Geodynamic and Geochemical Modeling of Mantle Processes along the Southwest Indian Ridge at 35°-40°E: A Hotspot-Mid-Ocean Ridge Interaction Region

Abstract:

Mantle convection can be regarded as the superposition of two convective models: a
plate mode and a plume mode. Geodynamic modeling of these regimes has granted
insight into surface features, and tells us about the mantle processes in a system largely
devoid of observables. Our study of the 35°–40°E segment of the Southwest Indian Ridge
(SWIR) seeks to link geochemical and geological observations with the underlying mantle
processes.
Both plate and plume modes interact and combine at the SWIR 35°–40°E segment. The
mid-ocean ridge itself is a manifestation of the plate tectonics mode of mantle convection.
The slow opening rate and obliquity of this segment should lead to low volcanic activity
along this segment. However, this segment is the point along the SWIR closest to the
Marion hotspot, a manifestation of the plume mode of mantle convection. When interacting
with the mid-ocean ridges, hotspots like the Azores, Iceland, Galápagos, and Rodriguez
produce distinctive patterns, such as propagating rifts, triple junctions, and enriched MORB
signatures. The Marion hotspot does not have a similar effect on the SWIR even though
it is associated with a bathymetric high and residual mantle Bouguer anomaly low. A
notable feature along the ridge is a V-shaped bathymetric anomaly around one of the non-
transform discontinuities (NTD).
As for the SWIR 10°-16°E area (Montési et al., 2011) geodynamics modeling predicts
magma focusing to highly segmented non-transform oblique segments (NTOS) along the
ridge. However, geophysical observations show a thinning crust at these regions. Modeling
without the segmentation along the oblique segments shows much better agreement with
the observations. So either the NTOS are a crustal structure that does not influence mantle
upwelling, melt extraction parameters vary along the ridge, or the density of the crust is
anomalous in NTOS due to a different fractionation history.
We will incorporate whole rock chemistry (including trace element, & REEs) constraints
to the evaluation of our hypotheses by modeling melt evolution and crystallization under
different conditions and comparing model predictions with collected samples.