Phase-A as boron carrier in the Earth's interior

Abstract

The interest of the scientific community on the role of Dense Hydrous Magnesium Silicates (DHMSs) as geochemical reservoir is constantly increasing, particularly for boron (B) which is widely considered as one of the best proxies to understand geological processes involving serpentinized materials. In particular, the genesis of blue B-bearing diamonds in the lower mantle has been proposed to be related to the deep recycling of serpentinized materials through DHMSs. However, it is still unknown whether DHMSs are capable to incorporate and transport significant amounts of B to great depth. In this Letter, we attempt to fill this knowledge gap by providing the results of a pilot experiment aimed to investigate the B content in Phase-A, which is the first DHMS mineral replacing antigorite along a prograde cold subduction path. By using a Walker-type Multi Anvil module, we synthetized Phase-A at 700 °C and 7.0 GPa starting from a stoichiometric composition modelled in the MSH system (2 SiO2: 7 MgO) and adding 1000 ppm of B as ultrapure boric acid (H3BO3) in H2O plus trace of NH4OH. The experimental product has been characterized through electron microprobe, single-crystal X-ray diffraction, and laser ablation ICP-MS. Structural analyses confirmed the crystallization of the Phase-A and chemical analyses show B contents of ca. 200 ppm that are structurally incorporated in the Phase-A, thus suggesting its role as viable B source at depth. Our preliminary results extend the current knowledge of the B cycle and open new perspectives to better disclose the deep recycling of elements at the origin of the geochemical heterogeneity of the Earth's mantle.