Plume-lithosphere interaction : geochemical evidence from upper mantle and lower crusal Xenoliths from the Kerguelen Islands

Abstract

This study is a geochemical investigation ofthe evolution of the Kerguelen plume, on the basis ofupper 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 ofthe metasomatic process were determined from trace element variations in clinopyroxene in connection to texture. This demonstrated that meltrock reaction and the precipitation ofnew clinopyroxenes occurred by metasomatic carbonatitic melts.It was also found that some ofthe 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-derivedbasaltic 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. 4Acknowledgments

An integrated study of microstructural, geochemical, and seismic properties of the lithospheric mantle above the Kerguelen plume (Indian Ocean)

“…Some other harzburgites from Kerguelen Islands plot off this trend and show higher Mg# of olivine for low olivine modal abundances. This deviation from the oceanic trend and the enrichment in orthopyroxene of these harzburgites have been explained as a result of partial melting in the stability field of spinel although at a higher pressure under which, orthopyroxene is stable during melting (>1.5 GPa [ Hassler , 1999]). In addition, the high Mg# of mineral phases (and Cr# for spinel, Table 1), the depletion in basaltic components (Al 2 O 3 , Na 2 O, TiO 2 , FeO; Figure 3) and the low HREE contents in clinopyroxenes (Figure 4) are characteristic features of a lithospheric mantle that has undergone extensive partial melting, close to exhaustion of clinopyroxene (>20% [ Grégoire et al , 1997, 2000b]).…”

“…Mg# of olivine versus modal olivine for harzburgites and dunites from Kerguelen Islands after Boyd [1989]. Data for Kerguelen [ Hassler , 1999; Grégoire et al , 2000a, 2000b; this study]; Samoan xenoliths from Hauri and Hart [1994] and Canary Islands xenoliths from Wulff‐Pedersen et al [1996]. The dashed line represents the evolution of the Mg# of olivine and the olivine modal content of a peridotite undergoing progressive partial melting at relatively shallow depth in the lithosphere (<1.5 GPa [ Boyd , 1989]).…”

The subantarctic lithospheric mantle

“…6). Two samples (one anhydrous dunite and one Phlbearing clinopyroxenite; Mattielli et al, 1999;Hassler, 1999) show very radiogenic 87 Sr/ 86 Sr (0.70730 and 0.70869) and unradiogenic 143 Nd/ 144 Nd (0.51215 and 0.51199) and are isotopically very different from the Kerguelen basalts, as they extend towards Sr-Nd isotopes found in continental crust material…”

“…Sr-Nd + Pb isotopes analysed for bulk rock (n=13) and clinopyroxene separates (n=10) from harzburgites, dunites, two lherzolites and one clinopyroxenite show very heterogeneous compositions (Fig. 6; Mattielli et al, 1996Mattielli et al, , 1999Hassler, 1999) (Mattielli et al, 1996(Mattielli et al, , 1999Hassler, 1998). Peridotites showing fingerprints of metasomatism by carbonatitic melts seem to extend to more enriched Sr-Nd isotopic compositions than those metasomatized by alkaline basaltic silicate melts.…”

“…6). A few samples show extreme Sr-Nd isotopic compositions; the most "depleted" end of the spectrum is represented by one little-metasomatized harzburgite that has clinopyroxene with unradiogenic 87 Sr/ 86 Sr of 0.70329 (OB93-78; Hassler, 1999), which falls in the field of Indian MORB (Fig. 6).…”