Basalts from MAR at 13°15′–13°40′N: What mixed?

We present new data on chemical composition of basalt glasses collected within the geochemical anomaly of the Mid-Atlantic ridge at 13°25′–13°40′N, where fresh quenched lavas occur dominantly in asymmetric off-axial structures. The collected samples indicate wide compositional ranges from strongly depleted D-MORB with 0.04 wt% K₂O, K₂O/TiO₂ = 0.08 and (La/Sm)_N = 0.69 to the most enriched E-MORB with 0.92 wt% K₂O, K₂O/TiO₂ = 0.42 and (La/Sm)_N = 2.45. Clear correlations are traced between incompatible element ratios (K₂O/TiO₂, La/Sm, Nb/Zr), concentrations of MgO, K₂O, H₂O, and ⁸⁷Sr/⁸⁶Sr ratio, suggesting variable contribution to the compositional diversity of the glasses from two distinct source materials, one of which is depleted mantle. Testing the hypothesis of pyroxenite-derived melts as an enriched end-member failed to reproduce the observed REE patterns, (Sr/Zr)_N, and (Ba/Nb)_N ratios, and the K₂O–K₂O/TiO₂ relations. Two alternative hypotheses – melts derived from melting remnants of continental crust (1) or oceanic core complex (OCC) gabbro (2) – are suggested based on the complementary character of the REE patterns of the basalt glasses and partial melts compositions calculated at 1000 °C/50 MPa from either bulk continental crust as estimated by Taylor and McLennan (1995) (1) or OCC gabbro sampled in the study area (2). Complementarity of the two sources with the D-MORB holds also for the other geochemical parameters: (Sr/Zr)_N vs (Ba/Nb)_N, (La/Sm)_N vs (Rb/Nb)_N, and for the K₂O–K₂O/TiO₂ relations. The first hypothesis calls upon partial melting of continental crustal material preserved in the vicinity of ridge-transform fault system, while the second suggests melt-rock interaction within the off-axial structures of the pre-existed gabbro assemblages, with concurrent melting of the wall-rock material and dissolution of the resulting melt in the intruded magma. Both scenarios, operating either separately or jointly, might contribute to the MORB geochemical variability documented in the present study.