Origin of low-MgO primitive intraplate alkaline basalts from partial melting of carbonate-bearing eclogite sources

Abstract

Alkaline basalts occur widely in intraplate settings and carbonate-bearing mantle sources such as carbonated peridotites are increasingly regarded to play a key role in their formation. Carbonated eclogites, most likely the products of subducted carbonate-bearing altered oceanic crust, are probable alternative ingredients in the mantle sources of many intraplate alkaline basalts, highlighting the importance of the subduction-driven deep carbon cycle. However, this widely proposed hypothesis remains enigmatic because the recognition of low-MgO primitive alkaline basalts predicted by experiments is scarce. Here we show that Cenozoic continental intraplate alkaline basalts occurring above the stagnant oceanic slab in the mantle transition zone beneath the Hannuoba region, eastern China, display geochemical features consistent with their origin as low-degree partial melts of carbonate-bearing eclogites. Their MgO contents correlate positively with CaO, Ba/Th and Ti/Eu, but negatively with Dy/Yb and εNd. Remarkably, the most primitive alkaline basalts are characterized by low MgO (<5.25 wt.%), low heavy rare earth elements and Sc contents, low CaO/Al2O3 (<0.41), low Ti/Eu (<3380), but Dy/Yb (>7.1) higher than those of ocean island basalts (OIBs). These features cannot be ascribed to differentiation from high-MgO alkaline basalts because significant amounts of crystallization of clinopyroxene and garnet did not occur during ascent. Differentiation also cannot account for the correlations of time-integrated Sr-Nd isotopes with MgO, Dy/Yb and Ba/Th. Instead, the linear correlations mainly reflect strong interaction between ascending primitive alkaline melts and the lithospheric mantle. The compositions of primitive alkaline basalts reflect the key control of garnet and clinopyroxene in the mantle residue (eclogites), and the Ti, Zr and Hf anomalies further indicate the critical effect of carbonates in the eclogite source. Partial melting of the carbonate-bearing eclogites likely occurred in the uppermost asthenosphere. The production of alkaline basalts with low MgO contents by partial melting of carbonate-bearing eclogite below the peridotite solidus in an intraplate setting has been overlooked and the magmas were instead often considered to be highly evolved. Recycled altered oceanic crust thus may not only cause metasomatism of the deep mantle but may also serve as a direct source of mafic melts. These results on natural rocks support the experiment-based model for subducted altered oceanic crustal material and also indicate its diverse fate in the mantle.