An Examination of the Evolution of the Santorini Volcanic Field Using a New Oxygen Fugacity Method
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Date
2006-06
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The Ohio State University
Abstract
Volcanism on Santorini has occurred over the last ~600,000 years. During this
interval most of the erupted lavas have evolved via processes such as fractional
crystallization, mixing and assimilation, and therefore do not provide good information
about their mantle source. The basalts that do occur on Santorini are parental to the
evolved magmas. Studies of the basalts potentially provide information about magma
genesis in the mantle beneath Santorini.
Santorini is located in the Aegean Sea above an active subduction zone. The rate
of subduction is ~ 5 cm per year so that over the lifespan of the volcanic field
approximately 30 km of oceanic lithosphere has been subducted into the mantle beneath
the volcanic arc. As with all convergent margins, subduction of lithosphere releases
fluids and produces melt in the overlying mantle wedge creating the magma responsible
for volcanism. These interactions should modify the composition of the mantle over
time. However, it is not clear whether the composition of the mantle wedge and hence
the compositions of basaltic magmas generated from the wedge vary through time.
Santorini provides a unique opportunity to address this question.
Data collected by previous workers for basalts from Santorini have been compiled
and analyzed. In particular, the compositions of olivine crystals in two basalts erupted
about 400,000 years apart have been used to define the pre-eruption temperatures and
redox states of the host magmas. The redox state of the magma, monitored by the oxygen
fugacity (fO2), is believed to reflect that of the mantle source region. The latter should be
sensitive to recycling of oxidized oceanic lithosphere via subduction. An increase in fO2
indicates greater interaction of subducted lithosphere with the mantle. A new method,
the Olivine-Melt method, developed at OSU, is being used to determine fO2.
Graphical methods were used to examine compositional zoning in olivine and to
constrain the compositions that were in equilibrium with the melt prior to eruption. This
primarily involves examining variations in Mg/Fe between core and rim in individual
olivine crystals.
Results indicate that the redox states of the basalts, and hence the redox states of
the mantle source region have not changed over 400,000 years. Moreover, the preeruptive
temperatures of the basalts have not changed over 400,000 years. Basalts with
similar compositions have been erupted over the entire lifespan of the volcanic field,
suggesting that these conclusions are valid for ~600,000 years. The results are consistent
with those being obtained using other geochemical data at Ohio State. This constitutes
the first study of the long-term compositions of basalts erupted at a convergent margin.
The results imply either that the mantle source region has remained constant with time, or
more probably that processes such as mixing homogenize the mantle-derived basalts in
chambers located at the base of the crust.