The magmatic evolution of the Whakamaru supereruption constrained by microanalytical study of plagioclase and quartz
Abstract
The Whakamaru eruption is one of the largest known eruptions to have originated from the hyper-productive Taupo Volcanic Zone, New Zealand. Rhyolitic volcanism is dominant in the Taupo Volcanic Zone. However, the petrogenesis of these magmas is controversial with competing end-member models of fractional crystallisation from parental basalt magmas and crustal anatexis proposed. Crystals hosted in volcanic rocks have the potential to document the magmatic evolution and their passage through the sub-volcanic system. Each magmatic process has the potential to deposit a chemical or textural signature within a crystal producing highly zoned crystals. Electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry have been used to determine the major, minor and trace element concentrations of plagioclase and quartz crystals of samples from throughout a 100 m section of the Whakamaru Ignimbrite. Coupled with cathodoluminescence images of quartz crystals, this has allowed the magmatic evolution of the Whakamaru Ignimbrite to be deciphered and diffusional timescales of magmatic processes to be constrained. Three plagioclase populations have been identified: Group 1 crystals with inherited cores of An 45-60, and Ba 115 - 650 ppm and rims of ca. An30 and Ba 450 - 800 ppm; abundant Group 2 plagioclase crystals that are oscillatory zoned with An30-40 and Ba 450 - 730 ppm and; rare Group 3 plagioclase crystals with low Sr 280 - 480 ppm, cores of An25-35 and rims of An20-25. The chemical zonation of these plagioclase crystals permits the identification of four distinct silicic melts involved in the generation of the final rhyolitic melt: (1) an andesitic protolith melt from which the cores of Group 1 crystals fractionated; (2) a greywacke melt or greywacke protolith melt responsible for the overgrowth rims of Group 1 plagioclase cores; (3) an evolved melt derived from the rejuvenation of a mature crystal mush body evidence from Group 3 plagioclase crystals; and (4) the final rhyolitic melt generated from the amalgamation in varying proportions of the first three melts and fractional crystallisation of a plagioclase-dominant crystal assemblage in an open-system. Diffusion modelling of the greyscale intensity of cathodoluminescence images (as a proxy for Ti content) for a selection of bright core-rim interfaces of quartz crystals, suggests that renewed quartz growth occurred < 80 years prior to eruption and continued towards the climatic eruption. In addition, Sr diffusion modelling across of Group 1 plagioclase crystals provide further evidence of short (< 100 years) residence times of plagioclase crystals prior to eruption. This provides strong evidence for the formation of large volumes of eruptible rhyolitic magma bodies in the years preceding climatic eruptions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V51A1644S
- Keywords:
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- 1036 GEOCHEMISTRY / Magma chamber processes;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 8410 VOLCANOLOGY / Geochemical modeling