Leonie Sieger, Bradley Peters, Andrea Giuliani, Maria Schönbächler

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 paper 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 St. Helena and Mangaia, however since there is no mantle plume underlying Isla Isabel it is unlikely that these signatures derive from recycled oceanic crust. We argue that Isla Isabel lavas were instead produced by mixing of depleted mantle-like material and continental lithospheric mantle that was metasomatized ≥600 Ma ago, an age that overlaps with the regional breakup of Rodinia. Such preservation of ancient tectonic events is remarkable, since the exposed geological record in continental México preserves a very 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.