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

通过全球深渊橄榄岩的 Hf-Nd-Os 同位素分析评估地幔超贫化区域的重要性

• 批准号: 508386553
• 负责人: Professor Dr. Andreas Stracke, Ph.D.
• 金额: --
• 依托单位: Institut für Mineralogie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/508386553?language=en
• 关键词: Assessing; importance; ultra; depleted; domains

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.

对海底地幔岩（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差异。如果超贫化橄榄岩，在深海橄榄岩的Hf-Os同位素比值确定，是普遍存在于地球的地幔，连续熔融必须耗尽地球的地幔中的不相容元素在一个更大的程度上比以前认为的。地幔不相容元素亏损的大小与时间积分的元素流出通量直接相关，而元素流出通量与洋壳生成速率和地幔通过浅地幔熔融区的加工速率成正比。因此，更大程度的地幔亏损意味着一个强对流的地幔。这反过来又会导致地幔和地壳之间更高的质量通量，这意味着驱动硅酸盐地球分化的引擎可以运行得更快，壳幔循环可以以比以前认为的更高的速度运行。因此，地幔亏损的程度反映了全球地球成分的演化和动力学。然而，到目前为止，缺乏Hf同位素数据的深海橄榄岩-21 Hf同位素分析的深海橄榄岩相比，>1000 Hf同位素分析的洋中脊玄武岩-严重阻碍了评估的重要性，在地球地幔中的超亏损领域，从而其整体程度的亏损不相容元素。因此，建议分析Hf-Nd，但也Os同位素比值在深海橄榄岩从不同的全球性的地方代表了广泛的变化，在扩展速率和海洋地壳增生风格。新的数据库将揭示超贫化地幔的普遍性或广泛性。通过岩相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-Os同位素数据也将产生一系列的最小地幔亏损年龄。每个时间间隔的最小Hf-Os耗尽年龄的分布与洋脊部分熔融产生的残余耗尽地幔的数量成比例。后者与地幔的加工速率成正比，从而为地幔对流的活力和硅酸盐地球分异速率提供了一定的边界条件。此外，我们的新的Hf-Nd-Os同位素数据的深海橄榄岩将提供一个非常需要的基线解释的同位素系统学的蛇绿岩，因此可以帮助解决的性质和时间的过程中所涉及的形成次大陆岩石圈。