Abstract
Subduction of oceanic crust buries an average thickness of 300–500 m of sediment that eventually dehydrates or partially melts. Progressive release of fluid/melt metasomatizes the fore-arc mantle, forming serpentinite at low temperatures and phlogopite-bearing pyroxenite where slab surface reaches 700–900 °C. This is sufficiently high to partially melt subducted sediments before they approach the depths where arc magmas are formed. Here, we present experiments on reactions between melts of subducted sediments and peridotite at 2–6 GPa/750–1100 °C, which correspond to the surface of a subducting slab. The reaction of volatile-bearing partial melts derived from sediments with depleted peridotite leads to separation of elements and a layered arrangement of metasomatic phases, with layers consisting of orthopyroxene, mica-pyroxenite, and clinopyroxenite. The selective incorporation of elements in these metasomatic layers closely resembles chemical patterns found in K-rich magmas. Trace elements were imaged using LA-ICP-TOFMS, which is applied here to investigate the distribution of trace elements within the metasomatic layers. Experiments of different duration enabled estimates of the growth of the metasomatic front, which ranges from 1–5 m/ky. These experiments explain the low contents of high-field strength elements in arc magmas as being due to their loss during melting of sedimentary materials in the fore-arc.
Highlights
In the plate tectonic cycle, oceanic crust is continuously subducted and remixed into the mantle
Partial melts of the subducted and metamorphosed sedimentary rocks eventually separate from their source, rise, react with, and hybridize the mantle wedge beneath arcs to generate subduction-related magmatism [5]
These arc magmas are generally known for their strong depletion in high field strength elements (HFSE), which is commonly attributed to residual rutile during sediment de-volatilization, trace element filtering by amphibole in the mantle wedge [6], mantle-magma interaction [7], or multiple melt-depletion events [8]
Summary
In the plate tectonic cycle, oceanic crust is continuously subducted and remixed into the mantle. The sediment-derived components of arc magmas are supplied by the residues of partially melted sediments and not the unmodified sediments as commonly assumed This focuses attention on a gap in earlier research indicated by a ‘?’ in Figure 1, namely experiments on the interaction between sedimentary rocks and peridotite in the fore-arc of subduction zones where pressures 675 ◦ C in the fore-arc (white arrow, Figure 1) [18,29], and subducted sedimentary components may partially devolatilize and melt to infiltrate and metasomatize the lithospheric mantle in the fore-arc Element fractionation during these fore-arc processes will change the fundamental geochemical character of the sedimentary component that proceeds to depths where arc magmas form.
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