NB AND PB IN OCEANIC BASALTS - NEW CONSTRAINTS ON MANTLE EVOLUTION
NB AND PB IN OCEANIC BASALTS - NEW CONSTRAINTS ON MANTLE EVOLUTION
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DOI：
10.1016/0012-821x(86)90038-5
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发表时间：
1986-08-01
影响因子：
5.3
通讯作者：
WHITE, WM
中科院分区：
文献类型：
--
作者：
HOFMANN, AW;JOCHUM, KP;WHITE, WM
大洋中脊玄武岩（MORB）和洋岛玄武岩（OIB）的Nb/U比值和Ce/Pb比值分别为47 ± 10和25 ± 5。这些比值也反映了两类大洋玄武岩的地幔源区特征，并从Nb/U = 30和Ce/Pb = 9的地幔-地幔比值中分离出这些源区。大陆地壳中的Nb/U和Ce/Pb比值更低，分别为10和4。因此，OIB不可能来自地幔的原始部分，也不可能来自原始地幔和亏损地幔的混合物，更不可能来自再生大陆地壳。大陆地壳和残余地壳从原始地幔中分离出来的部分（MORB + OIB源区）地幔分异的可能性约为50%，但不确定性使得不能排除全地幔分异的可能性。大陆地壳的大部分分离后，地幔的分异部分发生化学贫化，随后内部重新均匀化。这一亏损但化学成分均匀的地幔区随后分化为MORB和OIB源区。第一次（陆壳-幔源）分异作用对Nb/U和Ce/Pb比值有分馏作用，而第二次（MORB源-OIB源）分异作用对Nb/U和Ce/Pb比值无分馏作用。根据Hofmann和白色[1，2]的模式，我们认为大洋壳的形成和俯冲是造成次级分异的主要机制。它是迄今为止地球上最重要的正在进行的体积分异过程，在地球历史的过程中，创造了至少十倍于现今大陆地壳体积的海洋地壳。（尽管不相容元素亏损），但在初次分异后，以Sr，Nd，Hf，Pb同位素比值为例，K/Rb等微量元素比值的变化，是在次生分化过程中产生的。在此过程中，Nb和Ce的体分配系数分别与U和Pb的体分配系数非常相似。这与原生分异过程中U比Nb更不相容，Pb比Ce更不相容形成鲜明对比。
Nb/U ratios and Ce/Pb ratios are surprisingly uniform at47 ± 10and25 ± 5, respectively, in both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). We show that these ratios also characterize the mantle sources ofbothtypes of oceanic basalts, and that these mantle sources have been fractionated from the primitive-mantle ratios ofNb/U = 30 andCe/Pb = 9. The respective ratios in the continental crust are even lower, namelyNb/U = 10 andCe/Pb = 4. Therefore, OIB cannot be derived from a primitive portion of the mantle, from mixtures of primitive and depleted mantle, or from recycled continental crust. The portion of the primitive mantle from which the continental crust and the residual (MORB plus OIB source) mantle has been differentiated is estimated to be about 50%, but the uncertainties are such that whole-mantle differentiation cannot be ruled out.We propose the following simple model to satisfy the above new constraint on mantle composition: The differentiated part of the mantle, chemically depleted after separation of the major portion of the continental crust, was subsequently internally rehomogenized. This depleted but chemically homogeneous mantle region was then differentiated into MORB and OIB source regions. The primary (continental crust—mantle) differentiation fractionated the Nb/U and Ce/Pb ratios, but the secondary (MORB source-OIB source) differentiation did not. Following the model of Hofmann and White [1,2], we suggest that the mechanism chiefly responsible for the secondary differentiation is the formation and subduction of oceanic crust. It is volumetrically by far the most important ongoing differentiation process on Earth and, over the course of Earth history, has created at least ten times as much oceanic crust as the present-day volume of continental crust.Because the residual mantle was homogenized (though depleted in incompatible elements) after the primary differentiation, the isotopic and chemical heterogeneities exemplified by the isotope ratios of Sr, Nd, Hf, and Pb, and by trace element ratios such as K/Rb, were created during the secondary differentiation. During this process, the bulk partition coefficients of Nb and Ce were very similar to those of U and Pb, respectively. This is in contrast with the primary differentiation, during which U was more incompatible than Nb, and Pb more incompatible than Ce.