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Journal of Petrology | Volume 43 | Number 7 | Pages 1367-1387 | 2002
© Oxford University Press 2002
Flood Basalt from Mont Tourmente in the Central Kerguelen Archipelago: the Change from Transitional to Alkalic Basalt at 
25 Ma
1DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
2DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF MASSACHUSETTS, AMHERST, MA 01003, USA
3DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, CP 160/02, UNIVERSITÉ LIBRE DE BRUXELLES, AV. F. D. ROOSEVELT, 50, B-1050, BRUSSELS, BELGIUM
4LABORATOIRE DE GÉOLOGIE, UNIVERSITÉ JEAN MONNET, CNRS–UMR 6524, 23 RUE DU DOCTEUR PAUL MICHELON, 42023, SAINT-ÉTIENNE, CEDEX 2, FRANCE
The surface of the Cenozoic Kerguelen Archipelago, constructed on the Kerguelen Plateau in the southern Indian Ocean, is dominantly flood basalt. With the objective of understanding the Cenozoic history of the Kerguelen mantle plume, the age and geochemical characteristics of this flood basalt province are being determined by studying stratigraphic sections of basalt flows at several locations. Sections from the NW, north–central, east and SE parts of the archipelago have been studied. Here we report results for a 597 m succession of lavas from Mont Tourmente from the Plateau Central, a region of the archipelago that has not been studied in detail. Mont Tourmente lavas range from 
26 Ma dominantly transitional basalts in the lower 80% of the section to 25·3 Ma dominantly alkalic basalts in the upper part of the section. The timing of this change from transitional to alkalic volcanism within the Mont Tourmente section is consistent with that defined by the older 28–29 Ma transitional basalts in the north and the 25 Ma alkalic lavas erupted in the east. This change in basalt composition may be related to migration of the archipelago away from the plume or to increasing lithosphere thickness over the 5 Myr of flood basalt volcanism. The alkalic and transitional Tourmente lavas are nearly homogeneous in isotopic ratios of Sr, Nd and Pb. They have lower 87Sr/86Sr and higher 143Nd/144Nd compared with lavas from the eastern sections, which have been proposed to be characteristic of the Kerguelen plume. Consequently, the Mont Tourmente isotopic data may reflect heterogeneity within the plume or a constant proportion of a depleted component mixed with the high 87Sr/86Sr and low 143Nd/144Nd plume. In contrast to many of the Cretaceous Kerguelen Plateau lavas, there is no evidence in trace element abundances or Sr and Nd isotopic ratios that the Cenozoic Kerguelen Archipelago lavas were influenced by continental lithosphere.
KEY WORDS: Kerguelen Archipelago; Kerguelen plume; flood basalt; igneous geochemistry
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