Mineralogical evidence for recycled oceanic crust in the source of continental alkaline basalts

The addition of recycled oceanic crust (ROC) to the source of continental alkaline basalts is considered a key mechanism resulting in their geochemical variability. However, the mechanism by which these ROC materials are involved in the source of continental alkaline basalts remains highly debated. Here, we present mineralogical characteristics and geochemical compositions of clinopyroxene and olivine phenocrysts from the Kula alkaline basalts (KABs) in Anatolia. Our findings provide crucial mineralogical evidence for understanding the process of melting ROC within the source of continental alkaline basalts. Three distinct types of clinopyroxene phenocrysts have been identified in the KABs. Type-1 is homogeneous with no zoning and has an equilibrated OIB-like basaltic host magma, combined with P-T calculation, which is interpreted to form from magmatic crystallization and differentiation processes originating from an enriched asthenospheric source. Type-2 shows reverse-zoning with green-cores (GCPX) and exhibits a geochemical subdivision into two distinct types, denoted as Type-2a and Type-2b. The cores of Type-2a are characterized by low Mg#, elevated contents of Na2O and Al2O3, and enrichment in LREE, LILE (especially Sr) without HFSE (Nb, Ta, Zr, Hf) anomalies. In contrast, the cores of Type-2b exhibit significantly lower Mg# and Al2O3 contents, and depletion in LREE, LILE, HFSE and enrichment in HREE with notable negative Eu, Nb and Ta anomalies. We propose that the cores of Type-2a likely crystallized through an interaction between the ROC melt and the asthenosphere mantle, whereas the cores of Type-2b may have directly precipitated from the ROC melt. The rims of both Type-2a and Type-2b exhibit compositions akin to those of Type-1. Conversely, Type-3 has a normal core-rim zoning structure and its geochemical composition diametrically opposes that of Type-2a. These observations suggest a multistage magmatic process characterized by continuous replenishment. Notably, all three types of clinopyroxenes exhibit similar 87Sr/86Sr ratios (ranging from 0.7033 to 0.7039), indicating their derivation from different stages of the magmatic process without involvement of a metasomatized lithospheric mantle. Trace element modeling also shows that the source of the KABs could be generated by 2-14% partial melting of the metasomatized depleted mantle by ~30% additions of the ROC. Moreover, the olivine phenocrysts in the KABs are mainly characterized by relatively low Ca, Ni content and variable Fe/Mn ratios, consistent with a fertilized peridotite source, supporting the generation of the KABs from an enriched asthenosphere mantle. Our study provides robust mineralogical evidence delineating a continuous transition process, wherein the melt derived from the ROC evolves to an alkali basaltic composition during a multistage magmatic history, underscoring that the melt was not entirely exhausted during its reaction with the asthenosphere mantle.