Ocean Island Basalts in the Upper Mantle: Multiply Saturated Volatile-bearing Melts at 5 GPa.
Abstract
By sampling the upper mantle, ocean island basalts (OIB's) inform on the Earth's interior. However, during ascent, they are susceptible to changes in composition and to loss of volatiles, erupted compositions may hence not directly reflect primitive OIB's from the asthenosphere. The presence of volatiles in OIB's, including considerable CO2, is well documented, in particular from melt inclusions[1]. As carbonates are thermally unstable in the asthenospheric mantle, redox-induced carbonate melting at depths of ~ 250 km, where C0 converts to CO2[ 2] , is the most likely melting mechanism. H2O is thought to derive through partitioning with the upper mantle phases containing ~ 0.4 wt%[ 3] .
We aim to determine the asthenospheric parent melts of erupted OIB's through high-pressure, high-temperature experiments. We hence force volatile-added primitive OIB's into multiple saturation with a garnet lherzolite similar to KLB-1[ 4] . The peridotitic starting composition is preequilibrated in a 1 atm furnace at fO2 ~ QFM corresponding to the upper mantle oxygen fugacity, the experiments are run graphite-saturated. The basaltic starting compositions are averages of primitive Hawaii and Cape Verde basalts based on natural samples from the GEOROC database. They represent the two OIB endmember magma series - almost tholeiitic and fairly undersaturated alkali basalt, respectively. Volatile contents used in the basaltic starting material are 1 wt% H2O and 5 wt% CO2[ 1] . Multiple saturation experiments, equilibrating 40-70% peridotite with 60-30% basalt are used to force the liquid into saturation with the four major mantle phases. The experiments are conducted at adiabatic mantle temperatures (1380-1410 ° C, 3-5 GPa). A layered starting material arrangement is used, yielding melt pools that allow for large area microbeam analysis of quenched liquids in equilibrium with a peridotite mineral assemblage. Saturation in olivine, low- and high-Ca pyroxene and garnet was obtained after several iterations. At 5 GPa liquids are kimberlitic with ~ 30 wt% SiO2, as little as ~ 4 wt% Al2O3, 16-22 wt% MgO, 14-18 wt% CO2 and ~ 3 wt% H2O; multiply saturated melts of both basalt types are also fairly undersaturated at 3 GPa. [ 1] Dixon et al., 1997. [ 2] Rohrbach and Schmidt, 2011. [ 3] Hirschmann et al., 2005. [ 4] Takahashi, 1986.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V11E0129P
- Keywords:
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- 1033 Intra-plate processes;
- GEOCHEMISTRY;
- 7208 Mantle;
- SEISMOLOGY;
- 8137 Hotspots;
- large igneous provinces;
- and flood basalt volcanism;
- TECTONOPHYSICS;
- 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY