One-Sided Joint Inversion of Shear Velocity and Resistivity from the
PI-LAB Experiment at the Equatorial Mid-Atlantic Ridge
Abstract
The lithosphere-asthenosphere system is fundamental to our understanding
of mantle convection and plate tectonics. Seismic and electromagnetic
methods are our primary means of determining its structure and physical
properties. These independent constraints with different sensitivities
to Earth’s properties hold promise for understanding the system. Here we
use the shear velocity model from Rayleigh waves and the MT based
resistivity model from near the equatorial Mid-Atlantic Ridge.
Cross-plots of the models suggest a linear or near-linear trend that is
also in agreement with petrophysical predictions. We therefore map the
MT model to a new shear-wave starting model using the petrophysical
relationship, which is then used to re-invert for shear-wave velocity.
The resulting shear-wave velocity model fits the phase velocity data,
and the correlation coefficient between the shear velocity and
resistivity models is increased. Much of the model can be predicted by
expectations for a thermal half-space cooling model, although some
regions require a combination of higher temperatures, volatiles, or
partial melt. We use the petrophysical predictions to estimate the melt
fraction, melt volatile content, and temperature structure of the
asthenospheric anomalies. We find up to 4% melt, with the lowest
resistivities and shear velocities explained by up to 20% water or 20%
CO2 in the melt or ~1% nearly pure
sulfide melt, depending on the set of assumptions used. Melt is required
in punctuated anomalies over broad depth ranges, and also in channels at
the base of the lithosphere. Melt in the asthenosphere is dynamic, yet
persistent on geologic time scales.