The geochemical evolution of the Izu‐Bonin arc system: A perspective from tephras recovered by deep‐sea drilling

Abstract

Less than 30 Ma tephra glasses from the Izu‐Bonin (IB) arc consist dominantly of low‐K tholeiites that are highly depleted in high field strength elements (HFSE), light rare earth elements (LREE), and large ion lithophile elements (LILE). Collectively, these characteristics indicate minimal contributions from the subducted sediment, a dominantly aqueous fluid subduction component transfer mechanism, and highly depleted mantle sources. Subtle temporal variations are evident throughout the Oligocene‐Neogene. Late Oligocene tephras are geochemically heterogeneous, incorporating low‐K, LREE‐ and HFSE‐depleted tephras, as well as low‐medium‐K glasses with flat‐ to super‐chondritic REE patterns. The latter, have higher Nb/Zr, Th/Nb ratios, possibly indicating small contributions from a slab‐derived melt. More LILE‐ and LREE‐enriched tephras, with higher Nb/Zr and Th/Nb are also evident during the 18–15 Ma interval, coincident with the cessation of backarc spreading in the Shikoku Basin. Extremely HFSE‐depleted glasses are only apparent at the IB volcanic front at the end of backarc spreading, indicating a significant delay between the initiation of extension, and the appearance of more depleted HFSE characteristics at the volcanic front. Similar long time frames are required for replenishment, demonstrating long residence time of material within the mantle wedge. Oligocene tephras are geochemically distinct from coeval turbidites and from Eocene to Oligocene boninitic and tholeiitic series lavas, which are overall characterized by lower HREE and Sm/Zr, and commonly have more enriched HFSE characteristics. Lower Sm/Zr are interpreted to be generated in highly depleted source rocks within the mantle wedge, where orthopyroxene is the dominant residual mineral controlling HFSE‐REE distribution of the arc magma. Elevated Nb/Zr and Zr/Hf in Eocene‐Oligocene boninites and to a lesser extent some tholeiites arise from an additional melt component that is potentially sourced from the subducted slab. LREE‐enrichments in Eocene‐Oligocene IB boninitic and tholeiitic are in most cases correlated with lower 143Nd/144Nd, and are consistent with the mobilization of LREE in a slab‐derived fluid and/or melt. The Late Oligocene represents a major turning‐point in the geochemical evolution of the IB arc, possibly involving a shift from high‐degrees of partial melting (dominantly decompressional) within a shallower, laterally extensive, tensional environment to dominantly fluid‐fluxed melting of deeper mantle components. Resultant cooling of the subducted slab resulted in a cessation in slab‐melting.