SYNOPSIS: Competing models for what controls the segmentation and intensity of ridge crest processes are at odds on the scale of mantle and crustal magmatic segmentation, the distribution of hydrothermal venting with respect to a volcanic segment and the properties of the thermal boundary layer that transports energy between the magmatic and hydrothermal systems. The recent discovery of an axial magma chamber (AMC) reflector beneath the Endeavour segment of the Juan de Fuca ridge, as well as systematic along axis changes in seafloor depth, ridge crest morphology and hydrothermal venting provide an ideal target for testing conflicting hypotheses. In 2008 we will conduct a seismic experiment to investigate the 3-D structure of the crust and topmost mantle beneath the Endeavour segment, a RIDGE2000 Integrated Study Site (ISS). The scientific objectives are to: (1) Determine if the segmentation and intensity of the magma-hydrothermal systems at the Endeavour ridge are related to magma supply or to the magma plumbing between the mantle and crust, and (2) Constrain the thermal and magmatic structure underlying the Endeavour hydrothermal system in order to understand the patterns of energy transfer.
EXPERIMENT: An active source seismic tomography experiment, using an array of 64 three-component ocean-bottom seismometers, will image the 3-D seismic structure of the crust and topmost mantle along an 80-km-long section of the Endeavour ridge. The experiment will image four targets: (1) crustal thickness variations within 25 km of the axial high (0 to 900 kyr); (2) the 2-D (i.e., map view) structure of the uppermost mantle beneath the spreading axis; (3) the 3-D structure of the crustal magmatic system and (4) the detailed 3-D, shallow crustal thermal structure beneath the Endeavour vent field. The results of imaging will define the recent history of magma supply, the pattern of melt delivery from the mantle to the crust and the structure and segmentation of the subseafloor magmatic and hydrothermal systems. These measurements are essential to testing critically competing hypotheses for what regulates the intensity of ridge crest magmatic and hydrothermal processes.
Statement of Broader Impacts: As part of the RIDGE2000 science program, this project will be an important part of understanding the flow of energy from the mantle to the water column that supports unique ecosystems which are not dependent on photosynthesis and that drives hydrothermal circulation which produces concentrated mineral deposits. The research program will involve both graduate and undergraduate students, contributing to their technical training.
Statement of Intellectual Impact: This experiment will provide critical tests of competing models for what controls the segmentation and intensity of ridge crest processes. By imaging mantle and crustal structure at several scales it will define the pattern of melt injection from the mantle to the crust, the segmentation of crustal magmatic systems, the location of crustal heat sources and their relations to hydrothermal circulation.