Investigation of Ancient and Modern Melting Processes Preserved in Sub-oceanic Island Mantle

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Plate tectonic processes occurring at the surface are ultimately governed by convection occurring within Earth's mantle. Understanding mantle convection processes requires a fuller understanding of both compositional and thermal variations in the mantle. This project aims to address compositional variations in the mantle, through the study of mantle fragments, known as "xenoliths", erupted in lavas from the Canary Islands. These xenoliths preserve evidence that the mantle from which they originated experienced ancient melting events (>>1 Ga), but that the chemical evidence for these events have been partly obscured by later modification of the xenoliths during incorporation and eruption within the lavas. Goals of the project are to develop methods to identify ancient and modern melting processes preserved in oceanic mantle beneath ocean islands, like the Canary Islands, and to use this new information to understand compositional variation in the Earth's upper mantle. A more refined understanding of when and how mantle heterogeneities are formed and, in some cases, destroyed has profound impact on our understanding of mantle melting processes, melt composition and physical behaviour of the mantle. Science questions will be addressed by measuring Nd-Hf-Os isotopes, highly siderophile elements and major- and trace-element abundances in a suite of xenoliths to understand the long-term evolution and HSE distribution in the mantle and to examine processes involved in generation and preservation of heterogeneous mantle. The new data will also be used to evaluate melt interactions and metasomatism in samples. Our methods will test hypotheses for the formation, evolution and destruction of depleted mantle domains in oceanic or continental environments. This will include comparison of modern melting processes in the islands with possible ancient melting processes preserved in the xenoliths, to assess models for the generation of refractory mantle in subduction-zone, plume or ridge settings, or as global-scale melting events, and to explore models for highly-depleted, refractory mantle domains as sources of distinct isotopic and elemental signatures recorded in some intraplate magmas.