The lead isotope composition of the Earth´s mantle from double-spike analysis of abyssal peridotites

大洋玄武岩被广泛用作地幔成分的探针，但这种方法假定地幔熔体继承了其源区的同位素组成。最近的橄榄岩铅同位素数据被认为代表了大洋上部地幔熔融的残留物，其放射性成因成分比大洋玄武岩中观察到的要少得多。在熔融过程中，从地幔中非放射性成分中提取的铅不完全，这可能解释了这样一个悖论：如果地球是从球粒陨石物质中吸积而来的，那么块状地球显然具有比预测的更具放射性的铅同位素成分。它还可以解释大洋中脊玄武岩的铅同位素体系，这显然反映了富集型和亏损型地幔组分的混合。现有的深海橄榄岩--大洋中脊地幔熔融的残留物--的可靠铅同位素数据太少(3次分析)，无法检验这一假设。我们建议的研究将使用新的和现有的方法相结合的方法来获得一些关于深海橄榄岩的第一批可靠的铅同位素数据。我们将分析单斜辉石和尖晶石分离物，并使用顺序溶解方法选择性地溶解块状岩粉中的不同矿物相。铅同位素组成将采用202Pb205Pb双尖峰法进行高精度测量。我们的结果将被用来检验这一假设，即上地幔中存在一种“隐藏的”非放射性成因的铅成分，它可以平衡地球的铅收支。

Oceanic basalts are widely used as probes of mantle composition, however this approach assumes that mantle melts inherit the isotopic composition of their source. Recent Pb isotope data for peridotites presumed to represent melting residues of the upper oceanic mantle extend to much less radiogenic compositions than observed in oceanic basalts. Incomplete extraction of Pb from an unradiogenic component in the mantle during melting could potentially explain the paradox that the bulk Earth apparently has a Pb isotope composition that is more radiogenic than is predicted if the Earth accreted from chondritic material. It could also explain the Pb isotope systematics of mid-ocean ridge basalts, which apparently reflect mixing between enriched and depleted mantle components. Reliable existing Pb isotope data for abyssal peridotites - residues of mantle melting at mid-ocean ridges - are too few (3 analyses) to test this hypothesis. Our proposed study will use a combination of new and existing methods to obtain some of the first reliable Pb isotope data for abyssal peridotites. We will analyse clinopyroxene and spinel separates, and use sequential dissolution methods to selectively dissolve different mineral phases within bulk-rock powders. Pb isotope compositions will be measured to high precision using a 202Pb-205Pb double-spike method. Our results will be used to test the hypothesis that a 'hidden' unradiogenic Pb component, which can balance the Earth's Pb budget, is present in the upper mantle.