| Summary: | Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2001 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 ...
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