Unusually large Nb [sbnd]Ta depletions in North Chile ridge basalts at 36°50′ to 38°56′S: major element, trace element, and isotopic data

Abstract

We present new major and trace element data for 28 basalts and 10 basaltic glasses recovered from 16 locations from the North Chile Ridge (NCR) at 36°50′ to 38°56′S. This part of the Chile Ridge consists of three short ridge segments, which are characterized by deep (3200–4100 m) axial valleys. Chemical compositions of the basaltic glasses vary from primitive to moderately fractionated basalts (MgO = 9.54−7.28 wt%). All rocks are incompatible element depleted, mid-ocean ridge basalts (MORB) with average chondrite-normalized (La/Sm) N ratios of 0.46 ± 0.08. The radiogenic isotopic ratios of 10 representative samples display a narrow range in 87Sr 86Sr ratios from 0.70241 to 0.70249. 143Nd 144Nd ratios also vary within a small range from 0.51312 to 0.51318, and the 206Pb 204Pb ratios range from 18.2 to 18.6, with one exception which has a 206Pb 204Pb of 19.1. Overall, isotopic compositions are similar to average depleted MORB from the EPR (East Pacific Rise) and MAR (Mid-Atlantic Ridge), but 207Pb 204Pb ratios are significantly higher. Pb isotopic systematics of East Pacific MORB reflect large-scale heterogeneities, which are probably the result of long-lived differences in Th/U ratios in the mantle. Significant differences exist in the inferred primary melt compositions between the NCR basalts and depleted MORB from the South EPR and Galapagos Spreading Centre (GSC). For a given MgO content, basalt glasses from the NCR have systematically higher Na and Ti and lower Ca concentrations than those from the South EPR and the GSC. This has been interpreted as indicating relatively low average degrees of melting, which is possibly the result of cooling the shallow asthenosphere near transform offsets. NCR basalts are, on average, more primitive than basalts from the EPR and GSC, implying the lack of a robust magmatic system in this part of the Chile Ridge. This, together with the characteristic ridge topography and short average segment length, suggests that the magmatic system is short-lived, similar to slow-spreading ridges. This can be attributed to lower upwelling rates and, consequently, low magma supply rates near the transform offsets. The majority of the basalts from the NCR are unusually depleted in Nb and Ta relative to average depleted MORB. Trace element modelling shows that the distinct trace element characteristics of the NCR lavas could be the result of melting a mantle which has experienced a previous melting episode. This episode was possibly related to upwelling and melting of the mantle beneath the Pacific-Farallon ridge more than a million years ago.