Abstract:
This study is a geochemical investigation ofthe evolution of the Kerguelen plume, on the
basis of upper mantle and lower crustal xenoliths. Ultramafic xenoliths include harzburgites
predominant, a lherzolite, dunites and pyroxenites, whereas lower crustal xenoliths are
cumulate gabbros recrystallized under granulite facies conditions. On the basis of the whole
rock major element characteristics and trace element abundance patterns in clinopyroxenes,
the harzburgites were found to be residues of extensive melting at high pressures within the
Kerguelen plume. These were then recrystallized at low pressures and metasomatized by
plume generated melts. Details of the metasomatic process were determined from trace
element variations in clinopyroxene in connection to texture. This demonstrated that meltrock
reaction and the precipitation of new clinopyroxenes occurred by metasomatic
carbonatitic melts.
It was also found that some of the harzburgites had distinctly unradiogenic Os isotopic
compositions and were identified as originating from the sub-Gondwanaland lithosphere.
On the basis of major and trace element compositions, the granulite xenoliths were found to
be originally gabbroic cumulates formed from plume-derived basaltic melts emplaced at the
base of the crust by underplating and subsequently recrystallized isobarically under
granulite conditions. The Sr, Nd and Os isotopic compositions of the peridotite and
granulite xenoliths demonstrate that the Kerguelen plume is isotopically heterogeneous and
displays a temporal progression toward more enriched Sr and Nd isotopic compositions
from the Ninetyeast Ridge to granulite xenoliths to Kerguelen basalts and Heard Island
basalts.
