首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
The late Quaternary Laacher See Tephra (LLST, MLST, ULST: Lower,Middle, Upper Laacher See Tephra) of the East Eifel volcanicfield (W. Germany) is zoned from highly evolved, volatile-richand crystal-poor phonolite at its base towards a mafic, crystal-richphonolite at the top of the deposit. This is shown by phenocrystabundances, major and trace element chemistry of whole rocks,matrix-glass and some mineral compositions. MgO content in wholerocks ranges from 0.07 wt. per cent in lower LLST to 0.85 wt.per cent in ULST phonolite. Late mafic hybrid pyroclasts containup to 7.0 wt. per cent MgO. Na2O shows a reverse trend from11.7 wt. per cent in LLST to 5.1 per cent in ULST. Trace elementsare divided into depleted (compatible) elements (e.g. Sr, Sc,Co), and enriched (incompatible) elements (e.g. Zn, Zr, Nb).‘Semi-compatible’ elements (Ta, Y) show minimumconcentrations at an intermediate stratigraphic level (MLST).All compositional gradients are smooth showing a major compositionalinterface between LLST/MLST and ULST. Twelve phenocryst phasesoccur: sanidine, plagioclase, hauyne, amphibole, clinopyroxene,sphene, apatite, Ti-magnetite, biotite, nepheline, cancriniteand zircon. The latter three are restricted to LLST phonolite.There is a steep gradient in mineral abundances from the LLSTand MLST to the mafic ULST phonolite. Microprobe analyses definetwo phenocryst populations: (a) Fe, Na and Mn-rich evolved phenocrysts(hastingsite, aegirine-augite, Ab-rich plagioclase) which predominatewithin highly differentiated (LLST) phonolite; (b) Mg and Ti-richand Fe, Na, Mn-poor phenocrysts, which are most abundant inULST. This compositional zonation is interpreted as the resultof continuous eruption from a zoned magma column; highly evolvedearly magma representing the upper part and mafic late phonoliticmagma representing the lower part of a stratified magma chamberemplaced at shallow, crustal levels.  相似文献   

2.
Twenty six whole rocks, seven matrix and fifty three mineral separates from the compositionally zoned late Quaternary Laacher See tephra sequence (East Eifel, W Germany) were analyzed by instrumental neutron activation. These data document the chemical variation within the Laacher See magma chamber prior to eruption with a highly fractionated phonolite at the top and a more mafic phonolite at its base as derived from other data. Incompatible elements such as Zn, Zr, Nb, Hf, U, light and heavy rare earths are extremely enriched towards the top whereas compatible elements (e.g. Sr, Sc, Co, Eu) are strongly depleted. Semicompatible elements (Ta and some middle REE) are depleted at intermediate levels. This chemical variation is shown by whole rock and matrix data indicating the phonolite liquid was compositionally zoned regardless of phenocryst content. Hybrid rocks (phonolite-basanite) show the largest concentrations for compatible elements. All elements (except Rb) display continuous compositional variations with regard to the stratigraphic position of pumice samples. From these data we are able to distinguish three main units: An early erupted highly fractionated magma, the main volume of evolved phonolite and a mafic phonolite as the final products. The extreme variation of trace element distribution coef ficients (K) for 9 mineral phases with respect to stratigraphic position (resp. matrix composition) cannot be explained by conventional mechanisms. We postulate a significant modification of the trace element content of the phonolite melt by liquid-liquid controlled differentiation processes subsequent to and/or contemporaneous with (fractional) crystallization which caused disequilibrium between phenocrysts and host matrix. Therefore, our “distribution coefficients” deviate from equilibrium partition coefficients equivalent to the amount of this post crystallization modification of the matrix composition. The relationship between varying K and matrix composition is demonstrated by a ΔK-ΔM-diagram (variation of K versus variation of matrix, M). Different parts of this diagram relate to different parameters (T, P, polymerization, complex-building, equilibrium crystallization in a zoned magma column and post crystallization disequilibrium effects) which are responsible for the variation of distribution coefficients in general. The ΔK-ΔM-diagram may allow to distinguish between different processes affecting the distribution coefficients measured in natural volcanic rocks from a differentiating magma system.  相似文献   

3.
Zoned eruption deposits commonly show a lower felsic and an upper mafic member, thought to reflect eruption from large, stratified magma chambers. In contrast, the Montaña Reventada composite flow (Tenerife) consists of a lower basanite and a much thicker upper phonolite. A sharp interface separates basanite and phonolite, and chilled margins at this contact indicate the basanite was still hot upon emplacement of the phonolite, i.e. the two magmas erupted in quick succession. Four types of mafic to intermediate inclusions are found in the phonolite. Inclusion textures comprise foamy quenched ones, others with chilled margins and yet others that are physically mingled, reflecting progressive mixing with a decreasing temperature contrast between the end-members. Analysis of basanite, phonolite and inclusions for majors, traces and Sr, Nd and Pb isotopes show the inclusions to be derived from binary mixing of basanite and phonolite end-members in ratios of 2:1 to 4:1. Although, basanite and phonolite magmas were in direct contact, contrasting 206Pb/204Pb ratios show that they are genetically distinct (19.7193(21)–19.7418(31) vs. 19.7671(18)–19.7807(23), respectively). We argue that the Montaña Reventada basanite and phonolite first met just prior to eruption and had limited interaction time only. Montaña Reventada erupted from the transition zone between two plumbing systems, the phonolitic Teide-Pico Viejo complex and the basanitic Northwest rift zone. A rift zone basanite dyke most likely intersected the previously emplaced phonolite magma chamber. This led to eruption of geochemically and texturally unaffected basanite, with the inclusion-rich phonolite subsequently following into the established conduit.  相似文献   

4.
Volcanic activity in Askja central volcano and its fissure swarmin 1875 occurred in response to a crustal rifting episode inIceland, resulting in up to 70 km lateral flow of magma withinthe crust, caldera collapse and a plinian eruption of acid magma(0·2 km3 dense-rock equivalent). Petrologic studies ofthe predominantly rhyolitic and crystal-poor ejecta reveal thata complex array of other liquid compositions was also present,including icelandite (0.75 per cent) and basalt (1.9 per cent),as well as leucocratic xenoliths of trondhjemite type. Mineralgeothermometers indicate that the rhyolite evolved at 990 to1010 °C and 0·5 Kb PH2O, the icelandite at 1005 to1020 °C and at fO2 10–10 atm. and the basalt at 1140to 1170 °C. A petrologic model of Askja in 1875 consists of a density-stratifiedmagma chamber with a rhyolitic upper part and a lower part offerrobasalt, with an intervening layer of icelandite. The modelcalculations show that the icelandite can be derived from ferrobasaltby 50 per cent fractional crystallization, but one-stage fractionalcrystallization models cannot account for generation of theacid magma. Simple partial or complete fusion of the field-associatedtrondhjemite xenoliths cannot produce the acid magma. Instead,a more complex fusion, hybridization and fractional crystallizationmodel is presented, which is consistent with the available petrologicevidence. This model involves large-scale fusion of pre-existingtrondhjemite intrusions or reactivation of previously consolidatedroof-rock in the magma chamber followed by hybridization ofthe acid magma with 7 to 14 per cent basaltic magma. Finally,10 to 11 per cent fractional crystallization of the dacite hybridis required to produce the observed compositional range withinthe rhyolite ejecta. The 1875 explosive eruption was causedby the ascent of tholeiitic basalt magma from depth during crustalrifting. Influx of new basalt magma in 1874–75 triggeredconvective mixing and hybridization in the compositionally zonedmagma chamber.  相似文献   

5.
Calc-alkaline olivine andesite and two-pyroxene dacite of theTaos Plateau volcanic field evolved in an open magmatic system.mg-numbers of spatially and temporally associated ServilletaBasalt (54–61) and ohvine andesite (49–59) are comparableand preclude fractional crystallization of ferromagnesian mineralsas the major differentiation process. If Servilleta olivinetholeiite is assumed to be the parental magma type, enrichmentsof highly incompatible trace elements (up to 17 ?) oVer concentrationsin the basalts require that andesitic and dacitic magmas containa substantial proportion of assimilated crust. Isotopic compositionsof andesite and dacite, which have slightly higher 87Sr/86Srratios than the basalts but lower 143Nd/144Nd, 206Pb/204Pb,207Pb/204Pb, and 208Pb/204Pb ratios, are consistent with contaminationof parental basalt by old, low Rb/Sr, low U/Pb, and low Th/Pbcontinental crust. Concentrations of highly incompatible traceelements in andesite and dacite lavas are decoupled from majorelement compositions; the highest concentrat ions of these elementsoccur in andesitic, rather than dacitic compositions, and andesitelavas are more variable in trace element contents. Assimilationof heterogeneous crust concurrent with fractional crystallizationof varying mineral assemblages could cause this decoupled behavior.High mg-numbers in andesite and dacite, skeletal olivine phenocrysts,and reversely zoned pyroxene phenocrysts are manifestationsof mafic replenishment and magma mixing in the Taos Plateaumagmatic system. Taos Plateau volcanoes are monolithologic and are distributedin a semi-concentric zoned pattern that is a reflection of thecomplex subvolcanic magmatic system. A central focus of basaltshields developed above the main basaltic conduit system; thesemagmas contain 10–35% admixed andesitic and dacitic magma.Basalt shields are surrounded by a partial ring of olivine andesiteshield volcanoes, where replenishment of basaltic magma providedthe heat necessary for prolonged assimilation of crust, resultingin intermediate-composition lavas. Dacite shields are locatedaround the periphery of the more mafic volcanoes and reflecta decrease in mafic input on the fringes of the magmatic system.  相似文献   

6.
The Bandas del Sur Formation preserves a Quaternary extra-calderarecord of central phonolitic explosive volcanism of the LasCañadas volcano at Tenerife. Volcanic rocks are bimodalin composition, being predominantly phonolitic pyroclastic deposits,several eruptions of which resulted in summit caldera collapse,alkali basaltic lavas erupted from many fissures around theflanks. For the pyroclastic deposits, there is a broad rangeof pumice glass compositions from phonotephrite to phonolite.The phonolite pyroclastic deposits are also characterized bya diverse, 7–8-phase phenocryst assemblage (alkali feldspar+ biotite + sodian diopside + titanomagnetite + ilmenite + nosean–haüyne+ titanite + apatite) with alkali feldspar dominant, in contrastto interbedded phonolite lavas that typically have lower phenocrystcontents and lack hydrous phases. Petrological and geochemicaldata are consistent with fractional crystallization (involvingthe observed phenocryst assemblages) as the dominant processin the development of phonolite magmas. New stratigraphicallyconstrained data indicate that petrological and geochemicaldifferences exist between pyroclastic deposits of the last twoexplosive cycles of phonolitic volcanism. Cycle 2 (0·85–0·57Ma) pyroclastic fall deposits commonly show a cryptic compositionalzonation indicating that several eruptions tapped chemically,and probably thermally stratified magma systems. Evidence formagma mixing is most widespread in the pyroclastic depositsof Cycle 3 (0·37–0·17 Ma), which includesthe presence of reversely and normally zoned phenocrysts, quenchedmafic glass blebs in pumice, banded pumice, and bimodal to polymodalphenocryst compositional populations. Syn-eruptive mixing eventsinvolved mostly phonolite and tephriphonolite magmas, whereasa pre-eruptive mixing event involving basaltic magma is recordedin several banded pumice-bearing ignimbrites of Cycle 3. Theperiodic addition and mixing of basaltic magma ultimately mayhave triggered several eruptions. Recharge and underplatingby basaltic magma is interpreted to have elevated sulphur contents(occurring as an exsolved gas phase) in the capping phonoliticmagma reservoir. This promoted nosean–haüyne crystallizationover nepheline, elevated SO3 contents in apatite, and possiblyresulted in large, climatologically important SO2 emissions. KEY WORDS: Tenerife; phonolite; crystal fractionation; magma mixing; sulphur-rich explosive eruptions  相似文献   

7.
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.  相似文献   

8.
The Dry Valley Drilling Project (DVDP) cored three holes (DVDP1, 2 and 3) at Hut Point Peninsula (HPP) on Ross Island, Antarctica.DVDP 1 reached 201 m and penetrated 40 stratigraphic units,mainly basanite and Ne-hawaiite lava flows and associated pyroclasticrocks. DVDP 2 and 3 are nearly identical. DVDP 3 reached 381m and penetrated 15 stratigraphic units. The lower unit is a214 m thick basanite hyaloclastite, this is overlain by flowsand pyroclastic rocks of basanite, Ne-hawaiite, and then alternatingNe-mugearite and Ne-benmoreite flows. Phonolite stratigraphicallyoverlies the drill site. Primary olivine (Fo88-Fo74) is confined to the basanites, althoughxenocrysts occur in other rock types. Calcium-rich clinopyroxeneis ubiquitous but shows little variation in terms of Ca, Mg,Fe2+. Kaersutite is common in all lavas except the basanitesand shows very little compositional variation. It is believedto have crystallized at temperatures below 1100 °C and atpressures between 1·5 and 10 kb. Rhönite, a titanosilicateis a common groundmass phase in some basanite lavas. Oxide mineralsshow a wide range of compositions; spinel and chromian spineloccur as inclusions in olivine in the basanites. Titano-magnetiteoccurs in most samples examined. Labradorite is the predominantfeldspar in the basanite, with andesine and oligoclase importantin all other rock types. DVDP and HPP samples show a well-defined sequence of basanite-Ne-hawaiite-Ne-mugearite-Ne-benmoreite-phonolite,termed the DVDP lava lineage. Differentiation of the lineagehas been modeled using major element least squares mass balancemodels. Over 75 per cent of the evolution from basanite to phonoliteoccurs in the initial step from basanite to Ne-hawaiite, whichrepresents a 43 per cent residual. The phonolite is only a 25per cent residual from a basanite parent; thus even though Ne-mugeariteand Ne-benmoreite are common, they represent only minor stepsin the differentiation sequence. The mass balance models indicatethat olivine, clinopyroxene, kaersutite, opaque oxides and feldsparare the dominant fractionated phases. Trace element (includingrare earth element) contents calculated using the Rayleigh equationconfirm all models. * Present address: Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801.  相似文献   

9.
Mount Erebus, Ross Island, Antarctica, is an active, intraplate,alkaline volcano. The strongly undersaturated sodic lavas rangefrom basanite to anorthoclase phonolite, and are termed theErebus lineage (EL). The lavas are porphyritic with olivine(Fo88–51), clinopyroxene (Wo45–53En36–41Fs8–30),opaque oxides (Usp31–76), feldspar (An72–11), andapatite. Rare earth element (REE) contents increase only slightlywith increasing differentiation compared with other incompatibleelements. The light REE are enriched (LaN/YbN= 14–20)and there are no significant Eu anomalies. 87Sr/86Sr is uniformand low ({small tilde} 0.7030) throughout the EL, suggestingderivation of the basanites from a depleted asthenospheric mantlesource, and lack of significant crustal contamination duringfractionation of the basanite. Regular geochemical trends indicatethat the EL evolved from the basanites by fractional crystallization.Major element mass balance calculations and trace element modelsshow that fractionation of 16% olivine, 52% clinopyroxene, 14%Fe-Ti oxides, 11% feldspar, 3% nepheline, and 3% apatite froma basanite parent leaves 23.5% anorthoclase phonolite. Minor volumes of less undersaturated, more iron-rich benmoreite,phonolite, and trachyte are termed the enriched iron series(EFS). The trachytes have 87Sr/86Sr of 0.704, higher than otherEFS and EL rocks, and they probably evolved by a combined assimilation-fractionalcrystallization process. The large volume of phonolite at Mt. Erebus requires significantbasanite production. This occurs by low degrees of partial meltingin a mantle plume (here termed the Erebus plume) rising at arate of about 6 cm/yr.  相似文献   

10.
La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km3. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (76–77 wt % SiO2) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (66–70 wt % SiO2), dense (40–60% vesicularity)and crystal rich (30–40 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770–790°C with melt water contents of 3·1–4·4wt %. The pre-eruptive Toconao magma was cooler (730–750°C)and its melt more water rich (6·3–6·8 wt% H2O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (87Sr/86Sri 0·7094–0·7131;143Nd/144Nd 0·51222–0·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H2O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber  相似文献   

11.
Volcán San Pedro in the Andean Southern Volcanic Zone(SVZ) Chile, comprises Holocene basaltic to dacitic lavas withtrace element and strontium isotope ratios more variable thanthose of most Pleistocene lavas of the underlying Tatara–SanPedro complex. Older Holocene activity built a composite coneof basaltic andesitic and silicic andesitic lavas with traceelement ratios distinct from those of younger lavas. Collapseof the ancestral volcano triggered the Younger Holocene eruptivephase including a sequence of lava flows zoned from high-K calc-alkalinehornblende–biotite dacite to two-pyroxene andesite. Notably,hornblende–phlogopite gabbroic xenoliths in the daciticlava have relatively low 87Sr/86Sr ratios identical to theirhost, whereas abundant quenched basaltic inclusions are moreradiogenic than any silicic lava. The latest volcanism rebuiltthe modern 3621 m high summit cone from basaltic andesite thatis also more radiogenic than the dacitic lavas. We propose thefollowing model for the zoned magma: (1) generation of hornblende–biotitedacite by dehydration partial melting of phlogopite-bearingrock similar to the gabbroic xenoliths; (2) forceful intrusionof basaltic magma into the dacite, producing quenched basalticinclusions and dispersion of olivine and plagioclase xenocryststhroughout the dacite; (3) cooling and crystallization–differentiationof the basalt to basaltic andesite; (4) mixing of the basalticandesite with dacite to form a small volume of two-pyroxenehybrid andesite. The modern volcano comprises basaltic andesitethat developed independently from the zoned magma reservoir.Evolution of dacitic and andesitic magma during the Holoceneand over the past 350 kyr reflects the intrusion of multiplemafic magmas that on occasion partially melted or assimilatedhydrous gabbro within the shallow crust. The chemical and isotopiczoning of Holocene magma at Volcán San Pedro is paralleledby that of historically erupted magma at neighboring VolcánQuizapu. Consequently, the role of young, unradiogenic hydrousgabbro in generating dacite and contaminating basalt may beunderappreciated in the SVZ. KEY WORDS: Andes; dacite; gabbro; Holocene; strontium isotopes  相似文献   

12.
U-series disequilibria analyses have been combined with chemical and petrographic analyses in order to assess both the timescales and processes involved in the formation of the chemically zoned Fogo A trachytes. Least squares major element modelling demonstrates that the mafic trachytes could have evolved from a parental alkali basalt via trachybasalt with 70% fractionation of augite (35–36%), plagioclase (23%), magnetite (16%), kaersutite (15%), olivine (8%) and apatite (2–3%). Derivation of the mafic trachytes from a basanite parent is inconsistent with calculated fractionation paths. Major and trace element variations in 25 pumice samples collected from throughout the stratigraphic extent of the Fogo A deposit show that the trachytes represent the inverted, extrusive equivalent of a strongly chemically zoned magma chamber. The zonation is attributed to 70–75% Rayleigh fractional crystallization of the observed phenocryst phases. Wallrock assimilation and magma mixing did not contribute significantly to the observed chemical trends. The maximum age of the Fogo A trachytic magma based on radioactive disequilibria between 230Th and 238U is 300000 years. However, a calculated model age suggests that the time of evolution of the Fogo A trachytes from a parent alkali basalt is only 90000 years. Constant element variations and Th-isotopic ratios in Fogo C, Fogo A and 1563 A.D. trachytes suggest that a single long-lived trachytic magma chamber has been the source of at least the past 15.2 Ka of trachytic volcanism from Agua de Pao. After each eruption an evolved cupola reformed and became zoned prior to the next eruption. The maximum time necessary to form the zonation is 4600 years, the time between the Fogo A and 1563 A.D. eruptions. Low (226Ra)/(230Th)i ratios in the Fogo A and 1563 A.D. trachytes suggest that alkali feldspar fractionation continued up to the time of the respective eruptions.  相似文献   

13.
A detailed study of the pyroclastic deposits of the AD 79 ‘Pompei’Plinian eruption of Vesuvius has allowed: (1) reconstructionof the thermal, compositional and isotopic (87Sr/86Sr) pre-eruptivelayering of the shallow magma chamber; (2) quantitative definitionof the syn-eruptive mixing between the different magmas occupyingthe chamber, and its relationships with eruption dynamics; (3)recognition of the variability of mafic magma batches supplyingthe chamber. During the different phases of the eruption 25–30%of the magma was ejected as white K-phonolitic pumice, and 70–75%as grey K-tephri-phonolitic pumice. The white pumice resultsfrom the tapping of progressively deeper magma from a body (T= 850–900%C) consisting of two distinct layers mainlyformed by crystal fractionation. The grey pumice results fromsyn-eruptive mixing involving three main end-members: the phonolitic‘white’ magmas (salic end-member, SEM), mafic cumulates(cumulate end-member, CEM) and a crystal-poor ‘grey’phono-tephritic magma (mafic end-member, MEM), which was nevererupted without first being mixed with ‘white’ magma.Evidence is provided that mixing occurred within the chamberand was characterized by a transition with time from physicalmixing at a microscopic scale to chemical hybridization. TheMEM magma had a homogeneous composition and constant 87Sr86Srisotopic ratio, possibly as a result of sustained convection.No unambiguous liquidus phases were found, suggesting that theMEM magma was superheated (T = 1000–1100C); its verylow viscosity was a main cause in the establishment of a physicaldiscontinuity separating the white and the grey magmas. Thewhite-grey boundary layer possibly consisted of a multiply diffusiveinterface, periodically broken and recreated, supplying thephonolitic body through mixing of moderate amounts of fractionatedgrey melts with the overlying white magma. The presence of alarge overheated mass indicates the young, growing stage ofthe AD 79 chamber, whose main engine was the periodic arrivalof hot mafic magma batches. These were characterized by K-tephriticto K-basanitic compositions, high temperatures (>1150C),high volatile contents (20–25% H2O +Cl+F+S), low viscosities[(1+2 102 poises)] and relatively low densities (2500–2600kg/m3). The birth of the Pompei chamber followed the repeatedarrival of these batches (on average characterized by 87Sr/86Sr070729)into a reservoir containing a tephriticphonolitic, crystal-enriched,magma, a residue from the preceding ‘Avellino’ Plinianeruption (3400 BP).In fact, about half of magma ejected duringthe AD 79 eruption could have been inherited from pre-Avellinotimes. KEY WORDS: Vesuvius; magma chamber; magma mixing; compositional layering phonolites; magma supply; potassic magmas *Correponding author  相似文献   

14.
The 12·7–10·5 Ma Cougar Point Tuff in southernIdaho, USA, consists of 10 large-volume (>102–103 km3each), high-temperature (800–1000°C), rhyolitic ash-flowtuffs erupted from the Bruneau–Jarbidge volcanic centerof the Yellowstone hotspot. These tuffs provide evidence forcompositional and thermal zonation in pre-eruptive rhyolitemagma, and suggest the presence of a long-lived reservoir thatwas tapped by numerous large explosive eruptions. Pyroxene compositionsexhibit discrete compositional modes with respect to Fe andMg that define a linear spectrum punctuated by conspicuous gaps.Airfall glass compositions also cluster into modes, and thepresence of multiple modes indicates tapping of different magmavolumes during early phases of eruption. Equilibrium assemblagesof pigeonite and augite are used to reconstruct compositionaland thermal gradients in the pre-eruptive reservoir. The recurrenceof identical compositional modes and of mineral pairs equilibratedat high temperatures in successive eruptive units is consistentwith the persistence of their respective liquids in the magmareservoir. Recurrence intervals of identical modes range from0·3 to 0·9 Myr and suggest possible magma residencetimes of similar duration. Eruption ages, magma temperatures,Nd isotopes, and pyroxene and glass compositions are consistentwith a long-lived, dynamically evolving magma reservoir thatwas chemically and thermally zoned and composed of multiplediscrete magma volumes. KEY WORDS: ash-flow tuff; Bruneau–Jarbidge; rhyolite; Yellowstone hotspot; residence time  相似文献   

15.
More than ca 100 km3 of nearly homogeneous crystal-poor phonolite and ca 100 km3 of slightly zoned trachyte were erupted 39 ka during the Campanian Ignimbrite super eruption, the most powerful in the Neapolitan area. Partition coefficient calculations, equilibrium mineral assemblages, glass compositions and texture were used to reconstruct compositional, thermal and pressure gradients in the pre-eruptive reservoir as well as timing and mechanisms of evolution towards magma chamber overpressure and eruption. Our petrologic data indicate that a wide sill-like trachytic magma chamber was active under the Campanian Plain at 2.5 kbar before CI eruption. Thermal exchange between high liquidus (1199°C) trachytic sill and cool country rocks caused intense undercooling, driving a catastrophic and fast (102 years) in situ fractional crystallization and crustal assimilation that produced a water oversaturated phonolitic cap and an overpressure in the chamber that triggered the super eruption. This process culminated in an abrupt reservoir opening and in a fast single-step high decompression. Sanidine phenocrysts crystal size distributions reveal high differentiation rate, thus suggesting that such a sill-like magmatic system is capable of evolving in a very short time and erupting suddenly with only short-term warning.  相似文献   

16.
87Sr/86Sr ratios of Tertiary tholeiitic, basalts alkali olivine basalts and olivine nephelinites from Lower Saxony and Hessia and Quaternary leucite-nepheline tephrites from the Laacher See area are similar to those obtained from Hawaii and range from 0.7031 to 0.7054. Three trachytes and one phonolite from the Westerwald and one phonolite from the Laacher See area have higher values (0.7063 to 0.7093). Three Vesuvian lavas, three Somma lavas and two trachytes of the Phlegraic Fields show substantially higher ratios than the comparable basaltic rocks from N.W. Germany (0.7071 to 0.7102). Three peridotite nodules vary between 0.7048 and 0.7081. Three limestone composites of Paleozoic and Mesozoic age show values between 0.7129 and 0.7174.The 87Sr/86Sr ratios of the trachytes and phonolites from the Westerwald and Laacher See area are probably influenced by crustal material. Assimilation processes of limestones producing the olivine-nephelinites from N.W. Germany seem to be unlikely. A discussion of the origin of the Sr ratios in Vesuvian rocks and of mantle homogeneity is included.  相似文献   

17.
The West Farrington pluton in the North Carolina Piedmont isconcentrically zoned from gabbro-diorite near the chilled marginsto leucogranodiorite in the center. A crystallization modelfor the West Farrington pluton has been derived utilizing chemical,petrographic, field, and experimental data. The model involvessimple in situ fractional crystallization from the margins inward,with minimal contamination, crystal settling and floating, ormetasomatism. Rocks of the pluton can be considered as mixturesof early crystallizing minerals (liquidus or near-liquidus phases)and crystallized interstitial liquids. Relative percentage ofentrapped pore liquid increased with increasing degree of crystallization. The original tonalite magma began crystallizing Fe-Ti spinels,plagioclase, and hornblende within a short temperature interval.Crystallization of these minerals controlled fractionation trends.The initial water content in the magma was probably 2–3per cent; under such conditions water saturation would havebeen reached after about 60 per cent of the magma crystallized,assuming crystallization in the lower epizone at about 2000bars total pressure.  相似文献   

18.
Volcán Popocatépetl has been the site of voluminousdegassing accompanied by minor eruptive activity from late 1994until the time of writing (August 2002). This contribution presentspetrological investigations of magma erupted in 1997 and 1998,including major-element and volatile (S, Cl, F, and H2O) datafrom glass inclusions and matrix glasses. Magma erupted fromPopocatépetl is a mixture of dacite (65 wt % SiO2, two-pyroxenes+ plagioclase + Fe–Ti oxides + apatite, 3 wt % H2O, P= 1·5 kbar, fO2 = NNO + 0·5 log units) and basalticandesite (53 wt % SiO2, olivine + two-pyroxenes, 3 wt % H2O,P = 1–4 kbar). Magma mixed at 4–6 km depth in proportionsbetween 45:55 and 85:15 wt % silicic:mafic magma. The pre-eruptivevolatile content of the basaltic andesite is 1980 ppm S, 1060ppm Cl, 950 ppm F, and 3·3 wt % H2O. The pre-eruptivevolatile content of the dacite is 130 ± 50 ppm S, 880± 70 ppm Cl, 570 ± 100 ppm F, and 2·9 ±0·2 wt % H2O. Degassing from 0·031 km3 of eruptedmagma accounts for only 0·7 wt % of the observed SO2emission. Circulation of magma in the volcanic conduit in thepresence of a modest bubble phase is a possible mechanism toexplain the high rates of degassing and limited magma productionat Popocatépetl. KEY WORDS: glass inclusions; igneous petrology; Mexico; Popocatépetl; volatiles  相似文献   

19.
The island of Tristan da Cunha, located at 37?S, 12?W in theSouth Atlantic, is the largest of a group of three islands,the others being Nightingale and Inaccessible. Tristan da Cunhacomprises a continuous series of alkaline lavas ranging in compositionfrom ankaramitic basanite through phonotephnte and tephnphonoliteto phonolite. Moderately porphyritic basanite is the dominantrock type ({small tilde}on the island. Major and trace element variations in the lavas describe well-definedtrends with increasing differentiation which are generally consistentwith control by fractional crystallization of phenocryst phases.None of the lavas can be considered to be primary in composition,mg-numbers range from 40 to 62, and covariation of certain minorand trace elements (e.g., Sr, Ba, P) suggests the presence ofat least two distinct fractionation trends. Sr, Nd, and Pb isotopicanalyses of a subset of the lavas confirm previously publisheddata for the island, but show a slightly greater range: 87Sr/86Sr= 0{dot}70495–0{dot}70517; 143Nd/144Nd = 0{dot}51259–0{dot}51247;206Pb/204Pb = 18{dot}47–18-{dot}74. Quantitative modelling of the compositional variations suggeststhat the ankaramitic basanites are partial olivine + clinopyroxene+ titanomagnetite (?minor plagioclase) cumulates, with {smalltilde}40% crystal accumulation being required to account forthe most porphyritic varieties. The range in composition frombasanite to phonotephrite can be accounted for by up to 50%fractional crystallization of clinopyroxene, olivine, titanomagnetite,and plagioclase, with minor apatite and, in some models, amphibole.Average proportions of these phases in the fractionate are Cpx40, TiMgt 20, Plag 30, Oliv 10. As much as 20% amphibole fractionationis required in models involving the phonotephrites. The compositionsof the evolved tephriphonolites and phonolites are consistentwith extensive (up to 80%) fractional crystallization of aninitial basanitic magma with clinopyroxenc (1–4%), amphibole(17–23%), plagioclase (6–20%) alkali feldspar (0–13%),and titanomagnetite (4–6%)?minor apatite and sphene beingthe dominant fractionating phases. Inferred trace element and isotopic characteristics of the sourceregions of the Tristan lavas are distinct from those givingrise to the Walvis Ridge, Gough Island, or Discovery Seamountbasalts. Normalized trace element abundances of the Tristanlavas are more similar to those of Marion Island, whereas Sr-,Nd-, dnd Pb-isotopic ratios are most similar to nearby InaccessibleIsland lavas. If Tristan da Cunha is the present-day surfaceexpression of the upwelling mantle plume that previously gaverise to the Walvis Ridge, then the source material tapped bythe lavas is distinctly heterogeneous (or has changed with time)within the limits imposed by maintaining the general characteristicsof DUPAL-type mantle.  相似文献   

20.
The 1995–1999 eruption of the Soufriere Hills volcano,Montserrat, has produced a crystal-rich andesite containingquench-textured mafic inclusions, which show evidence of havingbeen molten when incorporated into the host magma. Individualcrystals in the andesite record diverse histories. Amphibolephenocrysts vary from pristine and unaltered to strongly oxidizedand pseudomorphed by anhydrous reaction products. Plagioclasephenocrysts are commonly reverse zoned, often with dusty sievetextures. Reverse zoned rims are also common on orthopyroxenephenocrysts. Pyroxene geothermometry gives an average temperatureof 858 ± 20°C for orthopyroxene phenocryst cores,whereas reverse zoned rims record temperatures from about 880to 1050°C. The heterogeneity in mineral rim compositions,zoning patterns and textures is interpreted as reflecting non-uniformreheating and remobilization of the resident magma body by intrusionof hotter mafic magma. Convective remobilization results inmixing together of phenocrysts that have experienced differentthermal histories, depending on proximity to the intruding maficmagma. The low temperature and high crystallinity are interpretedas reflecting the presence of a cool, highly crystalline magmabody beneath the Soufriere Hills volcano. The petrological observations,in combination with data on seismicity, extrusion rate and SO2fluxes, indicate that the current eruption was triggered byrecent influx of hot mafic magma. KEY WORDS: Montserrat; eruption; magma mixing; mafic inclusion; sieve texture  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号