Disentangling Partial Melting and Crustal Recycling Signatures in Ocean Island Basalts With Multivariate Statistics

Abstract

Ocean island basalts (OIB) provide valuable constraints on mass exchange between the mantle and crust. When appropriate compositional data transformations are applied, multivariate analyses of OIB compositions can be used to infer mantle source heterogeneity and elucidate its driving processes. We apply Principal Factor Analysis (PFA) to log-ratio transformed global OIB data, resolving latent variables (“factors”) which show that melt degree and recycled continental crust contributions are the dominant factors shaping OIB compositions. Recycled oceanic crust (ROC) signals are initially masked by variable degrees of partial melting. But by filtering for samples formed under similar melting conditions and reapplying PFA, we resolve an ROC factor, distinct upper and lower continental crust (UCC and LCC) factors, a depth-of-melting factor, and a factor unique to Hawaii. Visualizing OIB samples in factor space (“biplots”) establishes a new framework for evaluating crustal recycling processes. In general, continental materials (UCC + LCC) are subducted together with oceanic crust and overwhelmingly shape the record of mantle heterogeneity sampled by OIB. However, rare geodynamic conditions may promote recycling of atypical combinations of recycled materials. The Azores, for example, uniquely sample mantle with UCC (+ROC) components devoid of LCC, while St Helena and Austral-Cook (canonical “HIMU”) reflect the striking absence of recycled continental input. Moreover, Northern and Southern Hemisphere hotspots appear to sample different combinations of continental materials. Overall, the proportion and type of continental materials in OIB mantle sources apparently depends on their availability in a given paleotectonic environment and the physicochemical parameters of past subduction.