Zirconium stable isotope fractionation during intra-crustal magmatic differentiation in an active continental arc
Zirconium stable isotope fractionation during intra-crustal magmatic differentiation in an active continental arc
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DOI：
10.1016/j.gca.2023.11.023
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发表时间：
2023-11
影响因子：
5
通讯作者：
L. Zieman;M. Ibáñez-Mejia;F. Tissot;H. Tompkins;Natalia Pardo;E. Bloch
中科院分区：
文献类型：
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作者：
L. Zieman;M. Ibáñez-Mejia;F. Tissot;H. Tompkins;Natalia Pardo;E. Bloch
锆（Zr）稳定同位素变化发生在共存的富锆副相之间以及块体岩石尺度上，但岩浆分异过程中锆同位素分馏的岩石学机制尚不清楚。会聚大陆边缘的幼年岩浆生成和地壳内分异可能在发展Zr同位素变化中起着至关重要的作用，安第斯山脉的北方火山带是检验这一假设的理想环境。为了研究这些过程对Zr稳定同位素组成的影响，我们报告了来自以下岩石样品的δ94/90 ZrNIST：1）哥伦比亚第四纪Granatifera凝灰岩中的幼年弧玄武岩; 2）下地壳衍生的石榴辉石岩（即，榴辉岩），角闪石，辉长岩堆发现在同一个单位;和3）长英质火山产品从多纳胡安娜火山杂岩，英安质复合火山在靠近和部分覆盖的Granatifera凝灰岩。玄武岩的δ94/90 ZrNIST值为-0.025 ± 0.018 ‰ ~+0.003 ± 0.015 ‰（n = 8），属于洋中脊玄武岩。英安岩的δ94/90 ZrNIST值为+0.008 ± 0.013 ‰ ~+0.043 ± 0.015 ‰（n = 14），相对于花岗岩类和洋中脊玄武岩略偏正。相比之下，（超）镁铁质堆晶岩的δ94/90 ZrNIST值变化很大，主要为正值，范围为−0.134 ± 0.012 ‰至+0.428 ± 0.012 ‰（n = 15）。从8个（超）镁铁质堆晶岩中分析了主要成岩相的单个颗粒和矿物组分，包括石榴石（n = 21），角闪石（n = 9）和单斜辉石（n = 18）。矿物组分记录了高度可变的Zr同位素组成，矿物间分馏（Δ94/90 Zr石榴石-角闪石）高达2.067 ‰。Recentab initiocalculations的Zr-O键力常数在成岩阶段预测有限的矿物间Zr同位素分馏在高温环境中，这表明我们观察到的大分馏不是振动平衡过程的产物。相反，我们提出了一个方案，其中大Zr同位素分馏发展动力学，通过Zr分配系数的变化引起的温度变化引起的共存相之间的亚固相线Zr扩散。总的来说，在这个钙碱性大陆弧环境中，Zr同位素的变化与岩浆分异指数（例如，Mg#，SiO2），并不是一个简单的函数的分步结晶。此外，这里研究的石榴石单斜辉石堆晶岩代表了密度不稳定的下弧地壳物质;因此，具有同位素可变δ94/90 Zr的物质可以作为下地壳沉降的结果再循环到地幔中。
Zirconium (Zr) stable isotope variations occur among co-existing Zr-rich accessory phases as well as at the bulk-rock scale, but the petrologic mechanism(s) responsible for Zr isotope fractionation during magmatic differentiation remain unclear. Juvenile magma generation and intra-crustal differentiation in convergent continental margins may play a crucial role in developing Zr isotope variations, and the Northern Volcanic Zone of the Andes is an ideal setting to test this hypothesis. To investigate the influence of these processes on Zr stable isotope compositions, we reportδ94/90ZrNISTof whole rock samples from: 1) juvenile arc basalts from the Quaternary Granatifera Tuff, Colombia; 2) lower crust-derived garnet pyroxenites (i.e., arclogites), hornblendites, and gabbroic cumulates found in the same unit; and 3) felsic volcanic products from the Doña Juana Volcanic Complex, a dacitic composite volcano in close proximity to and partially covering the Granatifera Tuff. The basalts haveδ94/90ZrNISTvalues ranging from −0.025 ± 0.018 ‰ to +0.003 ± 0.015 ‰ (n = 8), within the range of mid-ocean ridge basalts. The dacites haveδ94/90ZrNISTvalues ranging from +0.008 ± 0.013 ‰ to +0.043 ± 0.015 ‰ (n = 14), slightly positive relative to the Granatifera and mid-ocean ridge basalts. In contrast, the (ultra)mafic cumulates have highly variable, predominantly positiveδ94/90ZrNISTvalues, ranging from −0.134 ± 0.012 ‰ to +0.428 ± 0.012 ‰ (n = 15). Individual grains and mineral fractions of major rock-forming phases, including garnet (n = 21), amphibole (n = 9), and clinopyroxene (n = 18), were analyzed from 8 (ultra)mafic cumulates. The mineral fractions record highly variable Zr isotopic compositions, with inter-mineral fractionation (Δ94/90Zrgarnet-amphibole) up to 2.067 ‰. Recentab initiocalculations of Zr–O bond force constants in rock-forming phases predict limited inter-mineral Zr isotope fractionation in high-temperature environments, suggesting that the large fractionations we observe are not the product of vibrational equilibrium processes. Instead, we propose a scenario in which large Zr isotopic fractionations develop kinetically, induced by sub-solidus Zr diffusion between coexisting phases via changes in Zr distribution coefficients that arise from changes in temperature. Altogether, Zr isotope variability in this calc-alkaline continental arc setting exhibits no correlation with indices of magmatic differentiation (e.g., Mg#, SiO2), and is not a simple function of fractional crystallization. Furthermore, the garnet clinopyroxenite cumulates studied here represent density-unstable lower arc crust material; consequently, material with isotopically variableδ94/90Zr can be recycled into the mantle as a consequence of lower crustal foundering.