Showing papers by "Colin J. N. Wilson published in 1992"
TL;DR: In this article, it was shown that the uniform major-element chemistry of the magma may relate to a fine balance between rates of eruption and supply and the higher density of the more evolved (Ferich) magmas which could be tapped only after caldera-forming events had removed significant volumes of less evolved but lighter magma.
75 citations
TL;DR: The age of Rotoehu Ash is estimated as 64 ± 4 ka from whole-rock K-Ar age determinations on older and younger lava flows on Mayor Island volcano as mentioned in this paper.
Abstract: The age of the Rotoehu Ash is estimated as 64 ± 4 ka from whole‐rock K‐Ar age determinations on older and younger lava flows on Mayor Island volcano. This age is compatible with an existing U/Th disequilibrium age of 71 ± 6 ka and both are significantly older than the currently accepted value of 50 ka.
74 citations
TL;DR: The Waimihia pumice deposit was erupted from Taupo volcano about 3.3 ka BP as mentioned in this paper, and three crystal-poor magma types: rhyolite, rhyodacite and andesite.
Abstract: The Waimihia pumice deposit was erupted from Taupo volcano about 3.3 ka BP. It comprises a plinian tephra fall deposit divisible into two volumetrically subequal fall units and a late-stage, volumetrically minor, near-vent ignimbrite. About 7.5km 3 of magma were erupted at an average rate of c. 8 × 10 8 kg s −1 . Whole rock, glass and mineral compositions define three crystal-poor magma types: rhyolite, rhyodacite and andesite. The proportions of the three magmas varies with stratigraphic position in the two fall units. The lower Waimihia fall unit shows a steady upward increase in rhyodacite from c . 92%) and is similar to other post-22 ka Taupo rhyolites in terms of major element, trace element and Sr isotope composition, mineralogy and Fe-Ti oxide temperature and oxygen fugacity. The rhyodacite ( c. 7.3%) formed by hybridization of rhyolite and andesite prior to the eruption, and occurs as grey pumices and with rhyolite in streaky pumices. Andesite ( c. 0.3%) occurs as black scoria, sometimes containing traces of rhyodacite; it is an unusual high TiO 2 tholeiitic andesite whose 87 Sr/ 86 Sr ratio of 0.7062 is indistinguishable from that of the rhyolite. The heterogeneous pumice and scoria clasts were formed in a second magma mixing process that was active when the magmas were being transported to the surface during the eruption. The order in which the magmas were erupted, and the decoupling of the peaks in rhyodacite and andesite production, are explained by withdrawal from a three-layer sill that had formed following the injection of c. 0.16 km 3 of andesite into a ≥ 7.5 km 3 sillshaped rhyolitic magma chamber.
67 citations
TL;DR: Using a continuous laser and resistance furnace, the authors measured ages on Quaternary plagioclase with an absolute precision of about ±30 ka and on quaternary sanidine with a relative precision of better than 1%.
Abstract: Using a continuous laser and resistance furnace, we have measured ages on Quaternary plagioclase with an absolute precision of about ±30 ka and on Quaternary sanidine with a relative precision of better than 1%. Such precision was achieved by using low-temperature heating steps to remove much of the nonradiogenic argon contamination. Plagioclase is one of the most common mineral phases in volcanic rocks; thus, these procedures will be widely applicable to many problems for which precise radiometric age control has not been available. We studied plagioclase and plagioclase-sanidine concentrates from the oldest and the three largest silicic ash-flow deposits of the Taupo Volcanic Zone, New Zealand, one of the world9s largest and most active volcanic systems. The results are in close agreement with new magnetostratigraphic data, suggest that existing fission-track age determinations significantly underestimate the age of older units, and shift the inception of Taupo Volcanic Zone volcanism back to at least 1600 ka. The improved precision has permitted the first correlations between proximal and distal units; previous correlations with deep-sea ash flows in the western Pacific require major revision.
56 citations