Geochemistry of volcanic glass from Oligocene detrital sediments at DSDP Site 296, Kyushu Palau Ridge: Interpreting the magmatic evolution of the early northern Izu–Bonin–Mariana Island Arc

Understanding the petrologic and geochemical evolution of island arcs is important for interpreting the timing and impacts of subduction and processes leading to the formation of a continental crust. The Izu–Bonin–Mariana (IBM) Arc, western Pacific, is an outstanding location to study arc evolution. The IBM first arc (45–25 Ma) followed a period of forearc basalt and boninite formation associated with subduction initiation (52–45 Ma). In this study, we present new major and trace element data for the IBM first arc from detrital glass shards and clasts from DSDP Site 296, located on the northernmost Kyushu Palau Ridge (KPR). We synthesize these data with published literature for contemporaneous airfall ash and tephra from the Izu–Bonin forearc, dredge and piston core samples from the KPR, and plutonic rocks from the rifted eastern KPR escarpment, locations which lie within or correlate with KPR Segment 1 of Ishizuka, Taylor, Yuasa, and Ohara (2011). Our objective is to test ways in which petrologic and chemical data for diverse igneous materials can be used to construct a complete picture of this section of the Oligocene first arc and to draw conclusions about its evolution. Important findings reveal that widely varying primary magmas formed and differentiated at various depths at this location during this period. Changes in key trace element ratios such as La/Sm, Nb/Yb, and Ba/Th show that mantle sources varied in fertility and in the inputs of subducted sediment and fluids over time and space. Plutonic rocks appear to be related to early K‐poor dacitic liquids represented by glasses sampled both in the forearc and volcanic fronts. An interesting observation is that the variation in magma compositions in this relatively small segment encompasses that inferred for the IBM Arc as a whole, suggesting that sampling is a key factor in inferring temporal, across‐arc, and along‐strike geochemical trends.