Arc lavas with heavier magnesium (Mg) isotope ratios (denoted as δ26Mg) compared to mid-ocean-ridge basalt (MORB) are commonly attributed to the involvement of subducted serpentinite. However, this hypothesis is challenged by the widespread absence of high-δ26Mg serpentinite in the subducting plates beneath Pacific arcs and fails to consider potential δ26Mg heterogeneity in ambient arc mantle. Here, we show that backarc lavas from the Mariana and Ryukyu subduction zones have MORB-like boron (B) isotope compositions and B/Nb ratios, indicative of little subducted serpentinite in their source. Such lavas have similarly high δ26Mg as their arc counterparts, suggesting the ambient arc mantle in this area is intrinsically rich in isotopically heavy Mg. Further analysis of metaserpentinites demonstrates that deserpentinization residues inherit protolith δ26Mg without significant isotope fractionation during dehydration, highlighting the important contribution of deserpentinized peridotite to the high-δ26Mg mantle. We propose that the high-δ26Mg arc lavas are not necessarily related to ongoing serpentinite subduction, especially in fast-spreading ridge systems. On the other hand, long-term recycling of ancient high-δ26Mg serpentinite provides one plausible mechanism for the heavy Mg mantle domain identified beneath the arc systems. This potentially widespread domain in the mantle greatly affects long-term Mg recycling and crust-mantle evolution.
Zhang et al. (Mon,) studied this question.