Geoscience Reference
In-Depth Information
1
Volatiles under High Pressure
HANS KEPPLER
Bayerisches Geoinstitut, Universit at Bayreuth, Bayreuth, Germany
Summary
melt may form near the 440 and 660 km seismic
discontinuities. Water and carbon have been ex-
changed during the Earth's history between the
surface and the mantle with typical mantle res-
idence times in the order of billions of years.
However, the initial distribution of volatiles be-
tween these reservoirs at the beginning of the
Earth's history is not well known. Nitrogen, noble
gases, sulfur and halogens are also continuously
exchanged between mantle, oceans and atmo-
sphere, but the details of these element fluxes are
not well constrained.
Hydrogen and carbon are the two most important
volatile elements in the Earth's interior, yet their
behavior is very different. Hydrogen is soluble in
mantle minerals as OH point defects and these
minerals constitute a water reservoir comparable
in size to the oceans. The distribution of water in
the Earth's interior is primarily controlled by the
partitioning between minerals, melts and fluids.
Most of the water is probably concentrated in the
minerals wadsleyite and ringwoodite in the tran-
sition zone of the mantle. Carbon, on the other
hand, is nearly completely insoluble in the sili-
cates of the mantle and therefore forms a separate
phase. Stable carbon-bearing phases are likely car-
bonates in the upper mantle and diamond or
carbides in the deeper mantle. Already minute
amounts of water and carbon in its oxidized form
(as carbonate or CO
2
) greatly reduce the solidus of
mantle peridotite. Melting in subduction zones is
triggered by water and both water and CO
2
con-
tribute to the melting below mid-ocean ridges and
in the seismic low-velocity zone. Redox melt-
ing may occur when oxygen fugacity increases
upon upwelling of reduced deep mantle, convert-
ing reduced carbon species to carbonate or CO
2
that strongly depress solidus temperatures. The
large contrast of water storage capacity between
transition zone minerals and the mineral assem-
blages of the upper and lower mantle implies that
1.1
Introduction: What Are Volatiles and Why
Are They Important?
Volatiles are chemical elements and com-
pounds that tend to enter the gas phase in
high-temperature magmatic and metamorphic
processes. Accordingly, one can get some idea
about the types of volatiles occurring in the
Earth's interior by looking at compositions of
volcanic gases. Table 1.1 compiles some typical
volcanic gas analyses. As is obvious from this
table, water and carbon dioxide are the two
most abundant volatiles and they are also most
important for the dynamics of the Earth's interior
(e.g. Bercovici & Karato, 2003; Mierdel
et al
.,
2007; Dasgupta & Hirschmann, 2010). Other,
less abundant volatiles are sulfur and halogen
