Melt Fraction and Volatile Content Estimates Using MELTS-constrained
Bayesian Magnetotelluric Inversions: Case Study from Uturuncu, Bolivia
Abstract
Estimating the melt fraction and volatile content of regions of partial
melt beneath volcanoes has important implications for volcanic hazards
since higher melt fraction, volatile-rich magmas are more buoyant and
have a lower viscosity, and thus are more susceptible to mobilization
and possibly eruption. Magnetotelluric (MT) data can be used to model
subsurface bulk resistivity structures through inversion algorithms and
can provide information on the distribution and amount of melt and
volatiles contained in the residing magma by converting bulk resistivity
to estimates of melt fraction, temperature, and water content. These are
often treated as independent variables but, in reality, they are
thermodynamically correlated. Thermodynamic models such as MELTS can be
used to constrain the possible combinations of melt fraction,
temperature, and water content such that MT interpretations are
petrologically consistent. Probabilistic Bayesian inversion that
incorporates these constraints can be used to find a distribution of
models and interpretations which fit the MT data and provide a better
understanding of the uncertainty in MT-derived estimates of melt
fraction. In this study, we apply MELTS-coupled 1-D Bayesian inversions
of MT data at Uturuncu Volcano to evaluate the constraints that MT data
can provide on melt fraction estimates. Uturuncu Volcano is a large
composite volcano in southern Bolivia at the center of the Altiplano
Puna Volcanic Complex (APVC), the result of a large ignimbrite flare-up
during the past 10 Ma. Previous geophysical studies have shown that the
APVC is underlain by the voluminous, laterally-extensive Altiplano Puna
Magma Body (APMB) at approximately 15-20 km depth below surface. The
APMB has previously been interpreted to have a wide range of melt
fractions anywhere from 4% to 45%, but MT results suggest anomalously
high water contents of up to 10 wt%. Initial results from
petrologically-consistent MT inversion modelling suggests that the
resulting low resistivity of the APMB beneath Uturuncu requires high
melt fractions (e.g. >90%) in near-saturated conditions.
This suggests that either high melt fraction near-saturated magma
reservoirs exist at depth or that a significant phase of saline fluids
in over-saturated low melt fraction conditions is present.