GEOCHEMISTRY: SEISMIC WAVES AND WATER IN THE MANTLE

http://scienceweek.com/2007/sw070126.htm

The following points are made by Nathalie Bolfan-Casanova (Science 2007
315:338):

1) Our understanding of plate tectonics relies on the concept of relatively
rigid rocky plates moving on a more ductile shallow mantle called the
asthenosphere (1). The word asthenosphere comes from the Greek "a-sthenos"
meaning "without strength." This lack of strength especially affects
seismic waves, which slow down when entering the asthenosphere. For
decades, Earth scientists have tried to understand the reason for this
seismic wave deceleration. New work (2) reports new experimental findings
on the maximum amount of water that can be stored by the shallow mantle.
These results may solve a number of riddles, including the cause of the
seismic slowdown.

2) Why is water important? Water not only is essential to life but also
controls the dynamics of Earth's interior (3). Since the 1990s, geologists
have recognized with increasing certainty that mantle minerals can hold
substantial amounts of water. This implies that the oceans may no longer be
the main water reservoir of Earth. But water does not necessarily have to
be fluid to be stored in the deep Earth. Rather, it dissolves as hydroxyl
(OH-) in anhydrous minerals (such as olivine, pyroxenes, garnet, and their
high-pressure forms) as a result of the association of a proton (H+) with
oxygen of the mineral lattice. This creates a defect in the lattice and
thereby speeds up the kinetics of physical properties that depend on the
concentration of defects. Even at very low concentrations -- lower than 1%
by weight -- the presence of water has many consequences for mantle
properties such as creep and electrical conductivity (4,5). When present in
minerals as a defect, water will enhance the deformation of rocks and make
them more ductile. Dissolved as H+ in minerals, water will also increase
the electrical conductivity of the mantle by adding mobile charges. Water
also lowers the melting point of mantle rocks and allows melting at greater
depths than in the absence of water.

3) To understand how water affects mantle properties, we need to know how
much water can be stored in mantle minerals and how this storage capacity
varies with increasing depth. Researchers have firmly established that the
solubility of water in minerals increases with pressure and water partial
pressure. The water storage capacity of Earth's upper mantle (extending
from the base of the crust down to the transition zone at 410 km depth) was
thought to increase monotonically with depth. Moreover, in a mantle
consisting of 60% by volume of olivine, this mineral was believed to be the
one that dictates the water budget.

4) The results of Mierdel et al (2) completely change the picture: Water
storage capacity in Earth's shallow mantle is controlled by orthopyroxene,
a less abundant phase than olivine, because water solubility in this phase
is more than two orders of magnitude higher than in olivine. The reason for
this is composition. The enhanced affinity of pyroxenes for water is indeed
aided by aluminum through the coupled substitution of 2Al3+ + 2H+ for 2Mg2+
+ Si4+,which is a very efficient way to store up to 1 weight % water in
MgSiO3 orthopyroxene. Mierdel et al (2) also show that the curve of water
saturation versus depth has a pronounced minimum between 100 and 200 km.
Indeed, the water storage capacity of pyroxene with substituted aluminum is
dependent on the acceptance of the large aluminum cation into the small
tetrahedral site of silicon, the size of which diminishes as a function of
increasing pressure because of atomic compaction. This leads to the drastic
change of water solubility in pyroxene at pressures between 3 and 5 GPa,
corresponding to depths of 100 to 175 km. Depending on the tectonic
environment and the temperature, the minimum in solubility is shallow in
the case of the oceanic mantle but deepens in the case of the colder
continental mantle.

References (abridged):

1. D. L. Anderson, Theory of the Earth (Blackwell Scientific, Boston,
1989).

2. K. Mierdel, H. Keppler, J. R. Smyth, F. Langenhorst, Science 315, 364
(2007).

3. N. Bolfan-Casanova, Mineral. Mag. 69, 229 (2005).

4. D. L. Kohlstedt et al., in Physics and Chemistry of Partially Molten
Rocks, N. Bagdassarov, D. Laporte, A. B. Thompson, Eds. (Kluwer Academic,
Dordrecht, Netherlands, 2000).

5. D. Wang et al., Nature 443, 977 (2006).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com


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