Sr–Nd–Pb–Hf Isotopic Constraints on the Mantle Heterogeneities beneath the South Mid-Atlantic Ridge at 18–21°S

Jinnan Liu,Weiliang Liu,Yun Zhong,Xu Zhang,Zhilei Sun,Bin Xia,Wei Li,Yaoliang Ma,Yao Guan

doi:10.3390/min10111010

Abstract

In an attempt to investigate the nature and origin of mantle heterogeneities beneath the South Mid-Atlantic Ridge (SMAR), we report new whole-rock Sr, Nd, Pb, and Hf isotopic data from eight basalt samples at four dredge stations along the SMAR between 18°S and 21°S. Sr, Nd, and Pb isotopic data from SMAR mid-ocean ridge basalts (MORBs) at 18–21°S published by other researchers were also utilized in this study. The SMAR MORBs at 18–21°S feature the following ratio ranges: 87Sr/86Sr = 0.70212 to 0.70410, 143Nd/144Nd = 0.512893 to 0.513177, 206Pb/204Pb = 18.05 to 19.50, 207Pb/204Pb = 15.47 to 15.71, 208Pb/204Pb = 37.87 to 38.64, and 176Hf/177Hf = 0.283001 to 0.283175. The 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 176Hf/177Hf ratios of these MORBs varied considerably along the SMAR axis. The variable compositions of the Sr–Nd–Pb–Hf isotopes, combined with the corresponding whole-rock major and trace elemental abundances reported in previous studies, suggest that the SMAR MORBs at 18–21°S were probably derived from a heterogeneous mantle substrate related to a mixture of depleted mantle (DM) materials with a small amount (but variable input) of HIMU (high-μ, where μ = 238U/204Pb)- and enriched (EMII)-type materials. The HIMU-type materials likely originated from the proximal St. Helena plume and may have been transported through “pipe-like inclined sublithospheric channels” into the SMAR axial zone. The EMII-type materials possibly originated from a recycled metasomatized oceanic crust that may have been derived from the early dispersion of other plume heads into the subcontinental asthenosphere prior to the opening of the South Atlantic Ocean. In addition, the contributions of subducted sediments, continental crust, and subcontinental lithospheric mantle components to the formation of the SMAR MORBs at 18–21°S may be nonexistent or negligible.

Highlights

Basaltic rocks that erupt along mid-ocean constituteFOR the dominant PEER volcanic rock Minerals10, ridges (MORs) xREVIEW type on the modern Earth (e.g., [1])
It is well established that mid-ocean ridge basalts (MORBs) chemical compositions of MORB suites have been interpreted to indicate complex mixing processes originate from the partial melting of shallow-convecting asthenospheric mantle that is characterized involving by mantle heterogeneities that vary greatly in composition
Previous studies indicated that the South Mid-Atlantic Ridge (SMAR) MORBs at 18–21◦ S show N-type mid-ocean ridge basalts (N-MORB)- and enriched-type mid-ocean ridge basalt (E-MORB)-like geochemical compositions that suggest a enriched-type mid-ocean ridge basalt (E-MORB)-like geochemical compositions that suggest a complex complex magma source in this area [9,28,30]

Summary

Methods

Samples were ground using an agate mortar and pestle to 200 mesh. Whole-rock Sr, Nd, Pb, and Hf isotopic analyses were performed at Nanjing FocuMS Technology Co., Ltd., China. Samples were placed on a hot plate until dry and were subsequently mixed with 1.5 mL of 0.2 N HBr + 0.5 N HNO3 prior to ion exchange purification. Lithophile elements, including Sr, Hf, and rare earth elements (REEs), were initially washed from the column using a mixture of 0.2 N HBr + 0.5 N HNO3 (collected), and Pb was eluted with purified water. During this process, a second anion exchange column was needed to further purify/separate the Pb fraction because of the impurities within. A second anion exchange column was needed to further purify/separate the Pb fraction because of the impurities within. (3) Sr, Hf, and REEs were separated using a Biorad AG50W–X8 cation exchange column (Bio-Rad Laboratories, Hercules, CA, USA)

Results

Discussion

Conclusion