BASALTIC LAVAS OF MOUNT ST. HELENS: EVIDENCE FOR DIVERSE MANTLE SOURCES AND INPUTS FROM MORE EVOLVED MAGMAS

Mafic magmas were important in the petrologic evolution of MSH, but eruption of basalt was restricted to the Castle Creek eruptive period ca. 2200-1900 yr B.P., during which time dacitic and andesitic magmas were also erupted (Clynne et al., USGS Fact Sheet 2005-3045). The basalts define two distinct groups: low-K tholeiites [LKT] and high-K basalts similar to intraplate basalts [IPB]. The most primitive lavas in each group (MgO up to ~7.5 wt %) have ~0.5 wt % K₂O [LKT] and ~1.5 wt % K₂O [IPB]. Geochemical characteristics preclude their derivation from a common mantle source. With decreasing MgO, both groups form continuous and curvilinear trends that converge toward the composition of MSH andesites (~1 wt % K₂O and 5 wt % MgO). Similar arrays are observed for a variety of trace elements (e.g., La, Ta, Ba, Sr) and suggest inputs from more silicic magmas.

New SEM/EMP studies of MSH basalts document that olivine core compositions span large ranges (up to 23 mole % Fo, with an average of ~13 mole %) within individual samples. Cores with higher Fo contents than predicted from whole-rock Mg# suggest that MSH basalts were likely derived from more primitive LKT and IPBs variants similar to those from nearby Indian Heaven and Battleground lava fields. Some samples contain olivine grains with cores of Fo_60-72 (lower than predicted from whole-rock Mg#s). Those grains exhibit embayments, reverse zoning, and/or overgrowth rims of orthopyroxene with Mg#s ~70-65. The diverse olivine populations in some MSH basalts indicate that they received variable and relatively small inputs from more silicic magmas. Another explanation is petrological cannibalism and incorporation of antecrysts derived from previously emplaced magma batches. Despite close proximity of their vents (within a few km of each other) and eruption within a short time interval (< 300 years), we see no evidence of significant mingling or mixing between the LKTs and IPBs. We conclude that they ascended from different magma source regions along narrowly separated, discrete plumbing systems. This conclusion brings into question the existence of a broad region of hybridization in the crust (MASH zone), as well as lateral migration of magma from east of MSH as suggested by geophysical studies (Hill et al., 2009, doi: 10.1038/NGEO661; Kiser et al., 2016, doi: 10.1130/G37591.1).