The magnetotelluric component of the Mantle Electromagnetic and Tomography
(MELT) Experiment measured the electrical resistivity structure of the mant
le beneath the fast-spreading southern East Pacific Rise (EPR). The data re
veal an asymmetric resistivity structure, with lower resistivity to the wes
t of the ridge. The uppermost 100 kilometers of mantle immediately to the e
ast of the ridge is consistent with a dry olivine resistivity structure ind
icating a mantle depleted of melt and volatiles. Mantle resistivities to th
e west of the ridge are consistent with a Low-melt fraction (about 1 to 2 p
ercent interconnected melt) distributed over a broad region and extending t
o depths of about 150 kilometers, The asymmetry in resistivity structure ma
y be the result of asymmetric spreading rates and a westward migration of t
he ridge axis and suggests distinct styles of melt formation and delivery i
n the mantle beneath the two plates.