Evolution of the Taupo Volcanic Center, New Zealand: petrological and thermal constraints from the Omega dacite
Artículo

Open/ Download
Publication date
2013Metadata
Show full item record
Cómo citar
Gelman, Sarah E.
Cómo citar
Evolution of the Taupo Volcanic Center, New Zealand: petrological and thermal constraints from the Omega dacite
Abstract
The 20 ka *0.1 km3 Omega dacite, which
erupted shortly after the 26.5 ka Oruanui super-eruption,
compositionally stands out among Taupo Volcanic Zone
(TVZ) magmas, which are overwhelmingly characterized
by rhyolites ([90 % by volume). The previously reported
presence of inherited zircons in this zircon-undersaturated
magma has provided unequivocal evidence for the
involvement of upper-crustal material in a 1–10 year
timescale prior to the Omega eruption. However, whether
this crustal involvement is characterized by wholesale,
melting of preexisting crust or subordinate bulk assimilation
into an already differentiated magma body remains
unclear. To disentangle these processes, we describe the
mineral chemistry of the major phases present in the Omega
dacite and determine intensive parameters describing
magma chamber conditions. Dominantly unimodal
populations of plagioclase (An50–60), orthopyroxene (Mg#
from 58 to 68), and clinopyroxene (Mg# from 65 to 73),
along with coexisting equilibrium pairs of Fe–Ti oxides,
constrain pre-eruptive temperatures to 850–950 C, a
pressure between *3 and 7 kbars, and an oxygen fugacity
of*NNO. MELTS thermodynamic modeling suggests that
this phase assemblage is in equilibrium with the bulk rock
and glass compositions of the Omega dacite at these estimated
P–T–fO2 pre-eruptive conditions. Combining these
petrological observations with insights into conductive
thermal models of magma–crust interactions, we argue that
the Omega dacite more likely formed in the mid-to-lower
crust via protracted processing through fractional crystallization
coupled with some assimilation (AFC). Incorporation
of crustal material is likely to have occurred at various
stages, with the inherited zircons (and potentially parts of
glomerocrysts) representing late and subordinate uppercrustal
assimilants. This petrogenetic model is consistent
with the presence of a differentiating crustal column, consisting
of a polybaric fractional crystallization and assimilation
history. On the basis of petrological, thermal, and
geophysical considerations, upper-crustal reservoirs, which
feed large-scale rhyolitic volcanism in the TVZ, most likely
take the form of large, long-lived crystal mush zones. Following
large eruptions, such as the Oruanui event, this mush
is expected to crystallize significantly (up to 70–80 vol%
crystals) due to syn-eruptive decompression. Hence, the
Omega dacite, immediately post-dating the Oruanui event,
potentially represents incoming deeper recharge of lessevolved
magma that was able to penetrate the nearly
solidified upper-crustal mush. Over the past 20,000 years,
similar intermediate recharge magmas have incrementally
reheated, reconstructed, and reactivated the upper-crustal
mush zone, allowing a gradual return to rhyolitic volcanism
at the Taupo Volcanic Center.
Quote Item
Contrib Mineral Petrol (2013) 166:1355–1374
Collections