Origin of cross‐chain geochemical variation in Quaternary lavas from the northern Izu arc: Using a quantitative mass balance approach to identify mantle sources and mantle wedge processes

Abstract

We present major, trace element, and Pb‐Sr‐Nd‐Hf isotope data for Quaternary basalt and basaltic andesite lavas from cross‐chain volcanoes in the northern Izu (N‐Izu) arc. Lavas from Izu‐Oshima, Toshima, Udonejima, and Niijima islands show consistent chemical changes with depth to the Wadati‐Benioff zone, from 120 km beneath Izu‐Oshima to 180 km beneath Niijima. Lavas from Izu‐Oshima at the volcanic front (VF) have elevated concentrations of large ion lithophile elements (LILEs), whereas rear‐arc (RA) lavas are rich in light rare earth elements (LREEs) and high field strength elements (HFSEs). VF lavas also have more radiogenic Pb, Nd, Sr, and Hf isotopic compositions. We have used the Arc Basalt Simulator version 3 (ABS3) to examine the mass balance of slab dehydration and melting and slab fluid/melt‐fluxed mantle melting and to quantitatively evaluate magma genesis beneath N‐Izu. The results suggest that the slab‐derived fluids/melts are derived from ∼20% sediment and ∼80% altered oceanic crust, the slab fluid is generated by slab dehydration for the VF magmas at 3.3–3.5 GPa/660°C–700°C, and slab melt for RA magmas is supplied at 3.4–4.4 GPa/830°C–890°C. The degree of fluxed melting of the mantle wedge varies between 17% and 28% (VF) and 6% and 22% (RA), with a slab flux fraction of 2%–4.5% (VF fluid) to 1%–1.5% (RA melt), and at melting depths 1–2.5 GPa (VF) and 2.4–2.8 GPa (RA). These conditions are consistent with a model whereby shallow, relatively low temperature slab fluids contribute to VF basalt genesis, whereas deeper and hotter slab melts control formation of RA basalts. The low‐temperature slab dehydration is the cause of elevated Ba/Th in VF basalt due mainly to breakdown of lawsonite, whereas deeper breakdown of phengite by slab melting is the cause of elevated K and Rb in RA basalts. Melting in the garnet stability field, and at lower degrees of partial melting, is required for the elevated LILEs, LREEs, and HFSEs observed in the RA basalts. Less radiogenic Sr, Nd, Hf, and Pb in RA basalts are all attributable to lesser slab flux additions. The low H2O predicted for RA basalt magmas (<1.5 wt %) relative to that in VF basalt magmas (5–8 wt %) is also due to melt addition rather than fluid. All these conclusions are broadly consistent with existing models; however, in this study they are quantitatively confirmed by the geochemical mass balance deduced from petrological ABS3 model. Overall, the P‐T‐X(H2O) structure of the slab and the mantle wedge exert the primary controls on arc basalt genesis.