142Nd/144Nd inferences on the nature and origin of the source of high 3He/4He magmas
142Nd/144Nd inferences on the nature and origin of the source of high 3He/4He magmas
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142Nd/144Nd 高 3He/4He 岩浆来源的性质和起源推断
DOI：
10.1016/j.epsl.2017.05.005
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发表时间：
2017
影响因子：
5.3
通讯作者：
De Leeuw G
中科院分区：
文献类型：
--
作者：
De Leeuw G
高精度的142Nd/144Nd测量来自巴芬湾地区的苦味岩，其中含有最高的3He/4He比值，但在陆源地幔岩石中测得的值与陆源标准岩石和其他现代地幔源岩中的测量值没有什么区别。巴芬岛熔岩以其异常高的3He/4He和182W/184W比值而区别于其他热点熔岩，但它们的锶、142、143Nd、Hf和Pb同位素特征与北大西洋MORB的测量值重叠。这些特征要么表明高3He/4He岩浆的地幔源区具有不相容元素亏损的亲石同位素特征，要么表明该源区的He同位素特征与岩浆中的亲石同位素示踪解耦。如果块体地球具有比现代陆相岩石更接近顽火辉石球粒陨石测得的平均值的142Nd/144Nd比值，那么142Nd-143Nd组合数据与岩浆源在地球形成期间或之后不久获得的不相容元素的亏损是一致的。如果地球初始142Nd/144Nd高于顽辉石球粒陨石的平均值，则与传统的解释一致，即亏损地幔储集层是在146Sm约4Ga消亡后，通过提取大陆地壳等不相容的富元素储集层而形成的。然而，这一解释未能解释高的3He/4He。苦苦岩的Nd同位素组成反映了北大西洋MORB不相容元素贫化源的主要贡献，叠加了来自He浓度至少比亏损地幔高10倍的地幔源区的一小部分(10-20%)贡献，以及显著高于典型地幔岩石的W同位素组成。
High-precision measurements of142Nd/144Nd in picrites from the Baffin Bay region that contain the highest3He/4He ratios yet measured in terrestrial mantle-derived rocks are indistinguishable from the value measured in the terrestrial standard and other modern mantle-derived rocks. The Baffin Island lavas are distinguished from other hotspot lavas by their unusually high3He/4He and182W/184W ratios, but their Sr,142Nd,143Nd, Hf, and Pb isotopic signatures overlap the values measured in North Atlantic MORB. These features imply either that the mantle source region of high3He/4He magmas carries the lithophile isotopic signatures of incompatible element depletion, or that the He isotope signature of this source is decoupled from the lithophile isotope tracers in the magmas. The coupled142Nd–143Nd data are consistent with the magma source acquiring the incompatible element depletion during, or shortly after, Earth formation if the bulk-Earth has a142Nd/144Nd ratio more similar to the average measured for enstatite chondrites than modern terrestrial rocks. If Earth's initial142Nd/144Nd was higher than the average of enstatite chondrites, the data are consistent with the traditional interpretation that the depleted-mantle reservoir was formed through the extraction of an incompatible-element-rich reservoir, such as continental crust, after the circa 4 Ga extinction of146Sm. This explanation, however, fails to account for the high3He/4He. The Nd isotopic composition of the picrites could reflect a dominant contribution from the incompatible element depleted source of North Atlantic MORB, overprinted by a small (10–20%) contribution from a mantle source with He concentrations at least ten times higher than the depleted mantle along with W isotopic compositions substantially higher than typical of mantle-derived rocks.