Basalts from the intratransform domain at the Doldrums Megatransform (7-8° N MAR) reveal mantle re-melting

In the Equatorial Mid Atlantic Ridge at 7-8°N the Doldrums Fracture Zone (FZ) is located: a complex transform system characterized by four ‘intratransform’ ridge segments (ITRs) bounded by five active transforms (Skolotnev at el., 2020). We determined major element, trace element and Sr, Nd, Hf and Pb isotope compositions for on-axis lavas erupted along the entire fracture domain. Basalts from the central ITRs are selectively enriched in alkalis (Na2O+K2O= 4.3 wt%; Na8 up to 3.7) and LREE (La/SmN=0.86 -0.97), in agreement with low melting degrees in a cold intra-transform regime. Despite this, they have the most depleted Sr and Pb isotope compositions in the entire Equatorial MAR (i.e. 87Sr/86Sr = 0.70237 and 206Pb/204Pb = 18), coupled with comparatively less depleted Nd and Hf isotope compositions, which instead plot in the field of regional MORB (143Nd/144Nd = 0.5131-0.5232; 177Hf/176Hf = 0.2831-0.2832). Hence, the magmatism at the ITRs was predominately sourced by a depleted mantle source, melted at very low melting degrees. On this basis we suggest that the upper mantle currently melting beneath the central portion of the Doldrums FZ has previously undergone partial melting at the MAR axis, consuming the most fertile, isotopically enriched heterogeneities.

Skolotnev, S.G., Sanfilippo, A., Peyve, A., Muccini F., Sokolov, S. Y., Sani, C., Dobroliubova, K.O., Ferrando, C…& Ligi, M., 2020. Large-scale structure of the Doldrums multi-fault transform system (7-8ºN Equatorial Atlantic): preliminary results from the 45th expedition of the R/V AN Strakhov. Ofioliti, 45(1), 25-41.