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标题：Petrology and geochemistry of South Mid-Atlantic Ridge (19°S) lava flows: Implications for magmatic processes and possible plume-ridge interactions
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作者：Haitao Zhang ; Xuefa Shi ; Chuanshun Li 等
期刊名称：Geoscience Frontiers
印刷版ISSN：1674-9871
出版年度：2020
卷号：11
期号：6
页码：1953-1973
DOI：10.1016/j.gsf.2020.06.007
语种：English
出版社：Elsevier
摘要：AbstractThe South Mid-Atlantic Ridge (SMAR) 19°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°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 (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 19°S 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) and206Pb/204Pb (18.444–19.477) ratios of these lavas indicate the mantle source beneath the SMAR 19°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°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°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.Graphical abstractDisplay OmittedHighlights•Mantle potential temperature is not being influenced by the heat flowing from the nearby plumes.•Combined effects of conductive cooling and local trend reaction lead to the partial melting cessation at a deeper depth.•Plume-ridge interaction may have terminated, but Saint Helena (< 10%) and Tristan (< 5%) signals are remaining.
关键词：KeywordsenMagmatic processMantle potential temperatureCrystallization pressurePlume-ridge interactionSouth Mid-Atlantic Ridge