Sampling Volatiles from the Mantle
How do we actually sample the volatiles? In
this respect, we view the volcanoes as transfer conduits for the
volatiles from their magmatic source to the surface, and we take
advantage of two distinct transfer media.
this thin section the brightly colored minerals are the olivines
set in a dark groundmass.
First, we use the magmas themselves in
the form of lavas which help build up the structure of
the volcano (lava
). Lavas contain crystals (or phenocrysts)
of various minerals such as olivine and pyroxene, which can trap
volatiles within their mineral structures. Back in the laboratory,
we can crush or drill into the crystals to release the volatiles
and thereby gain information of the volatile state of the magmatic
Secondly, we can use the abundant geothermal activity normally associated
with active volcanoes as the transfer medium. Because magmas are
hot, they act to drive hydrothermal activity by setting up circulation
cells drawing in colder groundwaters, heating them up and
discharging the hot waters at the surface as fumaroles, mud pots,
hot springs etc. (
hydrothermal activity ).
Volatiles from the magma are released to the hydrothermal system
and brought to the surface dissolved in the aqueous fluids. By collecting
these fluids, we can very effectively sample the magmatic volatiles
as long as we make a small correction for the volatiles originally
dissolved in the groundwaters.