Variation of the paleomagnetic and rock magnetic properties across a
~20 m thick andesitic lava flow (Tungurahua Volcano,
Ecuador): implications for paleointensity
Abstract
Data of the Earth’s magnetic field strength is diversely applicable from
dating archeological artefacts or lava flows to understanding early
earth evolution and the mechanisms of the geodynamo. Lava flows are
commonly used as a means to obtain records of this paleointensity.
Understanding the underlying paleomagnetic and rock magnetic properties
and how they vary across a flow is crucial to ensure collection of good
quality samples for analysis. In fact, the success of paleointensity as
well as paleomagnetic analysis is strongly dependent on the
rock-magnetic properties of the samples, and large variations may exist
between samples even of the same unit, related mainly to varying cooling
rates. The active Tungurahua volcano is one of the most prominent
features in the Ecuadorian Eastern Cordillera with numerous basaltic
andesite and andesitic lava flows exposed along its flanks. To appraise
the relation between volcanic emplacement processes and rock-magnetic
properties, we sampled a vertical transect in a ~20 m
thick lava flow at Tungurahua volcano. A total of 55 oriented in situ
sample from six sites distributed across Ulba Cascada lava flow were
collected for this purpose. We will present petrographical analysis of
each sample as a function of the depth within the lava flow, with an
emphasis on the textural and structural characteristics of magnetic
minerals as observed with transmitted, reflected light and scanning
electron microscopes. Moreover, a detailed analysis of rock-magnetic
properties such as magnetic susceptibility, hysteresis (remanent
magnetization, saturation remanent magnetization, coercivity and
back-field coercivity), accompanied with detailed stepwise alternating
field and thermal demagnetizations will allow us to determine the
direction of the magnetic field and assess the variations of magnetic
properties with respect to the position within a lava flow. We will also
discuss the correlations that may exist between grain size, oxidation
state of the magnetic minerals and the emplacement processes of a lava
flow, as well as the implications of all these results to paleointensity
determinations.