Deciphering the Sources of Fine-Grained, Late Miocene Volcaniclastic Density Current Deposits in the Manji-Enpo Volcano-Bound Basin (Unit V, IODP Expedition 350 Site U1437 - Izu-Bonin Rear Arc): Insights from Shard and Crystal Geochemistry, SEM Petrography, XRF Core Scanning, and Shipboard Data.

Abstract:

The rear arc of the Izu island arc hosts chains of large seamounts. Volcano-bounded basins between these chains that are inferred to have trapped tephra derived from the adjacent volcanoes, the Izu arc front to the east, and tephra blown in from Japan, together with hemipelagic sediment swept in by ocean currents from the west. IODP Expedition 350 recovered core from site U1437 in the basin between the Manji and Enpo seamount chains. The lithostratigraphy at U1437 is notable for being very fine-grained and for coarse volcaniclastic material being rare; this contrasts with volcanic aprons around many other seamounts and volcanic islands. Given the absence of easily demonstrable proximal sediment, and the diversity of sediment sources, detailed analysis of well-preserved volcaniclastic deposits is required to determine the provenance and eruption and transport mechanisms at play.

We sampled three representative volcaniclastic density current deposits from lithological Unit V (1120.11–1312.21 mbsf). Unit V is characterized by monomict, thinly bedded (<1 m) felsic tuffs intercalated with thick intervals of tuffaceous mudstone. The intervals sampled preserved erosional basal contacts and upon visual inspection were minimally altered. We are (SEM-BSE imaging) and collecting LA-ICP-MS, EMP, and SEM-EDX analyses of shards and crystals in serial thin sections of each interval. These analyses are complemented by shipboard data and new XRF core scans for whole-rock multi-element composition. With these data we will determine textural and compositional gradients and grain size variations, and attempt to constrain the degree of in-mixing of hemipelagic sediment. These data will help us constrain the tephra source or sources. Understanding these volcaniclastic intervals in Unit V is important for constraining the different sources and material inputs into the other lithological units at U1437, a topic of extensive study by other Expedition 350 participants, and to characterize an alternative stratigraphic architecture around a seamount.