Petrology and geochemistry of South Mid-Atlantic Ridge (19°S) lava flows: Implications for magmatic processes and possible plume-ridge interactions

The South Mid-Atlantic Ridge (SMAR) 19 degrees S segment, approximately located along the line of Saint Helena volcanic chain (created by Saint Helena mantle plume), is an ideal place to investigate the issue whether the ridge-hotpot interaction process affected the whole MAR. In this study, we present major and trace elemental compositions and Sr-Nd-Pb isotopic ratios of twenty fresh lava samples concentrated in a relatively small area in the SMAR 19 degrees S segment. Major oxides compositions show that all samples are tholeiite. Low contents of compatible trace elements (e.g., Ni = 239-594 ppm and Cr = 456-1010 ppm) and low Fe/Mn (54-67) and Ce/Yb (0.65-1.5) ratios of these lavas show that their parental magmas are partially melted by a spinel lherzolite mantle source. Using software PRIMELT3, this study obtained mantle potential temperatures (T-p) beneath the segment of 1321-1348 degrees C, which is lower relative to those ridges influenced by mantle plumes. The asthenospheric mantle beneath the SMAR 19 degrees S segment starts melting at a depth of similar to 63 km and ceases melting at similar to 43 km with a final melting temperature of similar to 1265 degrees C. The extent of partial melting is up to 16%-17.6% with an average adiabatic decompression value of 2.6%/kbar. The correlations of major oxides (CaO/Al2O3) and trace elements (Cr, Co, V) with MgO and Zr show that the parental magma experienced olivine and plagioclase fractional crystallization during its ascent to the surface. Sr-87/Sr-86 (0.702398-0.702996), Nd-143/Nd-144 (0.513017-0.513177) and Pb-206/(PD)-P-204 (18.444-19.477) ratios of these lavas indicate the mantle source beneath the SMAR 19 degrees S segment is composed of a three-component mixture of depleted MORB mantle, PREMA mantle, and HIMU mantle materials. The simple, binary mixing results among components from plume-free SMAR MORB, Saint Helena plume and Tristan plume show that asthenospheric mantle beneath the SMAR 19 degrees S segment may be polluted by both Saint Helena and Tristan plume enriched materials. The abovementioned mantle potential temperatures, together with the low Saint Helena (<10%) and Tristan (<5%) components remaining in the asthenospheric mantle at present, show that the physically ridge-hotspot interactions at SMAR 19 degrees S segment may have ceased. However, the trace element and Sr-Nd-Pb isotopically binary mixing calculation results imply that these lavas tapped some enriched pockets left when Saint Helena and/or Tristan plume were once on the SMAR during earlier Atlantic rifted history.