50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
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25km northeast of the volcano. Although not<br />
directly downwind of Mt Taranaki, this site holds<br />
>90 Holocene tephras, which have been sampled<br />
within the top 4 m of sediment. Ten new<br />
radiocarbon dates throughout the length of this core<br />
enable a very detailed sedimentation rate curve to<br />
be established and each individual tephra layer can<br />
be assigned an age based on its position within the<br />
core. This key site is correlated to syn-eruptive<br />
deposits in other locations, including those on the<br />
edifice, by physical and petrological characteristics<br />
as well as titanomagnetite mineral chemistry fingerprinting<br />
techniques. By using these tephras as age<br />
horizons, other proximal eruption deposits which<br />
are not in the distal record can also be assigned a<br />
relative age and added to the event database. The<br />
new database will be used to develop the first<br />
comprehensive probabilistic eruption forecast for<br />
ash eruptions from Mt Taranaki.<br />
A probabilistic temporal model is then required to<br />
handle the stochastic nature of the observed record,<br />
as well as the imprecision and inaccuracy inherent<br />
in the age data obtained via radiocarbon<br />
dating/sedimentation rates and that of the<br />
correlation uncertainties. Such models are usually<br />
assembled using techniques from renewal theory,<br />
(marked) point processes, or hidden (semi-)Markov<br />
models. Combining the resulting model with a<br />
quantitative assessment of the styles and properties<br />
of eruptions possible at Taranaki will allow the<br />
model parameters to be estimated, and the goodness<br />
of fit to the observed record assessed. A suitably<br />
well fitting model can then provide a probabilistic<br />
forecast of future eruptions at Taranaki.<br />
POSTER<br />
MASS TRANSFER IN SUBDUCTION ZONES:<br />
AN ELEMENTAL AND ISOTOPIC<br />
PERSPECTIVE<br />
Simon Turner & Rhiannon George<br />
GEMOC, Macquarie University, Sydney, NSW,<br />
2109, Australia<br />
(sturner*els.mq.edu.au)<br />
Little doubt remains that subduction zone lavas<br />
contain elements recycled from the subducting slab.<br />
However, whether the key agents of this mass<br />
transfer are fluids, supercritical fluids or melts has<br />
major implications for the thermal structure of the<br />
mantle wedge. The evidence for contributions from<br />
both subducted sediment and altered oceanic crust<br />
are compelling and in most arcs their relative<br />
proportions vary inversely. Thus, so-called “fluidrich”<br />
lavas with high Ba/Th and Sr.Th ratios have<br />
low 87 Sr/ 86 Sr and 10 Be/ 9 Be etc and the converse is<br />
true for the so-called “sediment-rich”lavas with<br />
elevated La/Sm. A complicating factor is that many<br />
individual arcs tend to be dominated by one end-<br />
member. Nevertheless, experimental partition<br />
coefficient data are consistent with the differences<br />
between the fluid and sediment components being<br />
formed in the presence of different residual<br />
mineralogies. Sediment fluids appear to be poor in<br />
incompatible elements, relative to those derived<br />
from altered oceanic crust and cannot easily<br />
replicate the sediment end-member. We suggest<br />
that subducted sediments dehydrate at relatively<br />
shallow levels and that these fluids are not strongly<br />
sampled by arc lavas. Altered oceanic crust may<br />
dehydrate more extensively and to greater depths<br />
and may be buffered against melting. Model melts<br />
of dehydrated sediment residues provide a much<br />
better simulation of the inferred sediment endmember<br />
but may require ~800 C at ~ 2GPa,<br />
consistent with recent temperature-dependant<br />
viscosity models. These general inferences are<br />
strongly supported by 10 Be and U-series isotope<br />
data which suggest that the sediment (melt) endmember<br />
is added 100’s kyr to several Myr prior to<br />
eruption whereas addition of fluid components<br />
continues until a few 1000 yrs prior to eruption.<br />
Thus, the fluid and sediment end-member<br />
contributions are separate in composition, space<br />
and time. These data argue strongly against the<br />
involvement of any single supercritical fluid.<br />
ORAL<br />
DEVELOPMENTS IN THE EAST COAST<br />
BASIN, NORTH ISLAND, <strong>NEW</strong> ZEALAND.<br />
Chris Uruski, Brad Field, Rob Funnell,<br />
Chris Hollis, Andy Nicol & Guy Maslen<br />
Institute of Geological & Nuclear Sciences<br />
PO Box 30368<br />
Lower Hutt, New Zealand<br />
(c.uruski*gns.cri.nz)<br />
The East Coast Basin saw oil production in the late<br />
19 th century from wildcat wells near oil seeps. By<br />
the mid-20 th century, geology was being applied to<br />
oil exploration, but with little success. In the late<br />
20 th century seismic techniques were added to the<br />
exploration arsenal and several gas discoveries<br />
were made. At each stage it was recognised that<br />
exploration in this difficult but tantalising basin<br />
required more information than was available.<br />
Continuing work by exploration companies as well<br />
as by the Institute of Geological & Nuclear<br />
Sciences (GNS) has begun to reduce the risk of<br />
exploration. Source rocks have been identified and<br />
sophisticated thermal models show that petroleum<br />
is being generated and expelled from them as<br />
witnessed by the more than 300 oil and gas seeps<br />
onshore. Numerous reservoir facies have been<br />
recognised from outcrop studies and depositional<br />
models are being refined. All components of<br />
petroleum systems have been demonstrated to be<br />
50 th <strong>Kaikoura</strong>05 -91- <strong>Kaikoura</strong> <strong>2005</strong>