HR: 11:00h
AN: V22A-03    [Abstracts]
TI: Tracking Development of the Taupo (New Zealand) Rhyolitic Magma Chamber Through Melt Inclusions
AU: * Arculus, R J
EM: Richard.Arculus@anu.edu.au
AF: Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Belfield, S
AF: Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Earl, K L
AF: Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200 Australia
AB: The Taupo Volcanic Centre is one of the most globally productive in terms of magma volumes, and is located in the North Island of New Zealand, in-land from the Hikurangi Trench and along strike from the Kermadec Arc. The largest eruption in the last 30 thousand years was the Oruanui (~300 km3). On the basis of whole-rock and mineral compositions, Sutton et al. (2000; Jl. Geol. Soc. London, 157, 537-552) recognize four post-Oruanui (~26.5 ka) magma types erupted from the Taupo Volcanic Centre; the largest (about 35 km3) of these erupted at 1.77 ka had a short magma chamber residence time (less than 103 years), possibly resulting in a lack of compositional zonation. We have analysed phenocrysts, matrix glasses, and glass (formerly melt) inclusions trapped within the dominant plagioclase, pyroxene, and Fe-Ti oxide phenocryst assemblages of 3 (rhyolitic) of these groups, using electron microprobe (major elements; 5 to 20 micron-diameter spots) and laser ablation, inductively coupled plasma mass spectrometry (LA-ICP-MS) (trace elements; 30 to 40 micron spots). The most significant of these results are: 1. for the trace alkali (Rb, Cs) and alkaline earth elements (Sr, Ba), there are large abundance ranges both within matrix and melt inclusions, by up to factors of two; 2. the range of individual LA-ICP-MS spot analyses encompasses the compositional range (by X-ray fluorescence) of bulk pumices; 3. distinctive (by individual eruption) and positive correlations between the alkalis, alkaline earths and light rare earth elements; 4. even within a specific eruption, there is more than one trace element correlation trend; 5. individual phenocrysts are compositionally zoned requiring some heterogeneity of former host melts. Using the experimentally-constrained criterion of Mn partitioning for equilibrium between coexisting ilmenite-magnetite solid solutions, it is possible to calculate sequentially: the equilibrium T-fO2 of homogenised host magmas (temperature ranges ~ 760 to 860oC; -log10fO2 ~ 15.8 to 12); the Fe2+/Fe3+ and hence Mg/Fe2+ of the host magma; and equilibrium orthopyroxene composition. For the 1.77 ka eruption, the equilibrium orthopyroxene Mg/(Mg+Fe2+) = 0.5 (cf. 0.55 to 0.45 phenocryst range). We interpret these results to indicate the preservation within the melt inclusion population of the post-Oruanui, Taupo eruption sequence, of the compositional record of individual melt components that became homogenized within the larger magmatic system feeding this eruption sequence. Multiple crustal sources must be involved in Taupo Zone rhyolite generation driven by mantle-derived basalt ingress. A physical issue is how the diverse melt inclusion-hosting phenocrysts can be so uniformly distributed in otherwise chemically unzoned bulk (host) rhyolitic magma.
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly