Geochemistry of Basalts from the Asymmetric Spreading Ridge Segment at 16.5̊N on the Mid-Atlantic Ridge

Abstract:

We report major, trace element, and isotope composition on basalts from the slow-spreading mid-Atlantic Ridge from 16̊25’N to16̊50’N. Spreading is asymmetric, and spreading west of the ridge is dominated by detachment faulting that result in oceanic core complexes with exposed mantle and lower crust. Samples were collected by dredging during the R/V Knorr 205 expedition in the spring of 2013. Samples are from a 60 km by 40 km area, mainly from the west side of the spreading axis. Major and trace element data were obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on approximately 100 basaltic samples. MORB range in Na₂O from 2.5-3.3 wt%, FeO from 8.3-12.8 wt%, SiO₂ from 45-52 wt%, TiO₂ from 1.0-2.2 wt% at 8 wt% MgO, [La/Yb]_C1 values ranging from 0.6 to 2.3 and [La/Sm]_C1 values range from 0.55-1.35. The majority of the analyzed samples are LREE enriched. K₂O/TiO₂ ranges from 0.07 to 0.41 at 8 wt% MgO. Most dredges are relatively uniform in composition, and a few dredges show both LREE enriched and LREE depleted patterns.

In contrast to the enriched trace element nature of the basalts, the isotopic compositions fall towards the depleted end of the MORB range with ⁸⁷Sr/⁸⁶Sr between 0.70234 and 0.70276, ε_Nd between 8.6 and 11.3, ε_Hf between 15.9 and 19.5 and ²⁰⁶Pb/²⁰⁴Pb ranging from 18.16 to 18.96. These isotopic compositions are among the most depleted in the Atlantic. Trace elements and isotopes are generally correlated with the most incompatible enriched basalts, having the radiogenic Sr and Pb isotopes and unradiogenic Nd and Hf.

Compared to the asymmetric spreading segment at 13.3°N on the mid Atlantic Ridge (Wilson et al., 2013), the LIL enrichments at 16.5°N are not as strong, and the isotopic compositions are more depleted. We propose that the observed incompatible element enrichments are a direct effect of low extents of melting of the depleted mantle, in agreement with the observed morphology and geology of the segment.

Reference: Wilson et al. G3 (2013) 14, 979–995, doi:10.1002/ggge.20046.