The Influence of Viscosity on Fountains in Magma Chambers

Geological observations suggest that basaltic magmas mix readilybut that rhyolites and basalts can erupt through the same volcanicvent without mixing. The implication is that viscosity may havean important influence on the mixing of magmas. This paper examines the influence of viscosity on magma mixingwhen a magma of low viscosity is injected as a turbulent fountaininto a chamber containing a magma of higher viscosity. Threeseries of experiments were carried out. In the first, the lowviscosity fluid was injected into the tank at a Reynolds number(Re₁) of about 1000 to ensure that flow within the fountainwas fully turbulent. Both fluids were maintained at the sametemperature and the viscosity of the host fluid was varied systematically.If the viscosities of the two fluids are similar, turbulencein the fountain leads to extensive mixing and to formation ofa density-stratified layer which collects at the bottom of thetank. However, if the viscosity ratio of the fluids used isgreater than 400, no detectable mixing occurs. Motion withinthe fountain is again fully turbulent but the high momentumof the input fluid is not transmitted to the more viscous hostfluid. Experiments carried out at intermediate viscosity ratiosresulted in intermediate amounts of mixing. Theoretical considerations suggest that the criterion for mixingin turbulent fountains, when a fluid of low viscosity v₁ isinjected into a fluid of much higher viscosity v₂ is wd>kv₂ where w is the flow velocity through an input pipe of diameterd and k is a constant. This relationship is believed to be ageneral one, applying equally to jets and plumes, although thevalues of k may vary with the nature of the flow. The secondseries of experiments was designed to test this relationshipand determine values of k for fountains. The results show that,if wd/v₂ > 70, the inflowing fluid mixes with the host fluidas there was no viscosity difference between them. However,if wd/v₂ < 7, little or no mixing occurs even if motion withinthe fountain is fully turbulent. In the third series of experiments the temperature of the inputfluid was 70?C above that of the host fluid. This led to heatingofa thin boundary layer of the host fluid, lowering the viscosityof the host fluid adjacent to the fountain and thus allowingadditional mixing between the fluids. This effect was most obviousin the experiment with the highest viscosity ratio (1229) wherea small amount of mixing was detected, compared with no mixingin the equivalent experiment in the first series. The criterion for mixing, wd> kv₂ can be applied to naturalmagmas. The results show that if a primitive basaltic magmais injected, as a turbulent fountain or plume, into a chambercontaining fractionated basaltic magma, the two magmas willmix readily. However, if basaltic magma is injected into a chambercontaining granitic melt, little or no mixing occurs.     

On Convective Style and Vigor in Sheet-like Magma Chambers

The well known absence of magrnatic superheat is held here tobe a direct reflection of the ease and efficiency of large Rayleighnumber (Ra) convection in evacuating all convectable heat fromthe magma. Magmatic temperature is thus continually bufferedat or below the convective liquidus where the temperature differencedriving convection is vanishingly small and the governing Rais also always small regardless of body size. It is furtherheld here that for bodies where side wall cooling is of lesserimportance, the more common perception of magma chambers isof cooling from above and below where both the initial, isothermal(i.e. isodensity), and final (solid) states are dynamicallystable and that convection is necessarily a transient processconnecting these states. Extensive theoretical and experimentalstudies of cooling from above show that regardless of boundaryconditions transient, small Ra convection is independent oflayer thickness. Instead, convection is driven by formationof a thin, cool, and dense sublayer along the top boundary,and the characteristic length scale of the governing Rayleighnumber, which is time-dependent, is the sublayer thickness (d<<L).All dynamic features of the flow, including heat transfer relyon this length scale and not body thickness; virtually any sheet-likemagmatic body appears infinitely thick to such convection. BecauseRa is small, this transient stage persists for most, if notall, of the period of solidification to mush, whence the bodyis dynamically dead. Under conditions of strongly variable viscosity, only the leadingpart of d (i.e. d'), forward of a critical rheological front,is unstable. Convection itself is restricted to a region whereviscosity changes by no more than a factor of about 3 to 10.Most of the cool, dense sublayer is rigid, immobile crust, unableto participate in convection and cool the body. Rapid advanceof this crust due to cooling inhibits convection by consuminginstabilities before maturation to finite amplitude. Inclusionof solidification in the stability analysis changes the lengthscale in the governing Rayleigh number (Ra_v) to K/V (thermaldiffusivity/advance velocity). Ra is subcritical for large V₀(early times) and only with time becomes supcrcritical. Thisis in striking contrast to the usual Ra_L, which is initiallythe largest it will ever be. Because of continual collapse ofthe unstable sublayer, convection may remain near the criticalRa_v. Convection is thus initially weak and, because the heatflux from the system monotonically decreases with time due tothe thickening conductive crust and cooling, it is preventedfrom becoming indefinitely strong and instead slowly diminisheswith time. Conduction through the advancing crust is balanced by latentheat of crystallization at the crystallization front and convectionoccurs in response to this cooling. Because convection is confinedto the nearly isoviscous, nonsuperheated magma, crust growthis unaffected by convection, even when it is artificially forcedat unnatural rates. The crusts of Hawaiian lava lakes reflectthis in growing at the same rate regardless of lake thicknessand, in numerical convective modeling, imposed Rayleigh number.Overall cooling is well approximated by that of a stagnant,purely conducting layer whose central temperature is constantuntil arrival of the slowly moving cooling front In fact, therate of change of a body's central temperature is a direct measureof the total rate of heat transfer (i.e. Nusselt number, Nu)from that region. This is shown to be very nearly zero for Hawaiianlava lakes, precluding all but the weakest of convective heattransfer within the magma itself. The maximum heat transfer in terms of Nusselt number of anyunheated body within a conductive medium and always kept perfectlywell mixed thermally, relative to the same stagnant body, isshown to be Nu=2 regardless of shape and size. This thermalevolution is closely followed by previous calculations thatassume a large Rayleigh number based on layer thickness. Thermal convection in unheated, sheetlike magma chambers isa transient, sluggish process governed by solidification andsmall scale, small Rayleigh number instabilities; thermallydriven convective turbulence, in the usual sense, is out ofthe question.     

Bubbling Magma Chambers,Cupolas, and Porphyry Copper Deposits

Porphyry copper deposits are the major source of copper and significant sources of molybdenum, gold, and other metals. They are associated with the near-surface intrusion of small stocks of intermediate composition. They can form when H₂O-unsaturated magma is emplaced into wall rock that is cool enough that steep lateral thermal gradients create a narrow solidification front. At depths less than ～4 km, cooling and crystallization cause fluid saturation to occur within sidewall magma that is mobile because it contains less than ～25% suspended crystals. After a sufficient volume of bubbles forms, mobile sidewall magma buoyantly rises instead of sinking. The bubbles expand as they decompress, and at depths of ～2 km they become large enough to rise on their own. separate from the upwelled magma, and charge the cupola at the top of the stock with magmatic fluid. The partially degassed magma sinks into the interior of the stock.

Upwelling of saturated sidewall magma entrains deeper-seated, nearly saturated magma, which decompresses and saturates as it rises. As the system cools, the depth of H₂O saturation and sidewall upwelling increases. Bubbles of copper-rich fluid are generated where the saturation front extends to depths of ～6 km or more. Overall, the system is cooling, but the upward advection of heat maintains the cupola region at roughly constant position for the life of convective upwelling along the sidewalls.

Porphyry copper ore deposits can form where draining of the fluid pocket beneath a cupola is steady and a large volume of magma is cycled through the system. Magma in the stock that escapes to intrude commonly has a porphyritic texture because crystal growth is enhanced, and nucleation is suppressed when the magma is H₂O saturated. Porphyry copper deposits of common size can form during the solidification of large stocks. Super-giant porphyry copper deposits can form where the saturation front propagates from a stock into an underlying batholithic chamber with a magma volume on the order of 1000 km³ and a top at depths of 10 to 15 km.     

Isotopic Studies of Processes in Mafic Magma Chambers: I. The Kiglapait Intrusion, Labrador

The initial Nd and Sr isotopic ratios of cumulate rocks andminerals of the Kiglapait intrusion are pertinent as indicatorsof the processes that affected the Kiglapair magma while itwas resident in the crust. A Sm–Nd mineral isochron indicatesthat the crystallization age of the intrusion is 1305?22 millionyears. The initial _Nd values range from—1?6 in the LowerZone to 6 in the Upper Zone and correlate with the anorthitecontent of plagioclase. The initial ₈₇Sr/⁸⁶Sr ratios are 0.70407to 0.70433 in the Lower Zone, and increase monotonically up-section(decreasing plagioclase anorthite content) to 0.7068 in theUpper Zone. These variations are attributed to assimilationof roof rock concurrently with crystallization. It is evidentthat replenishment of the chamber with uncontaminated magmawas important during the accumulation of the lower zone rocks,but did not occurduring crystallization of the Upper Zone. Amathematical model relating isotope ratio shifts to the relativerates of crystallization, assimilation, and replenishment ispresented. It is estimated that the rate of assimilation wasbetween 0.01 and 0.04 of the crystallization rate, and thatduring the accumulation of the Lower Zone, the rate of replenishmentwas about half of the crystallization rate. A formulation for‘assimilation efficiency’is presented that relatedthe actual amount of assimilation (determined from the isotopedata) to the thermodynamic maximum allowable amount. The assimilationefficiency of the Kiglapait intrusion is not tightly constrained,but appears to be of the order of 0.01 near the end of crystallizationand perhaps as high as 0.07 near the beginning. Further quantitativeestimates of assimilation efficiency in different intrusionsemplaced under different conditions may aid in understandingassimilation mechanisms and in assessing the role of assimilationin magmatic evolution in general. Many layered intrusions studiedto date have initial Nd isotope ratios close to the chondriticvalue (_Nd= 0), suggestive of mantle magma sources that are chemicallyundepleted, and thus different from the sources of mid-oceanridge basalt and island-arec basalt.     

长白山天池火山双岩浆房岩浆作用与互动式喷发

广义的长白山火山在我国境内包括天池火山、望天鹅火山、图们江火山和龙岗火山,是我国最大的第四纪火山岩分布区。长白山各个火山区的火山活动具有此起彼伏的穿时性特征,天池火山之下地壳和地幔两个岩浆房具有上下呼应、互动式喷发之特点。一方面来自地幔的钾质粗面玄武岩浆直接喷出地表,在天池火山锥体内外形成诸多小火山渣锥;另一方面钾质粗面玄武岩浆持续补给地壳岩浆房,发生岩浆分离结晶作用和混合作用,形成双峰式火山岩特征并触发千年大喷发。西太平洋板块俯冲-东北亚大陆弧后引张是长白山天池火山喷发的动力学机制。     

The Discovery of Deep High-Resistivity Block and Inadequately Consolidated Magma Chambers in Gaoligongshan Oblique Collisional Orogen and Its Tectonic Implications

<正>Objective The Gaoligongshan oblique collisional orogen is located in the southern section of the Hengduan Mountains,and belongs to one of the main Late Yanshanian-Himalayan oblique collisional orogens in the Sanjiang area.Many researchers have studied the geology,geochemistry and geophysics of this region,and many research achievements have been obtained from deep geophysical exploration of the region,especially using the magnetotelluric(MT)sounding technique.However,detailed understanding     

多期岩浆分离结晶作用中微量元素丰度关系

本文提出了多期岩浆分离结晶中元素丰度关系的数学表达式;阐述了微量元素正向演化和反向演化的机理;指出了在不同岩浆分离阶段一些微量元素丰度关系常数(R)有明显变化,可以此作为阶段划分的依据。由峨嵋山玄武岩系及基性-超基性层状岩体研究所得出的元素丰度关系式,适用于其他成分岩浆分离结晶成因的岩浆岩系。本文着重列举数据较齐全的智利安底斯山脉玄武岩、玄武安山岩、安山岩、英安岩和流纹岩岩系的资料作为系统验证。     

席状岩浆房中的岩浆冷却及动力稳定性

关于岩浆房中岩浆的结晶分异方式一直存在着争议,其基本矛盾在于对岩浆冷却历史及热不稳定性可能引起的对流形式的理解各不相同。本文在充分考虑席状岩浆房的基本物理特征的基础上,建立了稳定岩浆体系的动态冷却模型,并以攀枝花层状岩体为例,计算了相应的温度场、密度场及粘度场。同时,提出了一种基于能量守恒的、对热不稳定性进行分析的新方法。计算了热不稳定性可能引起的对流强度、对流区域及对流时间,并深入讨论了在一定的热不稳定状态下,不同的岩浆屈服强度对对流特征参量的影响。结果表明,在该岩体的整个固化过程中,如果曾发生过自发对流,其对流也是极微弱而短暂的,不会影响中下部岩浆房中岩浆的固化和结晶。这一结论与对该岩体的实际观察相符。     

Polybaric Geochemical Evolution of Two Shield Volcanoes from Santorini, Aegean Sea, Greece: Evidence for Zoned Magma Chambers from Cyclic Compositional Variations

Petrographic, mineralogic, and geochemical data are reportedfor lavas from two of the major shield volcanoes of the Santorinivolcanic complex (Skaros and Micro Profitis Ilias), both ofwhich were active prior to the well-known Minoan eruption withassociated caldera collapse. Field work and whole-rock chemicalanalyses indicate four cycles of eruptive activity within theSkaros sequence and three within the Micro Profitis Ilias (MP1) sequence. SiO₂ and LIL-element contents decrease from thebase to the top of all cycles except for the uppermost cycleof Skaros. Chemical variations within cycles are interpretedto result from eruption from compositionally and thermally zonedmagma chambers. Major oxide data and the results of least-squares, mass balancemodeling indicate that fractional crystallization played animportant role in the development of the observed chemical variations.However, observed systematic variations in groundmass compositionswithin each cycle, observed irregular variations in total phenocrystcontent and the results of density calculations require thatgeneration of the chemical zonation did not involve crystalsettling but reflects unstable density stratification, probablyresulting from sidewall crystallization. Some of the primitivelavas erupted on Santorini preserve phenocryst and xenocrystevidence for a stage of high-pressure fractional crystallization(involving removal of olivine, clinopyroxene, orthopyroxene,and Cr-spinel). Trace element data combined with petrographic data (i.e. theoccurrence of abundant phenocrysts with resorption textures)is taken as evidence that magma mixing was also important inthe development of cyclic variations. Most basaltic andesitesfrom Skaros appear to be hybrids derived by mixing of basaltand andesite/dacite. Mineralogic data demonstrate that mixingwas also important in the development of zonation in the chambersbeneath MPI, but trace element data cannot be explained by combinedfractionation and mixing alone. Specifically, incompatible,and compatible element abundances are lower than predicted iffractionation and mixing occurred and it is suggested that theanomalous trace element behaviour of especially LIL elementsreflects the simultaneous operation of assimilation, for whichthere is support from isotopic studies. It is concluded thatinter- cyclic chemical variations are explicable in terms offractionation, mixing and assimilation. The LIL element andhighly compatible element concentrations in the most primitivelavas erupted in each cycle of Skaros and MPI increase withtime, indicating that mixing became more important with time.     

Das Magma in den Vulkanherden

Ohne Zusammenfassung     

Lateral Magma Flow in Mafic Sill-complexes

正The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes.Delineating the structure and volume of plumbing systems is,however,difficult because:(1)active     

The Role of Open-System Processes in the Development of Silicic Magma Chambers: a Chemical and Isotopic Investigation of the Fogo A Trachyte Deposit, Sao Miguel, Azores

The processes operating in the development of chemical zonationin silicic magma chambers have been addressed with a Sr–Nd–Pb–Hf–Thisotope study of the chemically zoned trachyte pumice depositof the Fogo A eruption, Fogo volcano, Azores. Sr isotopic variationis observed in whole rocks, glass separates and sanidine phenocrysts(whole-rock ⁸⁷Sr/⁸⁶Sr: 0·7049–0·7061; glass⁸⁷Sr/⁸⁶Sr: 0·7048–0·7052; sanidine ⁸⁷Sr/⁸⁶Sr:0·7048–0·7062). Thorium isotopic variationis observed in glass separates, with (²³⁰Th/²³²Th)_o rangingfrom 0·8737 to 0·8841, and exhibiting a negativecorrelation with Sr isotopes. The Nd, Pb and Hf isotopic compositionsof the whole-rock trachytic pumices are invariant and indistinguishablefrom basalts flanking the volcano. The Sr isotope variationsin the whole rocks are proposed to be the result of three distinctprocesses: contamination of the Fogo A magma by assimilationof radiogenic seawater-altered syenite wall rock, to explainthe Sr and Th isotopic compositions of the glass separates;incorporation of xenocrysts into the trachytic magma, requiredto explain the range in feldspar Sr isotopic compositions; andpost-eruptive surface alteration. This study emphasizes theimportance of determining the isotopic composition of glassand mineral separates rather than whole rocks when pre-eruptivemagmatic processes are being investigated. KEY WORDS: Azores; open-system processes; Sr isotopes; trachytic pumices; zoned magma chambers     

Zum Thema Magmagestein und Magma

Ohne Zusammenfassung     

Evolution of the Nyiragongo Magma

This paper deals with the rocks, mainly foiditic in composition,of the Nyiragongo volcano and compares them with the mainlytephritic to phonolitic rocks of the neighbouring Nyamuragiravolcano and with the South Kivu basaltic rocks. All these rocksoccur in the Lake Kivu area of the Western Rift Valley of CentralAfrica. Two hundred and sixty-four chemical analyses consideredrepresentative were selected of these rock groups and were subjectedto statistical evaluation. The Nyiragongo magma is considered to be of mantle origin withoutsignificant crustal admixture. The Nyamuragira magma and theNyiragongo magma are both potassic in character and, despitebeing geographically closely related, display pronounced differencesin chemistry. The fairly extreme composition of the Nyiragongomagma is interpreted as a result of a locally strong transferand enrichment of mainly alkalis with a passive depletion ofsilica.     

岩浆密度及其重要意义

本文通过计算不同构造环境的原生玄武岩岩浆密度,首次提出了两种原生玄武岩浆密度类型:岛弧玄武岩浆(AB)密度类型(Ⅰ);WB MORB岩浆密度类型(Ⅱ)。指出Ⅰ型岩浆密度受控于岩浆起源的fO2和fH2O条件;Ⅱ型岩浆密度则与岩浆起源压力有关。原生岩浆密度可作为判定岩浆产出的地球动力学背景的重要标志之一。考察岩浆密度与压力的关系,指出原生玄武岩浆密度在一定程度上控制了岩浆房的发育部位(深度),制约了岩浆结晶的温压条件,并对岩浆侵位与喷发产生重要影响。在岛弧地区,岩浆密度与地壳厚度变化严格地控制着岛弧岩石系列(Tb-CA-Sh)的时空演变,决定了岩浆的演化途径和结晶条件。利用岩浆密度资料,反演并建立了义敦岛弧区的地壳双层结构模式。     

Caledonian Magma Genesis and Crustal Recycling

The isotopic compositions of Sr, Nd, Pb and O together withabundance data for Rb, Sr, Sm, Nd, U and Pb are reported forsamples from the component parts of the c. 400 Ma old EtiveComplex, temporally and spatially related Lorne and Glencoelavas, and the Dalradian country rocks into which the Complexhas been emplaced. These and published data available for otherCaledonian granites are used to evaluate the petrogenesis ofthe Etive Complex in particular, and the role of crustal recyclingin the generation and evolution of the Caledonian granites ingeneral. Nd-isotope compositions of Etive samples at 400 Ma range from – 9.9 to – 4.7 compared with–8.4 < – 3.2 for the associated volcanics investigated here, and an estimatedvalue for depleted mantle 400 Ma ago at approximately ? 7. Dalradiancountry rocks have – 23.4 < – 7.5 and two partially digested metasedimentaryxenoliths within the granite have values of –9. 3 and -4.0. Initial ⁸⁷Sr/ ⁸⁶Sr ratios forthe Etive Complex range from 0–7043 to 0–7079, whereasDalradian metasediment in the immediate vicinity of the granitehas an initial ratio of 0–726. Oxygen isotopes in theComplex have 7. 6 per mil <¹⁸O < 10.0 per mil, all inexcess of typical values of mantle oxygen and reflect a crustalcomponent. An upper limit of 25 per cent Dalradian assimilationis set by the Nd-Sr isotopic variations with the granites andxenoliths. The Etive complex parent magma prior to Dalradian xenolith assimilationis estimated to have values between – 10 and – 5. In order to satisfy the Srand Pb isotope composition, additional components from a deepersource within the lithosphere (lower crust or continental lithosphericmantle) with relatively unradiogenic Sr, Nd and Pb are required. The crustal residence ages of the Etive Complex average about1.5 Ga, similar to those of many other late and post-tectonicCaledonian granites. The generation of the Etive Complex andCaledonian granites in general has been dominated by recyclingof the continental lithosphere, rather than the addition ofnew material from asthenospheric sources.     

岩浆混合作用:来自岩石包体的证据

镁铁质岩浆与长英质岩浆之间的混合作用是导致壳幔混源花岗岩类形成的主要机制。暗色、细粒且具火成结构的岩石包体是指示岩浆混合作用存在的可靠证据。这些岩石包体具有下列特征:(1)包体常呈等轴状,表明包体岩浆曾以液态球滴状存在于寄主岩浆中;(2)由于基性岩浆温度恒高于酸性岩浆(温度超出约300℃),这类包体常具有淬冷边;(3)包体有时含有反向脉;(4)包体中能见到自寄主岩浆捕获的长石捕虏晶。进一步分析了三个典型的含暗色微粒包体的花岗质杂岩(平潭、普陀山、花山—姑婆山)研究实例,认为暗色微粒包体的形成,可用来自深部岩浆房的玄武质岩浆向浅部酸性岩浆房的注入作用来解释。     

Magma in forearcs: implication for ophiolite generation

Forearc areas (“non-volcanic” arcs) of contemporary island arcs at convergent plate boundaries contain magmatic rocks. Geological evidence, seismic profiles, heat flow data, density considerations and petrological and geochemical arguments suggest that a forearc tholeiitic association (FAT) (containing high-Mg calc-alkaline andesites) is present in “non-volcanic” arcs at some stage of island-arc development. The fractionated, as well as primitive magma, is unable to penetrate low-density sediments and underplates thick piles of unconsolidated accreting rocks. The underplating causes upwelling. The occurrence of magma in forearcs provides an alternative interpretation for the tectonic setting of some ophiolitic masses. Rather than “ocean-ridge formation” and later “obduction” it offers an autochthonous (island-arc bound and geologically-substantiated) interpretation for the ophiolite suite.     

Magmatic Conditions and Magma Ascent as Indicated by Hornblende Phase Equilibria and Reactions in the 1995-2002 Soufriere Hills Magma

A study of amphiboles and associated minerals in samples ofSoufrière Hills andesite erupted from 1995 to 2002 showssignificant compositional variations within hornblende phenocrysts,a separate set of small pargasitic crystals in the groundmass,and two types of reaction rims on the phenocrysts. The compositionof the amphiboles and coexisting phases defines the thermalhistory of the erupting magma. As many as seven zones (<200µm wide) in the hornblende phenocrysts begin with a sharpincrease in Mg and Si, and then change gradually to a more Fe-and Al-rich hornblende, a transition that is consistent witha temperature rise. Analyses of the hornblende phenocrysts andassociated Fe–Ti oxides verify previous conclusions thatthe pre-eruption magma was at 130 MPa and 830 ± 10°C,but was variably heated before eruption. The heating occurredwithin