The unexpected intersection of rhyolitic
magma and retrieval of quenched glass particles
at the Iceland Deep Drilling Project-1
geothermal well in 2009 at Krafla, Iceland,
provide unprecedented opportunities to
characterize the genesis, storage, and behavior
of subsurface silicic magma. In this study,
we analyzed the complete time series of glass
particles retrieved after magma was intersected,
in terms of distribution, chemistry,
and vesicle textures.
Detailed analysis of the particles revealed
them to represent bimodal rhyolitic magma
compositions and textures. Early-retrieved
clear vesicular glass has higher SiO2, crystal,
and vesicle contents than later-retrieved
dense brown glass. The vesicle size and distribution
of the brown glass also reveal several
vesicle populations. The glass particles vary
in δD from −120‰ to −80‰ and have dissolved
water contents spanning 1.3−2 wt%,
although the majority of glass particles exhibit
a narrower range. Vesicular textures
indicate that volatile overpressure release
predominantly occurred prior to late-stage
magma ascent, and we infer that vesiculation
occurred in response to drilling-induced decompression.
The textures and chemistry of
the rhyolitic glasses are consistent with variable
partial melting of host felsite. The drilling
recovery sequence indicates that the clear
magma (lower degree partial melt) overlays
the brown magma (higher degree partial
melt). The isotopes and water species support
high temperature hydration of these partial
melts by a mixed meteoric and magmatic
composition fluid. The textural evidence for
partial melting and lack of crystallization imply
that magma production is ongoing, and
the growing magma body thus has a high potential
for geothermal energy extraction.
In summary, transfer of heat and fluids
into felsite triggered variable degrees of felsite
partial melting and produced a hydrated rhyolite
magma with chemical and textural heterogeneities
that were then enhanced by drilling
perturbations. Such partial melting could
occur extensively in the crust above magma
chambers, where complex intrusive systems
can form and supply the heat and fluids required
to re-melt the host rock. Our findings
emphasize the need for higher resolution
geophysical monitoring of restless calderas
both for hazard assessment and geothermal
prospecting. We also provide insight into how
shallow silicic magma reacts to drilling, which
could be key to future exploration of the use
of magma bodies in geothermal energy.