Rifting and recharge as triggers of the mixed basalt–rhyolite Halarauður ignimbrite eruption (Krafla, Iceland)
Shane M. RooyakkersJohn StixKim BerloDaniele MorgaviMaurizio PetrelliMonika K. RusieckaSimon J. BarkerB. L. A. CharlierDavid A. NeaveFrancesco VetereDiego Perugini
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Keywords:
Silicic
Basaltic andesite
Dacite
Caldera
Igneous differentiation
Fractional crystallization (geology)
Phenocryst
Phenocryst
Dacite
Igneous differentiation
Hornblende
Basaltic andesite
Alkali basalt
Fractional crystallization (geology)
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Citations (88)
The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1987)
The Kushigatayama Subgroup, Kurosawa basalt-mudstone and Tsukimibashi andesite of the Nishiyatsushiro Group of middle Miocene are distributed in the eastern part of the Koma mountain and are composed of basaltic and andesitic lavas and pyroclastic rocks and small amounts of mudstones and acid tuffs. Most of the volcanic rocks were altered to low-grademetamorphic rocks ranging from laumontite to prehnite-pumpellyite facies. Volcanic rocks of the Kushigatayama Subgroup are composed of olivine basalt, diopsideolivine basalt, diopside-augite basalt, augite andesite, hypersthene-augite andesite and augitehornblende andesite. Almost of olivine phenocrysts are altered to saponite or chlorite/saponite mixed layered mineral. Phenocryst clinopyroxenes are diopside, endiopside, salite and augite and groundmass clinopyroxenes range augite through subcalcic augite to pigeonite. Mafic phenocryst content is 38.8 percent in maximum. Abnormal phenocryst content results from the presence ofmegacryst or cumulate. Most of volcanic rocks may belong to tholeiite series. Basalts having FeO*/MgO ratios(0.91-1.00) are very close to the primitive basalt in composition, although some basalts include cumulates. Differentiation from olivine basalt to andesite is mainly interpreted by fractionation of olivine, clinopyroxene and plagioclase. From whole rock chemical compositions, P2O5/MnO/TiO2 ratios in Mullen's diagram and chondrite-normalized REE concentration, basalts of this area as well as those of the Misaka and Tanzawa areas are obvious to be island arc tholeiite.
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Basaltic andesite
Diopside
Pigeonite
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We found high-Mg andesite (56.5 wt.% SiO2 and 7.2 wt.% MgO) from Mikasayama in Wassamu town, northern Hokkaido. Its K-Ar age is 11.1±0.8 Ma. The high-Mg andesite is characterized by co-existence of Fo-rich olivine (Fo90-85) and An-poor plagioclase (An64-38) phenocrysts. The mineralogical evidence suggests that the high-Mg andesite from Mikasayama was produced by mixing of primitive basalt magma, containing Mg-rich olivine and clinopyroxene phenocrysts, and hornblende dacite magma.
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Dacite
Basaltic andesite
Igneous differentiation
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Samples from several stratigraphic levels in the Miocene volcanic succession on Lesbos, Greece, contain complexly zoned clinopyroxene and feldspar phenocrysts. Electron microprobe analyses have been made of zoned clinopyroxenes and feldspars, and their inclusions; and phenocryst mineralogy is compared with host-rock chemical composition. The commonest pyroxene zoning sequence is cores of greenish salite or augite mantled by diopside which, in turn, may be rimmed by augite, with each type of pyroxene in optical and chemical discontinuity with the next. None of the pyroxenes has high pressure characteristics. The Ts content of phenocryst cores is correlated with the K-content of the host rock; and the most diopsidic overgrowths also occur in the most K-rich rocks. Augite rims occur only in Al-rich rocks. Most of the features of the pyroxenes can be accounted for by a hypothetical fractional crystallization scheme. However, two observations suggest that mixing of small magma batches may also be important. The earliest crystallized diopsides that rim salite are rich in Cr2O3, suggesting precipitation from an unfractionated magma. Anorthoclase (implying advanced crystallization of feldspars) occurs as cores to feldspar phenocrysts and as inclusions in clinopyroxene cores in several samples.
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Pyroxene
Fractional crystallization (geology)
Porphyritic
Diopside
Igneous differentiation
Nepheline
Sanidine
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Phenocrysts and xenocrysts of augite, subcalcic augite, pigeonite, orthopyroxene, and olivine in two glassy rhyodacite dredge samples from 95/sup 0/W on the GSC reflect a complex history of fractional crystallization and magma mixing. The pyroxene compositions can be grouped into clusters reflecting three major sources: a) Fenumber approx. 0.2 from basalt, b) Fenumber approx. 0.5 from rhyodacite, and c) Fenumber approx. 0.6 from rhyolite. Pyroxene Ti/Al ratios of 1:14, 1:7 and 1:3 have Fenumbers suggesting original crystallization from basalt, rhyodacite, and rhyolite melts respectively. These general compositional groups are typical of those produced during fractional crystallization of basalt to rhyolite. At relatively constant Fenumber the augites in any group display a wide spectrum of variation in Wo, Al, and Ti. Basaltic augite core to rim variations exhibit both increases and decreases in Ti at nearly constant Fenumber. A continuous variation in subcalcic augites is present from Fe-augite to Fe-pigeonite. These effects are likely kinetic, perhaps due to rapid cooling rates, but possibly due to supersaturation produced during mixing. Magma mixing may have played an important role in bringing together these contrasting components. The abundance of very-fine-grained basaltic xenoliths and xenocrysts, the glassy rhyolitic inclusions and associated xenocrysts, along with majormore » reverse zoning in Fenumber or major discontinuities in Fenumber in the ferromagnesian phases all point to coexisting melts of radically differing composition.« less
Pigeonite
Phenocryst
Pyroxene
Fractional crystallization (geology)
Igneous differentiation
Xenolith
Magma chamber
Basaltic andesite
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Crystallization experiments were conducted on dry glasses from the Unzen 1992 dacite at 100–300 MPa, 775–875°C, various water activities, and fO2 buffered by the Ni–NiO buffer. The compositions of the experimental products and natural phases are used to constrain the temperature and water contents of the low-temperature and high-temperature magmas prior to the magma mixing event leading to the 1991–1995 eruption. A temperature of 1050 ± 75°C is determined for the high-temperature magma based on two-pyroxene thermometry. The investigation of glass inclusions suggests that the water content of the rhyolitic low-temperature magma could be as high as 8 wt % H2O. The phase relations at 300 MPa and in the temperature range 870–900°C, which are conditions assumed to be representative of the main magma chamber after mixing, show that the main phenocrysts (orthopyroxene, plagioclase, hornblende) coexist only at reduced water activity; the water content of the post-mixing dacitic melt is estimated to be 6 ± 1 wt % H2O. Quartz and biotite, also present as phenocrysts in the dacite, are observed only at low temperature (below 800–775°C). It is concluded that the erupted dacitic magma resulted from the mixing of c. 35 wt % of an almost aphyric pyroxene-bearing andesitic magma (1050 ± 75°C; 4 ± 1 wt % H2O in the melt) with 65 wt % of a phenocryst-rich low-temperature magma (760–780°C) in which the melt phase was rhyolitic, containing up to 8 ± 1 wt % H2O. The proportions of rhyolitic melt and phenocrysts in the low-temperature magma are estimated to be 65% and 35%, respectively. It is emphasized that the strong variations of phenocryst compositions, especially plagioclase, can be explained only if there were variations of temperature and/or water activity (in time and/or space) in the low-temperature magma.
Phenocryst
Dacite
Igneous differentiation
Silicic
Melt inclusions
Pyroxene
Magma chamber
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Citations (174)
We found high-Mg andesite (56.5 wt.% SiO2 and 7.2 wt.% MgO) from Mikasayama in Wassamu town, northern Hokkaido. Its K-Ar age is 11.1±0.8 Ma. The high-Mg andesite is characterized by co-existence of Fo-rich olivine (Fo90-85) and An-poor plagioclase (An64-38) phenocrysts. The mineralogical evidence suggests that the high-Mg andesite from Mikasayama was produced by mixing of primitive basalt magma, containing Mg-rich olivine and clinopyroxene phenocrysts, and hornblende dacite magma.
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Dacite
Basaltic andesite
Igneous differentiation
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The Batur volcanic field (BVF), in Bali, Indonesia, underwent two successive caldera-forming eruptions that resulted in the deposition of silicic ignimbrites. The magmas erupted during and between these eruptions show a broad range of compositions from low-SiO2 andesite to high-SiO2 dacite. On the basis of their geochemistry and mineralogy these magmas may be assigned to six groups: (1) homogeneous andesites with phenocryst compositions essentially in equilibrium with the whole-rock composition; (2) remobilized crystal-rich low-SiO2 andesites with resorbed phenocrysts in equilibrium with the whole-rock composition; (3) mixed low-SiO2 dacite with a relatively large range of phenocryst compositions, with most phenocrysts slightly too evolved to be in equilibrium with the whole-rock; (4) extensively mixed low-SiO2 dacites with a very large and discontinuous range of phenocryst compositions, with most phenocrysts either more Mg-rich or more evolved than the equilibrium compositions; (5) remobilized crystal-rich low-SiO2 dacites with resorbed and euhedral phenocrysts; (6) homogeneous high-SiO2 dacites lacking evidence for magma mixing and showing narrow ranges of phenocryst compositions in equilibrium with the whole-rock composition. This range of silicic magmas is interpreted to reflect a combination of closed- and open-system fractional crystallization, magma mixing and remobilization of cumulate piles by heating. The variety of magmas erupted simultaneously during the caldera-forming eruptions suggests that the magmatic system consisted of several independent reservoirs of variable composition and degree of crystallization. The magmatic evolution of individual reservoirs varied from closed-system fractional crystallization to fully open-system evolution, thereby resulting in simultaneous production of magmas with contrasted compositions and mineralogy. Extensive emptying of the magmatic system during the caldera-forming eruptions led to successive or simultaneous eruption of several reservoirs.
Phenocryst
Silicic
Fractional crystallization (geology)
Igneous differentiation
Dacite
Caldera
Magma chamber
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Citations (41)
Phenocryst
Basaltic andesite
Hornblende
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Citations (450)
The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1973)
Hypersthene andesite, erupted on one of the volcanic centers of the Setouchi Petrographic Province, has been studied petrologically. This rock is shown as the typical calcalkaline andesite by the microscopic observations and the chemical analyses. The phenocrysts are composed of an abundance of labradorite, hypersthene, and augite, a small amount of hornblende and ore minerals. Relic of olivine, surrounded by a thick reaction rim of hypersthene, can be found in thin sections of the SiO2-poor variety sometimes. Orthopyroxene phenocrysts (En61.3-66.3) are hypersthenic in a narrow sense, and are clearly different from those (En73.5-89.1) of the phenocryst-poor andesites (Ujike, 1972) from the same area. Parallel-growth of hypersthene (inside) and augite (outside), which has been considered as a phenomenon caused by magmatic contamination process (Ota, 1958), appears in this andesite. The data obtained by high temperature melting experiments of natural rock compositions have been compiled from literature to show that magmatic chemistry, especially normative Wo/(Wo+En+Fs), would strongly control the order of crystallization of the two pyroxenes. In short, from melt being 100×Wo (Wo+En+Fs)_??_20 by weight, ortho- and clinopyroxenes have likely been to start crystallization simultaneously; from melt having the value smaller than 15, orthopyroxene has crystallized at a higher temperature than clinopyroxene very often. This compilation suggests that the augite must have nucleated at a slightly lower temperature than the hypersthene on the surface of the latter crystal from the hypersthene andesitic magma having low Wo component, and that no contamination process might be needed to explain the parallel-growth of the pyroxenes. This kind of parallel-growth may be found frequently in calc alkaline andesite since Wo content of average andesite is small.
Phenocryst
Pigeonite
Basaltic andesite
Andesites
Fractional crystallization (geology)
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