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

Autor: Fox, J. M., Falloon, T. J., Carey, R. J., Watson, S. J., Duncan, R. A., Olin, P. H., Arculus, R. J., Coffin, M. F.
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Zdroj: Geochemistry, Geophysics, Geosystems: G3; Nov2024, Vol. 25 Issue 11, p1-22, 22p
Abstrakt: 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. Plain Language Summary: Heard Island and the McDonald Islands are active volcanic islands in the southern Indian Ocean that together with the Kerguelen Archipelago were produced by the activity of the long‐lived Kerguelen mantle plume. We dredged lavas from four newly identified sea knolls around the islands. Our geochemical and geochronological analyses of samples from the dredges revealed that the sea knolls around the McDonald Islands are mainly young phonolites of different isotopic compositions from those of Heard Island and the Kerguelen Archipelago. Our new results allow us to distinguish two lava series, the McDonald Series and the Heard Series and provide isotopic evidence for the chemical zonation of the Kerguelen mantle plume since 53.4 Ma. Key Points: High resolution bathymetry obtained around the McDonald Islands on the Kerguelen Plateau reveals > 70 sea knolls for the first timeNewly determined ages and compositions of McDonald Islands sea knolls reveal they are mainly young (<89 Kyr) phonolitic lavasHf and Pb isotopic compositions of McDonald Islands sea knolls provide evidence for compositional zonation of the Kerguelen plume [ABSTRACT FROM AUTHOR]
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