HR: 15:15h
AN: V53F-07    [Abstracts]
TI: How can the water concentrations in nominally anhydrous minerals (NAMs) from mantle xenoliths be used to learn about the distribution of water in the mantle?
AU: Grant, K J
EM: K.Grant@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, UK BS8 1RJ United Kingdom
AU: * Kohn, S C
EM: Simon.kohn@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, UK BS8 1RJ United Kingdom
AU: Brooker, R A
EM: r.a.brooker@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, UK BS8 1RJ United Kingdom
AB: Water has a very large effect on the physical and chemical properties of the Earth's mantle. In recent years, progress has been made in understanding how water is stored in mantle phases and the maximum amount which can be dissolved in each phase has been constrained to some extent. However, the methods which are used to estimate the actual amount of water in different regions of the mantle remain rather crude. Although mantle xenoliths would ideally be convenient samples which record the water concentrations in their source regions, the very fast diffusion of H in silicates means that water could potentially be either lost or gained by interaction with the host magma during ascent. Methods are therefore required to assess the degree of xenolith-melt interaction which has occurred in each case. In this study we have performed high pressure experiments to obtain mineral-mineral and mineral-melt partitioning coefficients to use as the basis of such methods. Careful analysis of the OH stretching region in the FTIR spectra of the experimentally produced samples, and natural xenolith material, provides an additional, powerful method for comparing the pressure, temperature and chemical environment preserved by the xenoliths. Several series of experiments were performed. Firstly equilibrium partitioning experiments between forsterite and enstatite in the system MgO-SiO₂-H₂O and partitioning between forsterite, enstatite and melt in the system Na₂O-MgO-SiO₂-Al₂O₃-H₂O were used to study the effect of pressure, temperature and composition on the equilibrium water concentrations in the phases and the equilibrium hydrated defect structure, as observed by FTIR. D^H_opx/ol varies from about 3 to 30 depending on Al concentration and pressure. Finally, FTIR measurements of olivine and orthopyroxene in several xenoliths were made, and compared with experimentally hydrated crystals from the same xenoliths. Large (4 mm) crystals were annealed for different lengths of time to assess the kinetics of re-equilibration and, in some cases, the starting materials were finely ground before hydrothermal treatment to ensure that all recovered crystals were grown at known P and T and did not contain a defect structure inherited from the xenolith. The spectrum for recrystallized Ichinomegata olivine (in equilibrium with orthopyroxene) is significantly different from that for hydrated crystals from the same xenolith; peaks in the region 3500-3650 cm^-1 are of minimal intensity compared with those between 3300-3400 cm^-1. This result is consistent with results from the simple, synthetic systems, and implies that the spectra observed in most xenolithic olivine is not that for equilibrated, orthopyroxene-buffered conditions.
DE: 1025 Composition of the mantle
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3630 Experimental mineralogy and petrology
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 7208 Mantle (1212, 1213, 8124)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005