Publikationsdatum:
2014-03-05
Beschreibung:
[1] The concentration and distribution of volatiles in the Earth's mantle influences properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H 2 O, P, and F were measured in olivine, orthopyroxene and clinopyroxene by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H 2 O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and co-existing pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral-mineral partition coefficients. Clinopyroxenes have 60-670 ppm H 2 O, while orthopyroxenes have 10-300 ppm, which gives calculated olivine concentrations of 8-34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 10 19 Pa s at 1250 °C and ~15 km depth, compared to a dry effective viscosity of 2.5 × 10 21 Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20-220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for re-equilibration.
Print ISSN:
0148-0227
Thema:
Geologie und Paläontologie
,
Physik
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