The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the Mismatch Between Erupted Mid-Ocean Ridge Basalts and Lower Crustal Gabbros
The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the Mismatch Between Erupted Mid-Ocean Ridge Basalts and Lower Crustal Gabbros
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DOI：
10.3389/feart.2020.00025
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发表时间：
2020-02
期刊：
影响因子：
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通讯作者：
A. McCoy-West;M. Millet;K. Burton
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文献类型：
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作者：
A. McCoy-West;M. Millet;K. Burton
洋中脊玄武岩（MORB）的微量元素和同位素组成为所有寻求了解地幔演化的研究提供了重要的基石。在全球范围内有一个显着的过度富集的不相容的微量元素浓度的MORB相对于水平，这应该是由分离结晶。热和地球化学的限制表明，MORB需要在开放系统岩浆房的生成。然而，下大洋地壳岩石的岩石学表明，这些富集可能是通过反应性孔隙流（RPF）形成的。稳定同位素组成是过程依赖性的，因此提供了一个很好的机制来比较这些对比模型。本研究提出了第一个钕（Nd）稳定同位素组成的印度MORB和良好的特点辉长岩从下大洋地壳采样在西南印度洋脊（SWIR）（孔735 B）。印度MORB的平均δ 146 Nd为−0.025 ± 0.005‰，与太平洋MORB的组成完全相同。尽管全球MORB的源组成存在显著差异（即，143 Nd/144 Nd），它们不可区分的δ 146 Nd组成表明δ 146 Nd是通过一致的过程均匀化的（即，在全球洋脊网络上的开放系统岩浆房中重复的熔体添加）。与之形成鲜明对比的是，大洋辉长岩的δ 146 Nd值介于−0.026至−0.127‰之间，是陆地岩石中Nd稳定同位素自然变异的两倍。单斜辉石分离物具有可变的δ 146 Nd，但在相同的主要元素组成下，其同位素比辉长岩全岩重。δ 146 Nd的这种大的变化不能仅仅由岩浆矿物的分馏或聚集产生。孔735 B保存了RPF的广泛证据，RPF可以在晶体生长过程中诱导动力学同位素分馏。然而，单斜辉石中能产生的最大动力学同位素分馏只有约。0.02‰，因此，需要几个循环的溶解和再沉淀的同位素签名在晶界上解释的δ 146 Nd的范围观察到的辉长岩。洋壳的平均组成（δ 146 Nd = −0.067‰）与MORB之间的巨大差异，加上735 B孔上地壳熔体抽取的证据有限，导致了RPF中的熔体对MORB爆发的Nd同位素组成没有实质性贡献的结论。
The trace element and isotopic compositions of mid-ocean ridge basalts (MORB) provide an important cornerstone for all studies seeking to understand mantle evolution. Globally there is a significant over-enrichment in the incompatible trace element concentrations of MORB relative to levels which should be generated by fractional crystallization. Thermal and geochemical constraints suggest that MORB require generation in open system magma chambers. However, the petrology of lower oceanic crustal rocks suggests instead that these enrichments maybe formed through reactive porous flow (RPF). Stable isotope compositions are process dependent and therefore provide an excellent mechanism to compare these contrasting models. This study presents the first neodymium (Nd) stable isotope compositions of Indian MORB and well characterized gabbroic rocks from the lower oceanic crust sampled at the Southwest Indian Ridge (SWIR) (Hole 735B). Indian MORB is extremely homogenous with a mean δ146Nd of −0.025 ± 0.005‰ which is identical to the composition of Pacific MORB. Despite significant variability in the source composition of MORB globally (i.e., 143Nd/144Nd) their indistinguishable δ146Nd compositions suggests δ146Nd was homogenized through a consistent process (i.e., repeated melt addition in the open-system magma chambers across the global ridge network). In stark contrast, oceanic gabbros have δ146Nd ranging from −0.026 to −0.127‰, doubling the natural variability in Nd stable isotopes observed in terrestrial rocks. Clinopyroxene separates possess variable δ146Nd but are isotopically heavier than the gabbroic whole rocks at the same major element compositions. These large variations in δ146Nd cannot be generated solely by the fractionation or accumulation of magmatic minerals. Hole 735B preserves widespread evidence of RPF which could induce kinetic isotope fractionation during crystal growth. However, the maximum kinetic isotope fractionations that can be generated in clinopyroxene are only ca. 0.02‰, therefore several cycles of dissolution and reprecipitation of isotopic signatures at grain boundaries are required to explain the range of δ146Nd observed in the gabbros. The large disconnect between the average composition of the oceanic crust (δ146Nd = −0.067‰) and MORB, combined with limited evidence of melt extraction to the upper crust at Hole 735B, led to the conclusion that melts involved in RPF have not contributed in a substantial way to the Nd isotope composition of erupted MORB.