Title

Boron Isotopes Reveal Multiple Metasomatic Events in the Mantle Beneath the Eastern North China Craton

Document Type

Article

Publication Date

12-2016

Keywords

Boron isotope, North China Craton, Intracontinental basalts, Recycled oceanic crust

Digital Object Identifier (DOI)

https://doi.org/10.1016/j.gca.2016.08.027

Abstract

Linkages inferred between the geochemical heterogeneity of the mantle beneath eastern Eurasia and the stagnant Pacific slab documented geophysically in its mantle transition zone are as yet not clearly characterized. In this paper we report new elemental and isotopic data for boron (B) on a suite of well-characterized Cenozoic basalts (alkali basalts, basanites and nephelinites), with ocean island basalt (OIB)-like trace element signatures from western Shandong of the eastern North China Craton (NCC). Correlations between major elements (e.g., FeOTversus SiO2), trace elements (e.g., CeN/PbN versus BaN/ThN) and radiogenic isotopes (e.g., 206Pb/204Pb versus 87Sr/86Sr) suggest these basalts are derived via the mixing of melts from two mantle components: a fluid mobile element (FME; such as Ba, K, Pb and Sr) enriched component, which is most evident in the alkali basalts, and a FME depleted mantle component that is more evident in the basanites and nephelinites. The alkali basalts in this study have lower B concentrations (1.4–2.2 μg/g) but higher δ11B (−4.9 to −1.4) values than the basanites and nephelinites (B = 2.1–5.0 μg/g; δ11B = −6.9 to −3.9), and all the samples have nearly constant B/Nb ratios between 0.03 and 0.07, similar to the observed range in B/Nb for intraplate lavas. Our high-SiO2 samples have higher δ11B than that of our low SiO2 samples, indicating that the B isotopic differences among our samples do not result from the addition of a continental crustal component in the mantle source, or direct crustal assimilation during the eruption process. The positive B versus Nb correlation suggests the B isotopic compositions of the western Shandong basalts primarily reflect the pre-eruptive compositions of their mantle sources. Correlations among B, Nd and Sr isotope signatures of the western Shandong basalts differ from those among basalts from plume settings (e.g., Azores and Hawaii), and are inconsistent with models suggesting single-step metasomatic additions to the mantle. We thus call upon multiple metasomatic events in the mantle beneath the eastern NCC in order to interpret its observed radiogenic and boron isotopic variability. The heavier δ11B, FME-enriched mantle source developed during an older event, while metasomatism by melts from the stagnant Pacific plate in the MTZ led to the development of a FME-depleted mantle source at greater depths with a lower δ11B.

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Yes

Citation / Publisher Attribution

Geochimica et Cosmochimica Acta, v. 194, p. 77-90

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