Rapid open-system assembly of a large silicic magma body: time-resolved evidence from cored plagioclase crystals in the Oruanui eruption deposits, New Zealand |
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Authors: | B L A Charlier C J N Wilson J P Davidson |
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Institution: | (1) Department of Earth and Environmental Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK;(2) Geology-SGGES, and Institute of Earth Science and Engineering, University of Auckland, PB 92019 Auckland Mail Centre, Auckland, 1142, New Zealand;(3) Department of Earth Sciences, University of Durham, South Road, Durham, DH1 3LE, UK |
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Abstract: | Rhyolitic pumices in the 26.5 ka Oruanui eruption (Taupo volcano, New Zealand) contain an average of 10 wt% crystals. About
2 wt% of the crystal population is feldspar crystals that display bluish–grey cloudy cores, the colour being imparted by exsolved
needles of rutile. The volume of cloudy-cored feldspars thus amounts to ~1.0 km3 in a total magma volume of ~530 km3. The cored feldspars show great variability in detail, but in general have a rounded cloudy core bounded by a zone rich in
glass and mineral inclusions, that was then overgrown by a euhedral clear rim. Sr-isotopic variations in eight representative
crystals were measured on micromilled samples of selected growth zones in the cores and rims, and linked to feldspar compositions
through microprobe traverses. The cloudy cores range from 87Sr/86Sr = 0.70547 to 0.70657, with compositions of An43 to An78. The overgrowth rims display wider variations: inner parts show extreme ranges in composition (maxima 87Sr/86Sr = 0.70764 and An78), while outer parts in seven of eight crystals are zoned, with outward-decreasing Sr-isotopic and An values to figures that
are in accord with the bulk pumice and other, clear-feldspar values, respectively. The three parts of the crystals represent
distinct regimes. The cloudy cores are inherited from an intermediate plutonic protolith that has been subjected to melting.
The inner overgrowth rims were crystallised from a high temperature, relatively radiogenic melt derived from Mesozoic-Palaeozoic
metasedimentary rocks (“greywacke”). The outer euhedral rims reflect mixing into and continued growth within the growing Oruanui
magma body. The cloudy-cored feldspars also contain rare zircon inclusions. Twenty one zircons were recovered by HF digestion
of a bulk sample of cloudy feldspars and analysed by SHRIMP for U–Th isotopes with which to calculate model ages. Eighteen
of 21 crystals returned finite ages, the model-age spectrum of which is similar to the age spectra from free zircons in Oruanui
pumices. Assembly of the Oruanui magma body was not only rapid (over ~40 kyr, as shown by other data) but involved a wide
open system, with significant contributions from partly-melted intermediate-composition igneous intrusions (cloudy cores)
and greywacke melts (inner overgrowths) being introduced into the magma body up to the point of eruption. Such open system
behaviour contrasts with that proposed in models for comparably voluminous silicic magmas derived dominantly by fractionation
(such as the Bishop Tuff) where the magma and its crystal cargo were better insulated thermally and chemically from country-rock
interaction. |
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Keywords: | Oruanui eruption Sr isotopes 87Sr/86Sr Microsampling Magmatic processes Zircon |
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