Modeling the dehydrogenation of mantle olivine with implications for the water content of the Earth's upper mantle, and ascent rates of kimberlite and alkali basaltic magmas (Invited)
Abstract
Water has a strong influence on the physical and chemical properties of Earth materials and processes. Even small amounts (< 0.005 wt%) of H in nominally anhydrous minerals (NAMs) such as olivine greatly affect the melting temperature, electrical conductivity and ionic diffusivity, and rheological behaviour of the mantle. Direct information about the H content in NAMs comes from analyses of crystals in mantle xenoliths or in megacrysts transported to the Earth’s surface by ascending magmas in different tectonic settings. The maximum H content of olivine crystals from cratonic kimberlites is typically one or two orders of magnitude higher than those from alkali basalts of continental interiors or subduction-zones. This may reflect the water contents of the upper mantle from different tectonic settings but several natural and experimental observations preclude such a straightforward interpretation. Several studies report higher H concentrations in olivine centres than in their rims, and a positive correlation between grain size and maximum H contents. This can be interpreted as that H has diffused out of the crystal during the transport from the mantle to the surface. If this is the case, then the maximum H concentration and the length of the H zoning in olivine may provide crucial information for understanding magma styles and rates, as well as the source water content of the mantle and it host magma. We have constructed a diffusion model of H in olivine that accounts for many aspects of the process of H loss from the crystal, styles of magma transport, and degassing. This includes variable boundary conditions as the magma ascents to the surface, the depth at which magma is saturated in volatiles, and whether magma degassing occurs in equilibrium or disequilibrium conditions. The results show that aside from grains size, two other parameters control the preservation of H in the olivine are whether the crystals are xenocrysts or not, and the water content of the host magma. For a mean temperature of 1200 oC and a grain size of 0.3 to 1 cm we find that: (i) ascent rates of > 10 m/s are required to keep > 50 % of the initial content of hydrogen in the olivine xenocryst; (ii) the high water content and absence of H zoning of some olivine in kimberlites require ascent rates > 100 m/s. (iii) ascent rates of alkali basaltic magmas appear to be an order of magnitude lower than those from kimberlites. The ascent rates obtained from these calculations can be far or close to the mean ascent rate of a more realistic ascent path of an accelerating magma, so need to be treated with caution when compared to other estimates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V24C..06C
- Keywords:
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- 1025 GEOCHEMISTRY / Composition of the mantle;
- 1037 GEOCHEMISTRY / Magma genesis and partial melting;
- 8428 VOLCANOLOGY / Explosive volcanism;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies