Multi-stage evolution of the lithospheric mantle in the West Antarctic Rift System - a mantle xenolith study

Mantle xenoliths allow us to investigate the geochemical and dynamic evolution of the mantle beneath the western margin of Antarctica and reconstruct a timeline of geologic events that are obscured on the surface. For this study, mantle xenoliths, brought to the surface by recent volcanism, were collected along a transect from the rift shoulder and into the rift basin in the western margin of the West Antarctic Rift System (WARS), thus providing a recent snapshot of the lithospheric mantle after major episodes of rifting. The second chapter of my thesis focuses on determining the age and persistence of the mantle within the rift. The rhenium-osmium (Re-Os) isotope system has proven to be an invaluable tracer of the tectonic history of the lithospheric mantle and can constrain the age of melt extraction and subsequent stabilization of the lithospheric mantle. This allowed us to track the age of the lithospheric mantle across this rifted margin. Os isotopes, combined with major element compositions, reveal widespread Paleoproterozoic (1.7-2.4 Ga) stabilization of the lithosphere and subsequent preservation, suggesting the lithosphere has dynamically thinned in response to rifting. Major element data allowed us to place temperature (T) constraints on the mantle and characterize the thermal history in the WARS. This study also revealed the oldest lithosphere ages recorded in Antarctica (3.3 Ga) and is the first to report ages that coincide with adjacent crustal ages, thus confirming the coupled relationship between the lithospheric mantle and continental crust. An integral factor controlling the composition of magmas generated at Earth’s surface is the composition of the SCLM. Magmas generated at depth must pass through it, and subsequently may take on geochemical signatures of the lithosphere, or may leave behind geochemical imprints of the migrating magma in the SCLM. Trace elements provide a means to investigate both the depletion and re-enrichment history of the SCLM. The third chapter of my thesis investigates the metasomatic overprinting of the Paleoproterozoic SCLM. Metasomatism, which is the chemical alteration of a rock by a migrating melt and/or fluid, leaves behind diagnostic signatures of the metasomatizing agent (e.g. subduction related fluids or carbonated melts). This can occur cryptically, where a melt percolates through the rock, changing the composition of the rock, but not the lithology. Modal metasomatism produces new mineral phases that are not typically expected in the rock. In xenoliths, trace elements enable us to decode geochemical signatures, and determine the sources of metasomatism. The WARS lithosphere has experienced varying degrees of re-enrichment, broadly characterized by low high field strength element (HFSE) abundances and rare earth element (REE) enrichments that correspond with carbonatite metasomatism. In addition, the presence of secondary hydrous phases (e.g. amphibole and phlogopite) imparted distinct geochemical signatures, revealing that the SCLM beneath the WARS was modified by reactive porous flow with an evolving metasomatic fluid/melt. Widespread Cenozoic rift-related volcanism (