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.

In 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 generation ). 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 system.

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.

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