Petrology and geochemistry of South Mid-Atlantic Ridge (19S) lava flows:
Implications for magmatic processes and possible plume-ridge interactions
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Abstract
The South Mid-Atlantic Ridge (SMAR) 19S 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 ridgehotpot
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 19S segment. Major oxides compositions show that all samples are tholeiite. Low contents of compatible
trace elements (e.g., Ni ¼ 239–594 ppm and Cr ¼ 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 (Tp) beneath the segment of
1321–1348 C, which is lower relative to those ridges influenced by mantle plumes. The asthenospheric mantle
beneath the SMAR 19S segment starts melting at a depth of ~63 km and ceases melting at ~43 km with a final
melting temperature of ~1265 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.
87Sr/86Sr (0.702398–0.702996), 143Nd/144Nd (0.513017–0.513177) and 206Pb/204Pb (18.444–19.477) ratios
of these lavas indicate the mantle source beneath the SMAR 19S 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 19S 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 19S 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.
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