Volatile and Trace Element Partitioning for a Peralkaline Rhyolite Volcano: Mayor Island, New Zealand
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Mayor Island, New Zealand is a small isolated occurrence of a peralkaline rhyolite volcano 60km away from the calc-alkaline Taupo Volcanic Zone. During the volcano's 130,000 year sub-aerial history the erupted magma increases in peralkalinity, with the Agpaitic Index (AI) rising from 1 in the very oldest lavas to ~ 1.25 in the most recent eruptions, straddling the compositional divide between comendite and pantellerite. In addition to this an ~40% enrichment of REE's and certain trace elements (e.g. Zr,Y,Nb) is observed. Phenocryst phases present include alkali feldspar, quartz, aenigmatite, Na-hedenbergite, aegerine and fayalitic olivine. With the slight exception of the feldspar which has a slight bimodality in Na/K ratio the major element compositions of these phases remains stable throughout the evolution of the magma. Analysis of melt inclusions, matrix glass and phenocryst phases has allowed an estimation of (i) volatile element partitioning between melt and vapour and, (ii) non-volatile trace element partitioning between phenocrysts and melt. These data allow an assessment of the role that the phenocrysts have played in the evolving magma composition through fractional crystaUisation and they give insight into the effect of degassing on magma chemistry. Trace elements were measured using the secondary ion mass spectrometer (ion microprobe) analysis and the volatile components of both inclusion and matrix glass utilised FTIR (H20), electron microprobe (C1) and ion microprobe (H,F) techniques.Keywords:
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The Yamakogawa Rhyolite, which erupted in the early Quaternary period in central Kyushu, Japan, comprises seven units, three contain of which spatter and stretched pumice. Our fieldwork shows that these are the deposits of strombolian fire-fountains and rheomorphic tuff. Such deposits derived from silicic magma have been previously described and still are controversial. Some of the reasons given for their formation were exclusively peralkaline composition and high-magmatic temperature. The chemical analyses of the Yamakogawa Rhyolite show nonperalkaline composition and low-magmatic temperature. Moreover, the mineral assemblage of the Yamakogawa Rhyolite suggests that its water content was indistinguishable from other rhyolitic deposits. This is the first report that demonstrates that eruption of silicic magma as fire-fountain and pyroclastic flow with rheomorphism is not, necessarily, restricted to peralkaline composition, high-magmatic temperature and low-water content rhyolite.
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Mayor Island, New Zealand is a small isolated occurrence of a peralkaline rhyolite volcano 60km away from the calc-alkaline Taupo Volcanic Zone. During the volcano's 130,000 year sub-aerial history the erupted magma increases in peralkalinity, with the Agpaitic Index (AI) rising from 1 in the very oldest lavas to ~ 1.25 in the most recent eruptions, straddling the compositional divide between comendite and pantellerite. In addition to this an ~40% enrichment of REE's and certain trace elements (e.g. Zr,Y,Nb) is observed. Phenocryst phases present include alkali feldspar, quartz, aenigmatite, Na-hedenbergite, aegerine and fayalitic olivine. With the slight exception of the feldspar which has a slight bimodality in Na/K ratio the major element compositions of these phases remains stable throughout the evolution of the magma. Analysis of melt inclusions, matrix glass and phenocryst phases has allowed an estimation of (i) volatile element partitioning between melt and vapour and, (ii) non-volatile trace element partitioning between phenocrysts and melt. These data allow an assessment of the role that the phenocrysts have played in the evolving magma composition through fractional crystaUisation and they give insight into the effect of degassing on magma chemistry. Trace elements were measured using the secondary ion mass spectrometer (ion microprobe) analysis and the volatile components of both inclusion and matrix glass utilised FTIR (H20), electron microprobe (C1) and ion microprobe (H,F) techniques.
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The Yamakogawa Rhyolite, which erupted in the early Quaternary period in central Kyushu, Japan, comprises seven units, three contain of which spatter and stretched pumice. Our fieldwork shows that these are the deposits of strombolian fire-fountains and rheomorphic tuff. Such deposits derived from silicic magma have been previously described and still are controversial. Some of the reasons given for their formation were exclusively peralkaline composition and high-magmatic temperature. The chemical analyses of the Yamakogawa Rhyolite show nonperalkaline composition and low-magmatic temperature. Moreover, the mineral assemblage of the Yamakogawa Rhyolite suggests that its water content was indistinguishable from other rhyolitic deposits. This is the first report that demonstrates that eruption of silicic magma as fire-fountain and pyroclastic flow with rheomorphism is not, necessarily, restricted to peralkaline composition, high-magmatic temperature and low-water content rhyolite.
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Abstract The riebeckite‐bearing rhyolite of Winkston Hill, Peebles, is shown to be peralkaline and may be named a pantellerite. Trace element comparison with other more altered members of the Tweeddale lavas from Wrae Hill suggests that the whole suite is peralkaline. It is thought most unlikely that the suite was developed in association with an Upper Ordovician subduction zone, and it was perhaps erupted on an oceanic island.
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This chapter contains sections titled: Introduction Geologic Setting Evolution of the Volcanic Center Discussion
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Abstract The Caradoc volcanic rocks of the Lleyn Peninsula represent a major episode of rhyolite‐dominated eruption (total volume up to 560 km 3 ) within the Southern British Caledonides. New geochemical data reveal the original chemical characteristics of the basic, intermediate, and acid lavas. The basalts are interpreted as having predominantly within‐plate character and are associated with intermediate lavas (trachybasalts) and high‐Zr, originally peralkaline, trachytes, and rhyolites. A low‐Zr, originally high‐K, subalkalic group of rhyolites, and a basalt with transitional island arc tholeiite‐MORB characteristics are unrelated to these lavas. The volcanic rocks are interpreted to have been erupted onto a complex, rapidly evolving, active continental margin dominated by crustal tension.
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Bimodal associations of basalt and rhyolite of Upper Ordovician age which were erupted in a submarine environment occur within the Caledonian orogenic belt of South Britain at Parys Mountain (Anglesey), in Snowdonia (North Wales) and at Avoca (SE Ireland). The volcanic rocks have experienced hydrothermal alteration and low-grade metamorphism, and therefore immobile elements (e.g. Ti, Zr, Nb, Y) have been used to identify the original geochemical characteristics. The basalts have characters transitional between volcanic ‘arc’ and ‘within plate’ types consistent with eruption on an extensional part of an active continental margin. Two groups of rhyolites have been identified. A low-Zr group (Zr<500ppm), represented at all three locations, is interpreted as originally of high-K, subalkaline type. A high-Zr group (Zr>500ppm), represented at Snowdonia and Avoca, is interpreted as originally being peralkaline in composition; their high Zr/Nb ratios (>10) are typical of peralkaline rhyolites erupted above subduction zones. The bimodal nature of the associations and the peralkaline character of some rhyolites indicates magma production in a complex tectonic setting, transitional between an active continental margin/island arc and an extensional environment. Associated sulphide mineralization is volcanogenic and probably syn-sedimentary. High-level, rhyolitic magma chambers are thought to have driven convection of the hydrothermal fluids from which the sulphides precipitated.
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