Seminar Rachel Bezard

The mantle, as sampled by oceanic basalts, exhibits significant chemical and radiogenic isotope heterogeneity, most of which is commonly attributed to the recycling of crustal material into this reservoir over Earth’s history. However, the nature, age, distribution, and recycling conditions of these components remain strongly debated. Moreover, alternative processes such as intra-mantle metasomatism have been proposed to partly, or even entirely, account for mantle compositional diversity. Because the type, depth, and conditions of crustal recycling are linked to Earth’s tectonic regime, robust identification and characterization of recycled crust are essential for developing a clearer picture of Earth’s geodynamic evolution.

Over the past decade, stable molybdenum (Mo) isotopes have emerged as a promising tracer for this purpose. In this talk, new and published Mo isotope data for oceanic basalts are presented and evaluated. The ability of the Mo isotope system to trace and characterize recycled crustal components is first illustrated, before discussing the role of crustal recycling in generating the observed radiogenic isotope variability of the mantle. Finally, the Mo isotopic compositions of oceanic basalts are shown to provide new constraints on the nature, and, especially, the conditions under which crustal material was recycled into mantle sources during the Precambrian.