A parcel of lherzolite in an upwelling mantle beneath 8-10 ?N of the East Pacifi

Question

A parcel of lherzolite in an upwelling mantle beneath 8-10 ?N of the East Pacific Rise ( about 100km deep) contains Pb, Ce and Yb at levels common for depleted MORB mantle.

a) If the pathway of these elements in this setting stayed the same, discuss the processes that affect them over a time span of the next 200 million years

b) Compare how the same elements will be processed over the next 200 million years in a parcel of lherzolite beneath Central Iceland (100 km deep) and highlight how pathways may diverge for some of the elements in Iceland relative to the EPR.

Explanation / Answer

Melt generated by mantle upwelling is fundamental to the production of new oceanic crust at mid-ocean ridges, yet the forces controlling this process are debated. Passive-flow models predict symmetric upwelling due to viscous drag from the diverging tectonic plates, but have been challenged by geophysical observations of asymmetric upwelling that suggest anomalous mantle pressure and temperature gradients, and by observations of concentrated upwelling centres consistent with active models where buoyancy forces give rise to focused convective flow. Here we use sea-floor magnetotelluric soundings at the fast-spreading northern East Pacific Rise to image mantle electrical structure to a depth of about 160?kilometres. Our data reveal a symmetric, high-conductivity region at depths of 20-90?kilometres that is consistent with partial melting of passively upwelling mantle. The triangular region of conductive partial melt matches passive-flow predictions, suggesting that melt focusing to the ridge occurs in the porous melting region rather than along the shallower base of the thermal lithosphere. A deeper conductor observed east of the ridge at a depth of more than 100?kilometres is explained by asymmetric upwelling due to viscous coupling across two nearby transform faults. Significant electrical anisotropy occurs only in the shallowest mantle east of the ridge axis, where high vertical conductivity at depths of 10-20?kilometres indicates localized porous conduits. This suggests that a coincident seismic-velocity anomaly is evidence of shallow magma transport channels rather than deeper off-axis upwelling.

***  The plate tectonic processes, or “plate,” model for the genesis of melting anomalies (“hotspots”) attributes them to shallow-sourced phenomena related to plate tectonics. It postulates that volcanism occurs where the lithosphere is in extension, and that the volume of melt produced is related primarily to the fertility of the source material tapped.

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