crystallinity felsic magma; magma plumbing system; multistage magma mixing; upward dragging of injected magma; vent opening by low-viscosity magma     

Evolution of Mafic Alkaline Melts Crystallized in the Uppermost Lithospheric Mantle: a Melt Inclusion Study of Olivine-Clinopyroxenite Xenoliths, Northern Hungary

Olivine-clinopyroxenite xenoliths exhumed in alkali basalts(sensu lato) in the Nógrád–GömörVolcanic Field (NGVF), northern Hungary, contain abundant silicatemelt inclusions. Geothermobarometric calculations indicate thatthese xenoliths crystallized as cumulates in the upper mantlenear the Moho. These cumulate xenoliths are considered to representa period of Moho underplating by mafic alkaline magmas priorto the onset of Late Tertiary alkaline volcanism in the Carpathian–Pannonianregion. The major and trace element compositions of silicatemelt inclusions in olivine display an evolutionary trend characterizedby a strong decrease in CaO/Al₂O₃. The parental melt of thecumulates was a basanite formed by low-degree ( 2%) partialmelting of a garnet peridotite source. The compositional trendof the silicate melt inclusions, textural features, and modellingwith pMELTS show that the parental melt evolved by major clinopyroxeneand minor olivine crystallization followed by the appearanceof amphibole simultaneously with significant resorption of theearlier clinopyroxene and olivine. The resulting residual meltwas highly enriched in Al₂O₃, alkalis and most incompatibletrace elements. This type of melt is likely to infiltrate andreact with surrounding mantle peridotite as a metasomatic agent.It might also form high-pressure pegmatite-like bodies in themantle that might be the source of the amphibole and sanidinemegacrysts also found in the alkali basalts of the NGVF. Preferentialremelting of the later-formed (i.e. lower temperature) mineralassemblage (amphibole, sanidine, residual glass) might havesignificantly contaminated the host alkaline mafic lavas, increasingtheir Al₂O₃ and total alkali contents and, therefore, reducingtheir MgO, FeO and CaO content. KEY WORDS: silicate melt inclusions; geochemistry; petrogenesis; Nógrád–Gömör Volcanic Field; Pannonian Basin     

Assimilation and Fractional Crystallization Controlled by Transport Process of Crustal Melt: Implications from an Alkali Basalt-Dacite Suite from Rishiri Volcano, Japan

Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basalt–dacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO₂ correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (‘r value’). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t^–1/2), and was about 3 x 10^–2m/year at t = 10 years and 1 x 10^–2 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope     

High-Magnesian Andesite Produced by Two-Stage Magma Mixing: a Case Study from Hachimantai, Northern Honshu, Japan

High-magnesian andesite occurs at Hachimantai, northern Honshu,Japan. Disequilibrium zoning features indicate that the phenocrystminerals were derived from three different magmas. Chemicalcompositions and zoning profiles are accounted for by two-stagemagma mixing: the first mixing occurred between a crystal-freebasalt magma and a more differentiated olivine basalt magma;the second stage occurred by mixing between the resultant ofthe first-stage mixing and a hypersthene–augite andesitemagma. Mass balance of phenocryst crystals shows that end-membercompositions were c. 52·0 wt % SiO₂ and 10·1 wt% MgO for the mafic end-member and 57·0 wt % SiO₂ forthe felsic end-member of the second-stage mixing. Phenocrystminerals of the first-stage mixing end-member indicate the similarityof the end-member composition to that of basalts from nearbyvolcanoes. The counterpart aphyric magma in the first-stagemixing was more magnesian than the estimated mafic end-member.Calculations of the phase equilibria of similar basalts fromnearby volcanoes and comparison of results with previous phaseequilibrium experiments showed that the olivine basalt end-memberof the first stage was hydrous and situated at a depth wherethe pressure was less than 2 kbar. Two-pyroxene thermometryestimates are about 1050°C for the pyroxenes derived fromthe felsic end-member of the second-stage mixing, and about1180°C for groundmass pyroxenes. Crystallization temperaturesof 1170–1230°C are estimated for minerals from themafic end-member of the second-stage mixing based on phase equilibriumcalculations. These similar temperature estimates between thegroundmass and the mafic end-member imply achievement of thermalequilibrium between end-members preceding crystallization. Themagma plumbing system of the eastern Hachimantai is illustratedby a recent volcanic event, involving lateral dike intrusiontoward a pressure source. The encounter of a laterally migratingbasalt dike and an andesite magma chamber triggered the magmamixing that produced the high-magnesian andesite. The modelcan account for the relation between the petrological modeland surface distribution of volcanic rocks. The infrequencyof such mixing-derived high-magnesian andesite stems from therarity of high-magnesian basalt as a potential mixing end-memberin northern Honshu. KEY WORDS: high-magnesian andesite; Hachimantai; Northern Honshu; high-magnesian basalt; two-stage magma mixing     

Generation of Palaeocene Adakitic Andesites by Magma Mixing; Yanji Area, NE China

Palaeocene (c. 55–58 Ma) adakitic andesites from the Yanjiarea, NE China, are typically clinopyroxene-bearing sodic andesitescontaining 60· 9–62· 2% SiO₂ and 4·02–4· 36% MgO, with high Mg-number [100 Mg/(Mg+ Fe) atomic ratio] from 65· 5 to 70· 1. Whole-rockgeochemical features include high Cr (128–161 ppm) andNi (86–117 ppm) concentrations, extremely high Sr (2013–2282ppm), low Y (10–11 ppm) and heavy rare earth elements(HREE; e.g. Yb = 0· 79–1· 01 ppm), and mid-oceanridge basalt (MORB)-like Sr–Nd–Pb isotopic compositions[e.g. ⁸⁷Sr/ ⁸⁶Sr(i) = 0· 70298–0· 70316,_Nd(t) = +3· 8 to +6· 3 and ²⁰⁶Pb/ ²⁰⁴Pb = 17·98 – 18· 06], analogous to high-Mg adakites occurringin modern subduction zones. However, mineralogical evidencefrom clinopyroxene phenocrysts and microcrystalline plagioclaseclearly points to magma mixing during magma evolution. Iron-richclinopyroxene (augite) cores with low Sr, high Y and heavy REEcontents, slightly fractionated REE patterns and large negativeEu anomalies probably crystallized along with low-Ca plagioclasefrom a lower crustal felsic magma. In contrast, high Mg-numberclinopyroxene (diopside and endiopside) mantles and rims havehigher Sr and lower HREE and Y concentrations, highly fractionatedREE patterns (high La/Yb) and negligible Eu anomalies, similarto those found in adakites from subduction zones. The Yanjiadakitic andesites can be interpreted as a mixture between acrust-derived magma having low Mg-number and Sr, and high Yand HREE, and a mantle-derived high Mg-number adakite havinghigh Sr and low Y and HREE concentrations. During storage and/orascent, the mixed magma experienced further crustal contaminationto capture zircons, of a range of ages, from the wall rocks.The absence of coeval arc magmatism and an extensional tectonicregime in the Yanji area and surrounding regions suggest thatthese Palaeocene adakitic andesites were formed during post-subductionextension that followed the late Cretaceous Izanagi–Farallonridge subduction. Generation of these adakitic andesites doesnot require contemporaneous subduction of a young, hot oceanicridge or delamination of eclogitic lower crust as suggestedby previous models. KEY WORDS: magma mixing; adakitic andesites; Palaeocene; NE China     

Diverse Origins of Xenoliths from Seamounts at the Continental Margin, Offshore Central California

A diverse assemblage of small mafic and ultramafic xenolithsoccurs in alkalic lava from Davidson and Pioneer seamounts locatedat the continental margin of central California. Based on mineralcompositions and textures, they form three groups: (1) mantlexenoliths of lherzolite, pyroxenite, and dunite with olivineof >Fo₉₀; (2) ocean crust xenoliths of dunite with olivine<Fo₉₀, troctolite, pyroxene-gabbro, and anorthosite withlow-K₂O plagioclase; (3) cumulates of seamount magmas of alkalicgabbro with primary amphibole and biotite and anorthosites withhigh-K₂O plagioclase. The alkalic cumulates are geneticallyrelated to, but more evolved than, their host lavas and probablycrystallized at the margins of magma reservoirs. Modeling andcomparison with experimentally derived phases suggest an originat moderate pressures (0·5–0·9 GPa). Thehigh volatile contents of the alkalic host lavas may have pressurizedthe magma chambers and helped to propel the xenolith-bearinglavas directly from deep storage at the base of the lithosphereto the eruption site on the ocean floor, entraining fragmentsof the upper mantle and ocean crust cumulates from the underlyingabandoned spreading center. KEY WORDS: basaltic magmatism; continental margin seamounts; geothermobarometry; mineral chemistry; xenoliths     

Stress-driven Melt Segregation and Strain Partitioning in Partially Molten Rocks: Effects of Stress and Strain

The evolution of melt segregation in deforming partially moltenolivine-rich rocks has been studied in a series of laboratoryexperiments. During deformation, melt segregates into networksof anastamosing channels (or ‘bands’) surroundinglenses of melt-depleted material. We quantify the nature ofthe melt distribution in the samples, including thickness, angle,spacing, volume fraction, and melt fraction of melt-rich bands,to understand the dynamics of melt-network organization. Twoseries of experiments were designed to isolate the effects of(1) increasing shear strain (at similar stress levels), and(2) varying stress levels (deformed to similar shear strains).Melt-rich bands develop by a shear strain of unity. In samplesdeformed at varying stress levels, higher stress produces smallercharacteristic band spacings. We relate these variations tothe compaction length, _c, which varies only as a result of thereduction of matrix viscosity with increasing stress. Simpleapproaches to scaling from experimental to mantle conditionssuggest that stress-driven melt segregation can occur in theasthenosphere; if so, it will significantly affect rheological,transport and seismic properties, with enticing consequencesfor our understanding of plate–mantle interactions. KEY WORDS: melt segregation; rock rheology; magma transport; self-organization; mid-ocean ridges     

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     

A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland

The clinopyroxene–plagioclase–plagioclase dihedralangle, _cpp, in gabbroic cumulates records the time-integratedthermal history in the sub-solidus and provides a measure oftextural maturity. Variations in _cpp through the Layered Seriesof the Skaergaard intrusion, East Greenland, demonstrate thatthe onset of crystallization of clinopyroxene (within LZa),Fe–Ti oxides (at the base of LZc) and apatite (at thebase of UZb) as liquidus phases in the bulk magma is recordedby a stepwise increase in textural maturity, related to an increasein the contribution of latent heat to the total heat loss tothe surroundings and a reduction in the specific cooling rateat the crystallization front of the intrusion. The onset ofboth liquidus Fe–Ti oxide and apatite crystallizationis marked by a transient increase in textural maturity, probablylinked to overstepping before nucleation. Textural maturationat pyroxene–plagioclase–plagioclase triple junctionseffectively ceases in the uppermost parts of the Layered Seriesas a result of the entire pluton cooling below the closure temperaturefor dihedral angle change, which is 1075°C. Solidificationof the Layered Series of the Skaergaard intrusion occurred viathe upwards propagation of a mush zone only a few metres thick. KEY WORDS: magma; partial melting; asthenosphere; olivine; mantle     

Evidence of Magma Mixing in the 'Daly Gap' of Alkaline Suites: a Case Study from the Enclaves of Pantelleria (Italy)

The island of Pantelleria consists of trachytes, pantelleritesand minor mildly alkaline basalts. Rocks of intermediate composition(falling in the so-called ‘Daly Gap’) such as mugearites,benmoreites and mafic trachytes occur only in the form of enclavesin trachytes and pantellerites inside the main caldera of theisland (Caldera ‘Cinque Denti’), which collapsedduring the ‘Green Tuff’ ignimbrite eruption at 50ka. The enclaves include volcanic, subvolcanic and intrusiverock types. The enclaves in host trachyte contain traces ofglass; devitrified glass occurs within enclaves in host pantellerites.Minerals in the enclaves show regular compositional variationswith whole-rock silica content. Glass present in the medium-grainedsamples is interpreted to be the result of incipient melting.The major and trace element compositions of the enclaves showregular and linear variations between an evolved mafic magma(hawaiite) and a felsic end-member similar to the ‘GreenTuff’ trachyte. Fractional crystallization modelling ofcompatible and incompatible trace elements (V, Ni, Zr, La, Sm,Lu, Nb, Y, Th) does not reproduce the observed trends. Rocksof intermediate composition within the ‘Daly Gap’can be explained only by magma mixing between an already differentiatedmafic magma (hawaiite) and an anorthoclase-rich trachytic meltin the lower and higher parts, respectively, of a stratifiedmagmatic chamber. Medium-grained enclaves are interpreted asthe result of fragmentation of solidified mixing layers in theroof of the magma chamber during the eruption of the ‘GreenTuff’, when the collapse of the caldera took place. Diffusioncalculations suggest a residence time of <5 days for theenclaves in their host magmas. KEY WORDS: Daly Gap; enclaves; magma mixing; Pantelleria     

Petrogenesis of Cogenetic Silica-Oversaturated and -Undersaturated Syenites by Periodic Recharge in a Crustally Contaminated Magma Chamber: the Kangerlussuaq Intrusion, East Greenland

The Palaeogene Kangerlussuaq Intrusion (50 Ma) of East Greenlanddisplays concentric zonation from quartz-rich nordmarkite (quartzsyenite) at the margin, through pulaskite, to foyaite (nephelinesyenite) in the centre; modal layering and igneous laminationare locally developed but there are no internal intrusive contacts.This is an apparent violation of the phase relations in Petrogeny'sResidua System. We propose that this intrusion is layered, gradingfrom quartz syenite at the bottom to nepheline syenite at thetop. Mineral and whole-rock major and trace element data andSr–Nd–Hf–Pb isotope data are presented thatprovide constraints on the petrogenesis of the intrusion. Radiogenicisotope data indicate a continuously decreasing crustal componentfrom the quartz nordmarkites (⁸⁷Sr/⁸⁶Sr = 0·7061; _Ndi= 2·3; _Hfi = 5·2; ²⁰⁶Pb/²⁰⁴Pb_meas = 16·98)to the foyaites (⁸⁷Sr/⁸⁶Sr = 0·7043–0·7044;_Ndi = 3·8–4·9; _Hfi = 10·7–11·1;²⁰⁶Pb/²⁰⁴Pb_meas = 17·78–17·88); the foyaitesare dominated by a mantle isotopic signature. The average Mg-numberof amphibole cores becomes increasingly primitive, varying from26·4 in the nordmarkites to 57·4 in the pulaskites.Modal layering, feldspar lamination and the presence of hugebasaltic xenoliths derived from the chamber roof, now restingon the transient chamber floor, demonstrate bottom-upwards crystallization.The intrusion cannot, therefore, have formed in a system closedto magmatic recharge. The lack of gneissic xenoliths in thenordmarkites suggests that most contamination took place deeperin the crust. In the proposed model, the nordmarkitic magmaformed during crustal assimilation in the roof zone of a large,silica-undersaturated alkali basaltic/basanitic, stratifiedmagma chamber, prior to emplacement in the uppermost crust.The more primitive syenites, terminating with foyaite at thetop of the intrusion, formed as a consequence of repeated rechargeof the Kangerlussuaq Intrusion magma chamber by tapping lesscontaminated, more primitive phonolitic melt from deeper partsof the underlying chamber during progressive armouring of theplumbing system. KEY WORDS: Kangerlussuaq; East Greenland; syenite; crustal contamination; magma mixing     

Mechanism of Crystal Redistribution in a Sheet-like Magma Body: Constraints from the Nosappumisaki and Other Shoshonite Intrusions in the Nemuro Peninsula, Northern Japan

Processes of crystal separation in a magma heavily laden withcrystals without phase change are investigated from observationson frozen magma systems: Nosappumisaki and other shoshoniteintrusions in the Nemuro peninsula, Japan, for which the originof the crystals and the initial conditions are well constrained.The Nosappumisaki intrusion is 120 m in thickness and extendsfor more than 1·5 km. It exhibits a wide range of lithologicalvariation, principally as a result of crystal redistributionafter intrusion. Crystals in each lithology can be clearly dividedinto two kinds according to their composition and texture: thosepresent before the intrusion of the magma (‘phenocrysts’)and those that crystallized in situ after intrusion. From thevertical change in mode and size of ‘phenocrysts’,it is shown that (1) augite ‘phenocrysts’ were rapidlydeposited, with little overgrowth after intrusion, by significantcoagulation or clustering on a time-scale of more than a fewyears, and (2) plagioclase ‘phenocrysts’, definitelydenser than the melt but concentrated in the upper level, floatedby counter flow of massive deposition of augite ‘phenocrysts’.These results indicate that in a magma heavily laden with crystalsof a few millimeters in size (>20 vol. %), crystal–crystaland crystal–melt interaction play an important role inthe separation of crystals from the host melt. KEY WORDS: magma chamber; sill; crystal settling; plagioclase flotation; Nosappumisaki     

Distinguishing Melting of Heterogeneous Mantle Sources from Crustal Contamination: Insights from Sr Isotopes at the Phenocryst Scale, Pisgah Crater, California

Compositionally heterogeneous basaltic centers from a varietyof tectonic environments, including Pisgah Crater in the MojaveDesert region of California, exhibit secular changes in theirchemistry that might be explained by the sequential meltingof ultramafic to mafic mantle sources. We have analyzed phenocrystsfrom alkali basalts and hawaiites erupted at Pisgah Crater toinvestigate the effects of open-system modifications imposedon basaltic systems. We present ⁸⁷Sr/⁸⁶Sr data for individualphenocrysts of amphibole and clinopyroxene and the first publishedresults of single olivine grains, in addition to plagioclase.Each mineral phase exhibits a range in Sr isotope compositionthat may only partially overlap the isotopic composition ofthe other mineral phases, suggesting an interplay between twomagmatic end-members that continued up to the time of eruption.Limited ⁸⁷Sr/⁸⁶Sr variability in minerals from early and intermediatelavas indicates only moderate syn-crystallization open-systemmodification, whereas minerals in late-erupted lavas have muchhigher ⁸⁷Sr/⁸⁶Sr, consistent with extensive open-system modification.Rimward increases in ⁸⁷Sr/⁸⁶Sr of plagioclase confirm that thesechanges occurred within the stability field of plagioclase and,therefore, at crustal or near-crustal depths. The major elementcompositions of olivine-hosted melt inclusions indicate thatan Al-rich component of andesitic composition (⁸⁷Sr/⁸⁶Sr 0·7056),possibly derived from plagioclase-rich cumulates or pelites,was assimilated by magma generated from asthenosphere or younglithosphere with ⁸⁷Sr/⁸⁶Sr 0·7038. The results clearlydemonstrate the utility of measuring the ⁸⁷Sr/⁸⁶Sr of individualminerals and indicate that Pisgah Crater basalts probably acquiredisotopically enriched geochemical signatures from crustal contamination,rather than from mixing of heterogeneous mantle melts. KEY WORDS: assimilation; basalts; melt inclusions; minerals; Sr isotopes     

Overlap of Karoo and Ferrar Magma Types in KwaZulu-Natal, South Africa

A suite of mafic dykes from the Underberg region of southernKwaZulu-Natal (South Africa) were intruded at 178 Ma, coincidentin age with the major Okavango Dyke Swarm of Botswana, and alsocoincident with minor Karoo-related intrusions of the northernand central Lebombo. The dykes are all low-Ti–Zr tholeiites,they trend NW–SE and are presumed to continue into theKaroo central area of the Lesotho Highlands. In many respects,the Underberg dykes are similar to the majority of the low-Ti–Zrvolcanic and subvolcanic intrusions of the Karoo; however, their⁸⁷Sr/⁸⁶Sr and Nd isotope ratios are either ‘Ferrar-like’(⁸⁷Sr/⁸⁶Sr 0·710; Nd < –3) or transitional betweenKaroo low-Ti–Zr and Ferrar low-Ti magmas. A potentialFerrar source for at least some of the Underberg dykes is supportedby anisotropy of magnetic susceptibility analyses of the dykesuite, which demonstrate absolute flow direction from the SEto the NW, consistent with Gondwana reconstructions. The roleof crustal contamination and combined fractional crystallizationis also demonstrated to have played a key role in the petrogenesisof the Underberg dykes, involving a local upper crust contaminant.However, the composition of the ‘Ferrar-like’ dykescannot be easily explained by AFC processes, but they do demonstratethat melting of a lithospheric mantle source enriched to a smalldegree by subduction-derived fluid was also important. KEY WORDS: dyke; basalt; crustal contamination; large igneous province