Residual forearc peridotites recording the early magmatic stage of subduction initiation

Subduction initiation is fundamental to plate tectonics. Early basaltic magmatism preceding boninitic magmatism is key to understanding the onset and mechanism of subduction initiation. However, the mantle processes during the early basaltic magmatism remain poorly understood due to the scarcity of corresponding residual peridotites. To find the residual peridotites from the early stage of subduction initiation, we compiled spinel data and examined peridotites from the Izu-Bonin-Mariana and Tonga forearc regions that contain some of the lowest spinel Cr/Al ratios. These samples present small amounts of pargasitic amphibole and light rare earth elements depleted patterns in their clinopyroxenes and amphiboles. These features differ from depleted forearc peridotites associated with boninitic magma, abyssal peridotites, and backarc peridotites, but resemble lherzolites from the Oman ophiolite. Melting models can reproduce the compositions of clinopyroxene and the corresponding early basalts for each region. We suggest that the studied forearc peridotite samples are residues after early basaltic magma extractions, which potentially involved a common process of near fractional melting under moist conditions.