The Deep Lithosphere Filter and the Growth of Continental Arcs

William Marsh Rice University, Houston TX

Investigators

Abstract

Understanding the processes involved in making continents is of fundamental importance not only to understanding the whole earth system, but also to many aspects of human society. Continental crust is formed at convergent margins where an oceanic plate sinks below a more buoyant continental plate. Melting in the mantle wedge region between these two plates generates magmas that rise and ultimately become the building blocks for new continental crust. Much of the recent scientific focus has been on the shallow aspects of continental arcs, such as explosive eruptions at arc volcanoes and the formation of large ore deposits, given their importance to modern society. However, all of these shallow events are the end-result of processes that begin in the deep region (the lithosphere) beneath continental arcs, an area that has received comparatively less attention. For instance, the arc magmas that are sampled at the Earth's surface are the end products of a complex differentiation path that started where the mantle wedge underwent partial melting. Factors such as water and carbon dioxide content of magmas, which bear directly on the explosiveness of volcanic eruptions, are ultimately controlled by the amounts present in the mantle source. Investigating the deep continental arc lithosphere is challenging, but knowledge of what processes go on deep beneath continental arcs and their relationships to what is observed at the surface is of paramount importance to earth science. Although the deep lithosphere cannot be observed directly, xenoliths (foreign pieces of rock entrained by rapidly ascending magmas) that sample this region offer a unique window into the lower crust and upper mantle. Importantly, mantle xenoliths often represent melt residues, thus they are the complement to arc magmas. Lower crustal xenoliths represent arc magmas that have fractionated at depth ('cumulates') and may provide insights into the early differentiation paths of magmas. Thus, the first objective of this proposal is to evaluate the contribution of deep lithospheric fractionation to magmatic differentiation. In particular, what is the chemical composition of deep cumulates, and what effect does this have on the physical evolution of continental arcs? Because such cumulates are rich in dense minerals like garnet, they may be responsible for de-stabilizing the deep lithosphere beneath continental arcs. The second objective of this proposal is to place the geochemical characteristics of deep lithosphere xenoliths into the broad context of how continental arcs have evolved over time. In particular, how and when does arc lithosphere thicken (or thin)? Is thickening in arcs related to periods of unusually high magmatic flux, and if so, does this place a limit on how rapidly the deep lithosphere can grow beneath the arc before hitting the slab? Answering these questions requires using a consortium of diverse tools, ranging from geochemical instruments to physical modeling. All aspects of the study will contribute to a transformative view of arc and continental crust evolution because deep lithospheric processes were hitherto largely unrecognized as important for crustal differentiation. This research will be conducted by undergraduates and graduate students.