Constraining the Magmatic Budget of the EPR at 9oN Using Broadband Marine MT

In February 2004 we will be conducting a 60 site broadband magnetotelluric (MT) survey at the Ridge2000 integrated study site (ISS) on the East Pacific Rise (EPR) near 9^oN. Broadband marine MT instrumentation developed at Scripps Institution of Oceanography and used extensively by the petroleum industry will allow resolution of electrical conductivity structure at much shallower depths than traditional MT instrument systems. A small aperture (30km) transect of densely spaced MT sites across the ridge at the ISS "bull's eye" focus area at 9^o50'N will image the electrical conductivity structure of the crust and shallow mantle in the vicinity of the ridge axis. A large aperture (200km) MT transect across the ridge at 9^o30'N will target both crustal and upper mantle conductivity structures. We will also be deep-towing the new Scripps Underwater Electromagnetic Source Instrument (SUESI) to allow for controlled-source EM data which will provide constraints on the shallowest/resistive structures. Key issues we are addressing with our experiment are: 1. What is the distribution, vertical extent and volume fraction of melt present in the mid to lower crust? A dense transect of MT sites near the ridge axis at the ISS "bull's eye" focus area at 9^o50'N will delineate the region of crust containing partial melt and constrain the total amount of connected melt present. 2. To what extent does hydrothermal circulation play a role in heat removal from the crust? What link(s) exists between the hydrothermal activity and crustal magma at the ridge axis? How does hydrothermal circulation affect the lateral extent of the crustal melt? A wide array of MT sites will provide estimates of lateral electrical conductivity variations in the crust near the mid-ocean ridge, from which constraints on temperature, porosity and permeability variations in the region surrounding the mid to lower crustal melt accumulations at 9^o50'N can be made. 3. To what extent is melt accumulating at the base of the crust? Does a relatively thin layer of concentrated melt pond at the base of the crust, or does melt accumulate from a broader region of the mantle? A wide aperture line of MT sites up to 100 km from the ridge axis at 9^o30'N will help to distinguish between these hypotheses. 4. What, if any, connection exists between the crustal melt accumulations and the region of decompression melting deeper in the mantle? Again, the wide aperture line of MT sites will allow us to place constraints on the delivery of melt to the ridge by the crust-mantle system. Electrical conductivity images of the crust and upper mantle from this experiment will complement existing results from other geophysical methods used at this segment of the EPR (seismic refraction/reflection, compliance and gravity). Estimates of the amount and distribution of partial melt in both the crust and upper mantle, and of the temperature, porosity and permeability structure of the crust will provide useful constraints on the nature of the complex magmatic system, and will support the interpretation of a wide variety of other Ridge2000 EPR ISS experiments.