STEADY AND NONSTEADY STATE MAGMA CHAMBERS BELOW THE EAST PACIFIC RISE

Volcanic rocks collected on flowline traverses on the flanks of the Ea st Pacific Rise (EPR) document changes in axial magma chemistry with t ime, providing a record of the thermal history of axial magma chambers (AMC). We present data for a sample set df closely spaced (1-2 km) sa mples along EPR flowlines at three localities out to similar to 800 ka showing both steady (constant average temperature) and non-steady sta te behavior of magma chambers on time scales of 200-500 ka. Though bas ed on only three symmetrical traverses so far, it appears that magmati cally robust ridge locations (11 degrees 20'N and 9 degrees 30'N) have steady state chambers, whereas a magmatically starved axis (10 degree s 30'N) shows large temperature changes with time. These observations provide a new petrologic perspective to the ongoing debate regarding t he significance and causes of morphologic variations along the axis of the East Pacific Rise.