Global trends in novel stable isotopes in basalts: Theory and observations
Global trends in novel stable isotopes in basalts: Theory and observations
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玄武岩中新型稳定同位素的全球趋势：理论和观察
DOI：
10.1016/j.gca.2021.12.008
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发表时间：
2022
影响因子：
5
通讯作者：
Soderman C
中科院分区：
文献类型：
--
作者：
Soderman C
全球地幔熔体的地球化学表明，洋中脊玄武岩（MORB）和洋岛玄武岩（OIB）都具有源自上地幔和下地幔的岩性和温度异质性。最近，非传统稳定同位素被建议作为补充地幔异质性现有示踪剂（例如，主量元素和微量元素、放射性同位素）的新工具，因为矿物和氧化还原特异性平衡稳定同位素分馏效应可以将熔体的稳定同位素比率与其源矿物学和熔化程度联系起来。在这里，我们研究了五种稳定同位素系统（Mg-Ca-Fe-V-Cr），这些系统在模型或自然样本中显示出有望作为地幔温度和/或岩性异质性的示踪剂。我们使用定量模型，结合热力学自洽地幔熔融和平衡同位素分馏模型，探索这些元素在三种地幔岩性（橄榄岩、硅过剩和硅缺乏辉石岩）熔融过程中的同位素比行为，响应地幔矿物学、氧逸度、温度和压力的变化。我们发现，考虑到当前的分析精度，预计这里检查的稳定同位素系统不会通过平衡同位素分馏过程对地幔电位温度变化敏感。相比之下，在某些情况下，预计在喷发玄武岩的稳定同位素比中可以检测到源岩性非均质性，尽管考虑到当前的分析精度，地幔源中类 MORB 辉石岩的比例通常仅为 > 10%。镁和钙稳定同位素对含石榴石源岩性最敏感，而铁和铬稳定同位素对地幔源中类 MORB 辉石岩的存在可能敏感，尽管铬同位素的行为相对缺乏约束，需要进一步的工作才能有信心地应用于地幔熔体。当将橄榄岩和辉石岩熔体预测的平衡同位素分馏的幅度和方向与天然 MORB 和 OIB 数据进行比较时，我们发现自然数据的各个方面（包括 MORB 的平均 Mg-Ca-Fe-V 同位素组成、MORB 数据中的 Mg-Ca 同位素组成范围、OIB 的平均 Mg-Ca-Cr 同位素组成范围以及 OIB 数据中的 Mg-V-Cr 同位素组成范围）可以与橄榄岩和辉石岩来源部分熔融过程中的平衡同位素分馏相匹配，甚至某些 MORB 数据也需要辉石矿。然而，即使考虑到自然样本测量的分析不确定性，全球 MORB 和 OIB 数据集中看到的稳定同位素组成的范围表明，可能需要动态同位素分馏或修改回收地壳材料的同位素组成以使其与 MORB 不同的过程来解释所有自然数据。我们的结论是，这里考虑的五种稳定同位素系统有可能成为喷发玄武岩源岩性的其他地球化学示踪剂的强大补充示踪剂。然而，在这些新型稳定同位素能够明确地用于了解喷发玄武岩的来源异质性之前，需要不断提高分析精度，并结合地幔矿物和熔体之间同位素分馏的实验和理论预测。
The geochemistry of global mantle melts suggests that both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample lithological and temperature heterogeneities originating in both the upper and lower mantle. Recently, non-traditional stable isotopes have been suggested as a new tool to complement existing tracers of mantle heterogeneity (eg, major and trace elements, radiogenic isotopes), because mineral-and redox-specific equilibrium stable isotope fractionation effects can link the stable isotope ratios of melts to their source mineralogy and melting degree. Here, we investigate five stable isotope systems (Mg-Ca-Fe-V-Cr) that have shown promise in models or natural samples as tracers of mantle temperature and/or lithological heterogeneity. We use a quantitative model, combining thermodynamically self-consistent mantle melting and equilibrium isotope fractionation models, to explore the behaviour of the isotope ratios of these elements during melting of three mantle lithologies (peridotite, and silica-excess and silica-deficient pyroxenites), responding to changes in mantle mineralogy, oxygen fugacity, temperature and pressure. We find that, given current analytical precision, the stable isotope systems examined here are not predicted to be sensitive to mantle potential temperature variations through equilibrium isotope fractionation processes. By contrast, source lithological heterogeneity is predicted to be detectable in some cases in the stable isotope ratios of erupted basalts, although generally only at proportions of> 10% MORB-like pyroxenite in the mantle source, given current analytical precision. Magnesium and Ca stable isotopes show most sensitivity to a garnet-bearing source lithology, and Fe and Cr stable isotopes are potentially sensitive to the presence of MORB-like pyroxenite in the mantle source, although the behaviour of Cr isotopes is comparatively under-constrained and requires further work to be applied with confidence to mantle melts. When comparing the magnitude and direction of predicted equilibrium isotopic fractionation of peridotite and pyroxenite melts to natural MORB and OIB data, we find that aspects of the natural data (including the mean Mg-Ca-Fe-V isotopic composition of MORB, the range of Mg-Ca isotopic compositions seen in MORB data, the mean Mg-Ca-Cr isotopic composition of OIB, and the range of Mg-V-Cr isotopic compositions in OIB data) can be matched by equilibrium isotope fractionation during partial melting of peridotite and pyroxenite sources–with pyroxenite required even for some MORB data. However, even when considering analytical uncertainty on natural sample measurements, the range in stable isotope compositions seen across the global MORB and OIB datasets suggests that kinetic isotope fractionation, or processes modifying the isotopic composition of recycled crustal material such that it is distinct from MORB, may be required to explain all the natural data. We conclude that the five stable isotope systems considered here have potential to be powerful complementary tracers to other geochemical tracers of the source lithology of erupted basalts. However, continued improvements in analytical precision in conjunction with experimental and theoretical predictions of isotopic fractionation between mantle minerals and melts are required before these novel stable isotopes can be unambiguously used to understand source heterogeneity in erupted basalts.