Petrology and geochemistry of Adak Island plutonic xenoliths: implications for primitive magma generation and crustal differentiation in the Aleutian Island arc

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Emma S Sosa, Claire E Bucholz, Mattison H Barickman, Jill A VanTongeren, Jacob B Setera, Suzanne Mahlburg Kay, Robert W Kay
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引用次数: 0

Abstract

Abstract Deep crustal cumulates in arcs offer a window into the chemistry and crystallization conditions (P-T-H2O-fO2) of primitive basalts in the upper mantle and lower crust and can be studied in ancient exhumed terranes or in xenoliths erupted in young arc lavas. Here, we expand on previous studies and thoroughly characterize the extensive xenolith suites erupted from the Mt. Moffett and Mt. Adagdak volcanic centers (Adak Island, Central Aleutians), which range from primitive ultramafic cumulates to more evolved amphibole gabbros and hornblendites. We present detailed petrography as well as in situ trace and major element mineral chemistry. We use these data to calculate pressure, temperature, and fO2 estimates for the xenoliths, and compare these findings to experimental results to understand the crystallization sequence and P-T-H2O-fO2 under which the cumulates formed. The Moffett crystallization sequence is defined by early amphibole fractionation and an abrupt shift in oxide compositions from chromite to magnetite, while the Adagdak suite is characterized by simultaneous saturation of amphibole+plagioclase and oxide compositions that become increasingly aluminous before magnetite saturation. Olivine–spinel oxybarometry of the Adagdak xenoliths indicates that they are oxidized relative to MORB (FMQ +0.1 to +2.1). Highly fractionated REE and elevated Sr/Y ratios are observed in clinopyroxene from the most primitive cumulates, consistent with a contribution from a basaltic eclogite melt. This basaltic eclogite melt is hypothesized to come from partial melting of the slab or through melting of basalt introduced into the subarc mantle through forearc subduction erosion. These signatures are greatly diminished in the more evolved lithologies, which can be explained through fractionation of plagioclase and amphibole. Our findings support the presence of a complex magmatic plumbing system beneath Adak, with Mt. Moffett and Mt. Adagdak volcanic centers tapping compositionally distinct sources. More broadly, our results are consistent with studies suggesting that low-degree basaltic eclogite melts through slab melting or forearc subduction erosion contribute to arc magmas in the Aleutians, though the associated geochemical signatures are easily obscured by differentiation in the crust.
阿达克岛深成包体的岩石学和地球化学:对阿留申岛弧原始岩浆生成和地壳分异的启示
摘要弧内深部地壳堆积物为研究上地幔和下地壳原始玄武岩的化学和结晶条件(P-T-H2O-fO2)提供了一个窗口,可以在古代出土地体或年轻弧熔岩喷发的捕虏体中进行研究。本文在前人研究的基础上,对Moffett山和Adagdak山火山中心(阿留申群岛中部Adak岛)喷发的大量捕虏岩套进行了全面的描述,这些捕虏岩套的范围从原始超镁铁质堆积到更进化的角闪岩辉长岩和角闪岩。我们介绍了详细的岩石学以及原位微量元素和主要元素矿物化学。我们利用这些数据计算了捕虏体的压力、温度和fO2估算值,并将这些结果与实验结果进行比较,以了解堆积形成的结晶顺序和P-T-H2O-fO2。Moffett晶化序列由早期角闪洞分馏和氧化物成分从铬铁矿向磁铁矿的突变定义,而Adagdak晶化序列的特征是角闪洞+斜长石同时饱和,氧化物成分在磁铁矿饱和之前变得越来越铝。Adagdak包体的橄榄石-尖晶石氧压测定结果表明其相对于MORB呈氧化状态(FMQ +0.1 ~ +2.1)。在最原始堆积的斜辉石中观察到高分馏的REE和高Sr/Y比值,与玄武岩榴辉岩熔体的贡献一致。这种玄武岩榴辉岩熔融被认为是由于板块的部分熔融或玄武岩通过弧前俯冲侵蚀进入弧下地幔的熔融。这些特征在更进化的岩性中大大减弱,这可以通过斜长石和角闪洞的分馏来解释。我们的研究结果支持在阿达克火山下存在一个复杂的岩浆管道系统,莫菲特火山和阿达克火山中心从不同的成分来源中提取岩浆。更广泛地说,我们的结果与一些研究结果一致,这些研究表明,低程度的玄武岩榴辉岩通过板块融化或弧前俯冲侵蚀形成了阿留申群岛的弧岩浆,尽管相关的地球化学特征很容易被地壳的分异所掩盖。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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