Interactions Between Mantle Plumes and Mid-Ocean Ridges: Constraints from Geophysics, Geochemistry, and Geodynamical Modeling.

摘要

This thesis studies interactions between mid-ocean ridges and mantle plumes. 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 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. 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 indeed significantly curtail plume dispersion. Chapter 3, an investigation of helium isotope systematics along the western SWIR (9 deg - 24 deg E), reports uniformly low 3He/He ratios of 6.3-7.3 RIRa, compared to values of 8+1 R for normal mid-ocean ridge basalt. There is no evidence for influence of the Bouvet plume on this section of the SWIR from elevated 3He/4He ratios. Chapter 4 studies 3D mantle flow and temperature patterns beneath oceanic ridge- ridge-ridge triple junctions (TJs). For the slowest-spreading ridge, model results for non-hotspot-affected TJs like the Rodrigues TJ predict (1) an increase of temperature and upwelling velocity within a few hundred kilometers of the TJ to approach those of the fastest-spreading ridge, and (2) a strong component of along-axis flow directed away from the TJ. For hotspot-affected TJs like the Azores TJ, this along-axis flow is predicted to advect a hotspot thermal anomaly away from its deep-seated source.