Geochemistry and magmatic properties of eruption e

Authors: Victoria C Smith Phil Shane Ian A Nairn Catherine M Williams

Publish Date: 2006/05/19

Volume: 69, Issue: 1, Pages: 57-

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Abstract

Post10 ka rhyolitic eruptions from the Haroharo linear vent zone Okataina Volcanic Centre have occurred from several simultaneously active vents spread over 12 km Two of the three eruption episodes have tapped multiple compositionally distinct homogeneous magma batches Three magmas totalling ~8 km3 were erupted during the 95 ka Rotoma episode The most evolved Rotoma magma SiO2=765–779 wt Sr=96–112 ppm erupted from a southeastern vent and is characterised by a cummingtonitedominant mineralogy a temperature of 739±14°C and fO2 of NNO+052±011 The least evolved SiO2=750–764 wt Sr=128–138 ppm orthopyroxene+ hornblendedominant Rotoma magma erupted from several vents and was hotter 764±18°C and more reduced NNO+040±013 The ~11 km3 Whakatane episode occurred at 56 ka and also erupted three magmas each from a separate vent The most evolved SiO2=733–762 wt Sr=88–100 ppm Whakatane magma erupted from the southwestern Makatiti vent and is cummingtonitedominant cool 745±11°C and reduced NNO+034±008 The least evolved SiO2=728–741 wt Sr=132–134 ppm magma was erupted from the northeastern Pararoa vent and is characterised by an orthopyroxene+ hornblendedominant mineralogy temperature of 764±18°C and fO2 of NNO+040±013 Compositionally intermediate magmas were erupted during the Rotoma and Whakatane episodes are likely to be hybrids A single ~13 km3 magma erupted during the intervening 81 ka Mamaku episode was relatively homogeneous in composition SiO2=761–768 wt Sr=104–112 ppm temperature 736±18°C and oxygen fugacity NNO+019±012 Some of the vents tapped a single magma while others tapped several Deposit stratigraphy suggests that the eruptions alternated between magmas which were often simultaneously erupted from separate vents Both effusive and explosive activity alternated but was predominantly effusive 75 erupted as lava domes and flows The plumbing systems which fed the vents are inferred to be complex with magma experiencing different conditions in the conduits As the eruption of several magmas was essentially concurrent the episodes were likely triggered by a common event such as magmatic intrusion or seismic disturbanceVCS acknowledges funding from the Foundation for Research and Science and Technology FRST University of Auckland and New Zealand Federation of Graduate Women This investigation was also partially funded by FRST contract IAN001 Paul Wallace and Dr JC Mora are thanked for their constructive reviews We also thank T Kobayashi for providing Whakatane stratigraphy and Ritchie Sims electron microprobe and John Wilmshurst Xray fluorescence for help with data acquisitionMajor and trace element concentrations were determined by Xray fluorescence spectrometry XRF techniques using a Siemens SRS 3000 instrument Xray spectrometer with a Rh tube at the University of Auckland Two different techniques were used The first included majors by Norrish fusion and traces by Compton correction Major element beads were prepared with Norrish flux Lanthanum oxide heavy absorber flux in a ratio of 126 g of ignited sample to 675 g of flux similar to the technique described by Harvey et al 1973 Trace element briquettes were prepared with 10 g of rock powder and a liquid PVP/Methyl Cellulose binder The second method included majors and traces by the lowdilution fusion The sample beads were prepared with 1222 flux in a ratio of 2 g of ignited sample to 6 g of flux similar to the technique described by Mori et al 1999 Some Rotoma samples were prepared using both methods to establish consistencyGlass and mineral compositions were determined by an energy dispersive electron microprobe Jeol JXA840A at the University of Auckland The analyses were collected using Princeton GammaTech Prism 2000 Si Li EDS XRay detector An accelerating voltage of 15 kV beam current of 600 pA and 100s count time were used A 2μm focussed beam was used to analyse the minerals and a 15μm defocused beam for glass shards to minimise the loss of sodium Typical microprobe analyses of glass shards have totals of 92–96 The discrepancy from 100 is mostly due to secondary hydration Shane 2000Vent uncertain V2RT1 V5RT1 V16RT2 V16bRT2 V22RT1 V28RT2 V55RT2 V57RT2 V58RT2 V59RT2 V67RT2 V70RT1 V103RT2 V105RT2 V106RT2 V107RT2 V109RT1 V138RT2 V139RT2 V141RT2 V143RT2 V145RT2 R12RT1 R13RT1 R14RT1 R28aRT3 R60aRT1 R78/11RT2 R78/11gRT1 R78/12RT3 R78/14RT1 R78/14bRT1 R78/14eRT3 R78/15eRT2 R78/16RT1 R78/17RT1 R78/18RT1 R78/3RT1 R78/5RT2 R78/7RT1 R78/7aRT1 R78/9RT1 R79RT1 R82cRT1

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