Oceanic Crust Components in Intra-Continental Basalts: From the Core-Mantle Boundary or the Mantle Transition Zone?

Abstract:

Recent studies have revealed the ubiquitous oceanic crustal components in Cenozoic intra-continental basalts in eastern China (Zhang et al., 2009; Xu et al., 2012a, b). The main arguments in support of this include: (a) strong similarity of trace element patterns and Fe/Mn ratios of these continental basalts with respect to oceanic island basalts; (b) coupled high Fe₂O₃and water contents and low⁸⁷Sr/⁸⁶Sr ratio; (c) high d¹⁸O less than mantle values recorded in olivine and clinopyroxene phenocrysts; (d) pyroxenite-dominated source as constrained by comparison of olivine-hosted melt inclusions with experimental results. Whether these recycled oceanic components from the core-mantle boundary (CMB) or from the mantle transition zone is a matter of debate. In the case of eastern Asian continent, we favor for a provenance from the mantle transition zone given the following considerations: (a) Geophysical studies show a stagnantPacific slab within the mantle transition zone underneath the studied region. Such a sandwich structure prevailed at least since late Cretaceous thus ruling out the possibility that oceanic crustal components from the CMB; (b) Pb isotopes do not show the high time-integrated ²³⁸U/²⁰⁴Pb mantle component expectedfor a HIMU basalt, suggesting a young age of recycled components. (c) The same Indian MORB-like isotopiccomposition is found in the eastern China Cenozoic basalts and in the extinct Izanaghi–Pacific plate ofNWPacific. This suggests that the recycled oceanic components may have been derived from the seismically detected stagnantPacific slab within the mantle transition zone. We thus speculate that not all recycled oceanic components are carried to the surface by mantle plumes, but can be transported to the shallow level by other processes from the mantle transition zone.