The prevalence of plagioclase antecrysts and xenocrysts in andesite magma, exemplified by lavas of the Tongariro volcanic complex, New Zealand
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Basaltic andesite
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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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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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Andesite magmatism plays a major role in continental crustal growth, but its subduction-zone origin and evolution is still a hotly debated topic. Compared with whole-rock analyses, melt inclusions (MIs) can provide important direct information on the processes of magma evolution. In this article, we synthesize data for melt inclusions hosted by phenocrysts in andesites, extracted from the GEOROC global compilation. These data show that melt inclusions entrapped by different phenocrysts have distinct compositions: olivine-hosted melt inclusions have basalt and basaltic andesite compositions, whereas melt inclusions in clinopyroxene and othopyroxene are mainly dacitic to rhyolitic. Hornblende-hosted melt inclusions have rhyolite composition. The compositions of melt inclusions entrapped by plagioclase are scattered, spanning from andesite to rhyolite. On the basis of the compositional data, we propose a mixing model for the genesis of the andesite, and a two-chamber mechanism to account for the evolution of the andesite. First, andesite melt is generated in the lower chamber by mixing of a basaltic melt derived from the mantle and emplaced in the lower crust with a felsic melt resulting from partial melting of crustal rocks. Olivine and minor plagioclase likely crystallize in the lower magma chamber. Secondly, the andesite melt ascends into the upper chamber where other phenocrysts crystallize. According to SiO2-MgO diagrams of the MIs, evolution of the andesite in the upper chamber can be subdivided into two distinct stages. The early stage (I) is characterized by a phenocrystal assemblage of clinopyroxene + othopyroxene + plagioclase, whereas the late stage (II) is dominated by crystallization of plagioclase + hornblende.
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Cosigüina volcano, in northwestern Nicaragua, erupted violently on 20–24 January 1835, producing pumice, scoria, ash fall deposits, and pyroclastic flows with a bulk tephra volume of ∼6 km3. New geochemical data are presented for bulk-rocks, matrix glasses, melt inclusions and minerals from the 1835 deposits and a pre-1835 basaltic andesite tephra, with the aim of shedding light on the magmatic processes and associated timescales that led to the eruption. Our results reveal that the 1835 eruption was fed by a compositionally and thermally zoned magma reservoir situated ∼4 km (PH2O ∼100 MPa) beneath the volcano. Small volumes of crystal-poor dacite (<10 wt % phenocrysts, 63·8–64·8 wt % SiO2, ∼950°C) and silicic andesite (<10 wt % phenocrysts, 62·2 wt % SiO2, 960–1010°C) were erupted first, followed by relatively crystal-rich andesite (15–30 wt % phenocrysts, 57·4–58·8 wt % SiO2, 960–1010°C), which accounts for ∼90% of the erupted magma. The pre-1835 basaltic andesite (∼20 wt % phenocrysts, 52·4 wt % SiO2, 1110–1170°C) represents a mafic end-member for Cosigüina. The major and trace element compositions of the bulk-rocks, melt inclusions and matrix glasses suggest that (1) the pre-1835 basaltic andesite is a plausible parent for the 1835 magmas, (2) the 1835 andesite bulk-rocks do not represent true melts, but instead mixtures of silicic andesite liquid and a component of accumulated crystals dominated by plagioclase, and (3) the silicic andesite and dacite formed from the andesite magma through liquid extraction followed by fractional crystallization. Observed bimodal to trimodal crystal populations are consistent with a multi-stage, polybaric differentiation process, with calcic plagioclase (An75–90, An90–95) and magnesian clinopyroxene (Mg# = 67–75), plus olivine and magnetite, forming from mafic andesite, basaltic andesite and basalt in the lower crust. The calcic plagioclase exhibits sieve textures, which may be the result of H2O-undersaturated decompression during magma ascent to the upper crust; An50–65 plagioclase lacking a sieve texture, orthopyroxene (Mg# = 61 and 63–72), clinopyroxene (Mg# = 67), magnetite and apatite crystallized from andesite to dacite liquids in the shallow magma reservoir. An75–90 plagioclase comprising entire phenocrysts or cores with An50–65 rims in the 1835 magmas is cognate from earlier stages of differentiation and shows evidence of extensive diffusion of Mg when compared with similar An75–95 crystals hosted in the pre-1835 basaltic andesite. Using plagioclase–melt Mg partitioning and modelling of the Mg diffusion process, we constrain the residence time of these crystals in the silicic liquids to more than 100 years and less than 2000 years, with detailed analysis of three crystals yielding ∼400 years. We propose that magma reservoir zonation occurred on timescales of 102–103 years at Cosigüina. The occurrence of H2O-rich fluid inclusions in all 1835 samples and volatile element systematics in melt inclusions imply that the magmas were saturated with a vapour phase (H2O, S, ± CO2) during much of their evolution in the upper crust. Accumulation of free gas at the top of the magma reservoir may have led to overpressurization of the system, triggering the eruption. Catastrophic release of this exsolved vapour and syn-eruptive devolatilization of the melt injected several teragrams of S into the atmosphere. Our data, coupled with independent evidence from ice cores and tree rings, indicate that the Cosigüina eruption had a sizeable atmospheric impact comparable with or larger than that of the 1991 Pinatubo eruption. Stratigraphic evidence shows that Cosigüina has produced >15 compositionally zoned explosive eruptions in the past, suggesting that similar future eruptions are likely. The products of the 1835 eruption of Cosigüina share many features with compositionally zoned eruptive sequences elsewhere, such as the climactic eruption of Mount Mazama, the ad 79 'Pompei' eruption of Vesuvius and the 1912 eruption of Novarupta–Katmai.
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Pumice
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