I find it fascinating that a single rock has the ability to preserve geochemical imprints over billions of years, and with that it also preserves the history of Earth’s dynamic evolution. Radiogenic isotopes provide a means to access this information and shed light on the dynamic processes occurring beneath Earth’s surface. I am especially interested in using modern analytical techniques to measure Sr-Nd-Hf-Os-Pb isotopes and trace elements in peridotites, which record processes in Earth’s mantle.
My current research involves using a suite of analytical techniques to investigate the geochemical and dynamic evolution of the subcontinental lithospheric mantle (SCLM) beneath the West Antarctic Rift System. The SCLM undergoes compositional changes during tectonic events such as continental rifting, subduction, and orogeny, with each event leaving behind a geochemical imprint. Geochemical mapping of the lithosphere allows us to gain a better understanding of how the continents are structured, and enables us to piece together the tectonic evolution of the continents. Here we look at the Western Ross Sea area of the West Antarctic Rift System (WARS) as a case study.
My thesis focuses on using these geochemical imprints to unravel the geochemical and tectonic evolution of the SCLM in the WARS:
- Os isotope ratios can be used to date the melt depletion events in the asthenosphere that are considered to be equivalent to the stabilization age of the lithospheric mantle. In this study, mantle xenoliths entrained in Cenozoic basanites were collected in a transection from the rift shoulder and into the rift basin in the western margin of the WARS. Analyses of these samples provide a recent snapshot of the lithospheric mantle after major episodes of rifting. If we can trace the age of the lithospheric mantle across this margin, we can begin to understand its behavior in response to rifting and subsequent structure. We repeatedly observe ~1.7 Ga depletion events across the rifted margin, indicating the widespread Paleoproterozoic stabilization of the lithosphere and dynamic thinning of lithosphere across the western margin of the WARS.
- Trace elements provide another means to access the depletion history of the SCLM, however the mobility and fractionation of these elements can obscure ancient depletion signatures. Nevertheless, we can exploit the behavior of these trace elements. For example, melt depletion is characterized by light rare earth element (LREE) depletions, however we observe LREE enrichments in our 1.7 Ga xenoliths. Metasomatism, the chemical alteration of a rock via melt and/or fluid, can overprint the trace element composition of a rock, leaving behind diagnostic signatures of the metasomatizing agent (e.g. LREE re-enrichment by a carbonatitic melt).
- Sr-Nd-Hf-Pb isotopes can be used to constrain the role of the SCLM in the formation of WARS volcanism, as well as date metasomatic events the lithosphere. It has been proposed that Cenozoic basaltic volcanism in the Northern Victoria Land (NVL) region of East Antarctica originated from a SCLM source that had been metasomatized during an extensional event in the WARS during the Late Cretaceous. Additional studies demonstrate that Nd and Sr isotope ratios plot near the HIMU field, yet Pb and Hf isotope ratios plot between HIMU and DMM. This can be accounted for by varying proportions of lithospheric mantle source that has experienced Paleozoic to Mesozoic modification due to subduction related fluids and flux melting which can incorporate fluid mobile elements such as Pb from the mantle wedge. By measuring these same isotope systems on mantle xenoliths, we can begin to address whether SCLM is a source for the widespread Cenozoic volcanism in the WARS and when it acquired characteristic isotopic signatures.