Magnesium isotope composition of volcanic rocks from cold and warm subduction zones: Implications for the recycling of subducted serpentinites and carbonates

Magnesium (Mg) isotopes are regarded as a sensitive tracer to the contribution from subducted serpentinites and carbonates. However, the source, distribution, and controlling factors of the Mg isotope composition of arc magmas remain unclear. In this study, we investigated the intra-arc and inter-arc variations in Mg isotope compositions of volcanic rocks from two typical cold subduction zones [NE Japan (NEJ) and Izu arcs] and a typical hot subduction zone [SW Japan (SWJ) arc] to address the question. The volcanic rocks from the frontal-arc regions of NEJ and Izu have isotopically heavy Mg (δ26Mg = –0.20 to –0.08 ‰) compared to the mantle-like δ26Mg values of most of volcanic rocks from SWJ and the rear regions of NEJ and Izu arcs (–0.28 to –0.17 ‰). It is also worth noting that NEJ arc includes samples with δ26Mg values (–0.61 to –0.39 ‰) significantly lower than the mantle, but similar to the < 110 Ma intra-continental basalts from eastern China, which is the first observation in modern arc rocks. No obvious effects of post-eruptive alteration, fractional crystallization, partial melting, or the addition of silicate-rich sediment and oceanic crust components could be identified in the Mg isotope compositions of these volcanic rocks. By contrast, the correlations between the δ26Mg values and the proxy for serpentinite component (i.e., 11B/10B and Nb/B ratios) indicate that the component exerts a strong control on the Mg-isotopic signature of these arc rocks. Considering metamorphic reactions in subduction lithologies under P-T conditions postulated for these arcs, the variations in δ26Mg values of these arc magmas are unlikely to have been controlled by dehydration of serpentinites in subducted oceanic lithosphere (slab serpentinite). Instead, the high-δ26Mg values of frontal-arc rocks are delivered by the fluids from serpentinite formed in the lowermost part of the sub-arc mantle (mantle wedge serpentinite) in channelized flow. Comparatively, such a high-δ26Mg signature is invisible in volcanic rocks from rear-arc regions of NEJ and Izu, and the entire SWJ, suggesting that the major Mg carriers in subducted serpentinites (e.g., talc, chlorite, and serpentine) were broken down completely before subducted slabs reached the depth beneath these volcanoes. Moreover, the volcanic rocks with low δ26Mg values from the rear arc of NEJ are characterized by high La/Yb and U/Nb ratios as well as low Ti/Eu, Ti/Ti*, and Hf/Hf* ratios, suggesting the involvements of carbonates in their magma sources. The quantitative modeling suggests that < 20 % of sedimentary carbonate (dolomite) was recycled into their mantle source, revealing that Mg-rich carbonate could be incorporated into a deep mantle wedge at rear-arc depths of 150–400 km in subduction zones.