Abstract:
Between 9º-25º E on the ultraslow-spreading Southwest Indian Ridge lie two sharply
contrasting supersegments. One 630 km long supersegment erupts N-MORB that is
progressively enriched in incompatible element concentrations from east to west. The
second 400 km long supersegment contains three separate volcanic centers erupting EMORB
and connected by long amagmatic accretionary segments, where mantle is
emplaced directly to the seafloor with only scattered N-MORB and E-MORB erupted.
Rather than a major break in mantle composition at the discontinuity between the
supersegments, this sharp contrast in geometry, physiography, and chemistry reflects
“source” versus “process” dominated generation of basalt.
Robust along-axis correlation of ridge characteristics (i.e. morphology, upwelling rate,
lithospheric thickness), basalt chemistry, and crustal thickness (estimated from gravity)
provides a unique opportunity to compare the influence of spreading geometry and rate
on MORB generation. What had not been well established until now is the importance of
melting processes rather than source at spreading rates < 20 mm/yr. Along the
orthogonally spreading supersegment (14 mm/yr) moderate degrees of partial melting
effectively sample the bulk mantle source, while on the obliquely spreading
supersegment (7-14 mm/yr) suppression of mantle melting to low degrees means that the
bulk source is not uniformly sampled, and thus “process” rather than “source” dominates
melt chemistry.
