Boron recycling beyond arcs: the role of DHMSs

Since the late 1970s, significant efforts have been devoted to studying the phase transitions of hydrous minerals and their stability fields within the altered ultramafic portion of the subducting slab at high-T and high- to ultrahigh-P conditions. The deep subduction of hydrated ultramafic rocks in a cold thermal regime stabilizes the so-called dense hydrous magnesium silicates (DHMSs), which play a crucial role in influencing the deep-water cycle. In recent years, the role of the DHMSs as geochemical reservoirs has gained attention, particularly concerning boron (B), a key element for understanding geological processes involving serpentinized materials. For instance, the genesis of blue B-bearing diamonds in the lower mantle has been proposed as witness for the deep recycling of serpentinized materials via DHMSs (e.g., Regier et al., 2023). Investigating the geochemical behaviour of DHMSs remains a challenging task, hindering our understanding of the trace element budget transferred to depth through the cooler portions of the subducting slabs.

Here, we present the main results of a project aiming to address this knowledge gap using an experimental petrological approach that combines crystal-chemistry with in-situ geochemical investigations to unravel the potential of DHMSs to incorporate B (Cannaò et al., 2023). Using a Walker-type Multi Anvil apparatus, we synthetized high-P (olivine and humite) and ultrahigh-P (Phase-A, Mg-sursassite, Phase 11.5) phases in B-rich MSH and MASH systems. The synthetized phases were characterized for major and trace element concentrations with EMPA and LA-ICP-MS, respectively, while crystal-chemical investigations were conducted using single-crystal XRD and micro-Raman techniques. We document significant B enrichment, from hundreds to thousands of µg/g, in DHMSs, as well as in olivine and humite, suggesting that B can structurally be incorporated into these high-P and ultrahigh-P phases. These findings indicate that along cold prograde subduction paths, the destabilization of both antigorite and chlorite can transfer significant amounts of B to depth, either through olivine/humite or DHMSs. This work extends current knowledge of the B cycle and opens new perspectives to better disclose the deep recycling of elements, shedding light on the origin of the geochemical heterogeneity of the Earth’s mantle.

Cannaò, E., Milani, S., Merlini, M., Tiepolo, M., & Fumagalli, P. (2023). Phase-A as boron carrier in the Earth's interior. Lithos, 452, 107211.

Regier, M. E., Smit, K. V., Chalk, T. B., Stachel, T., Stern, R. A., Smith, E. M., Foster, G. L., Bussweiler, Y., DeBuhr, C., Burnham, A. D., Harris, J. W. & Pearson, D. G. (2023). Boron isotopes in blue diamond record seawater-derived fluids in the lower mantle. Earth and Planetary Science Letters, 602, 117923.