Determining the inputs to the Mariana Subduction Factory: Using core-log integration to reconstruct basement lithology at ODP Hole 801C

Abstract

[1] Calculating elemental mass balance across subduction zones enhances our understanding of global geochemical budgets and large-scale Earth processes. However, to accurately constrain the input flux, it is critical to know the lithological diversity and chemical characteristics of the downgoing oceanic plate. The west Pacific altered ocean crust that was drilled during Ocean Drilling Program (ODP) Leg 185 represents a significant component of the input to the Mariana Subduction Factory. The lithological sequence in Hole 801C consists of aphyric basalt, occurring as thick massive units, pillow units, and breccia units. The shallowest basalts are intercalated with sediments and two hydrothermal deposits. Core recovery was good for a basement hole (average 47%); however, over half the lithological section was unaccounted for. Downhole logging data provide a continuous record of physical, chemical, and structural properties of the rocks at the borehole wall, thus, when calibrated using available cored material, they can be used to reconstruct lithology in unrecovered intervals. Core-log integration results reveal a significant bias in core recovery, with infrequent retrieval of delicate breccia units and preferential recovery of more massive, competent, and less altered flow units. This is important because the breccia units are host to many of the key tracer elements used in mass balance calculations. The massive basalts exhibit high density, resistivity and velocity values and low porosity and gamma ray values. Formation MicroScanner (FMS) images of massive basalts are bright (reflecting their resistive nature) with a homogenous texture and regular fracture pattern. Breccia or pillow basalts are characterised by low resistivity, density and velocity, and high porosity and gamma ray values and unrecovered intervals displayed these same characteristics. The reconstructed log-based lithological sequence consists of thick massive flow units (27.4%), pillow units (33%), breccia units (31%), sediments (1.4%), and hydrothermal deposits (1.3%), with 5.9% unclassified due to unreliable tool response in intervals where hole conditions were poor. These findings have a significant bearing on the Subduction Factory recycling equation. The proportion of pillow basalts doubled and the amount of breccia increased six-fold from that reported using core description alone, demonstrating convincingly that core-log integration is essential to provide an accurate representation of the input flux. The log-based stratigraphy reconstructed for Hole 801C represents the first example of Jurassic fast-spread (160 km/m.y.) ocean crust and provides constraints on the relationships between crustal structure, age, alteration, and spreading rate.