No significant boron in the hydrated mantle of most subducting slabs

Abstract

Boron has become the principle proxy for the release of seawater-derived fluids into arc volcanics, linked to cross-arc variations in boron content and isotopic ratio. Because all ocean floor serpentinites so far analysed are strongly enriched in boron, it is generally assumed that if the uppermost slab mantle is hydrated, it will also be enriched in boron. Here we present the first measurements of boron and boron isotopes in fast-spread oceanic gabbros in the Pacific, showing strong take-up of seawater-derived boron during alteration. We show that in one-pass hydration of the upper mantle, as proposed for bend fault serpentinisation, boron will not reach the hydrated slab mantle. Only prolonged hydrothermal circulation, for example in a long-lived transform fault, can add significant boron to the slab mantle. We conclude that hydrated mantle in subducting slabs will only rarely contribute to boron enrichment in arc volcanics, or to deep mantle recycling.

Molybdenum isotopes unmask slab dehydration and melting beneath the Mariana arc

“…Serpentinite breakdown has long been believed to play a crucial role in element transfer in subduction zones 8 , 15 , 16 . However, whether this serpentinite formed in the downgoing oceanic lithosphere or in the fore-arc mantle has been difficult to resolve 27 , 28 . Mo isotopes may help distinguish the respective roles of these two serpentinite sources.…”

“…While the importance of serpentinite subduction in the subduction zone chemical cycle is now widely acknowledged, it remains a formidable task to discriminate among these competing models. Which serpentinite sources are responsible, and how they work in the subduction zones are still unclear 27 , 28 . Molybdenum (Mo) is mobile in fluids, like Pb and Ba, but it can also be partially retained in rutile during slab dehydration or melting.…”

Water in the Supra‐Subduction‐Zone Mantle of the Mariana‐Izu‐Bonin Forearc: Constraints From Peridotitic Orthopyroxene

“…Thus, the only plausible reason for B enrichment in the primary mantle minerals is supply of boron as the rocks still resided at mantle depth, providing for high enough temperatures to facilitate re‐equilibration. A possible scenario is that B‐enriched fluid was liberated by dehydration of abyssal serpentinite in a subducting footwall plate, which may contain up to 200 ppm B (Harvey et al., 2014; McCaig, 2018; Pabst et al., 2011; Vils et al., 2008). The released fluid, in turn, migrated to the overlying mantle wedge and re‐equilibrated with the mantle minerals.…”

Hydrothermal troctolite alteration at 300 and 400 °C: Insights from flexible Au-reaction cell batch experimental investigations

“…Considering this close association with troctolites and an alteration texture typical for rodingitization, massive occurrence of clinopyroxene in the 400 °C experiment might provide an alternative low T formation hypothesis for this unique dyke lithology. Secondary clinopyroxene in association with altered troctolites has been reported before at Hess Deep where it was interpreted as a late high-grade (>360 °C) hydrothermal overprint of a preceding chlorite-prehnite alteration that occurred lower temperatures (200 °C -Gillis et al 2014;McCaig et al 2018).…”

“…However, only few studies investigated the alteration of troctolitic rocks (Frost et al 2008;Beard et al 2009;Nakamura et al 2009;Nozaka et al 2017;McCaig et al 2018). Frost et al (2008) suggested that the troctolite alteration resembles a one-rock rodingitization, in which olivine represents the ultramafic and plagioclase the gabbroic component.…”