Assessing the importance of ultra-depleted domains in Earth’s mantle with Hf-Nd-Os isotope analyses of global abyssal peridotites

对海底地幔岩石、深海橄榄岩的同位素分析表明，地球地幔的组成可能与以前的估计有很大的不同。如果超贫橄榄岩，如深海橄榄岩的Hf-Os同位素比值所确定的那样，在地幔中普遍存在，那么持续的熔融一定使地幔中不相容元素的枯竭程度比以前想象的要大得多。地幔不相容元素耗竭的大小直接与时间积分的外向元素通量有关，而外向元素通量与海洋地壳的生成速率和地幔通过浅层地幔熔融区的加工速率成正比。因此，更大程度的地幔耗竭意味着强烈的地幔对流。这反过来又会导致地幔和地壳之间更大的质量通量，这意味着驱动硅酸盐土分化的引擎可能运行得更快，地壳-地幔循环的速度可能比之前想象的要快。因此，地幔损耗的程度反映了地球的全球成分演化和动力学。然而，到目前为止，深海橄榄岩的Hf同位素数据不足（与大洋中脊玄武岩的bbb1000 Hf同位素分析相比，深海橄榄岩的21 Hf同位素分析）严重阻碍了对地幔超贫域重要性的评估，从而影响了其不相容元素的总体枯竭程度。因此，建议分析来自全球不同地区的深海橄榄岩的Hf-Nd和Os同位素比率，代表了扩展速率和海洋地壳增生方式的广泛变异性。新的数据库将揭示超枯竭地幔的普遍程度或分布范围。通过岩石学和主要微量元素化学的仔细筛选，将确定最有希望进行同位素分析的样品。新的地球化学数据，结合Hf-Nd-Os同位素分析，还将确定受穿越或捕获熔体反应影响最小的样品。hf - o同位素数据也将得出地幔最小衰竭年龄的范围。各时间间隔内Hf-Os最小损耗年龄的分布与洋脊部分熔融产生的剩余损耗地幔的数量成正比。后者与地幔加工速率成正比，从而为地幔对流活力和硅酸盐土分异速率提供了一定的边界条件。此外，我们新的深海橄榄岩Hf-Nd-Os同位素数据将为解释蛇绿岩中橄榄岩的同位素系统提供急需的基线，从而有助于解决涉及次大陆岩石圈形成过程的性质和时间。

Isotopic analyses of submarine mantle rocks, abyssal peridotites, have shown that the composition of Earth’s mantle could differ substantially from previous estimates. If ultra-depleted peridotite, as identified by the Hf-Os isotope ratios in abyssal peridotites, is ubiquitous in the Earth’s mantle, continuous melting must have depleted Earth’s mantle in incompatible elements to a much larger extent than previously thought. The magnitude of incompatible element depletion of Earth’s mantle relates directly to the time-integrated outgoing elemental flux, which is proportional to the rate of oceanic crust generation and the rate of mantle processing through melting regions in the shallow mantle. A greater extent of mantle depletion thus implies a vigorously convecting mantle. This in turn would lead to a higher mass flux between Earth’s mantle and crust, which means that the engine that drives silicate earth differentiation could run faster and crust-mantle cycling could operate at a higher rate than previously thought. Hence, the extent of mantle depletion mirrors the global compositional evolution and dynamics of the Earth. Up to now, however, the scant Hf isotope data on abyssal peridotites –21 Hf isotope analyses on abyssal peridotites compared to >1000 Hf isotope analyses on mid ocean ridge basalts– severely hamper assessing the importance of ultra depleted domains in Earth’s mantle, and thus its overall extent of depletion in incompatible elements. It is therefore proposed to analyze Hf-Nd, but also Os isotope ratios in abyssal peridotites from different global localities representing a broad range of the variability in spreading rate and ocean crust accretion style. The new database will reveal how common, or wide-spread, ultra-depleted mantle is. Careful screening, by petrography and major-trace element chemistry, will identify the most promising samples for the isotope analyses. The new geochemical data, in conjunction with the Hf-Nd-Os isotope analyses, will also identify the samples least affected by reaction with traversing or trapped melts. The Hf-Os isotope data will also yield a range of minimum mantle depletion ages. The distribution of minimum Hf-Os depletion ages per time interval scales with the amount of residual depleted mantle generated by partial melting at ocean ridges. The latter is directly proportional to the mantle processing rate, and will thus give some boundary conditions for the vigor of mantle convection and the rate of silicate earth differentiation. Moreover, our new Hf-Nd-Os isotope data on abyssal peridotites will provide a much needed baseline for interpreting the isotope systematics of peridotites in ophiolites, and could therefore help resolving the nature and timing of the processes involved in formation of the subcontinental lithosphere.