McDonald Islands Phonolitic Lavas: Evidence for Zonation of the Kerguelen Plume

The McDonald Islands, together with Heard Island and the Kerguelen Archipelago, are volcanic islands on the mostly submerged Kerguelen Plateau, and the products of the long-lived Kerguelen mantle plume (at least 130 Myr; Coffin et al., 2002, https://doi.org/10.25919/jw5f-ad35). The first multibeam bathymetry data acquired around the Heard and McDonald islands reveal > 70 sea knolls surrounding the McDonald Islands and three sea knolls north of Heard Island. Rocks dredged from McDonald Islands sea knolls include fresh vesicular phonolitic lavas, phonolitic obsidian, phonolitic pillow fragments, and one basanite. These are the first phonolites sampled from the seafloor on the Kerguelen Plateau. Dredging of one sea knoll north of Heard Island recovered basaltic lavas. Lavas from the sea knolls are young, returning ⁴⁰Ar/³⁹Ar plateau ages of 73.7 ± 15.1 ka to 7.0 ± 2.7 ka for McDonald Islands sea knoll phonolites and 9.0 ± 1.3 ka for the Heard Island sea knoll. We define a new magma series, the McDonald Series, characterized by low εHf (−3.9 to −4.4) and lower Δ²⁰⁷Pb/²⁰⁴Pb (4.5–4.8) and Δ²⁰⁸Pb/²⁰⁴Pb (79–85) than all other lavas on the Kerguelen Plateau. This newly defined series is the product of a relatively young (Pleistocene-Holocene) phase of volcanism produced by a distinct component of the Kerguelen mantle plume. We propose that McDonald Series phonolites together with 53.4 Ma lavas previously dredged from Ninetyeast Ridge provide evidence for zonation of the Kerguelen mantle plume.