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1.
Disequilibrium phenocryst assemblages in the Younger Andesitesand Dacites of Iztacc?huatl, a major Quaternary volcano in theTrans-Mexican Volcanic Belt, provide an excellent record ofepisodic replenishment, magma mixing, and crystallization processesin calc-alkaline magma chambers. Phenocryst compositions andtextures in ‘mixed’ lavas, produced by binary mixingof primitive olivine-phyric basalt and evolved hornblende dacitemagmas, are used to evaluate the mineralogical and thermal characteristicsof end-members and the physical and chemical interactions thatattend mixing. Basaltic end-members crystallized olivine (FO90–88) andminor chrome spinel during ascent into crustal magma chambers.Resident dacite magma contained phenocrysts of andesine (An45–35),hypersthene (En67–61), edenitic-pargasitic hornblende,biotite, quartz, .titanomagnetite, and ilmenite. On reachinghigh-level reservoirs, basaltic magmas were near their liquidiat temperatures of about 1250–1200?C according to theolivine-liquid geothermometer. Application of the Fe-Ti-oxidegeothermometer-oxygen barometer indicates that hornblende dacitemagma, comprising phenocrysts (<30 vol. per cent) and coexistingrhyolitic liquid, had an ambient temperature between 940 and820?C at fO2s approximately 0?3 log units above the nickel-nickeloxide buffer assemblage. Mixing induced undercooling of hybridliquids and rapid crystallization of skeletal olivine (Fo88–73),strongly-zoned clinopyroxene (endiopside-augite), calcic plagioclase(An65–60); and orthopyroxene (bronzite), whereas low-temperaturephenocrysts derived from hornblende dacite were resorbed ordecomposed by hybrid melts. Quartz reacted to form coronas ofacicular augite and hydroxylated silicates were heated to temperaturesabove their thermal stability limit ({small tilde}940?C foramphibole, according to clinopyroxene-orthopyroxene geothermometry,and {small tilde}880?C for biotite). Calculations of phenocrystresidence times in hybrid liquids based on reaction rates suggestthat the time lapse between magma chamber recharge and eruptionwas extremely short (hours to days). It is inferred that mixing of magmas of diverse compositionis driven by convective turbulence generated by large differencesin temperature between end-members. The mixing mechanism involves:(1)rapid homogenization of contrasting residual liquid compositionsby thermal erosion and diffusive transfer (liquid blending);(2) assimilation of phenocrysts derived from the low-temperatureend-member; and (3) dynamic fractional crystallization of rapidlyevolving hybrid liquids in a turbulent boundary layer separatingbasaltic and dacitic magmas. The mixed lavas of lztacc?huatlrepresent samples of this boundary layer quenched by eruption.  相似文献   

2.
为揭示华北克拉通北缘中晚三叠世解放营子花岗闪长岩的岩浆混合机制,对寄主岩石和镁铁质包体中斜长石和角闪石开展了电子探针分析.分析结果显示,多斑和少斑包体边部的斜长石斑晶发育An值增加的突变环带,环带的An值为32～46,明显高于核部和边部斜长石的An值(18～31),而核部的An值与寄主岩石中斜长石的An值一致,该特征指...  相似文献   

3.
Voluminous andesite and dacite lavas of Daisen volcano, SW Japan,contain features suggesting the reverse of normal fractionation(anti-fractionation), in the sense that magma genesis progressedfrom dacite to andesite, accompanied by rises in temperature.A positive correlation exists between phenocryst content (0–40vol. %) and wt % SiO2 (61–67%). Phenocryst-rich dacitescontain hornblende and plagioclase that are generally unaltered,clear, and euhedral. However, phenocryst-poor rocks containsieve-textured plagioclase, resorbed plagioclase, and opacitein which hornblendes are pseudomorphed. Some Daisen rocks containtwo coexisting pyroxenes. Many orthopyroxene phenocrysts fromtwo-pyroxene lavas have high-Ca overgrowth rims (up to 50 µm),a feature consistent with crystallization from a higher-temperaturemagma than the core. Rim compositions are similar from phenocrystto phenocryst in individual samples. Temperatures of 800–900°Care obtained from the cores, whereas temperatures of 1000–1100°Care indicated for the rims. Lavas ranging from aphyric andesite(  相似文献   

4.
High-temperature–pressure experiments were carried outto determine the chlorine–hydroxyl exchange partitioncoefficient between hornblende and melt in the 1992 Unzen dacite.Cl in hornblende and melt was analyzed by electron microprobe,whereas OH in hornblende and melt was calculated assuming anionstoichiometry of hornblende and utilizing the dissociation reactionconstant for H2O + O = 2(OH) in water-saturated melt, respectively.The partition coefficient strongly depends on the Mg/(Mg + Fe)ratio of hornblende, and is expressed as ln K1 = (Cl/OH)hb/(Cl/OH)melt= 2·37 – 4·6[Mg/(Mg + Fe)]hb at 2–3kbar and 800–850°C. The twofold variation in Cl contentin the oscillatory zoned cores of hornblende phenocrysts inthe 1991–1995 dacite cannot be explained by the dependenceof the Cl/OH partition coefficient on the Mg/(Mg + Fe)hb ratio,and requires c. 80% variation of the Cl/OH ratio of the coexistingmelt. Available experimental data at 200 MPa on Cl/OH fractionationbetween fluid and melt suggest that c. 1·2–1·8wt % degassing of water from the magma can explain the required80% variation in the Cl/OH ratio of the melt. The negative correlationbetween Al content and Mg/(Mg + Fe) ratio in the oscillatoryzoned cores of the hornblende phenocrysts is consistent withrepeated influx and convective degassing of the fluid phasein the magma chamber. KEY WORDS: chlorine; element partitioning; hornblende; oscillatory zoning; Unzen volcano  相似文献   

5.
The evolution of large bodies of silicic magma is an importantaspect of planetary differentiation. Melt and mineral inclusionsin phenocrysts and zoned phenocrysts can help reveal the processesof differentiation such as magma mixing and crystal settling,because they record a history of changing environmental conditions.Similar major element compositions and unusually low concentrationsof compatible elements (e.g. 0·45–4·6 ppmBa) in early-erupted melt inclusions, matrix glasses and bulkpumice from the Bishop Tuff, California, USA, suggest eutectoidfractional crystallization. On the other hand, late-eruptedsanidine phenocrysts have rims rich in Ba, and late-eruptedquartz phenocrysts have CO2-rich melt inclusions closest tocrystal rims. Both features are the reverse of in situ crystallizationdifferentiation, and they might be explained by magma mixingor crystal sinking. Log(Ba/Rb) correlates linearly with log(Sr/Rb)in melt inclusions, and this is inconsistent with magma mixing.Melt inclusion gas-saturation pressure increases with CO2 fromphenocryst core to rim and suggests crystal sinking. Some inclusionsof magnetite in late-erupted quartz are similar to early-eruptedmagnetite phenocrysts, and this too is consistent with crystalsinking. We argue that some large phenocrysts of late-eruptedquartz and sanidine continued to crystallize as they sank severalkilometers through progressively less differentiated melts.Probable diffusive modification of Sr in sanidine phenocrystsand the duration of crystal sinking are consistent with an evolutionaryinterval of some 100 ky or more. Crystal sinking enhanced thedegree of differentiation of the early-erupted magma and pointsto the importance of H2O (to diminish viscosity and enhancethe rate of crystal sinking) in the evolution of silicic magmas. KEY WORDS: crystal settling; differentiation; melt inclusions; rhyolite; trace elements  相似文献   

6.
The 1995 eruption of Fogo (Cape Verde Islands) differed from previous eruptions by the occurrence of evolved lavas, the SW-orientation of vents, and pre-eruptive seismicity between Fogo and the adjacent (~20 km) island of Brava. We have conducted a thermobarometric and chemical study of this eruption in order to reconstruct its magma plumbing system and to test for possible connections to Brava. The bimodal eruption produced basanites (5.2–6.7 wt% MgO) and phonotephrites (2.4–2.8 wt% MgO) that are related by fractional crystallization. Clinopyroxene-melt-barometry of phenocrysts yields pressure ranges of 460–680 MPa for the basanites and 460–520 MPa for the phonotephrites. Microthermometry of CO2-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yields systematically lower pressure ranges of 200–310 MPa for basanites and 270–470 MPa for phonotephrites. The combined data indicate pre-eruptive storage of the 1995 magmas within the lithospheric mantle between 16 and 24 km depth. During eruption, the ascending magmas stalled temporarily at 8–11 km depth, within the lower crust, before they ascended to the surface in a few hours as indicated by zonations of olivine phenocrysts. Our data provide no evidence for magma storage at shallow levels (<200 MPa) or lateral magma movements beneath the Fogo-Brava platform. Sr–Nd–Pb isotope ratios of samples from Brava differ significantly from those of the 1995 and older Fogo lavas, which rules out contamination of the 1995 magmas by Brava material and indicates different mantle sources and magma plumbing systems for both islands.  相似文献   

7.
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   

8.
We have investigated the evolution of an active silicic magma-feedingsystem beneath Usu volcano, Japan, where eight eruptions havebeen recorded since AD 1663. All magmatic products contain similartypes of plagioclase and orthopyroxene phenocrysts that consistof homogeneous cores with uniform compositions, and a zonedmantle that increases in size with time. The compositions ofplagioclase and orthopyroxene phenocrysts vary gradually andregularly with time, as do the bulk-rock compositions. The textureof these phenocrysts also changes systematically, caused byprogressive crystal growth, dissolution and diffusion. On thebasis of these observations, we conclude that the same magma-feedingsystem has persisted at Usu volcano since AD 1663. Compositionalvariation of magnetite phenocrysts differs from that of plagioclaseand orthopyroxene, because magnetite has large diffusion coefficientsand should represent magmatic conditions immediately beforethe eruption. Most pumices from Usu volcano contain two typesof magnetite phenocryst, each with a different composition andcrystallization temperature, indicating that two magmas mixedbefore each eruption (approximately several days before). Theend-members changed with time: rhyolite + basaltic andesite(1663); dacite ± rhyolite (1769, 1822, 1853); dacite± dacite (1977, 2000). The temperature of the magma apparentlyincreases with time, and the increase can be explained by sequentialtapping from a magma chamber with a thermal and chemical gradientin addition to injection of high-temperature magma. KEY WORDS: continuous existence of magma chamber; dacite; dissolution and diffusion of phenocrysts; magma mixing; magnetite  相似文献   

9.
Zircon U–Pb geochronology results indicate that the John Muir Intrusive Suite of the central Sierra Nevada batholith, California, was assembled over a period of at least 12 Ma between 96 and 84 Ma. Bulk mineral thermochronology (U–Pb zircon and titanite, 40Ar/39Ar hornblende and biotite) of rocks from multiple plutons comprising the Muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure. Assembly of intrusive suites in the Sierra Nevada and elsewhere over millions of years favors growth by incremental intrusion. Estimated long-term pluton assembly rates for the John Muir Intrusive Suite are on the order of 0.001 km3 a−1 which is inconsistent with the rapid magma fluxes that are necessary to form large-volume magma chambers capable of producing caldera-forming eruptions. If large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites, it is doubtful that the intrusive suites represent cumulates left behind following caldera-forming eruptions.  相似文献   

10.
Crystallization experiments were conducted on dry glasses fromthe Unzen 1992 dacite at 100–300 MPa, 775–875°C,various water activities, and fO2 buffered by the Ni–NiObuffer. The compositions of the experimental products and naturalphases are used to constrain the temperature and water contentsof the low-temperature and high-temperature magmas prior tothe magma mixing event leading to the 1991–1995 eruption.A temperature of 1050 ± 75°C is determined for thehigh-temperature magma based on two-pyroxene thermometry. Theinvestigation of glass inclusions suggests that the water contentof the rhyolitic low-temperature magma could be as high as 8wt % H2O. The phase relations at 300 MPa and in the temperaturerange 870–900°C, which are conditions assumed to berepresentative of the main magma chamber after mixing, showthat the main phenocrysts (orthopyroxene, plagioclase, hornblende)coexist only at reduced water activity; the water content ofthe post-mixing dacitic melt is estimated to be 6 ± 1wt % H2O. Quartz and biotite, also present as phenocrysts inthe dacite, are observed only at low temperature (below 800–775°C).It is concluded that the erupted dacitic magma resulted fromthe mixing of c. 35 wt % of an almost aphyric pyroxene-bearingandesitic magma (1050 ± 75°C; 4 ± 1 wt % H2Oin the melt) with 65 wt % of a phenocryst-rich low-temperaturemagma (760–780°C) in which the melt phase was rhyolitic,containing up to 8 ± 1 wt % H2O. The proportions of rhyoliticmelt and phenocrysts in the low-temperature magma are estimatedto be 65% and 35%, respectively. It is emphasized that the strongvariations of phenocryst compositions, especially plagioclase,can be explained only if there were variations of temperatureand/or water activity (in time and/or space) in the low-temperaturemagma. KEY WORDS: Unzen volcano; magma mixing; experimental study  相似文献   

11.
Phenocryst zoning patterns are used to identify open-systemmagmatic processes in the products of the 2001 eruption of ShiveluchVolcano, Kamchatka. The lavas and pumices studied are hornblende–plagioclaseandesites with average pre-eruptive temperatures of 840°Cand fO2 of 1·5–2·1 log units above nickel–nickeloxide (NNO). Plagioclase zoning includes oscillatory and patchyzonation and sieve textures. Hornblendes are commonly unzoned,but some show simple, multiple or patchy zoning. Apatite microphenocrystsdisplay normal and reverse zoning of sulphur. The textural similarityof patchy hornblende and plagioclase, together with Ba–Srsystematics in patchy plagioclase, indicate that the cores ofthese crystals were derived from cumulate material. Plagioclase–liquidequilibria suggest that the patchy texture develops by resorptionduring H2O-undersaturated decompression. When H2O-saturatedcrystallization recommences at lower pressure, reduced pH2Oresults in lower XAn in plagioclase, causing more Al-rich hornblendeto crystallize. Plagioclase cores with diffuse oscillatory zoning,and unzoned hornblende crystals, probably represent a populationof crystals resident in the magma chamber for long periods oftime. In contrast, oscillatory zoning in the rims of plagioclasephenocrysts may reflect eruption dynamics during decompressioncrystallization. Increasing Fe/Al in oscillatory zoned rimssuggests oxidation as a result of degassing of H2O during decompression.A general lack of textural overlap between phenocryst typessuggests that different phenocryst populations were spatiallyor temporally isolated during crystallization. We present evidencethat the host andesite has mixed with both more felsic and moremafic magmas. Olivine and orthopyroxene xenocrysts with reactionor overgrowth rims and strong normal zoning indicate mixingwith basalt. Sieve-textured plagioclase resulted from mixingof a more felsic magma with the host andesite. The mineralogyand mineral compositions of a mafic andesite enclave are identicalto those of the host magma, which implies efficient thermalquenching, and thus small volumes of intruding magma. Mixingof this magma with the host andesite results in phenocryst zoningbecause of differences in dissolved volatile contents. We suggestthat small magma pulses differentiated at depth and ascendedintermittently into the growing magma chamber, producing incrementalvariations in whole-rock compositions. KEY WORDS: patchy zoning; magma mixing; Shiveluch  相似文献   

12.
Heterogeneous andesitic and dacitic lavas on Cordn El Guadalbear on the general problem of how magmas of differing compositionsand physical properties interact in shallow reservoirs beneathcontinental arc volcanoes. Some of the lavas contain an exceptionallylarge proportion (<40%) of undercooled basaltic andesiticmagma in various states of disaggregation. Under-cooled maficmagma occurs in the silicic lavas as large (<40 cm) basalticandesitic magmatic inclusions, as millimeter-sized crystal-clotsof Mg-rich olivine phenocrysts plus adhering Carich plagioclasemicrophenocrysts (An50–70), and as uniformly distributed,isolated phenocrysts and microphenocrysts. Compositions andtextures of plagioclase phenocrysts indicate that inclusion-formingmagmas are hybrids formed by mixing basaltic and dacitic melts,whereas textural features and compositions of groundmass phasesindicate that the andesitic and dacitic lavas are largely mechanicalmixtures of dacitic magma and crystallized basaltic andesiticmagma. This latter observation is significant because it indicatesthat mechanical blending of undercooled mafic magma and partiallycrystallized silicic magma is a possible mechanism for producingthe common porphyritic texture of many calc-alkaline volcanicrocks. The style of mafic-silicic magma interaction at CordonEl Guadal was strongly dependent upon the relative proportionsof the endmembers. Equally important in the Guadal system, however,was the manner in which the contrasting magmas were juxtaposed.Textural evidence preserved in the plagioclase phenocrysts indicatesthat the transition from liquid-liquid to solid-liquid mixingwas not continuous, but was partitioned into periods of magmachamber recharge and eruption, respectively. Evidently, duringperiods of recharge, basaltic magmas rapidly entrained smallamounts of dacitic magma along the margins of a turbulent injectionfountain. Conversely, during periods of eruption, dacitic magmagradually incorporated small parcels of basaltic andesitic magma.Thus, the coupled physical-chemical transition from mixed inclusionsto commingled lavas is presumably not coincidental. More likely,it probably provides a partial record of the dynamic processesoccurring in shallow magma chambers beneath continental arevolcanoes. KEY WORDS: Chile; commingling; magma mixing; magmatic inclusions *Present address: Department of Earth Sciences, Montana State University, Bozeman, MT 59717, USA  相似文献   

13.
Mafic microgranular enclaves (MMEs) are widespread in the Horoz pluton with granodiorite and granite units. Rounded to elliptical MMEs have variable size (from a few centimetres up to metres) and are generally fine-grained with typical magmatic textures. The plagioclase compositions of the MMEs range from An18?CAn64 in the cores to An17?CAn29 in the rims, while that of the host rocks varies from An17 to An55 in the cores to An07 to An33 in the rims. The biotite is mostly eastonitic, and the calcic-amphibole is magnesio-hornblende and edenite. Oxygen fugacity estimates from both groups?? biotites suggest that the Horoz magma possibly crystallised at fO2 conditions above the nickel?Cnickel oxide (NNO) buffer. The significance of magma mixing in their genesis is highlighted by various petrographic and mineralogical characteristics such as resorption surfaces in plagioclases and amphibole; quartz ocelli rimmed by biotite and amphibole; sieve and boxy cellular textures, and sharp zoning discontinuities in plagioclase. The importance of magma mixing is also evident in the amphiboles of the host rocks, which are slightly richer in Si, Fe3+ and Mg in comparison with the amphiboles of MMEs. However, the compositional similarity of the plagioclase and biotite phenocrysts from MMEs and their host rocks suggests that the MMEs were predominantly equilibrated with their hosts. Evidence from petrography and mineral chemistry suggests that the adakitic Horoz MMEs could be developed from a mantle-derived, water-rich magma (>3 mass%) affected by a mixing of felsic melt at P >2.3?kbar, T >730°C.  相似文献   

14.
Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2 content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.  相似文献   

15.
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   

16.
西藏甲玛矿区斑岩内石英和长石斑晶的阴极发光(CL)特征及元素含量变化有效记录了岩浆演化、混合及补给事件.石英斑晶的显微生长结构表明,原始岩浆经历过2次铁镁质岩浆混合作用.根据石英斑晶中Ti含量的变化可知,在2次溶蚀前后,石英结晶温度分别增高了约110℃和80℃.此外,斜长石斑晶的反环带及其Ba、Sr、Fe等元素的浓度梯...  相似文献   

17.
The Fish Canyon Tuff, Colorado, forms one of the largest (3000km3 known silicic eruptions in Earth history. The tuff is ahomogeneous quartz latite consisting of 40% phenocrysts (plagioclase,sanidine, biotite, hornblende, quartz, magnetite, apatite, sphene,and ilmenite) in equilibrium with a highly evolved rhyoliticmelt now represented by the matrix glass. Melt inclusions trappedin hornblende and quartz phenocrysts are identical to the newlyanalyzed matrix glass composition indicating that hornblendeand quartz crystallized from a highly evolved magma that subsequentlyexperienced little change. This study presents experimentalphase equilibrium data which are used to deduce the conditions(P, T, fO2, fH2O, etc.) in the Fish Canyon magma chamber priorto eruption. These new data indicate that sanidine and quartzare not liquidus phases until 780?C temperatures are achieved,consistent with Fe-Ti oxide geothermometry which implies thatthe magmatic temperature prior to eruption was 760?30?C. NaturalFe-Ti oxide pairs also suggest that log fO2 was -12.4 (intermediatebetween the Ni-NiO and MnO-Mn3O4 oxygen buffers) in the magmachamber. This fO2.102 is supported by the experimentally determinedvariations in hornblende and melt Mg-numbers as functions offO2 A new geobarometer based on the aluminum content of hornblendesin equilibrium with the magmatic assemblage hornblende, biotite,plagioclase, quartz, sanidine, sphene, ilmenite or magnetite,and melt is calibrated experimentally, and yields pressuresaccurate to ?0.5 kb. Total pressure in the Fish Canyon magmachamber is inferred to have been 2.4 kb (equivalent to a depthof 7.9 km) based on the Al-content of natural Fish Canyon hornblendesand this new calibration. This depth is much shallower thanhas been proposed previously for the Fish Canyon Tuff. Variationsin experimental glass (melt) composition indicate that the magmawas water-undersaturated prior to eruption. XH2O in the fluidphase that may have coexisted with the Fish Canyon magma isestimated to have been 0.5 by comparing the An-content of naturalplagioclases to experimental plagioclases synthesized at differentXH2O and Ptotals. This ratio corresponds to about 5 wt.% waterin the melt at depth. The matrix glass chemistry is reproducedexperimentally under these conditions: 760?C, 2.4 kb, XH2O=0.5,and log fo2=NNO+2 log units. The fugacity of SO2 (91 b) is calculatedfrom the coexistence of pyrrhotite and magnetite. Maximum CO2fugacity (2520 b) is inferred assuming the magma was volatilesaturated at 2.4 kb.  相似文献   

18.
The temperatures at which pigeonite began to crystallize intholeiitic basalts and andesite of Hakone volcano range from1123 to 1019 ?C based on the three-pyroxene geothermometer ofIshii (1975). These values are lower than the temperatures ofother tholeiitic magmas of similar solidification index, suchas the magmas of Funagata-yama, O-shima, and Akita-komagatakevolcanoes and the Skaergaard intrusion. The cores of Ca-poorpyroxene phenocrysts formed in the Hakone subvolcanic magmareservoir are invariably orthopyroxene; the groundmass and microphenocrystpyroxenes crystallized in the lava during ascent and extrusionof magma are generally pigeonite, but orthopyroxene is alsopresent occasionally. The subvolcanic temperature-compositionpath [T-XFe path; XFe = Fe/(Mg + Fe), atomic ratio] of the Ca-poorpyroxenes does not intersect the pigeonite eutectoid reactionline (PER-line) of Ishii & Takeda (1974), whereas the lava-flowpaths characteristically intersect it at XFe values between0. 31 and 0–52. These two different kinds of T XFe pathsmay be interpreted as reflecting different solidus temperatures,and the content of volatiles, particularly water, appears tobe the controlling factor. It is suggested that the water contentof the magma increased because of fractional crystallizationof anhydrous minerals from a relatively hydrous magma in thesubvolcanic reservoir; then each erupted batch of the fractionatedmagma had its water content lowered by degassing as it ascendedto the volcano. Pigeonite is common in the groundmass of basalticrocks throughout the world, both extrusive and intrusive, butit is rare as phenocrysts in the former, a feature that mayalso be explained by the different solidus temperatures. Calc-alkalirocks in Hakone volcano appear to be derived by fractionationof hydrous tholeiitic magma.  相似文献   

19.
The Younger Andesites and Dacites of Iztacc?huatl volcano, Mexico,constitute a medium-K calcalkaline rock suite (58–66 wt.per cent SiO2) characterized by high Mg-numbers (100Mg/(Mg+0?85Fe2+=55–66) and relatively high abundances of MgO (2?5–6?6wt. per cent), Ni(17–158 p.p.m.), and Cr (42–224p.p.m.). Chemical stratigraphy plots of eruptive sequences indicatethe existence of a plexus of long-lived dacite magma chambersperiodically replenished by influxes of basaltic magma ascendingfrom depth. Short-term geochemical evolution after batch influxwas dictated by magma mixing and eventual dilution of the basalticcomponent by ‘quasi-steady state’ hornblende dacitemagma. The chemical data support textural and mineralogicalevidence for rapid homogenization of originally diverse magmasby convective blending of residual liquids accompanied by dynamicfractional crystallization (Nixon, 1988). Internally-consistent mixing calculations used to derive thecomposition of basaltic magma influx incorporate analyticaluncertainties and the observed range of salic end-member compositions.Mafic end-members are basalts to basaltic andesites (52–54wt. per cent SiO2) with Mg-numbers (73–76), MgO (9–11wt. per cent), Ni (250 p.p.m.), and Cr (340–510 p.p.m.)concentrations, and liquidus olivine compositions (Fo90–88),appropriate for unfractionated partial melts of mantle peridotite.The majority of model compositions are Ol-Hy-normative, similarto those of primitive basaltic lavas on the flanks of Iztacc?huatland in the Valley of Mexico. However, calculated magma batchesrange from weakly Qz-normative to strongly Ne-normative. Bothcalculated and analyzed basaltic compositions are distinguishedby highly variable abundances of alkalies and incompatible traceelements, notably Rb, Ba, Sr, P, Zr, and Y. Initial 87Sr/86Sr ratios for Iztacc?huatl lavas (0?7040–0?7046;n=24) are comparable to those for primitive basaltic rocks (0?7037–0?7045;?=4) and indicate that (1) mantle source regions are isotopicallyheterogeneous; and (2) contamination of iztacc?huatl magma chambersby radiogenic crustal rocks was not a significant factor inthe evolution of calc-alkaline andesites and dacites. The replenishment of Iztacc?huatl dacite reservoirs by Ne-normativemagmas late in the history of cone growth precludes exhaustionof mantle source regions by progressive partial melting. Thewaning stages of volcanic activity at Iztacc?huatl appear toreflect the inability of dense basaltic influxes to successfullypenetrate a large high-level chamber of low density hornblendedacite magma.  相似文献   

20.
The main goal of this investigation is estimating volume of volatile emission, atmospheric and climatic impact of the Kurile Lake caldera-forming eruption, one of the Earth’s largest Holocene explosive eruptions. The volatile content of magma before the eruption was estimated by comparing H2O, S, Cl and F contents in natural quenched glassy melt inclusions trapped by plagioclase phenocrysts. The volatile content of igneous rocks after eruption was estimated by comparing concentrations of degassed matrix glasses. As a result of KO-eruption not more than (3.7–4.2) × 1012 kg of water, (4.3–4.9) × 1010 kg of chlorine, (8.6–9.8) × 109 kg of fluorine and (2.6–2.9) × 1010 kg of sulphur were injected into the atmosphere. This eruption had to produce an important climatic impact.  相似文献   

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