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1.
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   

2.
Major and trace element data for a sequence of peralkaline silicic lavas and pyroclastic flows, exposed in the caldera wall of the Paisano volcano, west Texas, document systematic fractional crystallization during magmatic evolution and an open system, magma mixing event in the upper parts of the sequence. Stratigraphically lowest flows are comendite and comenditic quartz trachyte lavas and ash flow tufts. Overlying these units is a trachyte with compositional, textural and mineralogical features indicating that it is the product of magma-mixing; similar flows occur in other parts of the volcano at the same stratigraphic level. This composite trachyte is considered to be a mixture of mugearitic or mafic trachytic magma, derived from a similar source region which yielded the earlier caldera wall flows. Trace element concentrations of the post-trachyte comenditic quartz trachyte lavas suggest they were erupted from a chamber whose magma was diluted by an influx of mugearitic or mafic trachytic magma during a magma mixing event.Rayleigh fractionation calculations show that the comendites and comenditic quartz trachytes can be derived from a parental mugearite magma by 88% to 93% fractionation of dominantly plagioclase and alkali feldspar, with lesser amounts of clinopyroxene, magnetite and apatite. Zircon was not a significant fractionating phase. The composition, mineralogy and depth of the source region(s) which generated these magmas cannot be constrained from the present data set.  相似文献   

3.
This paper is a companion to Clark (1988; hereafter Part I) which described the evolution of the Tejeda Magmatic System (TMS), a Miocene caldera complex, Gran Canaria, Spain, based on geochronologic, paleomagnetic and field data. In this study, petrochemical data are used to corroborate the history out-lined in Part I. Geochemical discriminant analysis shows that whereas the Extra-Caldera (EC) Mogan/Fataga volcanics are separated by a composition gap, no composition gap exists within the Intra-Caldera (IC) sequence. IC ignimbrites change rapidly but progressively from pantellerites and comendites to comenditic trachytes and finally to trachytes in a 0.47 Ma time interval. Significantly, the lower pantelleritic part of the IC series is similar to the EC pantellerites (units B, C and D) as expected based on results from Part I. The appearance of a compositional gap in the EC sequence is the result of flows having been trapped within the caldera during the 0.47 Ma Mogan-Fataga transition interval. The transitional IC sequence may be geochemically modelled by mixing of Mogan comendites and Fataga trachytes. The mixing was most probably induced by the high discharge of magma from the compositionally-zoned Tejeda magma body. The rate of change in erupted composition is best explained by imagining a continuous influx of Fataga or parental Fataga magma into a chamber whose previous silicic component (Mogan composition) was no longer being replenished and that the two magmas did not convectively mix prior to eruption. Repose times between successive eruptions in the lower to middle Mogan (from P1/T1 to A) were of order 30 000 a; the upper Mogan pantellerites and comendites/comenditic trachytes (B to F?) erupted once every 125 000 years or so. The longer repose time for the upper units is consistent with their more differentiated character.  相似文献   

4.
Mafic to intermediate enclaves are evenly distributed throughoutthe dacitic 1991–1995 lava sequence of Unzen volcano,Japan, representing hundreds of mafic recharge events over thelife of the volcano. This study documents the morphological,textural, chemical, and petrological characteristics of theenclaves and coexisting silicic host lavas. The eruptive productsdescribed in this study appear to be general products of magmamingling, as the same textural types are seen at many othervolcanoes. Two types of magmatic enclaves, referred to as Porphyriticand Equigranular, are easily distinguished texturally. Porphyriticenclaves display a wide range in composition from basalt toandesite, are glass-rich, spherical and porphyritic, and containlarge, resorbed, plagioclase phenocrysts in a matrix of acicularcrystals and glass. Equigranular enclaves are andesitic, non-porphyritic,and consist of tabular, medium-grained microphenocrysts in amatrix glass that is in equilibrium with the host dacite magma.Porphyritic enclaves are produced when intruding basaltic magmaengulfs melt and phenocrysts of resident silicic magma at theirmutual interface. Equigranular enclaves are a product of a moreprolonged mixing and gradual crystallization at a slower coolingrate within the interior of the mafic intrusion. KEY WORDS: mafic enclaves; quenched mafic inclusions; magma mingling; Unzen volcano; Unzen Scientific Drilling Project; resorbed plagioclase  相似文献   

5.
Mafic inclusions present in the rhyolitic lavas of Narugo volcano,Japan, are vesiculated andesites with diktytaxitic texturesmainly composed of quenched acicular plagioclase, pyroxenes,and interstitial glass. When the mafic magma was incorporatedinto the silica-rich host magma, the cores of pyroxenes andplagioclase began to crystallize (>1000°C) in a boundarylayer between the mafic and felsic magmas. Phenocryst rim compositionsand interstitial glass compositions (average 78 wt % SiO2) inthe mafic inclusions are the same as those of the phenocrystsand groundmass glass in the host rhyolite. This suggests thatthe host felsic melt infiltrated into the incompletely solidifiedmafic inclusion, and that the interstitial melt compositionin the inclusions became close to that of the host melt (c.850°C). Infiltration was enhanced by the vesiculation ofthe mafic magma. Finally, hybridized and density-reduced portionsof the mafic magma floated up from the boundary layer into thehost rhyolite. We conclude that the ascent of mafic magma triggeredthe eruption of the host rhyolitic magma. KEY WORDS: mafic inclusion; stratified magma chamber; magma mixing; mingling; Narugo volcano; Japan  相似文献   

6.
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   

7.
A suite of peralkaline trachytes from Longonot volcano, Kenya,which erupted during the last 6000 years, has been analysedfor major and trace elements, Pb and Nd isotopes, and U–Th–Radisequilibria. The lavas are divided into three stratigraphicgroups of trachytes (Lt2a, Lt2b and Lt3), and hybrid lavas,designated LMx1 and LMx2, which, respectively, pre-date andpost-date the Lt2 lavas. Major and trace elements are consistent,with up to 37% within-group fractional crystallization of predominantlyalkali feldspar. The parental magma for the different trachytegroups had a more mafic composition—probably hawaiitic.Nd and Pb isotopes show minimal variation, both within and betweenmagma groups, and indicate that up to 10% comendite magma fromthe neighbouring Olkaria volcanic field may have intermixedwith the Longonot magma. (230Th/238U) disequilibria indicatethat limited U/Th fractionation occurred during the past 10kyr, whereas (226Ra/230Th) disequilibria reflect the effectof alkali feldspar fractionation >8 kyr ago in the Lt2a lavas,between 3 and 7 kyr ago in the Lt2b lavas and in the past 3kyr for the Lt3 lavas. (226Ra/230Th) disequilibria in the Lt2blavas are interpreted using a model that combines the equationsof radioactive decay and in-growth with Rayleigh crystallizationto give fractionation rates of about 0·2 x 10–4/yearfor the evolution of hawaiite to trachyte, but more rapid ratesof up to 3 x 10–4/year for fractionation within the trachytesequence. (226Ra/230Th) from two whole-rock–alkali feldsparpairs are interpreted to show the crystals formed at 5800 yearsBP (Lt2b) and 2800 years BP (Lt3), implying that phenocrystformation continued almost up to the time of eruption. The resultsstrongly indicate that fractionated magmas can be stored forperiods on the order of 1000–2500 years prior to eruption,whereas other magmas were erupted as fractionation was proceeding. KEY WORDS: trachyte; magma chambers; u-series; Kenya  相似文献   

8.
Usu volcano, located in northern Japan, has erupted seven timessince AD 1663. Before these seven eruptions, the volcano hada long repose period ( 5000 yr). The 1663 eruption was thefirstand by far the largest among the seven, producing nearlyaphyric rhyolitic pumice. Small mafic inclusions (‘micro-clots’J,consisting of glass, quenched crystals and abundant vesiclesoccur in the pumice. On the basis of petrological studies ofthe microclots, it is concluded that these are quenched meltsof a mafic magma injected into the rhyolite. The products ofthe 1769 eruption (and those of the following five eruptions)were dacites with abundant (10–15 vol %) microphenocrysts.According to crystal size distribution (CSD) analysis, the newmicrophenocrysts appear to have crystallized at a considerablyhigher cooling rate ( 300 times) than the phenocrysts in the1663 eruptive products. The contrasting petrologic featuresof the aphyric rhyolite and the following microphenocryst-richdacites can be explained by mixing and rapid cooling of a maficmagma injected during the 1663 eruption. We estimate the sizeof the magma chamber beneath Usu volcano just after the 1663eruption, using numerical calculations for a cooling magma chamber.If the magma chamber was sill-like, its thickness is estimatedto have been several hundreds of meters. KEY WORDS: Usu volcano; Japan; magma chamber evolution *Corresponding author. Present address: Geomechanics, Earthquake Research Institute, The University of Tokyo, I-I-I Yayoi, Bunkyo-ku, Tokyo 113, Japan  相似文献   

9.
U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and are associated with small volumes of hawaiite. The mafic rocks are characterised by high (230Th/232Th) (≥1.06) with low (238U/230Th) ratios (≤0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly lower (230Th/232Th) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (238U/230Th) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar to single crystal laser fusion 40Ar/39Ar ages from sanidine phenocrysts (16–38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (230Th/232Th)0 ratios (0.69–1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano. Received: 29 May 1996 / Accepted: 24 November 1997  相似文献   

10.
Processes of crystal separation in a magma heavily laden withcrystals without phase change are investigated from observationson frozen magma systems: Nosappumisaki and other shoshoniteintrusions in the Nemuro peninsula, Japan, for which the originof the crystals and the initial conditions are well constrained.The Nosappumisaki intrusion is 120 m in thickness and extendsfor more than 1·5 km. It exhibits a wide range of lithologicalvariation, principally as a result of crystal redistributionafter intrusion. Crystals in each lithology can be clearly dividedinto two kinds according to their composition and texture: thosepresent before the intrusion of the magma (‘phenocrysts’)and those that crystallized in situ after intrusion. From thevertical change in mode and size of ‘phenocrysts’,it is shown that (1) augite ‘phenocrysts’ were rapidlydeposited, with little overgrowth after intrusion, by significantcoagulation or clustering on a time-scale of more than a fewyears, and (2) plagioclase ‘phenocrysts’, definitelydenser than the melt but concentrated in the upper level, floatedby counter flow of massive deposition of augite ‘phenocrysts’.These results indicate that in a magma heavily laden with crystalsof a few millimeters in size (>20 vol. %), crystal–crystaland crystal–melt interaction play an important role inthe separation of crystals from the host melt. KEY WORDS: magma chamber; sill; crystal settling; plagioclase flotation; Nosappumisaki  相似文献   

11.
Products of the Pomici di Base plinian eruption of Somma-Vesuvius consist of pumice and scoria fall deposits overlain by lithic-rich phreatomagmatic deposits. The plinian fall, which represents most of the magma volume involved in the eruption, ranges in composition from trachyte (SiO2 = 62.5 wt%) to latite (SiO2 ≈ 58 wt%) in the lower one-third of the deposit, whereas the upper two-thirds of the total thickness consists of latitic scoriae with fairly uniform composition (SiO2 ≈ 55–56 wt%). All the products have very low content of phenocrysts (from 4 wt% in trachyte pumice to 1 wt% in the latite scoriae), most of which are not in equilibrium with the host rock. Minerals not in equilibrium, both in trachytic and latitic rocks, consist of discrete crystals of sanidine and plagioclase wetted by trachytic glass and felsic aggregates with interstital trachytic glass. Trends of major and trace elements are consistent with crystal-liquid fractionation processes and rule out syn-eruptive mixing processes between latitic and trachytic magmas. We suggest that discrete crystals and crystal aggregates not in equilibrium with the host rock represent fragments of the crystallising boundary layer at the upper walls of the magma chamber, which were wrenched and admixed into the magma during the ascent. This process diversifies the mineral assemblage and increases the crystal content of the rocks. We propose that diffusive crystallization processes operating at the wall of the chamber allowed the formation of a two-fold layered reservoir with a more mafic, homogeneous lower body and a more evolved, compositionally graded upper body. Around one-quarter of crystals adhering to the upper part of the magma chamber were admixed into the magma during the eruption. The absence of significant syn-eruptive mixing processes and the major role played by diffusive crystallization are consistent with a low aspect ratio magma chamber (width/height <1). Received: 23 March 1998 / Accepted: 11 December 1998  相似文献   

12.
新疆西准噶尔夏尔莆岩体岩浆混合的岩相学证据   总被引：11，自引：0，他引：11  
夏尔莆岩体由寄主岩石、微粒镁铁质包体和中基性岩墙群组成,具丰富、典型的岩浆混合岩相学特征.野外露头,寄主岩石中暗色矿物分布不均并发育暗色矿物集合体、微小的镁铁质包体和不均匀混合条带;包体具有明显的塑性变形,与寄主岩石或界线截然或渐变过渡,常发育反向脉和寄主岩石中的长石巨晶(捕虏晶);中基性岩墙群与微粒镁铁质包体紧密共生并延伸方向基本一致,发育寄主岩石中的长石捕虏晶,被寄主岩的反向脉横切.在镜下,包体与寄主岩混合带中均发育斜长石异常环带和多种不平衡矿物共生现象,包体中发育针状磷灰石.这些特征表明镁铁质包体和中基性岩墙群来源于与寄主岩石同一岩浆事件的基性岩浆,并与其发生了强烈的岩浆混合作用.岩相学特征为夏尔莆岩体岩浆混合成因提供了重要佐证.  相似文献   

13.
Aniakchak caldera, Alaska, produced a compositionally heterogeneousignimbrite 3400 years ago, which changes from rhyodacitic atthe base to andesitic at the top of the eruptive sequence. Interpretationsof compositionally heterogeneous ignimbrites typically includeeither in situ fractional crystallization of mafic magma andgeneration of a stratified magma body or replenishment of asilicic magma chamber by mafic inputs. Another possibility,silicic replenishment of a more mafic chamber, exists. Geochemicalcharacteristics of the caldera-forming rhyodacite and severallate pre-caldera rhyodacites indicate independent origins foreach, within a maximum of 5000 years prior to caldera formation.Isotopic considerations preclude derivation of the caldera-formingrhyodacite from the caldera-forming andesite. However, the caldera-formingrhyodacite can be explained as the residual liquid of a mostlycrystallized basalt, with addition of crustal material. TheAniakchak andesite probably formed in a shallow chamber by successivemixing events involving small volumes of basalt and rhyodacite,together with contamination. The pre-caldera rhyodacites representerupted portions of intruding silicic magma, whereas anotherportion homogenized with the resident mafic magma. The caldera-formingevent reflects a large influx of rhyodacite, which erupted beforesignificant mixing occurred and also triggered draining of muchof the andesitic magma from the chamber. KEY WORDS: Aniakchak; caldera-forming eruption; geochemistry; ignimbrite; silicic replenishment  相似文献   

14.
塔里木西南缘铁克里克地区广泛发育早古生代中酸性侵入岩,本文对其中布雅岩体及其暗色包体进行系统的岩石学、年代学及岩石地球化学研究,确定了岩石成因及其构造属性。LA-MC-ICP-MS锆石U-Pb年代学研究表明,寄主石英二长闪长岩结晶年龄为432.6±2.5 Ma(MSWD=1.5),暗色包体结晶年龄为432.4±6.4 Ma(MSWD=0.031),二者形成时代相同,均为志留纪早期岩浆活动的产物。地球化学特征表明,布雅暗色包体应来源于地幔的部分熔融,而寄主岩石岩浆具有壳源岩浆的性质并经历了幔源岩浆不均匀的混合。野外及岩相学特征均显示暗色包体为铁镁质岩浆注入长英质岩浆快速冷凝形成的,是幔源岩浆底侵下地壳形成的岩浆混合作用的产物。它们是塔里木南缘早古生代碰撞造山作用晚期的岩浆记录。  相似文献   

15.
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  相似文献   

16.
In situ Sr-isotope and microchemical studies were used to determine the provenance of K-feldspar megacrysts hosted in mafic alkaline potassic, ultrapotassic rocks and in differentiated rocks from two nearby volcanic apparatus in central Italy.

At Monte Cimino volcanic complex, mafic leucite-free ultrapotassic megacryst-bearing rocks of olivine latitic composition are associated with evolved latite and trachyte. Here, latites and trachytes straddle the sub-alkaline field. Age-corrected 87Sr/86Sr values (Sri) of the analysed Cimino olivine latites vary from 0.71330 and 0.71578 and strongly increase at constant Mg value. Latite and trachyte have lower Sri than olivine latites ranging between 0.71331 and 0.71361. Sri of K-feldspar megacrysts from olivine latites are between 0.71352 and 0.71397, but core and rim 87Sr/86Sr ratios within individual megacryst are indistinguishable. In all the mafic rocks, the megacrysts are not in isotopic equilibrium with the hosts. K-feldspar megacrysts from both the latite and trachyte have similar Sr-isotope compositions (Sri=0.71357–0.71401) to those in the olivine latites. However, Sri of megacryst in the trachyte vary significantly from core to rim (Sri from 0.71401 to 0.71383). As with the olivine latites, the K-feldspar megacrysts are not in isotopic equilibrium with bulk rock compositions of the latite or trachyte.

At Vico volcano, megacryst-bearing rocks are mafic leucite-free potassic rocks, mafic leucite-bearing ultrapotassic rocks and old trachytic rocks. The mafic leucite-bearing and leucite-free rocks are a tephri-phonolite and an olivine latite, respectively. A megacryst in Vico trachyte is isotopically homogeneous (Sri CORE=0.71129, RIM=0.71128) and in equilibrium with the host rock (Sri bulk ROCK=0.71125). Sri of megacryst from tephri-phonolite is clearly not in isotopic equilibrium with its host (Sri bulk ROCK=0.71158), and it increases from core (Sri=0.71063) to rim (Sri=0.71077). A megacryst in Vico olivine latite is isotopically homogeneous (Sri CORE=0.71066, RIM=0.71065), but not in equilibrium with the host rock (Sri bulk ROCK=0.71013).

The Sr isotope microdrilling technique reveals that Cimino megacrysts were crystallised in a Cimino trachytic magma and were subsequently incorporated by mixing/mingling processes in the latitic and olivine latitic melts. A model invoking the presence of a mafic sub-alkaline magma, which was mixed with the olivine latite, is proposed to justify the lack of simple geochemical mixing relation between Cimino trachytes and olivine latites. This magmatological model is able to explain the geochemical characteristics of Cimino olivine latites, otherwise ascribed to mantle heterogeneity.

The similarity of core Sri of megacrysts hosted in Vico tephri-phonolite and olivine latite suggests that the K-feldspar megacrysts are co-genetic. Isotopic equilibrium between megacryst and Vico host trachyte indicates that the trachyte is the parent of this megacryst. On the contrary, the megacrysts hosted in tephri-phonolite and olivine latite do not derive from the old trachytic magma because no diffusion process may explain the core to rim Sr isotope increase of the xenocryst hosted in the tephri-phonolite. The megacrysts hosted in the Vico mafic rocks might derive from a trachytic melt similar in composition to the old Vico trachytes.  相似文献   


17.
Volcán Ceboruco, Mexico, erupted ~1,000 years ago, producing the Jala pumice and forming a ~4-km-wide caldera. During that eruption, 2.8 to 3.5 km3 of rhyodacite (~70 wt% SiO2) magma and 0.2 to 0.5 km3 of mixed dacite (~67 wt% SiO2) magma were tapped and deposited as the Jala pumice. Subsequently, the caldera was partially filled by extrusion of the Dos Equis dome, a low-silica (~64 wt% SiO2) dacite dome with a volume of ~1.3 km3. Petrographic evidence indicates that the Jala dacite and Dos Equis dacite originated largely through the mixing of three end-member magmas: (1) rhyodacite magma, (2) dacite magma, and (3) mafic magma. Linear least-squares modeling and detailed modal analysis indicate that the Jala dacite is predominantly a bimodal mixture of rhyodacite and dacite with a small additional mafic component, whereas the Dos Equis dacite is composed of mostly dacite mixed with subordinate amounts of rhyodacite and mafic magma. According to Fe–Ti oxide geothermometry, before the caldera-forming eruption the rhyodacite last equilibrated at ~865 °C, whereas the dacite was originally at ~890 °C but was heated to ~960 °C by intrusion of mafic magma as hot as ~1,030 °C. Zoning profiles in plagioclase and/or magnetite phenocrysts indicate that mixing between mafic and dacite magma occurred ~34–47 days prior to eruption, whereas subsequent mixing between rhyodacite and dacite magmas occurred only 1–4 days prior to eruption. Following the caldera-forming eruption, continued inputs of mafic magma led to effusion of the Dos Equis dome dacite. In this case, timing between mixing and eruption is estimated at ~93–185 days based on the thickness of plagioclase overgrowth rims.Editorial responsibility: T.L. Grove  相似文献   

18.
The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km3 of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO2. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and 87Sr/86Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO2)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO2) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO2 content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics  相似文献   

19.
Miocene Ignimbrite ‘A’ on Gran Canaria containsthree compositional endmember fiamme types(two rhyolites andone trachyte) each of which crystallized distinct feldspar.Various textural and compositional criteria are interpretedas reflecting a complex scenario within the magma chamber inwhich the crystals formed. About 25–30% of the feldsparphenocrysts contain evidence for magma mixing in the form of(1) partial to severe dissolution–resorption rims, (2)distinct zones of drastically different compositions and (3)overgrowth textures on formerly resorbed crystals. Four majortypes of zoning in the oligoclase to anorthoclase feldsparsof ignimbrite ‘A’ include a normal and a reverselyzoned type and two complexly zoned types. The feldspars withnormal and reverse zonation show only minor compositional amplitudesbetween individual zones (  相似文献   

20.
      东江口花岗岩及闪长质包体分别获得了218 Ma 和224 Ma 的形成年龄,闪长质包体中存在岩浆不平衡结构并发育与寄 主花岗岩相同的钾长石斑晶及淬冷形成的针状磷灰石,揭示了花岗岩形成过程中曾发生二元岩浆混合作用。这种混合作用 已造成寄主花岗岩和闪长质包体化学组成的趋同,同时使得它们的Sr-Nd-Pb 同位素组成发生强烈均一化。但暗色闪长质包 体锆石具有较宽的εHf（t ）值（-4.58~3.31）,保留了二端元岩浆源区的特征。秦岭早中生代同期闪长质包体锆石εHf（t ）＞ 10 及寄主花岗岩锆石εHf（t ）＜ -10 的差异表明,它们分别来自相对亏损地幔源区和中元古代滞留于地壳的幔源基性物质, 而两个源区的岩浆,自224 Ma 以来发生强烈混合作用,形成大规模的壳幔混合花岗岩体。  相似文献   

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