Lithospheric Mantle beneath East Antarctic (Oasis Jetty)

Kogarko L

- Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, Russia

We report petrographic, major and trace (including rare earth) element data for 46 spinel and garnet peridotite xenoliths from East Antarctic (Oasis Jetty) lithospheric mantleљ beneath Lambert-Amery Rift. The age of hosting ultrabasic alkaline dykes is cretaceous (Layba et al.1987). Investigated spinel and garnet peridotiteљ xenoliths representsљ about 80 km thick cross of subcontinental litospheric mantle. High equilibration temperatures (820-1217^о) of the xenoliths suggests influence of the ascending plume.

Distinct correlations is observed between contents ofљ MgO and other petrogenetic elements CaO, Al2O3, FeO, Na2O, Co, Ni, V. Comparison of these correlations with estimated trends of batch and fractional melting of initially fertile mantle peridotite (Niu, 1997) demonstrate, that Jetty peridotites plot at lower values for SiO2 and other components.

Garnets are characterized by unusual high LIL element concentrations (Ba/La=4000) and clinopyroxenes from the spinel peridotite suite are highly enriched in LREE [(La/Yb)_N =14] and in HFSE.

The decoupling of major and REE elements and observed mineral assemblages( which include Sr- and LREE-rich apatite,Ti- Ba-rich phlogopite, titanite, perovskite, henrymeierite) indicate that the upper mantle of East Antarctic has been affected by very strong metasomatism. Henrymeierite was found in mantle material for the first time (Kogarko et al, 2007).

Our isotopic data showed complete absence of isotope equilibrium among the rock forming minerals in the xenoliths – Opx, Cpx and Grt. Garnets are characterised by the most radiogenic Sr isotope composition (0.709024-0.714127), whereas Cpx exhibits the lowest ⁸⁷Sr/⁸⁶Sr ratios ( 0.702861-0.703711). On the one hand garnets are depleted in terms of Sm-Nd system and on the other hand they are significantly enriched in radiogenic Sr. Such anomalous signature in Antarctic mantle rocksљ confirm the influx of metasomatising fluid ( or melt) from the source with a time integrated LIL-elements enrichment such as sources of kimberlites and lamproites (high radiogenic Sr and low radiogenic Nd ) to the initially depleted mantle substrate.

It is probable, that that the interaction of primary dolomitic melts-fluids with mantle material according to reactions :2Mg₂Si₂O₆ + CaMg(CO₃)₂ = 2Mg₂SiO₄ + CaMgSi₂O₆ + 2CO₂ (1)

3CaMg(CO₃)₂ + CaMgSi₂O₆ = 4CaCO₃ +2Mg₂SiO₄+CO₂ (2)

resulted in wehrlitization and carbonatization. The fluids responsible for this metasomatism were probably derived fromљ the plume that arrived beneath the region at this time.

References

1.                    љKogarko L.,Kurat G.,Ntaflos T. Henrynrymeyerite in metasomatised upper mantle of Eastern Antarctica (2007) Can Min V45,part 3

2.                    Laiba, A.A., Andronnikov, A.V., Egorov, L.S. & Fedorov, L.V. (1987): Stock and dyke bodies of alkaline ultrabasic composition in Jetty Peninsula (Prince Charles Mountains, East Antarctica). in Ivanov, V.L. & Grikurov, G.E. (eds) Geological-geophysical investigations in Antarctica.љ Leningrad. 35-47.

3.                    Niu Y. (1997) Mantle Melting and Melt Extraction from Abyssal Peridotites: J.Petrology,V 38,8, 1047-1074.