Abstract:
This thesis studies interactions between mid-ocean ridges and mantle plumes using
geophysics, geochemistry, and geodynamical modeling. Chapter 1 investigates the effects
of the Marion and Bouvet hotspots on the ultra-slow spreading, highly-segmented
Southwest Indian Ridge (SWIR). Gravity data indicate that both Marion and Bouvet
impart high-amplitude mantle Bouguer anomaly lows to the ridge axis, and suggest that
long-offset transforms may diminish along-axis plume flow. Building upon this
observation, Chapter 2 presents a series of 3D numerical models designed to quantify the
sensitivity of along-axis plume-driven mantle flow to transform offset length, spreading
rate, and mantle viscosity structure. The calculations illustrate that long-offset transforms
in ultra-slow spreading environments may significantly curtail plume dispersion. Chapter 3
investigates helium isotope systematics along the western SWIR as well as near a global
array of hotspots. The first part of this study reports uniformly low 3HetHe ratios of
6.3-7.3 RlRa along the SWIR from 9°-24°E, compared to values of 8±1 Ra for normal
mid-ocean ridge basalt. The favored explanation for these low values is addition of
(U+ Th) into the mantle source by crustal and/or lithospheric recycling. Although high
HetHe values have been observed along the SWIR near Bouvet Island to the west, there
is no evidence for elevated 3HetHe ratios along this section of the SWIR. The second part
of Chapter 3 investigates the relationship between 3HetHe ratios and geophysical
indicators of plume robustness for nine hotspots. A close correlation between a plume's
flux and maximum 3HetHe ratio suggests a link between plume upwelling strength and
origination in the deep, relatively undegassed mantle. Chapter 4 studies 3D mantle flow
and temperature patterns beneath oceanic ridge-ridge-ridge triple junctions (TJs). In non-hotspot-
affected TJs with geometry similar to the Rodrigues TJ, temperature and upwelling
velocity along the slowest-spreading of the three ridges are predicted to increase within a
few hundred kilometers of the TJ, to approach those of the fastest-spreading ridge. Along
the slowest-spreading branch in hotspot-affected TJs such as the Azores, a strong
component of along-axis flow directed away from the TJ is predicted to advect a hotspot
thermal anomaly away from its deep-seated source.
