Precambrian mantle metasomatism preserved by the Nd-Hf isotopic compositions of modern HIMU-type intraplate basalts

Abstract

The presence of HIMU (high-²³⁸U/²⁰⁴Pb) signatures in ocean island basalts has long been used to argue that ancient oceanic crust has been tectonically recycled into the mantle sources of plume-derived volcanic hotspots such as St. Helena or Mangaia. However, alternative hypotheses regarding the origins of HIMU signatures have also been put forward. This study addresses the origins of HIMU-like Pb isotopic signatures in Isla Isabel, a small (∼1 km²) intraplate volcanic island located off the western coast of México, southeast of the southern tip of Baja California. The Nd-Hf isotopic signatures of Isla Isabel are nearly identical to the classical HIMU-type hotspots St. Helena and Mangaia; however, the physical and geochemical characteristics of Isla Isabel are inconsistent with the derivation of its lavas from deeply recycled oceanic crust. We argue that Isla Isabel lavas were instead produced by mixing of local depleted mantle and continental lithospheric mantle that experienced a prior enrichment event, likely in a ≥ 600 Ma-old paleo-subduction setting. Such preservation of ancient tectonic events is remarkable, since the exposed geological record in continental México preserves only a limited record of geological events older than the Mexican Cordillera (<165 Ma). Isla Isabel, therefore, illustrates that the origins of HIMU-type intraplate lavas are not limited to ancient recycled oceanic crust. Rather, they can also preserve information about the evolution of the upper mantle through large-scale tectonic cycles, even when these events have been otherwise erased from the surficial rock record.