Abstract
How serpentinites in the forearc mantle and subducted lithosphere become involved in enriching the subarc mantle source of arc magmas is controversial. Here we report molybdenum isotopes for primitive submarine lavas and serpentinites from active volcanoes and serpentinite mud volcanoes in the Mariana arc. These data, in combination with radiogenic isotopes and elemental ratios, allow development of a model whereby shallow, partially serpentinized and subducted forearc mantle transfers fluid and melt from the subducted slab into the subarc mantle. These entrained forearc mantle fragments are further metasomatized by slab fluids/melts derived from the dehydration of serpentinites in the subducted lithospheric slab. Multistage breakdown of serpentinites in the subduction channel ultimately releases fluids/melts that trigger Mariana volcanic front volcanism. Serpentinites dragged down from the forearc mantle are likely exhausted at >200 km depth, after which slab-derived serpentinites are responsible for generating slab melts.
Highlights
How serpentinites in the forearc mantle and subducted lithosphere become involved in enriching the subarc mantle source of arc magmas is controversial
The compositions of Mariana arc basalts indicate the involvement of two slab-derived components: an aqueous fluid enriched in fluid mobile elements (FMEs), characterized by high B/Nb, Ba/Nb, and Pb/Ce; and a hydrous melt, characterized by high Th/Nb and La/Sm but low B/Nb, Ba/Nb, and Pb/Ce2–4
The conclusion that altered mafic oceanic crust (AMOC) and marine sediment act as respective sources for the aqueous fluid and hydrous melt is inconsistent with Mariana arc lavas having unradiogenic 87Sr/86Sr (0.7031–0.7041), and fresh mid-ocean ridge basalt (MORB)-like Pb isotopes[7,8,9,10], and mantle-like oxygen isotopic compositions[11]
Summary
How serpentinites in the forearc mantle and subducted lithosphere become involved in enriching the subarc mantle source of arc magmas is controversial. We report molybdenum isotopes for primitive submarine lavas and serpentinites from active volcanoes and serpentinite mud volcanoes in the Mariana arc These data, in combination with radiogenic isotopes and elemental ratios, allow development of a model whereby shallow, partially serpentinized and subducted forearc mantle transfers fluid and melt from the subducted slab into the subarc mantle. The conclusion that AMOC (average 87Sr/86Sr = ~0.70455) and marine sediment (average 87Sr/86Sr = ~0.7106) act as respective sources for the aqueous fluid and hydrous melt is inconsistent with Mariana arc lavas having unradiogenic 87Sr/86Sr (0.7031–0.7041), and fresh mid-ocean ridge basalt (MORB)-like Pb isotopes[7,8,9,10], and mantle-like oxygen isotopic compositions[11]. This study reports new Mo elemental and isotopic data for well-characterized and very fresh lavas from Pagan and NW Rota-1 volcanoes in the Mariana arc, and serpentinite mud samples from the Asùt Tesoru (formerly Big Blue) Seamount in the Mariana forearc
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