地球核幔相互作用的W同位素研究进展

A review of the W isotope research for the Earth’s core-mantle interaction

地球核幔相互作用的研究难点在于无法获得实际样品。洋岛玄武岩和溢流玄武岩被认为是地幔柱减压熔融的产物,携带了核幔边界的物质信息,可作为研究地球核幔相互作用的样品。^(182)Hf-^(182)W同位素体系的特殊化学性质,使W同位素成为研究核幔相互作用的重要工具。本文介绍了W同位素示踪的基本原理,并回顾了核幔相互作用的W同位素研究进展。目前已发表的数据表明,全球洋岛玄武岩具有W元素丰度富集(67×10^(-9)~855×10^(-9))、^(182)W同位素亏损(μ^(182)W=-0.1~-16.1)的特征,由此推断洋岛玄武岩可能来源于核幔平衡源区。Baffin Bay和Ontong Java Plateau溢流玄武岩则具有^(182)W富集(μ^(182)W=23.4)的特征,可能来源于早期地幔源区。洋岛玄武岩和溢流玄武岩的μ^(182)W差异可能是由地幔柱头尾异质性引起。此外,引起μ^(182)W异常的其他原因可能有:原始地幔源区的隔离保存、后增生薄层物质的部分混入和低效的核幔分异作用等。目前,W同位素的核幔交换机制仍不清楚。

The challenge of studying the Earth’s core-mantle interaction is that the samples at the boundary between core and mantle cannot be obtain by current technology.Ocean island basalts and flood basalts are considered to be the products of decompression-melting of mantle plumes,which carry the materials in the core-mantle boundary and are the best samples for researching the core-mantle interaction in the Earth.The isotope of ^(182)W becomes an important tool for researching the Earth’s core-mantle interaction because the unique geochemical properties of ^(182)Hf-^(182)W isotope system.This paper introduces the basic principle of W isotope as tracer and reviews the research progress of W isotope in the Earth’s core-mantle interaction The published data has shown that the global oceanic island basalts are characterized by W content enrichment(67×10^(-9)~855×10^(-9))and ^(182)W isotope depletion(μ^(182)W=-0.1~-16.1),which suggest the ocean island basalts might be derived from the“Core-Mantle Equilibrated Reservoir”.On the other hand,the flood basalts from the Baffin Bay and Ontong Java Plateau are characterized by enrichment of^(182)W(μ^(182)W=23.4),which indicate these samples might be derived from the“Early Formed Mantle Reservoir”.The difference ofμ^(182)W between ocean island basalts and flood basalts might be caused by the heterogeneity of mantle plumes(eg.head-positive vs.tail-negative).In addition,theμ^(182)W anomaly could also be caused by other reasons,such as it might be from isolated preservation of primitive mantle reservoirs,or partial mixing of the late veneer,or inefficient differentiation of the Earth core-mantle,and so on.However,the core-mantle exchange mechanism of W isotopes is still controversial.