9:15 AM - 9:30 AM
[SGC33-02] Hydrogen isotopic fractionation between the SiO2 phase and silicate melt in the lower mantle.
Hydrogen isotopic fractionation between minerals and water-rich fluids is known to be large even at high temperatures. Recently it has been reported that the SiO2 phase is the most important water carrier in subducting former oceanic plates in the deep lower mantle. Deuterium-poor melt inclusions were found in the picritic basalts at Baffin Island, which derived from a plume originating from the lower mantle, suggesting a possibility that primordial materials containing deuterium-poor water exist in the lower mantle (Hallis et al., 2015). In order to understand the hydrogen isotopic fractionation process in the lower mantle, we conducted melting experiments under high pressures and determined the hydrogen isotope fractionation factor between the SiO2 phase and silicate melt.
Laser-heated diamond-anvil cell experiments were conducted in the lower mantle pressure range using a hydrated MORB glass containing H2O and D2O as a starting material. After a melting experiment at 40 GPa, the sample was recovered, and its cross section was prepared at the center of a laser-heated hot spot using a focused ion beam. The melting texture showed a quenched melt pool at the center being surrounded by the SiO2 phase. It contrasts previous melting experiments on anhydrous MORB material, in which CaSiO3 perovskite (davemaoite) is the liquidus phase over the entire pressure range of the lower mantle (Tateno et al., 2018), indicating that the presence of water expands the liquidus field of SiO2.
In addition, we conducted a series of melting experiments on the same sample at 30 to 70 GPa and about 4000 K, and the recovered sample was examined by the secondary ion mass spectrometry (SIMS) to obtain the deuterium/hydrogen (D/H) ratios for a melt pool and a surrounding SiO2 layer. A comparison of the D/H ratios revealed that the latter contained relatively large amounts of deuterium. Based on these experimental results, we will show the hydrogen isotope fractionation factor between the SiO2 phase and silicate melt and discuss the hydrogen isotopic fractionation upon dehydration and melting in the lower mantle.
Laser-heated diamond-anvil cell experiments were conducted in the lower mantle pressure range using a hydrated MORB glass containing H2O and D2O as a starting material. After a melting experiment at 40 GPa, the sample was recovered, and its cross section was prepared at the center of a laser-heated hot spot using a focused ion beam. The melting texture showed a quenched melt pool at the center being surrounded by the SiO2 phase. It contrasts previous melting experiments on anhydrous MORB material, in which CaSiO3 perovskite (davemaoite) is the liquidus phase over the entire pressure range of the lower mantle (Tateno et al., 2018), indicating that the presence of water expands the liquidus field of SiO2.
In addition, we conducted a series of melting experiments on the same sample at 30 to 70 GPa and about 4000 K, and the recovered sample was examined by the secondary ion mass spectrometry (SIMS) to obtain the deuterium/hydrogen (D/H) ratios for a melt pool and a surrounding SiO2 layer. A comparison of the D/H ratios revealed that the latter contained relatively large amounts of deuterium. Based on these experimental results, we will show the hydrogen isotope fractionation factor between the SiO2 phase and silicate melt and discuss the hydrogen isotopic fractionation upon dehydration and melting in the lower mantle.