HR: 15:00h
AN: V53F-06    [Abstracts]
TI: Grain boundaries, a possible water reservoir in the Earth's mantle?
AU: * Sommer, H
EM: somhol@rice.edu
AF: Rice University, 6100 Main St., Houston, TX 77005 United States
AU: Lee, C
EM: ctlee@rice.edu
AF: Rice University, 6100 Main St., Houston, TX 77005 United States
AU: Gasharova, B
EM:
AF: ANKA/Inst. Synchotron Radiation, Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldsh, D-76344 Germany
AU: Regenauer-Lieb, K
EM: regenaue@uni-mainz.de
AF: Institute for Geoscience, U Mainz, Johann-Joachim Becherweg 21, Mainz, D-55099 Germany
AU: Peslier, A
EM: Anne.Peslier@mail.uh.edu
AF: Texas Center for Superconductivity, U Houston, Houston, TX 77204 United States
AB: We present novel high-resolution synchrotron FTIR evidence for strongly heterogeneous distribution of water within natural olivine samples from about 125 km depth in the Earth's mantle. It has long been known that there can be substantial amounts of water in nominally water free minerals, such as olivine. However, it has been suggested that grain boundaries may also act as significant reservoirs of incompatible trace elements, and if so, there may be more water in the Earth's mantle than previously thought. The difficulties in quantifying the pristine water content in grain boundaries in natural mantle samples, such as xenoliths, are twofold. First, water may be lost (or added) during entrainment in the erupting host lava, and second, the spatial resolution for measuring water on grain boundaries is insufficient with SIMS or conventional FTIR. We have overcome both problems by examining grain boundaries of fully embedded mineral inclusions within olivine and by using synchrotron FTIR at ANKA (Karlsruhe; Germany). We focus on cracks and mineral inclusions as spinel, fully embedded in thick, doubly-polished sections of single oriented olivine grains. The inclusions were carefully selected to avoid any interferences from outcropping cracks or intersections with the polished surfaces of the olivine grain. The host olivine and the fully embedded cracks and mineral inclusions were mapped in transmitted-light mode. Single-element MCT detector was used to analyze a large number of overlapping spots using an aperture typically of 6 micron diameter and a step size of 2 microns in a grid pattern accessed by an automated X-Y stage. Using this setup with synchrotron IR radiation each spot analysis has the quality of a conventional benchtop instrument with an equivalent spot size larger than 50 æm. The resulting map has a diffracted limited X-Y resolution, which is about 3 æm in the spectral range of O-H stretching vibrations. Quantitative analysis of polarized IR spectra from the three principal crystallographic directions in olivine revealed about 130 ppm H2O in the olivine matrix. The mappings of the embedded inclusions show that the amount of water in olivine increases continuously up to about 800 ppm H/Si as the grain boundaries are approached. Although the horizontal spatial resolution of the measurement is high, we cannot avoid some smearing through the focal depth of the sample. In case of spinel inclusion, this leads to a peak of water when measuring the center of the inclusion, which is overlain and underlain by olivine. Measurements of other sections and at different focal depths suggest that the water is located around the grain boundary itself. In principle, coupled substitution with Fe3+ in olivine due to equilibration with spinel, could lead to increased water content around spinel inclusions. However, our Fo90 olivine samples cannot contain more than 0.005 moles of Fe3+, which would correspond to 250 ppm H/Si, whereas our measurements show grain boundary water contents up to 800 ppm H/Si. We conclude that water enrichment may be a genuine effect of preferential partitioning into grain boundaries.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3640 Igneous petrology
DE: 3904 Defects
DE: 8162 Rheology: mantle (8033)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005