Modelling arc recycling in the oceanic mantle using radiogenic isotope systems (OCEANS)

Project funded by the Spanish Ministry of Science and Innovation’s Program Proyectos de Generación de Conocimiento 2021

The presence on Earth of continental crust and active plate tectonics are unique features in the Solar System. However, the origin of the continents remains highly debated, hinging on the contribution of arc magmatism to continental growth in subduction zones. This contribution requires the removal of dense lithospheric cumulates of ultramafic pyroxenites, also known as delamination. Despite its importance for continental growth models and the thermo-chemical evolution of the Earth, the impact of such introduction of pyroxenitic heterogeneities in the convective mantle has not been considered. The heterogeneity of the mantle is instead mostly regarded as the result of oceanic subduction. In fact, while the mantle sources of oceanic basalts are known to be chemically and isotopically heterogeneous, the nature and origin of these heterogeneities and the role of arc delamination are unknown. Yet, recycling of arc pyroxenites most likely have dramatic consequences because of their peculiar physico-chemical characteristics, particularly for the isotopic compositions of the basaltic lavas produced in mid-ocean ridges. Furthermore, the presence of arc-derived material in the oceanic mantle is increasingly suspected and recycled arc rocks have recently been discovered in the mid-Atlantic ridge.

The project OCEANS thus aim to explore the exciting possibility that arc delamination could establish an unforeseen link between long-term continental growth and mantle heterogeneity. To that end, OCEANS will target the best pyroxenite exposures from a deep arc section worldwide in the Kohistan region of Pakistan and perform high-precision analysis of two key radiogenic isotopes, the Sm-Nd and Lu-Hf systems. Such analyses, never performed on these challenging rocks, are now made possible by technological developments in multi-collection mass spectrometry (MC-ICP-MS). These data will provide the inputs for a novel modelling approach based on reactive transport and formulations dedicated to pyroxenite melting to simulate the isotopic consequences of pyroxenite recycling in a mixed source. The pyroxenite’s distribution will also be constrained by drone mapping of some of the best mixed peridotite-pyroxenite outcrops in the world: the Cabo Ortegal Complex and the Ronda Peridotite massif of Spain.

The PI, Dr. Tilhac, a JdC post-doctoral researcher at CSIC, has extensive experience in mantle pyroxenites and radiogenic isotope systems, which will be complemented by the expertise of Dr. Bosch, an international expert of the Lu-Hf isotope system. OCEANS will also benefit from the contribution of Dr. Lambart, an experimental petrologist working on pyroxenite melting. Lastly, this project will make use of an isotopic database compiled by the PI as part of a multi-disciplinary program in collaboration with Dr. Begg, a senior geologist and mining consultant who will provide geophysical constraints to the case studies. The project will be carried out at the Instituto Andaluz de Ciencias de la Tierra (IACT), a mixed research unit of the Universidad de Granada and CSIC. The IACT hosts an internationally recognized research group in mantle geochemistry and benefits from world-class analytical facilities essential for the success of OCEANS.