Mt. Shasta andesite and dacite lavas contain high MgO (3.5-5 wt.%),
very low FeO*/MgO (1-1.5) and 60-66 wt.% SiO2. The range of major
and trace element compositions of the Shasta lavas can be explained
through fractional crystallization (similar to50-60 wt.%) with
subsequent magma mixing of a parent magma that had the major element
composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic
and trace element characteristics of the Mt. Shasta stratocone lavas
are highly variable and span the same range of compositions that is
found in the parental basaltic andesite and PMA lavas. This variability
is inherited from compositional variations in the input contributed
from melting of mantle wedge peridotite that was fluxed by a
slab-derived, fluid-rich component. Evidence preserved in phenocryst
assemblages indicates mixing of magmas that experienced variable
amounts of fractional crystallization over a range of crustal depths
from similar to25 to similar to4 km beneath Mt. Shasta. Major and trace
element evidence is also consistent with magma mixing. Pre-eruptive
crystallization extended from shallow crustal levels under degassed
conditions (similar to4 wt.% H2O) to lower crustal depths with
magmatic H2O contents of similar to10-15 wt.%. Oxygen fugacity varied
over 2 log units from one above to one below the Nickel-Nickel Oxide
buffer. The input of buoyant H2O-rich magmas containing 10-15 wt.% H2O
may have triggered magma mixing and facilitated eruption.
Alternatively, vesiculation of oversaturated H2O-rich melts could also
play an important role in mixing and eruption.