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a Mixing events b
Normalized density
0 0
Edifice Figure 3. Run-up to the
3 3
1999 eruption of Shishaldin
volcano, simplified after
Depth below summit (km) 12 Eruption 12 Entrapment depth Last storage depth crystal clocks (green stars)
6
6
Rasmussen et al. (2018c).
(a) The timing of mixing
events inferred from
9
9
coincides with long-period
(LP) and volcano-tectonic
(VT) seismicity leading up
15
determined from melt
Seismicity 15 to the eruption. (b) Depths
18 18 inclusions indicating that
magmas coalesced at <3
LP km depth (in the edifice).
VT Bin = 1 km
21 21
Melt inclusion depths
05/98 07/98 09/98 11/98 01/99 03/99 05/99
We conducted a focused study of the 1999 eruption of Shishaldin the maximum observed water contents, which minimizes the influence
volcano (Rasmussen et al., 2018c), which is compelling for multiple of diffusive leakage of water. Water contents are variable (~2-5 wt.%)
reasons. This sub-Plinian eruption had an unusually long phase of and correlate positively with geophysically determined magma storage
seismic activity preceding the eruption, and, despite the 43 million m depths, falling along the water-saturation curve (Fig. 4). The maximum
3
of tephra ejected, InSAR data recorded no discernable eruption-related water contents of melt inclusions often correlate with non-volatile
deformation. We established a close temporal link between increased trace elements, indicating diffusive leakage is not a major factor. Thus,
seismicity, stress field changes inferred from shear-wave splitting our data support a model in which intrinsically drier magmas (like
analysis, and magma mixing recorded by crystal clocks, confirming those that feed Shishaldin) degas and crystallize shallower than wet
that precursory seismicity tracked the priming of the magmatic magmas, resulting in shallower storage prior to eruption. So, what
system for eruption by delivery of new magma (Fig. 3). Our study controls primary water content?
was the first to connect timescale information recorded in chemical
zonation patterns in crystals with depth information recorded in melt Now in the final leg of our pursuit to understand the slab-volcano
inclusions, which we used along with geophysical data to interrogate connection, we are focusing on the extent to which slab depth relates
a previously enigmatic magmatic plumbing system. We found that a to the composition of arc magmas (Rasmussen et al., 2018a). We have
shallow magmatic system located largely within the edifice (<3 km collected major, trace, and volatile element data in melt inclusions and
below the summit) persists between eruptions. Prior to the 1999 bulk rock samples from the eight target volcanic centers in our corridor
eruption, the shallow magmatic system was recharged with deep (Fig. 1). These data exhibit systematic trends with slab depth (Fig. 5).
(>20 km) magma. InSAR observations at Shishaldin are insensitive to
deformation emanating from the shallow and deep parts of the system, Figure 4. Relationship between the maximum water
in part explaining the lack of an observed deformation signal. These content of melt inclusions and geophysically determined
results improve our understanding of eruption triggers and magmatic magma storage depths, which are referenced to the
plumbing systems. But this work raises the question, is the magmatic volcano summits (Rasmussen et al., 2018b).
system at Shishaldin unusually shallow?
0
Broadening our scope, we evaluated magma storage depths
throughout our corridor (Rasmussen et al., 2018b). Geophysical 2 Fisher
constraints indicate these depths vary significantly (~2-8 km below Okmok
the edifice; Fig. 4). The cause of such variability is poorly understood. Shishaldin
Some have argued for the importance of intrinsic (e.g., buoyancy, 4 Seguam
viscosity) controls (Annen et al., 2006), while others have emphasized Cleveland
the importance of extrinsic (e.g., crustal structure) controls (Chaussard Depth (km) 6 Westdahl
and Amelung, 2014). We investigated the influence of magmatic water Fisher
(deep)
content, a key intrinsic variable, on magma storage depth. Water is 8 Makushin Akutan
thought to be important because decompression-induced degassing
during magma ascent results in an increase in melt viscosity and 10 H2O saturation
magma crystallinity, both promoting stalling. We estimated magmatic
water content by measuring large suites of melt inclusions and taking 12
1 2 3 4 5 6
H O (wt%)
12 • GeoPRISMS Newsletter Issue No. 42 Spring 2019 2