SPATIAL DISTRIBUTION AND COMPOSITIONAL VARIATIONS OF MAGMATIC INCLUSIONS IN THE MOUNT HELEN DOME, LASSEN VOLCANIC CENTER: IMPLICATIONS FOR MAGMA CHAMBER DYNAMICS

The Mt. Helen (249 ka) dome is one of numerous silicic lavas erupted during late-stage activity at the Lassen volcanic center (LVC), southernmost Cascades. Similar to other LVC silicic lavas, Mt. Helen contains undercooled inclusions of more mafic magma (magmatic inclusions). Using 1 m² grids with 441 evenly spaced (5 cm) points, we determined spatial variations in the fraction and maximum size of inclusions in the dome by point-counting at ~100 outcrops separated by ~100 m. In addition, major and trace element compositions of inclusions and host dacite at 15 locations were obtained.

Important results include the following. (1) Inclusion fractions range from 3 to 19 vol%, with values > 10 vol% generally located along the dome margins. (2) The largest inclusions are ~1 m across, although inclusion fraction and maximum size are not correlated. (3) Inclusions are variably mixed magmas (56-61 wt% SiO₂) that contain up to 50% host dacitic magma. (4) Inclusions are also variably fractionated, as evidenced by large ranges in Ni (79-11 ppm) and Cr (87-7 ppm) contents. (5) Textural features, such as crenulate margins and concentric zonation in crystal and vesicle size, suggest many inclusions chilled following entrapment in cooler, more silicic host magma. (6) Host rock compositions are markedly homogeneous across the dome (65.4 ± 0.4 wt% SiO₂), indicating that the extent of inclusion disaggregation was uniform (~15%), despite large spatial variations in inclusion fraction. (7) Correlations between Ni and Cr contents in inclusions and adjacent hosts indicate that the effect of inclusion disaggregation on host dacitic magma was local (i.e., < 50 m).

Collectively, the field and geochemical data from Mt. Helen challenge the notion that magmatic inclusion-bearing silicic lavas erupt from stratified magma chambers characterized by lower, compositionally uniform mafic hybrid layers. Instead, in order to preserve the large compositional variations in inclusions observed within this single eruptive unit, the data require a more complex scenario. We envision this process to include numerous, possibly discrete cycles of mafic replenishment, mafic-silicic magma mixing, and incorporation into and chilling of hybrid mafic magma in overlying silicic magma, culminating with partial disaggregation of diverse composition inclusions in conduits during eruptive ascent.