Petrology of Fresh Submarine Lavas from Recent Phonolitic Volcanism on the Central Kerguelen Plateau, Southern Indian Ocean

The Kerguelen mantle plume underlies the volcanically-active Heard and McDonald islands on the Kerguelen Plateau, southern Indian Ocean. Basanite-trachybasalt-trachyte lavas erupt at the Big Ben volcano on Heard Island. Three volcanic islands; (McDonald Island, Meyer Rock, and Flat Island) comprise the McDonald islands, active between 1992-1997. Spatial, temporal, and geochemical patterns of volcanism across the Kerguelen Plateau since 30 Ma are complex.

In 2016, multibeam bathymetry revealed of >70 submarine sea knolls surround the McDonald Islands. Rocks dredged from the sea knolls included fresh vesicular phonolite lavas, phonolite obsidian, and phonolite pillow fragments. Phonolites on the McDonald Islands produced K/Ar ages of <82 ka (Clarke et al 1983); the sea knolls have ages 44 ka to 18 ka (⁴⁰Ar/³⁹Ar).

A limited range of MgO contents (3 to 1 wt%) is present. Significant enrichments and depletions in both major and trace elements exist as MgO decreases, due to fractional crystallisation: e.g., Zr increases from 740 to 2400 ppm, whereas Rb decreases from 1370 to 150 ppm. Ratios of highly incompatible trace elements such as Ce/Pb) decrease from 11 to 5 but changes of isotopic ratios are small (⁸⁷Sr/⁸⁶Sr increases from 0.70570 to 0.70572 and eNd decreases from -1.66 to -1.60).

New Sr, Nd, Pb, and Hf isotopic results from the submarine phonolites are within the range of lavas from McDonald Island reported by Barling et al. (1990). The Sr and Nd phonolite data are also within the range of Heard Island lavas but have lower ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, suggesting different plume components are involved at Heard compared with McDonald.

The geochemistry of the McDonald Island and adjacent submarine phonolites requires a distinct mantle plume source and parental magmas that are not present at Heard Island. Our results demonstrate the current expression of the Kerguelen plume source is heterogeneous, with McDonald phonolites also experiencing prolonged fractional crystallisation processes.