14 research outputs found
The First 10 Million Years of RearâArc Magmas Following Backarc Basin Formation Behind the Izu Arc
Abstract IODP Site U1437 is located in the Izu rearâarc region, approximately 330Â km west of the IzuâBonin trench axis. The oldest four units (Units IV through Unit VII) include volcaniclastic sediment and in situ hyaloclastites. They have ages of about 6â15Â Ma, shortly after cessation of Shikoku backarc basin opening. Three magma types are identified by their distinct geochemistry; they are similar types to those found in the modern Izu arc (Rear Arc Seamount Chain [RASC]âtype, Riftâtype, and volcanic front [VF]âtype). RASCâtype has the most enriched Nd and Hf isotope and fluidâimmobile trace element ratios and dominates from 9 to 6Â Ma. Riftâtype, dominant from 15 to 9Â Ma, is similar to VFâtype in NdâHf isotopes but has the least radiogenic Sr and Pb, and intermediate La/Yb and Nb/Yb, indicating a more fertile mantle source and less hydrous slab component than VFâtype. Less common and randomly distributed VFâtype sediments have the most radiogenic Sr and Pb, and the highest Ba/(Th, LREE [light rare earth element]) ratios, and are interpreted to be distally derived. The genesis of mafic Unit VII samples (~15Â Ma) was modeled using the Arc Basalt Simulator. Results are most similar to those for basalts in the modern rift environment indicating the addition of ~1% of a meltârich slab component generated at ~125Â km, to a Philippine Sea Plate ambient mantle that was more depleted than DMM (depleted MORB mantle). The initial postâShikoku basin magmatism in the Izu rearâarc generated Riftâtype magmas for about 6Â million years before the distinctive RASCâtype magmatism began, which then became increasingly enriched