Date of Award

Spring 2017

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

Julia G Bryce

Second Advisor

Samuel B Mukasa

Third Advisor

William C Clyde

Abstract

The East African Rift System (EARS) and the West Antarctic Rift System (WARS) are two of the largest continental rift systems on Earth, but the processes governing rift dynamics remain controversial. The large volume and distinct chemistry of Cenozoic rift-related lavas, combined with geophysical evidence for low-density mantle underlying both rifts, have traditionally been interpreted as evidence for anomalously hot mantle plumes actively rising from the deep mantle beneath both regions. However, in light of increasing evidence that these mantle structures may also be chemical in nature, alternative explanations highlight the role of heterogeneous, easily-fusible mantle components in driving magma genesis in the absence of significant thermal anomalies. These heterogeneous mantle domains may be linked to the complex tectonic histories of continents, which often involve multiple stages of accretion and associated recycling of materials between the crust and the mantle. In this dissertation, detailed chemical investigations of rift-related volcanic rocks provide critical new insight into the nature of the mantle underlying the two active rifts. In particular, the role of volatiles such as water and carbon dioxide is highlighted due to the ability of these components to enhance mantle melting during rift-related decompression.

In Chapter II, the first geochemical information from submarine lavas in the Ross Embayment of West Antarctica are reported alongside subaerial lavas from islands and mainland localities, which together provide evidence that volatilized, recycled mantle domains generated during ancient long-lived subduction along the paleo-Pacific margin of Gondwana are key components in the temporally evolving source of Cenozoic magmas (Aviado et al., 2015). In Chapter III, the first rift-wide study of magmatic volatiles recorded in olivine-hosted melt inclusions confirms that the West Antarctic mantle is enriched in water and carbon dioxide on a wide scale, and links the production of hydrated and carbonated components to subduction-related metasomatic processes. These results provide a compelling link between continental assembly, the production of easily fusible, heterogeneous chemical domains in the sub-continental lithospheric mantle (SCLM) and upper mantle, and rifting plus associated magmatism. These links are further explored in Chapter IV, in which the trace element and radiogenic isotope (Sr-Nd-Hf-Pb) systematics of mantle xenoliths from the East African Rift System (EARS) illustrate that the SCLM bears witness to a complex history involving Proterozoic melt depletion events, Pan African continental assembly, late-stage metasomatism, and plume impingement. These results demonstrate that SCLM serves as an important long-lived host of heterogeneous recycled mantle domains that are sampled throughout multiple episodes of convergence and breakup.

Collectively, these chapters suggest that ongoing rifting and magmatism in the WARS and the EARS are in part tied to shallow mantle processes involving the heterogeneous SCLM, rather than having exclusively deep plume origins. These shallow processes emerge as a consequence of the complex, multistage evolution of continents.

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