Drilling the input to the subduction factory: ODP Leg 185

The aim of Leg 185 of the Ocean Drilling Program was to core two sites in Mesozoic crust in the western Pacific, and so determine inputs into the western Pacific subduction factory. In order to characterise these geochemical fluxes, detailed geochemical data are required on the cored samples. These data must be integrated with logging information to reconstruct the entire drilled section of crust, and with seismic data to extend the fluxes regionally. In terms of the Subduction Factory initiative, Leg 185 results bear directly on the problem of the forcing functions on factory output, and the volatile cycle through the factory. Forcing functions include convergence vectors, thickness of the upper plate, slab temperature, and sediment transport to depth. The Izu-Mariana margin is an excellent area to examine these functions because of the large geochemical signal along-strike in the volcanic arc. These are illustrated where significant differences in Ba/Na are observed in the basalts formed in the Izu-Bonin and the Marainas systems. Similarly Pb-isotope variations are strikingly different and probably controlled by the sediment input to the system. Leg 185 results will also provide critical new data for the volatile cycle. The volatile emissions (H₂O, CO₂, SO₂ and CI) from arc volcanics derive from the basaltic portion of the downgoing plate and the subducted sediments, yet this volatile input is virtually unknown for any convergent margin. Results from Leg 185 will demonstrate how alteration zones and carbonate veins are organized in ancient fast-spreading crust, and so enable the first estimates for volatiles in the upper oceanic crust and sediments near a subduction zone with which to compare directly to volcanic and fore-arc volatile outputs. The oldest oceanic crust on Earth is subducting into the Izu-Mariana arc system, and in addition to providing geochemical data to input into the subduction equation, the two sites studied provide important constraints on the nature and history of Mesozoic ocean crust. Site 801 is in the Pigafetta Basin, which is in the Jurassic Quiet Zone (JQZ) and is dated as ~170 Ma (Pringle, 1992). It is the oldest crust drilled by ODP or the Deep Sea Drilling Project (DSDP). The second site, Site 1149 in the Nadezhda Basin, is on the same flow line as Site 801 but is on magnetic Anomaly M11 and, as such, has an estimated age of ~ 135 Ma. Both sites originated at spreading centers in the Southern Hemisphere and then migrated northward, but at different times and durations. Thus, in addition to the 'Subduction Factory experiment', Leg 185 scientists had an unparalleled opportunity to (1) assess the paleoequatorial sedimentation history of the Pacific Ocean since Mesozoic time, (2) place limits on the ages of the oldest magnetic anomalies in the ocean basins, and (3) study the nature of the JQZ.