Continental Lithospheric Source for Oceanic Island Magmas Depleted by Ta, Nb, Th and U

A. Yu. Borisova Vernadsky Institute of Geochemistry, Kosygin str., 19, Moscow, 117975, Russia


According to the reconstructions (Paul et al., 1990; Sborshov et al., 1991) the Aphanasey Nikitin Rise was constructed on juvenile oceanic crust of Late Cretaceous age close to Indian ridge. Nevertheless the olivin-porphyritic suite of the Aphanasey Nikitin Rise have unusual for oceanic magmatism geochemical features: relative depletions of Ta, Nb, Th, U, Ti relative to REE and Rb relative to the adjacent Ba.

Thus the goal of this work is the comprehensive understanding of the manner of these elements depletions in oceanic island magmas.

Samples and analytical methods

The Aphanasey Nikitin Rise basalts were collected in 20th cruise of scientific vessel "Mst. Keldusch" by submersible apparatus "MIR" in 1990 (Sborshov et al., 1991).

Mineral and glasses analyses were conducted using a CAMEBAX - Microbeam microanalyser at VIG, Moscow and Cameca IMS 4f according to technique details (Shimizu and Hart, 1982). The major element and Nb, Y, Zr contents analysed on whole rock powders using XRF at GEOMAR, Kiel. The concentration of Rb, Sr, Nd, Sm and Sr, Nd isotopic composition were analyzed by (isotope dilution-) thermal ionization mass spectrometry using standard procedures (Sushevskaya et al., 1996) in IGGP, St. Peterburg. Concentration of Th, Ta, Hf were measured by INAA in GI, Moscow and the other were published (Almuchamedov et al., 1993).


Composition of the spinel inclusions and melt inclusions in olivine microphenocrysts correspond to typical for oceanic tholeiite. Estimated T-FO2 parameters of primary melt differentiation correspond to MW buffer equilibrium and are characteristic of oceanic tholeiitic melts differentiation conditions. However, high K2O(1-1.6wt%), P2O5 (0.3-0.5wt%) contents (while MgO=9-10wt% content) of estimated parental melt are characteristic of nor oceanic tholeiites neither enriched tholeiites of Indian Ocean. Moreover high level of REE contents ((La/Yb)n = 5.9-6.5) correspond to that of most enriched P-type Indian MORB and Indian OIB. Composition of estimated primary melt is characterised by high La/Ta (32-38) and La/Nb (2.0-2.2) ratios which are characteristic of the most Ta and Nb depleted of Kerguelen Plateau basalts. The melt show highest La/Th (16-21) and La/U (46-74) ratios relative to those of plateau basalts of Indian Ocean however the melt is characterized by near-primitive mantle Th/Ta (1.6-2.3) value.


Depletions of the melt could have one of three probable origins. The Ta and Nb depletions was caused by processes connected with the influence of sub-continental (continental) lithospheric layer. The latter can be represented either depleted harzburgite (Kelemen et al., 1993), either metasomatized peridotite with minor contents of rutile or phlogopite (Bodinier et al., 1995; Grenough, 1988). But such processes usually fractionate only Ta and Nb from La but do not Th and U from La. Otherwise the anomalous geochemical features of the melt could correspond to anomalous composition of plume substance represented by mantle part of continental lithospheric mantle. But composition of continental lithospheric mantle does not possess the necessary composition for the melt (McDonough, 1990). At last, Th, U and Rb depletions of the melt could be caused by contamination of lower continental crust. In fact, it is established that substantial loss of Rb, U and Th occurs during granulite facies metamorphism (Taylor and McLennan, 1985). Indeed, continental flood basalts contaminated by lower continental crust (Thompson et al., 1983) possess all depletions of the melt.

Thus the most plausible model of the melt depletions origin is model close to continental flood basaltic melts forming. In fact, primary melt and liqidus phases compositions suggest that general source of the melt was astenosphere source. Whereas the anomalous features of the melt infer necessary influence of both lower continental crust by contamination (fractionation: Th, U, Rb) and continental (subcontinental) lithosphere mantle by transformation of melt during its ascent through lithospheric mantle (fractionation: Ta, Nb). In this case it is nececcary to suggest that the melts of oceanic structures possessing such geochemical features could be constructed on the fragment of continental lithosphere.


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