Paleomagnetic and Rock Magnetic Study to Determine the Emplacement
Temperatures of the ~ 3580 BC Chachimbiro Pyroclastic
Deposits, Ecuador
Abstract
Two rock magnetic methods can be used to determine the emplacement
temperature of pyroclastic deposits. The first is by looking at the
unblocking temperature spectra of the thermoremanent magnetization (TRM)
and the second is through the repeatability of thermomagnetic behavior.
Chachimbiro volcanic complex is an andesitic-dacitic stratovolcano
located at the northern zone of the Ecuadorian volcanic arc. The lateral
blast eruption that occurred at 3640-3510 BC originated from a ∼650 m
wide and ∼225 m high rhyodacite dome. This satellite lava dome, located
∼6 km to the east of the main vent, erupted, resulting in a large
pyroclastic density current (PDC). PDCs are hot mixtures of lithic
fragments, gas and pumice, varying in size from fine ash up to metric
blocks that descend the flanks of a volcano at great speeds, being the
primary cause of death during explosive eruptions. The resulting PDC
from this violent laterally directed explosion covered an area of 62
km^2, with the thickest parts of the deposit displaying as much as 15
m. We collected ~63 oriented block samples from 6
locations; their distances varying between 1.8 km to 6.7 km away from
the source. Here we present the emplacement temperatures of the
Chachimbiro pyroclastic deposits and the potential factors controlling
them. Our rock-magnetic results indicate low titanium Ti-magnetite as
the main magnetization carrier; maghemite being present in trace
amounts. We have recognized that, based on the unblocking analysis of
the TRM, the overall temperatures vary from 250 °C to 450 °C depending
on the clast size and type. In general, our results suggest a minimum
temperature of ~250 °C, with a large portion of the
juvenile clasts having temperatures up to about ~450 °C.
Furthermore, the analysis and the comparison of the Curie temperature
executed in ~30 samples, against the emplacement
temperatures obtained through the typical paleomagnetic studies will be
presented. This work highlights the usefulness of paleomagnetism and
rock magnetism to evaluate the emplacement temperatures of PDCs, thereby
allowing to better assess the associated risk.