The Missing Half of the Subduction Factory: Preliminary Results from the Izu Rear ARC, IODP Expedition 350, Site U1437

Abstract:

IODP Expedition 350 was the first in the rear part of the Izu-Bonin-Mariana intra-oceanic arc. The morphology of the Izu rear arc is dominated by a series of ~16 – 3 Ma basaltic to rhyolitic seamount chains up to ~50 km long, which strike at a high angle to the arc front, and differ from arc front rocks by being more similar to averaged continental crust (i.e. enriched in alkalis, Ba, Th, U and LREE). Site U1437 lies at 2117 mbsl in a ~20 km-wide volcano-bounded basin between the Manji and Enpo rear arc seamount chains, ~90 km west of the arc front.

Site U1437 was drilled to 1804 mbsf. The section is much more mud-rich than expected, with ~60% tuffaceous mud for the section as a whole, 89% in the uppermost 433 m, and high sedimentation rates of 100-260 m/my for the upper 1300 m. The proportion (40%) and grain size of tephra are much smaller than expected for an intra-arc basin, composed half of ash/tuff, and half of lapilli tuff of fine grain size. These were deposited by suspension settling through water and from density currents, in relatively distal settings. Volcanic blocks are only sparsely scattered through the lowermost 25% of the section (1320 to 1804 mbsf), which includes hyaloclastite, in situ quench-fragmented blocks, and a rhyolite peperite intrusion (i.e., proximal deposits). The lowest biostratigraphic datum is ~6.5 Ma at 867 mbsf; the lowest paleomagnetic datum ~9 Ma is at ~1300 mbsf; nannofossils at ~1400 mbsf indicate a range of 10.97–11.85 Ma; and rhyolite peperite at ~1390 mbsf has yielded a preliminary U-Pb zircon concordia intercept age of 13.6 +1.6/-1.7 Ma. Alteration resulted in destruction of fresh glass below ~750 mbsf, but minerals are less altered. Our preliminary interpretation is that arc front vs. rear arc sources can be distinguished in the upper (relatively distal) 1320 m, while the lower (proximal) 25% of the section may be heterogeneous, suggesting that the rear arc magmas only fully compositionally diverged after ca. 13 Ma.