The effect of host magma infiltration on the Pb isotopic systematics of lower crustal xenolith: An in-situ study from Hannuoba, North China

Abstract

The Pb isotopic compositions of granulite xenoliths are crucial to constrain the formation and evolution of the lower continental crust (LCC) and are potentially the key to resolve the first Pb paradox of the Earth. However, it remains unclear how host magma infiltration affects the Pb isotopic compositions of LCC xenoliths and how different this effect is for different xenoliths in composition (from mafic to felsic). To address these issues, here we provide an in-situ chemical and PbSr isotopic study on feldspars and clinopyroxenes for a suite of fresh mafic and felsic granulite xenoliths from the Hannuoba region in the northern margin of the North China Craton (NCC), which have been previously well characterized for bulk-rock chemical and Sr–Nd–Pb isotopic compositions. Our study shows that the in-situ plagioclase and bulk-rock Pb isotopic compositions of the Hannuoba mafic granulite xenoliths are significantly decoupled. Such decoupling is demonstrated to have been caused by host magma infiltration through contributing the main Pb budget into the grain boundaries and cracks. However, this process appears to have relatively less or negligible effect on the Pb isotopic systematics of feldspar-rich intermediate-felsic granulite xenoliths, which is reflected by their consistent in-situ (feldspar) and bulk-rock Pb isotopic compositions. Such different effects on Pb isotopic systematics for different LCC xenoliths in composition are highly related to the Pb contents or abundances of feldspar in LCC xenoliths. Furthermore, our data suggest that the effect of host magma infiltration on the Pb isotopic systematics of LCC xenoliths may be not easily eliminated by the leaching technique and thus it should be prudent to use the bulk-rock Pb isotopic data of low-Pb LCC and potentially upper mantle xenoliths, such as peridotite, pyroxenite and garnet−/pyroxene-rich mafic granulite. Instead, in-situ analyses of minerals (e.g., feldspar, clinopyroxene) could provide a more convenient and reliable method to obtain the pristine Pb isotopic compositions of these low-Pb xenoliths.