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| Titel |
Hydrogen diffusion in forsterite and implications for determining the water content of the mantle |
| VerfasserIn |
Michael Jollands, Joerg Hermann, Hugh O'Neill, José Alberto Padrón-Navarta |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095099
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| Publikation (Nr.) |
 EGU/EGU2014-10541.pdf |
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| Zusammenfassung |
| The retention of mantle hydrogen concentrations in “nominally anhydrous” minerals in
mantle peridotites depends on the rates of exhumation versus diffusion. The infrared spectra
of the hydroxyl stretching band in olivine of spinel lherzolite xenoliths are dominated by the
substitution mechanism associated with trace Ti4+ (Berry et. al., 2005), suggesting the
possibility that hydrogen diffusion rates may be dependent on Ti concentration and valence in
these olivines.
A single slab of synthetic forsterite was doped with ca. 400ppm Ti by annealing at 1500Ë
C, 1atm, QFM-5.6 in the presence of an MgTiO3-Mg2Ti2O5mix. This forsterite then served
as the target for hydrogen diffusion-in experiments, with the point defect population
homogenous, elevated, and defined by Ti concentration and the Ti3+/Ti4+ ratio in the
high-temperature pre-annealed experiments.
The doped forsterite was then cut into cubes, which were placed inside a mix of
forsterite+enstatite or forsterite+periclase in a thick walled silver capsule along with a
saturating quantity of water and an oxygen fugacity buffer assemblage (Re-ReO2), and taken
to run conditions (15kbar, 650-950°C) in an end-loaded piston cylinder.
The crystals were recovered from the capsules and analysed by FTIR. Progressive
‘decoration’ of the homogenous titanium defect population, from crystal rim to core, yielded
diffusion profiles of hydrogen associated to different defects. The spectra show that Ti occurs
as both as Ti3+ and Ti4+. The ratio of OH associated with each of these Ti species remains
constant along the diffusion pathways. There is no evidence for re-equilibration of the
Ti3+/Ti4+ ratio with the external Re-ReO2 oxygen buffer, which would be expected to
eliminate Ti3+.
In all cases, hydrogen diffusion was anisotropic, fastest along the c-axis and slowest along
a-axis. Hydrogen diffusion is slightly faster than in previous studies considering hydrogen
diffusion-in (Demouchy & Mackwell, 2006), but significantly faster than hydrogen
diffusion-out associated with titanium in forsterite (Padron-Navarta et al., in press). Thus, the
relative hydration states of olivines versus their surrounding assemblage must be considered
when determining whether olivine water contents represent true mantle values after
exhumation; diffusion-in of hydrogen will alter olivine significantly faster than
diffusion-out.
Our results show that at typical conditions for spinel lherzolite xenoliths in the
lithospheric mantle of 950°C, 15kbar, an anhydrous Ti-doped forsterite cube 1x1x1mm
would be fully hydrated by an external hydrous fluid in a matter of hours.
The effect of Ti on H diffusion (either in or out) during exhumation must be considered
when using mantle olivines to yield information regarding the water content of the
mantle.
References: Berry, A.J., Hermann, J., O’Neill, H.St.C., Foran, G.J., 2005. Fingerprinting
the water site in mantle olivine. Geology 33, 869-872; Demouchy, S., Mackwell, S., 2006.
Mechanisms of hydrogen incorporation and diffusion in iron-bearing olivine. Physics and
Chemistry of Minerals 33, 347-355; Padrón-Navarta J.A., Hermann, J., O’Neill, H. St. C.
Site-specific hydrogen diffusion rates in forsterite. Earth and Planetary Science Letters (in
press). |
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