Reliability of detrital marine sediments as proxy for continental crust composition: The effects of hydrodynamic sorting on Ti and Zr isotope systematics
Reliability of detrital marine sediments as proxy for continental crust composition: The effects of hydrodynamic sorting on Ti and Zr isotope systematics
复制标题
碎屑海相沉积物作为陆壳成分代表的可靠性：流体动力分选对钛和锆同位素体系的影响
DOI：
10.1016/j.gca.2021.05.030
复制
发表时间：
2021-05
影响因子：
5
通讯作者：
M. Klaver;S. MacLennan;M. Ibáñez-Mejia;F. Tissot;P. Vroon;M. Millet
中科院分区：
文献类型：
--
作者：
M. Klaver;S. MacLennan;M. Ibáñez-Mejia;F. Tissot;P. Vroon;M. Millet
碎屑沉积物的同位素组成记录为估计太古宙以来可蚀上地壳的组成提供了一个有价值的代用物。由于岩浆分异，钛（Ti）和锆（Zr）等难熔元素的同位素组成呈现出系统的变化。因此，碎屑沉积物中这些元素的同位素组成可以潜在地用来推断其源区的平均组成（例如，sio2含量），即使元素系统被风化和成岩过程所混淆。这种方法的一个关键前提是沉积物的同位素组成相对于它们的原岩保持不偏倚。在沉积过程中，同位素分选在多大程度上可以发生，特别是具有对比同位素组成的重矿物组合的水动力分选，仍然知之甚少。我们研究了这些过程对东地中海（EMS）一套碎屑沉积物的Ti和Zr同位素组成的影响。这些沉积物是两种主要物源成分的二元混合物，撒哈拉沙尘和尼罗河沉积物，具有强烈对比的矿物学和地球化学特征。EMS沉积物具有明显的锆石水动力分选证据，表现为Zr/ al2o3含量变化较大，εHf相对于陆相εNd-εHf阵列存在偏差。然而，我们的新数据并没有显示出锆石水动力分选或沉积物物源导致的明显的Zr同位素变化。尽管这与理论模型预测的可忽略不计的平衡锆熔体Zr同位素分馏相一致，但它与最近的观察结果形成对比，表明动态Zr同位素分馏可能是火成岩的共同特征。对于EMS沉积物，通过水动力分选，Zr同位素组成的变化可以忽略不计，这意味着细粒样品准确地反映了其来源的组成。然而，撒哈拉和尼罗河沉积物中几乎重叠的Zr同位素组成意味着，在这种情况下，Zr同位素的分辨率不足以作为有用的物源代理。钛的表现则不同。在撒哈拉和尼罗河物源组分之间观察到Ti同位素组成的微小但可解决的系统差异。具有强烈撒哈拉沙尘特征的样品显示了一些钛同位素证据，表明氧化物与锆石的流体动力学分选，但推断尼罗河沉积物的作用更强。EMS沉积物样品的回归表明，与其原岩（埃塞俄比亚洪水玄武岩）相比，尼罗河衍生组分的Ti同位素组成具有强烈的分馏性。虽然尼罗河沉积物中的Ti基本上以未改变的浓度携带，并通过同位素组成推断，从其来源到三角洲，但铁-钛氧化物的大规模水动力分选发生在沿岸细胞中。这一过程导致残余细粒沉积物中TiO2/ al2o3含量降低，Ti同位素组成向较重的成分转变。碎屑沉积物中这种“氧化物效应”的可能性对地壳演化模型具有启示意义，该模型使用钛同位素作为长英质地壳比例的代表，并可以解释在页岩记录中观察到的分散现象。
The isotopic composition of the detrital sediment record harbours a valuable proxy for estimating the composition of the erodible upper crust since the Archaean. Refractory elements such as titanium (Ti) and zirconium (Zr) can display systematic variations in their isotopic composition as a result of magmatic differentiation. Hence, the isotopic composition of such elements in detrital sediments could potentially be used to infer the average composition (e.g., SiO2content) of their source region, even when elemental systematics are obfuscated by weathering and diagenetic processes. A key premise of this approach is that the isotopic composition of sediments remains unbiased relative to their protolith. To what extent isotopic fractionation can occur during sedimentary processes, notably the hydrodynamic sorting of heavy mineral assemblages with contrasting isotopic compositions, remains poorly understood. We investigate the effects of such processes on the Ti and Zr isotope composition of a suite of detrital sediments from the Eastern Mediterranean Sea (EMS). These sediments are binary mixtures of two main provenance components, Saharan dust and Nile sediment, with strongly contrasting mineralogical and geochemical signatures.The EMS sediments display clear evidence for hydrodynamic sorting of zircon, expressed as a large variation in Zr/Al2O3and deviation of εHf relative to the terrestrial εNd-εHf array. Our new data, however, do not show pronounced Zr isotope variation resulting from either hydrodynamic sorting of zircon or sediment provenance. Although this agrees with theoretical models that predict negligible equilibrium zircon-melt Zr isotope fractionation, it contrasts with recent observations suggesting that kinetic Zr isotope fractionation might be a common feature in igneous rocks. For the EMS sediments, the negligible shift in Zr isotope composition through hydrodynamic sorting means that fine-grained samples accurately reflect the composition of their source. The nearly overlapping Zr isotope compositions of Sahara- and Nile-derived sediment, however, mean that Zr isotopes, in this case, have insufficient resolution to be a useful provenance proxy.Titanium behaves differently. A small but resolvable, systematic difference in Ti isotope composition is observed between the Sahara and Nile provenance components. Samples with a strong Saharan dust signature show some Ti isotope evidence for hydrodynamic sorting of oxides in tandem with zircon, but a much stronger effect is inferred for Nile sediment. Regression of the EMS sediment samples shows that the Ti isotope composition of the Nile-derived component is strongly fractionated compared to its protolith, the Ethiopian flood basalts. Whereas Ti in Nile sediment is carried in essentially unmodified concentration, and by inference isotope composition, from its sources to the delta, large-scale hydrodynamic sorting of Fe-Ti oxides occurs in the littoral cell. This process causes a decrease in TiO2/Al2O3of the residual fine-grained sediment fraction and shifts its Ti isotope composition to heavier compositions. The potential of such an “oxide effect” in detrital sediments has implications for crustal evolution models that use Ti isotopes as a proxy for the proportion of felsic crust and can account for the observed scatter in the shale record.