Evolving magma storage conditions beneath Mount St. Helens inferred from chemical variations in melt inclusions from the 1980-1986 and current (2004-2006) eruptions

Major element, trace element, and volatile concentrations in 187 glassy melt inclusions and 25 groundmass glasses from the 1980-86 eruption of Mount St. Helens are presented, together with 103 analyses of touching FE-Ti oxide pairs from the same samples. These data are used to evaluate the temporal evolution of the magmatic plumbing system beneath the volcano during 1980-86 and so provide a framework in which to interpret analyses of melt inclusions from the current (2004-2006) eruption.

Major and trace element concentrations of all melt inclusions lie at the high SiO₂ end of the data array defined by eruptive products of the late Quaternary age from Mount St. Helens. For several major and trace elements, the glasses define a trend that is oblique to the whole-rock trend, indicating that different mineral assemblages were responsible for the two trends. The whole-rock trend can be ascribed to differentiation of hydrous basaltic parents in a deep-seated magma reservoir, probably at depths great enough to stabilize garnet. In contrast, the glass trends were generated by closed-system crystallization of the phenocryst and microlite mineral assemblages at low pressures.

The dissolved H₂O content of the melt inclusions from 1980-86, as measured by the ion microprobe, ranges from 0 to 6.7 wt. percent, with the highest values obtained from the plinian phase of May 18, 1980. Water contents decrease with increasing SiO₂, consistent with decompression-driven crystallization. Preliminary data for dissolved CO₂ in melt inclusions from the May 18 plinian phase from August 7, 1980, indicate that X_H2O in a vapor phase was approximately constant at 0.80, irrespective of H₂O content, suggestive of closed-system degassing with a high bubble fraction or gas streaming through the subvolcanic system. Temperature and f
_O2
estimates
for touching Fe-Ti oxides show evidence for heating during
crystallization owing to release of latent heat. Consequently,
magmas with the highest microlite crystallinities record the
highest temperatures. Magmas also become progressively
reduced during ascent and degassing, probably as a result of
redox equilibria between exsolving S-bearing gases and magmas. The lowest temperature oxides have f
_O2
≈ NNO, similar
to high-temperature fumarole gases from the volcano. The
temperature and f
_O2
of the magma tapped by the plinian phase
of May 18, 1980, are 870-875°C and NNO+0.8, respectively.
The dissolved volatile contents of the melt inclusions
have been used to calculate sealing pressures; that is, the
pressure at which chemical exchange between inclusion and
matrix melt ceased. These are greatest for the May 18 plinian
magma (120 to 320 MPa); lower pressures are recorded by
samples of the preplinian cryptodome and by all post-May 18
magmas. Magma crystallinity, calculated from melt-inclusion
Rb contents, is negatively correlated with sealing pressure,
consistent with decompression crystallization. Elevated
contents of Li in melt inclusions from the cryptodome and
post-May 18 samples are consistent with transfer of Li in a
magmatic vapor phase from deeper parts of the magma system to magma stored at shallower levels. The Li enrichment
attains its maximum extent at ~150 MPa, which is ascribed to
separation of a single vapor phase into H₂
O-rich gas and dense
Li-rich brine at the top of the magma column.
There are striking correlations between melt-inclusion
chemistry and monitoring data for the 1980-86 eruption. Dissolved SO₂
contents of melt inclusions from any given event,
multiplied by the mass of magma erupted during that event, correlate with the measured flux of SO₂
at the surface, suggesting that magma degassing and melt-inclusion sealing are
closely related in time and space.
Textural and chemical evidence indicates that melt inclusions became effectively sealed (physically or kinetically)
shortly before eruption. Thus by converting pressure to depth
using a density model and edifice-loading algorithm for the
volcano, changing depths of magma extraction with time can
be tracked and compared to the seismic record. The plinian
eruption of May 18, 1980, involved magma stored 5-11 km
below sea level; this is inferred to be the subvolcanic magma
chamber. The preceding eruptions, including the May 18,
1980, blast, involved magma withdrawal from the cryptodome
and conduit down to 5 km below sea level. Subsequent 1980
eruptions tapped magma down to depths of ≤10 km below
sea level. Tapping of magma stored deeper than 2 km below
sea level stopped abruptly at the end of 1980, coincident
with the onset of extensive shallow seismicity and a change
from explosive to effusive eruption style from 1981 to 1986.
Overall, the 1980-86 eruption is consistent with the evisceration of a thin, vertically extensive body of magma extending
from 5 to at least 11 km below sea level and connected to the
surface by a thin conduit. In the absence of sustained high
magma-supply rates from depth, decompression crystallization of magma ascending through the system leads eventually
to plugging of the conduit.
The current eruption of Mount St. Helens shares some
similarities with the 1981-86 dome-building phase of the
previous eruption, in that there is extensive shallow seismicity
and extrusion of highly crystalline material in the form of a
sequence of flows and spines. Melt inclusions from the current eruption have low H₂
O contents, consistent with magma
extraction from shallow depths. Highly enriched Li in melt
inclusions suggests that vapor transport of Li is a characteristic
feature of Mount St. Helens. Melt inclusions from the current
eruption have subtly different trace-element chemistry from
all but one of the 1980-86 melt inclusions, with steeper rareearth-element (REE) patterns and low U, Th, and high-fieldstrength elements (HFSE), indicating addition of a new melt
component to the magma system. It is anticipated that increasing involvement of the new melt component will be evident as
the current eruption proceeds.