Garnet versus amphibole: Implications for magmatic differentiation and slab melting

Geology Pub Date : 2023-11-30 DOI:10.1130/g51637.1
Yajie Gao, Hugh St. C. O’Neill, J. Mavrogenes
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Abstract

The garnet signature in the rare earth element (REE) abundances in adakites has been considered a key genetic indicator of these controversial rocks, whose proposed origins include direct melting of subducted oceanic crust (“slab melts”). We show that the garnet signature may be quantified using the shape coefficients of chondrite-normalized REE patterns. We applied this method to a global data set of Cenozoic and Quaternary volcanic samples described as “adakites.” The results indicate that many, but not all, suites of rocks labeled as adakites have undergone fractional crystallization of garnet, starting from parental melts attributable to partial melts of garnet-bearing sources. The extreme garnet signatures seen in many examples require hybrid sources, consisting of subducted sediment as well as igneous oceanic crust; however, extensive deep-crustal differentiation obscures the major and trace-element characteristics of these sources, casting doubt on their identification as primitive slab melts.
石榴石与闪石:对岩浆分异和板块熔融的影响
阿达克岩中稀土元素(REE)丰度中的石榴石特征一直被认为是这些颇具争议的岩石的一个关键成因指标,这些岩石的成因包括俯冲洋壳的直接熔融("板块熔融")。我们的研究表明,石榴石特征可以利用软玉归一化 REE 模式的形状系数来量化。我们将这种方法应用于被称为 "adakites "的新生代和第四纪火山岩样本的全球数据集。结果表明,许多(但并非所有)被标记为 "adakites "的岩石套件都经历了石榴石的部分结晶,这些结晶始于可归因于含石榴石源的部分熔融的母熔体。在许多例子中看到的极端石榴石特征需要混合来源,包括俯冲沉积物和火成岩海洋地壳;然而,广泛的深层岩石分异掩盖了这些来源的主要元素和痕量元素特征,使人怀疑它们是否是原始板块熔体。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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