Jed L Mosenfelder1,
Thomas G. Sharp2, Paul D. Asimow1,
George R. Rossman1
1Division of Geological andPlanetary Sciences, California
Institute of Technology
Pasadena, California 91125-2500, U.S.A.
2Department of Geology, Arizona State University, Tempe, Arizona, USA
ABSTRACT
Constraints on water storage capacity and actual
content in the mantle must be derived not only from experimental
studies, but also from investigation of natural samples. Olivine is one
of the best-studied, OH-bearing “nominally anhydrous”
minerals, yet there remain multiple hypotheses for the incorporation
mechanism of hydrogen in this phase. Moreover, there is still debate as
to whether the mechanism is the same in natural samples vs.
experimental studies, where concentrations can reach very high values
(up to ~0.6 wt% H2O) at high pressures and temperatures. We present new
observations and review IR and TEM data from the literature that bear
on this question. Hydrogen incorporation in natural olivine clearly
occurs by multiple mechanisms, but in contrast to some previous
assertions we find that there are strong similarities between the IR
signatures of experimentally annealed olivines and most natural
samples. At low pressures (lower than ~2 GPa) in both experiments and
natural olivines, hydrogen incorporation might be dominated by a
humite-type defect, but the nature of the defect may vary even within a
single sample; possibilities include point defects, planar defects and
optically detectable inclusions. IR bands between 3300 and 3400 cm-1,
ascribed previously to the influence of silica activity, are apparently
related instead to increased oxygen fugacity. At higher pressures in
experiments, the IR band structure changes and hydrogen is probably
associated with disordered point defects. Similar IR spectra are seen
in olivines from xenoliths derived from deeper parts of the mantle
(below South Africa and the Colorado Plateau) as well as in olivines
from the ultra-high pressure metamorphic province of the Western Gneiss
Region in Norway.
Earth's
Deep Water Cycle, AGU Geophysical Monograph