Barium isotope evidence of a fluid-metasomatized mantle component in the source of Azores OIB
Introduction
Ocean island basalts serve as effective probes for deciphering the compositional variability and evolution of the mantle. Enriched geochemical signatures relative to the depleted upper mantle, observed in many OIB, can potentially be explained by the addition of recycled materials (e.g., Zindler and Hart, 1986). Various element/element ratios (e.g., Nb/Zr, La/Sm, and Sm/Zr; Workman and Hart, 2005) and radiogenic isotopes (e.g., Sr, Pb and Nd; White, 1985; Hart and Hauri, 1992) have been used to identify different enriched components, but distinguishing the origins of these components remains a challenge. With the recent development of analytical methods, a growing diversity of metal stable isotopes have been used to identify the different components of subducted slabs and to constrain the mechanism of crustal recycling in the mantle (e.g., Teng et al., 2017).
Barium is a fluid-mobile element during slab dehydration (Kessel et al., 2005), which makes it a sensitive indicator for fluid-rock reactions and to track fluids in subduction zones. Barium is also highly incompatible during mantle partial melting, so crustal materials usually have much higher Ba abundances than the mantle (Sun and McDonough, 1989; Rudnick and Gao, 2003). Seawater and crustal materials (e.g., oceanic/continental crust and glacial diamictites) show much larger Ba isotope variations (δ138/134Ba of −0.63 to +0.95‰) than MORB and the upper mantle (δ138/134Ba of +0.03 to +0.14‰) (e.g., Horner et al., 2015; Nan et al., 2018, Nan et al., 2022; Nielsen et al., 2018; Li et al., 2020; Deng et al., 2021). Although the Ba isotope signatures of most marine sediments are similar to the average of MORB, the altered oceanic crust (AOC) exhibits a large variation in δ138/134Ba (−0.23 to +0.40‰; Nan et al., 2017; Nielsen et al., 2018; Wu et al., 2020). Therefore, the addition of different components from subducted slabs could produce distinct Ba isotope signatures in the mantle.
Barium isotopes have been applied to discern the contribution of different recycled components in subduction zones (Wu et al., 2020; Gu et al., 2021). Tonga arc lavas exhibit a positive correlation between Ba isotope ratios and Ba/Th, indicating the contribution of a fluid-dominated source with heavy Ba isotopes (Wu et al., 2020). High-pressure veins and retrograded amphibolites from the Dabie orogen, central China, have large variations in δ138/134Ba (−0.17 to +0.46‰), further supporting that metasomatic fluids resulting from dehydration of different subduction components may have distinct Ba isotope signatures (Gu et al., 2021). Nielsen et al. (2020) proposed that the addition of subducted sediments and AOC could produce variable δ138/134Ba values (−0.07 to +0.11‰) in the Aleutian and Ryukyu arc lavas. If these enriched components were subducted and recycled into the mantle source of the OIB, they could be recorded by Ba isotope signatures. In this study, we applied Ba isotopes to investigate the origin of enriched components in the mantle sources of the Azores archipelago. The results of this study provide important insights into using Ba isotopes to trace subduction related materials in the mantle.
Section snippets
Geological setting and samples
The Azores archipelago is a group of nine islands located between ∼37–40°N and ∼24–32°W in the vicinity of the Mid-Atlantic Ridge (MAR) and the triple junction between the North American, African and Eurasian plates (Fig. 1). Two islands, Corvo and Flores, lie to the west of the ridge; the other seven islands, including Faial, Graciosa, Pico, São Miguel, São Jorge, Santa Maria, and Terceira, lie to the east of the ridge. To the east of the ridge is the Terceira Rift, a spreading center along
Analytical methods
Barium purification procedures were performed in an ISO-Class 6 clean laboratory of the CAS Key Laboratory of Crust-Mantle Materials and Environments at the University of Science and Technology of China (USTC). The details of the purification procedure are described in Nan et al. (2018). All reagents used were high purity acids that were double-distilled and diluted with ultrapure 18.2 MΩ·cm water. Barium was purified from the matrix using AG50W-X12 (200–400 mesh, Bio–Rad, USA) cation exchange
Results
The major element, trace element, 87Sr/86Sr and 206Pb/204Pb data are from Yu (2011) (Table 1 and Table S1). The Azores samples have SiO2 ranging from 43.5 to 53.2 wt% and MgO from 2.5 to 17.0 wt%, and exhibit a trend of decreasing Ni with decreasing MgO (Fig. 3b). The primitive magmas in the Azores have been inferred to have MgO contents of ∼12 wt% (Beier et al., 2006). Samples with MgO > 12 wt% are considered to result from the accumulation of olivine and cpx, and samples with MgO < 12 wt% are
The effect of weathering processes on Ba isotopes
The potential effects of any source-to-surface and weathering processes on the Ba isotope compositions of the Azores samples should be considered before using Ba isotopes to constrain mantle signatures. Strong chemical weathering can fractionate Ba isotopes (Gong et al., 2019). All Azores samples analyzed in this study are very fresh, and any visible weathered surfaces were removed during sample collection and/or by sawing prior to crushing. There is no evidence of secondary mineral phases in
Conclusions
This study reports Ba isotope compositions for basalts from five islands (Faial, Pico, São Jorge, São Miguel, and Terceira) of the Azores archipelago. These samples have large variations in radiogenic Sr-Nd-Pb-Hf isotope compositions, reflecting that they have different enriched components in their mantle sources. Except for samples from Terceira, all samples from Faial, Pico, São Jorge, and São Miguel have homogeneous and MORB-like Ba isotopes, indicating that those enriched components in
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (42173003, 42173018, 42073007, 41873007, 41873005, 41873009), Anhui Provincial Natural Science Foundation (2208085J33), the US National Science Foundation (NSF EAR #0510598), the West Light Foundation of the Chinese Academy of Sciences (xbzg-zdsys-202108), and the Janet and Elliot Baines Professorship to Elisabeth Widom.
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