The fluid dynamics of a basaltic magma chamber replenished by influx of hot,dense ultrabasic magma

This paper describes a fluid dynamical investigation of the influx of hot, dense ultrabasic magma into a reservoir containing lighter, fractionated basaltic magma. This situation is compared with that which develops when hot salty water is introduced under cold fresh water. Theoretical and empirical models for salt/water systems are adapted to develop a model for magmatic systems. A feature of the model is that the ultrabasic melt does not immediately mix with the basalt, but spreads out over the floor of the chamber, forming an independent layer. A non-turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion. Both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock. The convection continues until the two layers have almost the same temperature. The compositions of the layers remain distinct due to the low diffusivity of mass compared to heat. The temperatures of the layers as functions of time and their cooling rate depend on their viscosities, their thermal properties, the density difference between the layers and their thicknesses. For a layer of ultrabasic melt (18% MgO) a few tens of metres thick at the base of a basaltic (10% MgO) magma chamber a few kilometres thick, the temperature of the layers will become nearly identical over a period of between a few months and a few years. During this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling. Olivines only settle after the two layers have nearly reached thermal equilibrium. At this stage residual basaltic melt segregates as the olivines sediment in the lower layer. Depending on its density, the released basalt can either mix convectively with the overlying basalt layer, or can continue as a separate layer. The model provides an explanation for large-scale cyclic layering in basic and ultrabasic intrusions. The model also suggests reasons for the restriction of erupted basaltic liquids to compositions with MgO<10% and the formation of some quench textures in layered igneous rocks.     

Textural and compositional evidence for magma mixing and its mechanism,Abu volcano group,southwestern Japan

Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO^*/MgO vs SiO₂ diagram. One trend is characterized by FeO^*/MgO-enrichment with a slight increase in SiO₂ content (Fe-type trend), whereas the other shows a marked SiO₂-enrichment with relatively constant FeO^*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.     

Compositional and thermal convection in magma chambers

Magma chambers cool and crystallize at a rate determined by the heat flux from the chamber. The heat is lost predominantly through the roof, whereas crystallization takes place mainly at the floor. Both processes provide destabilizing buoyancy fluxes which drive highly unsteady, chaotic convection in the magma. Even at the lowest cooling rates the thermal Rayleigh number Ra is found to be extremely large for both mafic and granitic magmas. Since the compositional and thermal buoyancy fluxes are directly related it can be shown that the compositional Rayleigh number Rs (and therefore a total Rayleigh number) is very much greater than Ra. In the case of basaltic melt crystallizing olivine Rs is up to 10⁶ times greater than Ra. However compositional and thermal buoyancy fluxes are roughly equal. Therefore thermal and compositional density gradients contribute equally to convection velocities in the interior of the magma. Effects of thermal buoyancy generated by latent heat release at the floor are included.The latent heat boundary layer at the floor of a basaltic chamber is shown to be of the order of 1 m thick with very low thermal gradients whereas the compositional boundary layer is about 1 cm thick with large compositional gradients. As a consequence, the variation in the degree of supercooling in front of the crystal-liquid interface is dominated by compositional effects. The habit and composition of the growing crystals is also controlled by the nature of the compositional boundary layer. Elongate crystals are predicted to form when the thickness of the compositional boundary layer is small compared with the crystal size (as in laboratory experiments with aqueous solutions). In contrast, equant crystals form when the boundary layer is thicker than the crystals (as in most magma chambers). Instability of the boundary layer in the latter case gives rise to zoning within crystals. Diffusion of compatible trace elements through the boundary layer can also explain an inverse correlation, observed in layered intrusions, between Ni concentration in olivine and the proportion of Ni-bearing phases in the crystallizing assemblage.     

The 1996 Eruption of Karymsky Volcano, Kamchatka: Historical Record of Basaltic Replenishment of an Andesite Reservoir

The simultaneous eruption in 1996 of andesite from Karymskyvolcano and of basalt from the Academy Nauk vent 6 km away appearsto provide a case of mafic recharge of an andesite reservoirfor which the time of recharge is exactly known and direct samplesof the recharging magma are available. The explosive phreato-magmaticeruption of basalt was terminated in less than 24 h, whereasandesite erupted continuously during the following 4 years.Detailed petrological study of volcanic ash, bombs and lavasof Karymsky andesite erupted during the period 1996–1999provides evidence for basaltic replenishment at the beginningof the eruptive cycle, as well as a record of compositionalvariations within the Karymsky magma reservoir induced by basalticrecharge. Shortly after the beginning of the eruption the compositionof the matrix glass of the Karymsky tephra became more maficand then, within 2 months, gradually returned to its originalstate and remained almost constant for the following 3 years.Further evidence for basaltic replenishment is provided by thepresence of xenocrysts of basaltic origin in the andesite ofKarymsky. A conspicuous portion of the plagioclase phenocrystsin the Karymsky andesite has calcic cores, with compositionsand textures resembling those of plagioclases in the AcademyNauk basalt. Similarly, the earlier portion of the andesiteof the eruption sequence contains rare olivines, which occuras resorbed cores in pyroxenes. The composition of the olivinematches that of olivines in the Academy Nauk basalt. The sequenceof events appears to be: (1) injection of basaltic magma intothe Karymsky chamber with immediate, compensating expulsionof pre-existing chamber magma from the Karymsky central vent;(2) direct mixing of basaltic and andesitic magmas with dispersalof phenocrysts associated with the basalt throughout the andesiteso that newly mixed magma appeared at the vent within 2 months;(3) re-establishment of thermal and chemical equilibrium withinthe reservoir involving crystallization in the new hybrid liquid,which returned the melt composition to ‘normal’,formed rims on inherited calcic plagioclase, and caused theresorption of dispersed olivine xenocrysts. Taken together,these findings indicate that the Karymsky magma reservoir wasrecharged by basalt at the onset of the 1996 eruptive cycle.The rapidity and thoroughness of mixing of the basalt with thepre-existing andesite probably reflects the modest contrastin temperature, viscosity, and density between the two magmas. KEY WORDS: Karymsky; Kamchatka; magma mixing; andesite; volcanic glass; plagioclase     

Differentiation of the Cuthbert Lake ultramafic dikes and related mafic dikes

In the Cuthbert Lake region of north-central Manitoba, northeasterly trending ultramafic-mafic dikes, part of the Molson dike swarm, show a range of composition from gabbro to olivine-hornblende pyroxenite to hornblende peridotite. The major dike which is ultramafic in composition is 60 m thick. Olivine and chromian spinel were the earliest cumulus phases formed in a subcrustal magma chamber before the emplacement of the dikes. Orthopyroxene and clinopyroxene were formed following emplacement at about 1120° C. Plagioclase and hornblende were the latest phases to crystallize from the intercumulus melt. Mineralogical and chemical variations across the major dike are interpreted to have resulted from flow differentiation of multiple injections of magma carrying suspended olivine crystals. Olivine phenocrysts changed their compositions from about Fo₈₇ to values ranging from Fo₈₀to Fo₇₃ as a function of the amount of intercumulus melt. The composition of this melt is estimated to have been basaltic. A mafic dike, about 10 m thick and occurring about 20 m away from the main ultramafic dike, is believed to have been formed from magmas that were tapped from an upper layer overlying the olivine-rich zone in a subcrustal magma chamber. Separation must have occurred when clinopyroxene and plagioclase appeared on the liquidus.Geological Survey of Canada Contribution 36486     

Timescales of convection in magma chambers below the Mid-Atlantic ridge from melt inclusions investigations

Closed hopper and complex swallowtail morphologies of olivine microcrysts have been described in the past in both mid-oceanic ridge basalts and subaerial tholeitic volcanoes and indicate fluctuations in magma undercooling. We describe similar morphologies in a Mid-Atlantic ridge pillow basalt (sample RD87DR10), and in addition we estimate the duration of temperature fluctuations required to produce these textures as follows: (1) Pairs of melt inclusions are arranged symmetrically around the centre of hopper crystals and each pair represents a heating–cooling cycle. Using the literature olivine growth rates relevant to the observed morphologies, and measuring the distance between two successive inclusions, we estimate the minimum time elapsed during one convection cycle. (2) The major element composition of melt inclusions (analysed by electron microprobe) was found to be in the range of the boundary layer measured in the glass surrounding the olivines, irrespective of their size. Several major elements demonstrate that this boundary layer results from rapid quenching on the seafloor, and not from crystal growth at depth, implying the inclusions had the same composition as the surrounding magma when they were sealed. Using diffusivity of slow diffusing elements such as Al₂O₃, we estimate the minimum time required for inclusion formation. These two independent approaches give concordant results: each cooling–heating cycle lasted between a few minutes and 1 h minimum. Thus, these crystals probably recorded thermal convection in small magmatic bodies (a dyke or shallow magma chamber) during the last hour or hours before eruption.     

The dynamics of xenolith assimilation

Xenolith assimilation has been simulated with experiments involving melting wax spheres into hot water and aqueous solutions and characterised by a theoretical analysis. Both the cases of neutrally buoyant stationary spheres and spheres sinking through the hot host fluid are examined. Melt generated on a sphere's surface flows (compositional convection) in two regimes; as a sheet over one hemisphere and then detaching as finger plumes from the other. Positional variations in the melting rate are dominantly controlled by differences in the melt layer thickness which influence the thermal gradient and heat flux across the layer. The theoretical model predicts melt layer thicknesses and the heat flux from the surrounding fluid. Calculated melting rates agree well with experimental measurements. Partial melting of non-eutectic compositions produces a layer of crystal-melt mush at the xenolith's surface. The theoretical analysis is extended to account for variation in rheological properties across the mush layer. When stoped into typical magmas, xenoliths of common continental crust lithologies are predicted to melt at rates in the order of 2 mm/hour. Thicknesses of the mobile mush layer around xenoliths are predicted to be typically a few centimetres. Relatively mafic lithologies can melt quicker than silicic compositions because, although they are typically more refractory, their lower melt viscosities result in thinner mush layers and so higher heat fluxes. Especially quick melting of water-saturated lithologies occurs as a consequence of both the reduction in melting temperatures and melt viscosities. Due to hot ambient conditions in the lower continental crust the assimilation of xenoliths into underplated basaltic magma can be very rapid. For granulites and mafic-granulites the predicted melting rates are up to 17 mm/hour. Fast rates of melting and efficient mixing of melt into the host magma indicate that assimilation of xenoliths will have a significant influence on the compositional and thermal evolution of magmas. Received: 7 November 1996 / Accepted: 5 January 1998     

Water content of primitive low-K tholeiitic basalt magma from Iwate Volcano,NE Japan arc: implications for differentiation mechanism of frontal-arc basalt magmas

The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78–85) correlates positively with the An content of coexisting plagioclase phenocrysts (An_85–92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4–5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa.     

Olivine: a monitor of magma evolutionary paths in kimberlites and olivine melilitites

Petrographic and chemical criteria indicate that the overwhelming majority of olivines in kimberlites are probably cognate phenocrysts. The implied low volume of xenocryst olivines requires that primitive kimberlite magmas are highly ultrabasic liquids. Two chemically distinctive olivine populations are present in all of the kimberlites studied. The dominant olivine population, which includes large rounded olivines and smaller euhedral crystals, is Mg-rich relative to late-stage rim compositions. It is characterized by a range in 100 Mg/(Mg + Fe) and uniform Ni concentration, reflecting Rayleigh-type crystallization during magma evolution. The most Mg-rich of these olivines are considered to be similiar to those in the mantle source rocks. The second compositional population, generally very subordinate, though markedly more abundant in the megacrystrich Monastery kimberlite, is Fe-rich relative to rim compositions. This group of olivines crystallized from evolved liquids in equilibrium with iron-rich megacrysts, both entrained by the kimberlite magma during ascent. Differences between the chemical fields of Fe-rich olivines in Group I and Group II kimberlites point to relatively deeper derivation of the latter suite. Olivine chemistry can be used to characterize kimberlite magma sub-types, and may prove to be a useful tool for evaluating the diamond potential of kimberlites.     

Evolution of melt composition during intrusion of basalts into a silicic magma chamber

The article describes heat exchange between basaltic and rhyolite melts accompanied by fractional crystallization of phases in a basaltic melt. A numerical model has been developed for the homogenization mechanism of magma composition during intrusion of basaltic magma batches into felsic magma chambers. The results of numerical modeling demonstrate that the time needed for cooling the basalts and their fractionation to rhyolite melts is much shorter than the time required for chemical interaction based on diffusive mechanisms.     

加勒比海小安德列斯岛弧Kick’emJenny海底火山岩的斜长石成分环带:示踪大洋岛弧岩浆房的演化

我们对采自于加勒比海地区小安德列斯岛弧(Lesser Antilles Arc)Kick’em Jenny(KEJ)海底火山玄武岩中的斜长石斑晶进行了矿物形态和成分分析。利用电子探针(EMPA)和LA-ICP-MS测定了具有环带结构的斜长石斑晶中主量元素的空间分布,同时也利用LA-ICP-MS分析了斜长石中Sr的分布。结果表明,在不同的矿物斑晶中,元素含量均表现出和环带结构相联系的空间分布变化。斜长石斑晶中最主要的结构为韵律环带以及熔蚀结构,所测定的矿物边缘都存在An值从由内向外迅速降低的致密韵律环带,可能反映了快速结晶时的不平衡;而晶体内部的稀疏韵律环带结构是由岩浆填充或对流活动导致的。部分斜长石的熔蚀层An值由内向外升高,反映了高Ca岩浆填充的过程。这说明斜长石斑晶的矿物形态和元素环带可以用来制约俯冲带海底火山岩浆从源区上升到岩浆房再到喷发的复杂过程,包括岩浆演化、熔体多次填充、熔体与结晶矿物之间的反应、以及矿物再熔融等。这对于理解海底火山的喷发以及岛弧岩浆岩的演化有重要意义。     

Mixing of rhyolite,trachyte and basalt magma erupted from a vertically and laterally zoned reservoir,composite flow P1, Gran Canaria

The 14.1 Ma composite welded ignimbrite P1 (45 km³ DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km³) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite magma group (6 km³); (3) a minor fraction of Na-poor trachyandesite (<1 km³); and (4) nearly aphyric basalt (26 km³) zoned from 4.3 to 5.2 wt% MgO. We distinguish three sites and phases of mixing: (a) Mutual mineral inclusions show that mixing between trachytic and rhyolitic magmas occurred during early stages of their intratelluric crystallization, providing evidence for long-term residence in a common reservoir prior to eruption. This first phase of mixing was retarded by increasing viscosity of the rhyolite magma upon massive anorthoclase precipitation and accumulation. (b) All component magmas probably erupted through a ring-fissure from a common upper-crustal reservoir into which the basalt intruded during eruption. The second phase of mixing occurred during simultaneous withdrawal of magmas from the chamber and ascent through the conduit. The overall withdrawal and mixing pattern evolved in response to pre-eruptive chamber zonation and density and viscosity relationships among the magmas. Minor sectorial variations around the caldera reflect both varying configurations at the conduit entrance and unsteady discharge. (c) During each eruptive pulse, fragmentation and particulate transport in the vent and as pyroclastic flows caused additional mixing by reducing the length scale of heterogeneities. Based on considerations of magma density changes during crystallization, magma temperature constraints, and the pattern of withdrawal during eruption, we propose that eruption tapped the P1 magma chamber during a transient state of concentric zonation, which had resulted from destruction of a formerly layered zonation in order to maintain gravitational equilibrium. Our model of magma chamber zonation at the time of eruption envisages a basal high-density Na-poor trachyandesite layer that was overlain by a central mass of highly phyric rhyolite magma mantled by a sheath of vertically zoned trachyte-trachyandesite magma along the chamber walls. A conventional model of vertically stacked horizontal layers cannot account for the deduced density relationships nor for the withdrawal pattern.     

The chemistry of appinitic rocks associated with the Ardara pluton,Donegal, Ireland

Seventeen chemical analyses are given of rocks from appinitic intrusions associated with the Ardara granitic pluton in Donegal, northwest Ireland. The analyses show a series of basic and ultrabasic compositions, the high Al₂O₃, low MgO members of which are closely comparable to basalts, apart from their high water contents. The Ardara appinites are compared with other Caledonian appinitic rocks, and with basic and ultrabasic igneous rocks of other associations, from which the appinitic rocks are shown to be chemically quite distinct. The formation of the appinites is ascribed to crystal accumulation in a basaltic magma enriched in water. The granites associated with appinitic rocks also have distinctive compositions, indicative of formation under a high water pressure.     

长白山火山岩浆柱岩浆上升作用过程

长白山火山岩浆柱是一个在长白山区地下总体呈串珠状排列的向东南倾斜的层状富岩浆集合体,岩浆柱宽度宽者300～500 km,窄者30～50 km,深度延伸可达上千km。在这个岩浆柱内,热物质聚集与挥发份富集可以发生部分熔融而形成不同成分与密度的岩浆,岩浆聚集上升至某个深度时的停滞聚集又可形成水平向扩展的岩浆房,压力作用下岩浆房内岩浆演化出密度较轻的岩浆则可进一步上升直至喷出地表。天池火山的母岩浆粗面玄武岩来自地幔岩浆库,由其演化形成的碱型系列粗面岩类和碱流岩类岩石则来自地壳岩浆房。拉斑玄武岩系列的偏酸性岩石来源的地壳岩浆房与碱型系列碱流岩来源的地壳岩浆房深度位置也不相同。天池火山造盾玄武岩TiO2含量和SiO2含量之间反相关关系不能单纯用岩浆房分异结晶来解释,TiO2含量较高的样品代表了源区地幔的较低熔融程度的熔体,而低程度熔融的岩浆来源于更深的位置。玄武质岩浆“熔融结束”的深度随时间的增加而增加的过程控制了岩浆形成深度随时间的增加而增加并且岩浆形成速率随时间的增加而降低的规律。天池火山碱流质岩浆房千年大喷发时岩浆超压极大值Δpmax=625 MPa,层状岩浆房半径35 km,喷出岩浆层厚700 m,喷出岩浆体积30 km3;粗面质喷发的岩浆房超压极大值Δpmax=15 MPa以上。天池火山千年大喷发时临界喷发熔体黏度μcritm>27×1010 Pa·s-1,碱流质岩浆是从一个粗面质岩浆母体经几万年的结晶分异时间演化得来的。气象站寄生火山活动喷发前临界熔体黏度μcritm=12×1011 Pa·s-1,这极高的熔体黏度与喷发物中含有大量晶体与气泡相吻合。千年大喷发级别的大规模喷发周期上万年,远大于小规模喷发几百年以内的时间周期。天池火山作用造盾阶段因为玄武岩都直接喷出了地表,多数传导与扩散的岩浆热都没有用于加热深地壳,所以早期加热效率不高。在1～16 Ma之后造锥阶段在深地壳内形成残余的部分熔融带并阻止了玄武岩的喷发,系统的热效率变得很高,残余熔体生产率也就得到了加速。全新世造伊格尼姆岩喷发阶段大量的演化的碱流质残余熔体因重力不稳定而侵入上地壳内,并且形成大得足以引起造破火山口喷发的岩浆房。     

Fluid and melt inclusions in the Mesozoic Fangcheng basalt from North China Craton: implications for magma evolution and fluid/melt-peridotite reaction

Melt inclusions and fluid inclusions in the Fangcheng basalt were investigated to understand the magma evolution and fluid/melt-peridotite interaction. Primary silicate melt inclusions were trapped in clinopyroxene and orthopyroxene phenocrysts in the Fangcheng basalt. Three types of melt inclusions (silicate, carbonate, and sulfide) coexisting with fluid inclusions occur in clinopyroxene xenocrysts and clinopyroxene in clinopyroxenite xenoliths. In situ laser-ablation ICP-MS analyses of major and trace element compositions on individual melt inclusions suggest that the silicate melt inclusions in clinopyroxene and orthopyroxene phenocrysts were trapped from the same basaltic magma. The decoupling of major and trace elements in the melt inclusions indicates that the magma evolution was controlled by melt crystallization and contamination from entrapped ultramafic xenoliths. Trace element patterns of melt inclusions are similar to those of the average crust of North China Craton and Yangtze Craton, suggesting a considerable crustal contribution to the magma source. Calculated parental melt of the Fangcheng basalt has features of low MgO (5.96 wt%), high Al₂O₃ (16.81 wt%), Sr (1,670 ppm), Y (>35 ppm), and high Sr/Y (>40), implying that subducted crustal material was involved in the genesis of the Fangcheng basalt. The coexisting fluid and melt inclusions in clinopyroxene xenocrysts and in clinopyroxene of xenoliths record a rare melt-peridotite reaction, that is olivine + carbonatitic melt₁ (rich in Ca) = clinopyroxene + melt₂ ± CO₂. The produced melt₂ is enriched in LREE and CO₂ and may fertilize the mantle significantly, which we consider to be the cause for the rapid replacement of lithospheric mantle during the Mesozoic in the region.     

Sugarloaf Mountain,central Arizona,USA: A small-scale example of Miocene basalt-rhyolite magma mixing to yield andesitic magmas

Sugarloaf Mountain is a 200-m high volcanic landform in central Arizona, USA, within the transition from the southern Basin and Range to the Colorado Plateau. It is composed of Miocene alkalic basalt (47.2–49.1?wt.% SiO₂; 6.7–7.7?wt.% MgO) and overlying andesite and dacite lavas (61.4–63.9?wt.% SiO₂; 3.5–4.7?wt.% MgO). Sugarloaf Mountain therefore offers an opportunity to evaluate the origin of andesite magmas with respect to coexisting basalt. Important for evaluating Sugarloaf basalt and andesite (plus dacite) is that the andesites contain basaltic minerals olivine (cores Fo_76-86) and clinopyroxene (～Fs_9-18Wo_35-44) coexisting with Na-plagioclase (An_48-28Or_1.4–7), quartz, amphibole, and minor orthopyroxene, biotite, and sanidine. Noteworthy is that andesite mineral textures include reaction and spongy zones and embayments in and on Na-plagioclase and quartz phenocrysts, where some reacted Na-plagioclases have higher-An mantles, plus some similarly reacted and embayed olivine, clinopyroxene, and amphibole phenocrysts.Fractional crystallization of Sugarloaf basaltic magmas cannot alone yield the andesites because their ～61 to 64?wt.% SiO₂ is attended by incompatible REE and HFSE abundances lower than in the basalts (e.g., Ce 77–105 in andesites vs 114–166?ppm in basalts; Zr 149–173 vs 183–237; Nb 21–25 vs 34–42). On the other hand, andesite mineral assemblages, textures, and compositions are consistent with basaltic magmas having mixed with rhyolitic magmas, provided the rhyolite(s) had relatively low REE and HFSE abundances. Linear binary mixing calculations yield good first approximation results for producing andesitic compositions from Sugarloaf basalt compositions and a central Arizona low-REE, low-HFSE rhyolite. For example, mixing proportions 52:48 of Sugarloaf basalt and low incompatible-element rhyolite yields a hybrid composition that matches Sugarloaf andesite well ? although we do not claim to have exact endmembers, but rather, viable proxies. Additionally, the observed mineral textures are all consistent with hot basalt magma mixing into rhyolite magma. Compositional differences among the phenocrysts of Na-plagioclase, clinopyroxene, and amphibole in the andesites suggest several mixing events, and amphibole thermobarometry calculates depths corresponding to 8–16?km and 850° to 980?°C. The amphibole P-T observed for a rather tight compositional range of andesite compositions is consistent with the gathering of several different basalt-rhyolite hybrids into a homogenizing ‘collection' zone prior to eruptions. We interpret Sugarloaf Mountain to represent basalt-rhyolite mixings on a relatively small scale as part of the large scale Miocene (～20 to 15 Ma) magmatism of central Arizona. A particular qualification for this example of hybridization, however, is that the rhyolite endmember have relatively low REE and HFSE abundances.     

Genesis and evolution of subduction-zone andesites: evidence from melt inclusions

Andesite magmatism plays a major role in continental crustal growth, but its subduction-zone origin and evolution is still a hotly debated topic. Compared with whole-rock analyses, melt inclusions (MIs) can provide important direct information on the processes of magma evolution. In this article, we synthesize data for melt inclusions hosted by phenocrysts in andesites, extracted from the GEOROC global compilation. These data show that melt inclusions entrapped by different phenocrysts have distinct compositions: olivine-hosted melt inclusions have basalt and basaltic andesite compositions, whereas melt inclusions in clinopyroxene and othopyroxene are mainly dacitic to rhyolitic. Hornblende-hosted melt inclusions have rhyolite composition. The compositions of melt inclusions entrapped by plagioclase are scattered, spanning from andesite to rhyolite. On the basis of the compositional data, we propose a mixing model for the genesis of the andesite, and a two-chamber mechanism to account for the evolution of the andesite. First, andesite melt is generated in the lower chamber by mixing of a basaltic melt derived from the mantle and emplaced in the lower crust with a felsic melt resulting from partial melting of crustal rocks. Olivine and minor plagioclase likely crystallize in the lower magma chamber. Secondly, the andesite melt ascends into the upper chamber where other phenocrysts crystallize. According to SiO₂-MgO diagrams of the MIs, evolution of the andesite in the upper chamber can be subdivided into two distinct stages. The early stage (I) is characterized by a phenocrystal assemblage of clinopyroxene + othopyroxene + plagioclase, whereas the late stage (II) is dominated by crystallization of plagioclase + hornblende.     

The Generation of Granitic Magmas by Intrusion of Basalt into Continental Crust

When basalt magmas are emplaced into continental crust, meltingand generation of silicic magma can be expected. The fluid dynamicaland heat transfer processes at the roof of a basaltic sill inwhich the wall rock melts are investigated theoretically andalso experimentally using waxes and aqueous solutions. At theroof, the low density melt forms a stable melt layer with negligiblemixing with the underlying hot liquid. A quantitative theoryfor the roof melting case has been developed. When applied tobasalt sills in hot crust, the theory predicts that basalt sillsof thicknesses from 10 to 1500 m require only 1 to 270 y tosolidify and would form voluminous overlying layers of convectingsilicic magma. For example, for a 500 m sill with a crustalmelting temperature of 850 ?C, the thickness of the silicicmagma layer generated ranges from 300 to 1000 m for countryrock temperatures from 500 to 850?C. The temperatures of thecrustal melt layers at the time that the basalt solidifies arehigh (900–950?C) so that the process can produce magmasrepresenting large degrees of partial fusion of the crust. Meltingoccurs in the solid roof and the adjacent thermal boundary layer,while at the same time there is crystallization in the convectinginterior. Thus the magmas formed can be highly porphyritic.Our calculations also indicate that such magmas can containsignificant proportions of restite crystals. Much of the refractorycomponents of the crust are dissolved and then re-precipitatedto form genuine igneous phenocrysts. Normally zoned plagioclasefeldspar phenocrysts with discrete calcic cores are commonlyobserved in many granitoids and silicic volcanic rocks. Suchpatterns would be expected in crustal melting, where simultaneouscrystallization is an inevitable consequence of the fluid dynamics. The time-scales for melting and crystallization in basalt-inducedcrustal melting (10²–10³ y) are very short compared tothe lifetimes of large silicic magma systems (>10⁶ y) orto the time-scale for thermal relaxation of the continentalcrust (> l0⁷ y). Several of the features of silicic igneoussystems can be explained without requiring large, high-level,long-lived magma chambers. Cycles of mafic to increasingly largevolumes of silicic magma with time are commonly observed inmany systems. These can be interpreted as progressive heatingof the crust until the source region is partially molten andbasalt can no longer penetrate. Every input of basalt triggersrapid formation of silicic magma in the source region. Thismagma will freeze again in time-scales of order l0²–10³y unless it ascends to higher levels. Crystallization can occurin the source region during melting, and eruption of porphyriticmagmas does not require a shallow magma chamber, although suchchambers may develop as magma is intruded into high levels inthe crust. For typical compositions of upper crustal rocks,the model predicts that dacitic volcanic rocks and granodiorite/tonaliteplutons would be the dominant rock types and that these wouldascend-from the source region and form magmas ranging from thosewith high temperature and low crystal content to those withhigh crystal content and a significant proportion of restite.     

Density changes during the fractional crystallization of basaltic magmas: fluid dynamic implications

The dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes. In this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization. Removal of olivine and pyroxene, whose values of fractionation density are larger than the density of the magma, causes the density of residual liquid to decrease. Removal of plagioclase, with fractionation density less than the magma density, can cause the density of residual liquid to increase. During the progressive differentiation of basaltic magma, density decreases during fractionation of olivine, olivine-pyroxene, and pyroxene assemblages. When plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum. Calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects.In the closed-system evolution of basaltic magma, several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization, as well as on the geometry of the chamber. In an early stage of the evolution, where olivine and/or pyroxenes are the fractionating phases, compositional stratification can occur due to side-wall crystallization and replenishment by new magma, with the most differentiated magma tending to accumulate at the roof of the chamber. When plagioclase becomes a fractionating phase a zone of well-mixed magma with a composition close to the density minimum of the system can form in the chamber. The growth of a zone of constant composition destroys the stratification in the chamber. A chamber of well-mixed magma is maintained while further differentiation occurs, unless the walls of the chamber slope inwards, in which case dense boundary layer flows can lead to stable stratification of cool, differentiated magma at the floor of the chamber.In a basaltic magma chamber replenished by primitive magma, the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma. Successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber. In a chamber containing well-mixed, plagioclase — saturated magma, the primitive magma can be either denser or lighter than the resident magma. In the first case, the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma. Mixing then occurs between magmas of different temperatures and compositions. In the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma.     

Concentric zoning in the Tunk Lake pluton,coastal Maine

The Tunk Lake pluton of coastal Maine, USA is a concentrically zoned granitic body that grades from an outer hypersolvus granite into subsolvus rapakivi granite, and then into subsolvus non-rapakivi granite, with gradational contacts between these zones. The pluton is partially surrounded by a zone of basaltic and gabbroic enclaves, interpreted as quenched magmatic droplets and mushes, respectively, as well as gabbroic xenoliths, all hosted by high-silica granite. The granite is zoned in terms of mineral assemblage, mineral composition, zircon crystallization temperature, and major and trace element concentration, from the present-day rim (interpreted as being closer to the base of the chamber) to the core (interpreted as being closer to the upper portions of the chamber). The ferromagnesian mineral assemblage systematically changes from augite and hornblende with augite cores in the outermost hypersolvus granite to hornblende, to hornblende and biotite, and finally, to biotite only in the subsolvus granite core of the pluton. Sparse fine-grained basaltic enclaves that are most common in the outermost zone of the pluton suggest that basaltic magma was present in the lower portions of the magma chamber at the same time that the upper portions of the magma chamber were occupied by a granitic crystal mush. However, the slight variations in initial Nd isotopic ratio in granites from different zones of the pluton suggest that contamination of the granitic melt by basaltic melt played little role in generating the compositional gradation of the pluton. The zone of basaltic and gabbroic chilled magmatic enclaves, and gabbroic xenoliths, hosted by high-silica granite, that partially surround the pluton is interpreted as mafic layers at the base of the pluton that were disrupted by invading late-stage high-silica magma. These mafic layers are likely to have consisted of basaltic lava layers and basalt that chilled against granitic magma to produce coarse-grained gabbroic mush. Basaltic and gabbroic magmatic enclaves and gabbroic xenoliths are hornblende-bearing, suggesting that their parent melts were relatively hydrous. The water-rich nature of the underplating mafic magmas may have prevented extensive invasion of the granitic magma by these magmas, owing to the much greater viscosity of the granitic magma than the mafic magmas in the temperature range over which magma interaction could have occurred.