Early-Stage Melt-Rock Reaction in a Cooling Crystal Mush Beneath a Slow-Spreading Mid-Ocean Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge)
Early-Stage Melt-Rock Reaction in a Cooling Crystal Mush Beneath a Slow-Spreading Mid-Ocean Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge)
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DOI：
10.3389/feart.2020.579138
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发表时间：
2020-11
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通讯作者：
Alessio San fi lippo;C. MacLeod;R. Tribuzio;C. Lissenberg;A. Zanetti;W. Bohrson;M. Grégoire
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作者：
Alessio San fi lippo;C. MacLeod;R. Tribuzio;C. Lissenberg;A. Zanetti;W. Bohrson;M. Grégoire
辉长岩的显微结构和化学证据表明，熔体可能会与晶体框架，因为他们通过迁移下的洋中脊的水晶泥反应，然而，这个过程的矿物和熔体的成分演变的重要性仍然是一个有争议的问题。在这里，我们提供了新的见解，在何种程度上熔融岩石反应过程中可以发生在大洋辉长岩进行详细的研究，隐藏的反应熔体迁移保存在一个显然不起眼的，均匀的橄榄石辉长岩从深内的一节深的深成下盘的亚特兰蒂斯银行核心复杂的西南印度洋海岭（国际海洋发现计划孔U1473 A）。高分辨率的化学地图显示，矿物分区增加，并成为极端的厘米宽的频带内，其特征是不相容的微量元素浓度升高，并产生极端的更多/更少的不相容元素的比例。我们表明，无论是被困熔体的结晶，也不扩散可以解释这些观察。相反，采取新的方法校正矿物熔体分配系数的温度和组成，我们表明，这些化学变化可以产生的熔体的晶间反应性多孔流动，因为它迁移通过糊状框架，其组成演变的熔体岩石反应，因为它逐步本地化到一个厘米级的反应通道。我们建议，这里报道的情况下，可能代表，在微观世界中，一个保存的快照的通用机制，熔体可以渗透通过原始镁铁质（橄榄石辉长岩）晶体泥，并通过近无处不在的反应运输朝着更进化的组合物进行修改。
Microtextural and chemical evidence from gabbros indicates that melts may react with the crystal framework as they migrate through crystal mushes beneath mid-ocean ridges; however, the importance of this process for the compositional evolution of minerals and melts remains a matter of debate. Here we provide new insights into the extent by which melt-rock reaction process can occur in oceanic gabbros by conducting a detailed study of cryptic reactive melt migration as preserved in an apparently unremarkable, homogeneous olivine gabbro from deep within a section of the plutonic footwall of the Atlantis Bank core complex on the Southwest Indian Ridge (International ocean discovery program Hole U1473A). High-resolution chemical maps reveal that mineral zoning increases toward and becomes extreme within a cm-wide band that is characterized by elevated incompatible trace element concentrations and generates extreme more/less incompatible element ratios. We demonstrate that neither crystallization of trapped melt nor diffusion can account for these observations. Instead, taking the novel approach of correcting mineral-melt partition coefficients for both temperature and composition, we show that these chemical variations can be generated by intergranular reactive porous flow of a melt as it migrated through the mush framework, and whose composition evolved by melt-rock reaction as it progressively localized into a cm-scale reactive channel. We propose that the case reported here may represent, in microcosm, a preserved snapshot of a generic mechanism by which melt can percolate through primitive mafic (olivine gabbro) crystal mushes, and be modified toward more evolved compositions via near-pervasive reactive transport.