Petrology and geochemistry of South Mid-Atlantic Ridge (19S) lava flows: Implications for magmatic processes and possible plume-ridge interactions
Petrology and geochemistry of South Mid-Atlantic Ridge (19S) lava flows: Implications for magmatic processes and possible plume-ridge interactions
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南大西洋中脊 (19S) 熔岩流的岩石学和地球化学：对岩浆过程和可能的羽流-山脊相互作用的影响
DOI：
10.1016/j.gsf.2020.06.007
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发表时间：
2020
影响因子：
8.9
通讯作者：
Renjie Zhao
中科院分区：
文献类型：
--
作者：
Haitao Zhang;Xuefa Shi;Chuanshun Li;Quanshu Yan;Yaoming Yang;Zhiwei Zhu;Hui Zhang;Sai Wang;Yili Guan;Renjie Zhao
南大西洋中脊（SMAR） 19°S段大致位于圣赫勒拿岛火山链（由圣赫勒拿岛地幔柱形成）沿线，是研究海脊-火盆相互作用过程是否影响整个大西洋的理想场所。本研究在SMAR 19°S段相对较小的区域内集中了20个新鲜熔岩样品的主微量元素组成和Sr-Nd-Pb同位素比值。主要氧化物组成表明样品均为拉斑岩。相容微量元素（Ni = 239 ~ 594 ppm, Cr = 456 ~ 1010 ppm）含量低，Fe/Mn（54 ~ 67）和Ce/Yb（0.65 ~ 1.5）比值低，表明其母岩浆部分被尖晶石—辉沸石地幔源熔融。利用PRIMELT3软件，得到1321 ~ 1348°C段的地幔位温（Tp），相对于受地幔柱影响的脊而言，Tp较低。SMAR 19°S段下的软流层地幔在~63 km深度开始融化，在~43 km深度停止融化，最终融化温度为~1265℃。部分熔融程度达16% ~ 17.6%，平均绝热减压值为2.6%/kbar。主要氧化物（CaO/Al2O3）和微量元素（Cr, Co， V）与MgO和Zr的对比表明，母岩浆在上升过程中经历了橄榄石和斜长石的分离结晶。87Sr/86Sr（0.702398 ~ 0.702996）、143Nd/144Nd（0.513017 ~ 0.513177）和206pb /204Pb（18.444 ~ 19.477）比值表明，SMAR 19°S段地幔源由贫MORB地幔、PREMA地幔和HIMU地幔物质三组分混合组成。无羽流SMAR MORB、圣赫勒拿岛羽流和特里斯坦羽流组分的简单二元混合结果表明，SMAR 19°S段软流圈地幔可能同时受到圣赫勒拿岛羽流和特里斯坦羽流富集物质的污染。上述地幔位温，加上目前软流圈地幔中残留的低Saint Helena（<10%）和Tristan（<5%）分量，表明SMAR 19°S段的物理脊-热点相互作用可能已经停止。然而，微量元素和Sr-Nd-Pb同位素二元混合计算结果表明，这些熔岩在早期大西洋裂谷历史中利用了圣赫勒拿岛和/或特里斯坦羽流曾经在SMAR上留下的一些富集袋。
The 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.