Majoritic garnets monitor deep subduction fluid flow and mantle dynamics

Abstract

The ultradeep mantle rocks of western Norway display three generations of majoritic garnet. The first two derive from incompatible element–depleted transition-zone mantle and exsolved pyroxene components during Archean upwelling, accretion to subcratonic lithosphere (M1 stage), and isobaric cooling until the Middle Proterozoic (M2). A subsequent Scandian (430–390 Ma) subduction cycle initiated diamond crystallization (M3). Here we report a third majoritic garnet crystallized at grain boundaries and in microfractures, and stable with pyroxene, phlogopite, and spinel in the M3 assemblage. The trace element signatures of M3 minerals indicate crustal metasomatism, phlogopite being the main large ion lithophile element repository. These features imply majorite crystallization from crust-derived subduction fluids at 200 km depth. Our finding fixes the deepest occurrence of free subduction fluid phases and indicates that garnet is a reliable monitor of deep mantle evolution and fluid-mediated chemical recycling.