Geochemistry of Walvis Ridge basalts@en

The proposed origins for the Enriched Mantle I component are many and various and some require an arbitrary addition of an exotic component, be it pure sediment or an enriched melt from the subcontinental lithosphere. With Pitcairn, Walvis Ridge is the 'type-locality' for the Enriched Mantle I (EMI) component. We analyzed basalts from DSDP Site 525A, Site 527 and Site 528 on the Walvis Ridge with the aim to constrain the history of its source. The isotopic compositions we measured for the three sites overlap with the values obtained by Richardson et al. (1982a) and extend towards less radiogenic Sr and more radiogenic Pb and Nd isotopic compositions. We used our new trace element and radiogenic isotope (Hf, Nd, Pb and Sr) characterization in combination with the literature data to produce the simplest possible model that satisfies the trace element and isotopic constraints. Although the elevated 207Pb/204Pb with respect to 206Pb/204Pb predicts an ancient origin for EMI, none of the proposed origins had modeled it as such. The data is consistent with the EMI composition being formed by the addition of a melt to a mantle with bulk Earth-like composition followed by melt extraction of a low degree melt. The timing of these two events is such that the metasomatism has to have taken place prior to 4 Ga and the subsequent melt removal before 3.5 Ga. This confirms the expectation of an ancient character for the EMI component. […]富集地幔I（Enriched Mantle I，简称EMI）组分的成因假说繁多且各异，部分假说需要额外引入外源性组分——无论是纯沉积物，抑或是来自大陆岩石圈地幔（subcontinental lithosphere）的富集熔体。沃尔维斯脊与皮特凯恩海岭同为富集地幔I（EMI）组分的模式产地。本次研究对沃尔维斯脊上深海钻探计划（Deep Sea Drilling Program，简称DSDP）525A站位、527站位及528站位的玄武岩开展分析，旨在约束其源区的演化历史。我们测得的三个站位的同位素组成与Richardson等人（1982a）的测定结果重合，且呈现出放射成因Sr同位素比值降低、放射成因Pb及Nd同位素比值升高的变化趋势。我们结合新获得的微量元素与放射成因同位素（Hf、Nd、Pb及Sr）特征数据与已发表文献数据，构建了可同时满足微量元素与同位素约束条件的最简模型。尽管207Pb/204Pb相较于206Pb/204Pb比值偏高的现象，预示EMI具有古老成因，但目前所有已提出的成因假说均未按此逻辑对其进行建模。本研究数据与以下成因假说相符：EMI组分的组成是通过向具有整体地球（bulk Earth）类似组成的地幔中添加熔体，随后提取低程度熔融产生的熔体而形成。这两次事件的时间约束为：地幔交代作用（metasomatism）需发生于4 Ga之前，而后续的熔体移除过程则需发生于3.5 Ga之前。该结果证实了EMI组分具有古老成因特征的预期。[…]