Tiny droplets of ocean island basalts unveil Earth's deep chlorine cycle.

Fully characterising the exchange of volatile elements between the Earth's interior and surface layers has been a longstanding challenge. Volatiles scavenged from seawater by hydrothermally altered oceanic crust have been transferred to the upper mantle during subduction of the oceanic crust, but whether these volatiles are carried deeper into the lower mantle is poorly understood. Here we present evidence of the deep-mantle Cl cycle recorded in melt inclusions in olivine crystals in ocean island basalts sourced from the lower mantle. We show that Cl-rich melt inclusions are associated with radiogenic Pb isotopes, indicating ancient subducted oceanic crust in basalt sources, together with lithophile elements characteristic of melts from a carbonated source. These signatures collectively indicate that seawater-altered and carbonated oceanic crust conveyed surface Cl downward to the lower mantle, forming a Cl-rich reservoir that accounts for 13-26% or an even greater proportion of the total Cl in the mantle.

The tremendous potential of deep-sea mud as a source of rare-earth elements.

Potential risks of supply shortages for critical metals including rare-earth elements and yttrium (REY) have spurred great interest in commercial mining of deep-sea mineral resources. Deep-sea mud containing over 5,000 ppm total REY content was discovered in the western North Pacific Ocean near Minamitorishima Island, Japan, in 2013. This REY-rich mud has great potential as a rare-earth metal resource because of the enormous amount available and its advantageous mineralogical features. Here, we estimated the resource amount in REY-rich mud with Geographical Information System software and established a mineral processing procedure to greatly enhance its economic value. The resource amount was estimated to be 1.2 Mt of rare-earth oxide for the most promising area (105 km2 × 0-10 mbsf), which accounts for 62, 47, 32, and 56 years of annual global demand for Y, Eu, Tb, and Dy, respectively. Moreover, using a hydrocyclone separator enabled us to recover selectively biogenic calcium phosphate grains, which have high REY content (up to 22,000 ppm) and constitute the coarser domain in the grain-size distribution. The enormous resource amount and the effectiveness of the mineral processing are strong indicators that this new REY resource could be exploited in the near future.

Genesis of ultra-high-Ni olivine in high-Mg andesite lava triggered by seamount subduction.

The Kamchatka Peninsula is a prominent and wide volcanic arc located near the northern edge of the Pacific Plate. It has highly active volcanic chains and groups, and characteristic lavas that include adakitic rocks. In the north of the peninsula adjacent to the triple junction, some additional processes such as hot asthenospheric injection around the slab edge and seamount subduction operate, which might enhance local magmatism. In the forearc area of the northeastern part of the peninsula, monogenetic volcanic cones dated at <1 Ma were found. Despite their limited spatiotemporal occurrence, remarkable variations were observed, including primitive basalt and high-Mg andesite containing high-Ni (up to 6300 ppm) olivine. The melting and crystallization conditions of these lavas indicate a locally warm slab, facilitating dehydration beneath the forearc region, and a relatively cold overlying mantle wedge fluxed heterogeneously by slab-derived fluids. It is suggested that the collapse of a subducted seamount triggered the ascent of Si-rich fluids to vein the wedge peridotite and formed a peridotite-pyroxenite source, causing the temporal evolution of local magmatism with wide compositional range.