50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

and voluminous than at any on Earth. The high
frequency of episodes appears to reflect the thin
crust, high heat flow, and active tectonic regime of
the central North Island, New Zealand. The two
contemporaneously active calderas, OVC and TVC,
that have been active in the last 50 kyr display
contrasting volcanic and magmatic histories.
Following the last caldera-forming episode at ~50
ka (Rotoiti) from OVC, intra-caldera episodes have
tapped progressively more evolved (~71- 77 wt %
SiO2), shallower (>400- 150 MPa), and cooler (940
-730°C) magmas. Directly following the calderaforming
episode, eruptions were rhyodacites and
derived from deep, hot magmas, containing
clinopyroxene. The most recent OVC eruption
episodes (post-26.5 ka) were from shallow, cool
magmas, and are dominated by hydrous mineral
phases. In general, the frequency of episodes has
decreased and volume of eruptions appear to have
increased over time at OVC. At TVC, pre-26.5 ka
eruption episodes were relatively infrequent, and
were derived from shallow (~100 MPa), cool
(~750 o C) magmas, containing hydrous mineral
phases. After the caldera-forming 26.5 ka Oruanui
episode at TVC, initial eruption episodes were
dacitic, infrequent, and derived from small volume,
hot (>900°C), and possibly deep (~400 MPa)
magmas. At 12 ka TVC rhyolitic activity recommenced,
episodes since have been more
frequent, hotter (~790 -850°C) and it is likely that
they progressively tapped deeper (~140 -300 MPa)
magmas. Following caldera-formation, erupted
magmas at both OVC and TVC were the least
evolved, hottest (>900°C), and were likely from
deep (~400 MPa) chambers. This suggests that
these magmas rose rapidly after formation and
exploited new conduits generated during the
caldera collapse.
POSTER
VOLATILE AND TRACE ELEMENT
CONCENTRATIONS IN MELT INCLUSIONS
AND MATRIX GLASSES FROM POST-25 KA
TARAWERA RHYOLITES, OKATAINA
VOLCANIC CENTRE
V.C. Smith 1 ,I.A.Nairn 2 & P. Shane 1
1 Department of Geology, The University of
Auckland, Private Bag 92019, Auckland.
2 45 Summit Road, Rotorua RD5.
(v.smith*auckland.ac.nz)
Three of the four rhyolitic episodes (21.4 ka
Okareka, 17.7 ka Rerewhakaaitu, 13.8 ka Waiohau,
and ~AD1315 Kaharoa) from Tarawera in the last
25 kyr have each tapped two or more rhyolite
magmas. The relationships and properties of all
these Tarawera rhyolites are being examined by
ion-microprobe analyses (at University of
Edinburgh) of the volatile contents and trace
element compositions of quartz-hosted melt
inclusions and their associated matrix glasses. The
melt inclusions provide information on dissolved
volatile concentrations, minimum pressures of
crystallisation, and degassing processes in the preeruption
magmas. The trace-element compositions
of the inclusions and matrix melt can reveal magma
variation and mixing processes. Very little was
previously known about volatile abundances and
trace elements in the melt phases of Okataina
rhyolites.
More than 70 melt inclusions (>20 µm) have been
analysed in this study. Volatile contents vary
significantly between and within the magmas; the
wide ranges indicating that inclusions were trapped
at different stages of degassing-induced
crystallisation. H2O contents range from 0.7 to 5.4
wt %, and CO2 concentrations between 30-660
ppm. Most of the CO2 variation is seen between
magmas, with typical variation within a magma