Lithospheric mantle heterogeneity across the continental-oceanic transition, northwest Ross Sea, Antarctica: new evidence from oxygen isotopes

Oxygen isotopes and whole rock chemistry from alkali basalt and basanite in the northwest Ross Sea, Antarctica offer new insight on source heterogeneity across the transition from continental to oceanic lithosphere in a magma-poor rifted margin. In situ SIMS analysis of olivine (Fo 79-90) from the most primitive lavas (MgO ≥ 8 wt%, Mg# 53-70, Ni= 115-338 ppm, Cr= 244-540 ppm) yield an average δ¹⁸O = 5.18 × 0.60 ‰ (2σ, n=30) for alkali basalt and 5.25 × 0.44 ‰ (2σ, n=52) for basanite (× 0.28 ‰, 2σ precision on a homogeneous olivine standard). These are similar to the range for olivine from mantle peridotite and HIMU type oceanic basalts (δ¹⁸O= 5.0 to 5.4 ‰ and 4.9 to 5.2 ‰, respectively [1]), but with greater variability. Lavas in this region experienced little differentiation, have minimal evidence of crustal contamination (⁸⁷Sr/⁸⁶Sr < 0.7030, ¹⁴³Nd/¹⁴⁴Nd > 0.5129), and olivine show no correlation between δ¹⁸O and Fo content, further suggesting that the δ¹⁸O values are source related. Whole-rock chemistry of alkali basalt and basanite are spatially distributed. In general, alkali basalt is found in thicker continental lithosphere with lower Sr (477-672ppm) and Nb/Y (1.2-2.4) than basanite. Basanite is found in oceanic and thinned continental lithosphere with higher Sr (642-1131 ppm) and Nb/Y (2.4-3.6). Variation in degree of silica-undersaturation and Nb/Y can be explained by varying degree of partial melting. While alkali basalt and basanite can result from varying degrees of partial melting of similar source compositions, the presence of amphibole in mantle xenoliths have lead workers in this region to propose contributions from a metasomatic source [2, 3, 4] with variable ²⁰⁶Pb/²⁰⁴Pb ratios [5]. A negative correlation between Nb/Y and δ¹⁸O in both rock types suggests that varying degrees of partial melting are tapping sources with different δ¹⁸O values; lower degree melts have δ¹⁸O ≤ 5.0 ‰ and higher degree melts have δ¹⁸O > 5.3 ‰. Based on this evidence we propose three different mantle source domains in this region: 1) lower δ¹⁸O metasomatic veins in the continental and oceanic lithospheric mantle, 2) ambient continental lithospheric mantle with lower Sr and higher δ¹⁸O (for alkali basalt), and 3) ambient oceanic lithospheric mantle with higher Sr and δ¹⁸O akin to continental lithosphere (for basanite). Low degree melts will preferentially tap the more easily fusible veins while higher degree melts incorporate greater proportions of the adjacent ambient mantle. [1] Eiler (2001) RiMG, 43(1), 319-364. [2] Gamble and Kyle (1987) J. Petrol., 28(5), 755-779. [3] Coltorti et al. (2004) Lithos, 75(1), 115-139. [4] Perinelli et al. (2006) CMP, 151(3), 245-266. [5] Castillo et al. (2011) AGU Fall meeting.