Iceland exemplifies the potential for generation of abundant silicic magma in the absence of mature island arc or preexisting continental crust. Zircon ages (U-Th and U-Pb) and isotope compositions (Hf and O), combined with whole-rock isotope data (Nd, Hf, Pb), provide insight into the petrogenesis and mantle heritage of these silicic magmas. Zircon and whole-rock samples represent the past 15 Ma of Iceland’s geologic evolution, geographic extent (marginal fjordlands to neovolcanic zones), and modern tectonic settings (on-rift, propagating-rift, off-rift). The generation of Icelandic silicic magma has been influenced by hydrothermally altered crust, via assimilation and/or anatexis, throughout Iceland’s history. This is shown by consistently depleted O isotopes in zircon (median δ18O +3.1‰; >98% below +5.3‰), and silicic rocks. Zircon δ18O values appear to have become lower and more diverse since ca. 0.7 Ma (median +1.9‰). This decrease may reflect lower δ18O of meteoric waters involved in hydrothermal alteration during the Pleistocene and/or more volumetrically significant contributions from low δ18O altered crust. Zircon O compositions from historically active volcanoes confirm that the role of altered crust is greater in on-rift than in off-rift settings; diversity in δ18O at volcanoes in propagating rift settings suggests highly variable contributions from altered crust. The silicic record (whole-rock and zircon) exhibits a correlation between geographic position and isotope composition that seems to be independent of local tectonic setting. Silicic samples of all ages collected above 65° N have more radiogenic whole-rock Hf and Nd isotopic compositions, and less radiogenic Pb, than samples collected in southern Iceland; published isotopic data for basalts suggest a similar time-independent latitudinal trend. The persistence of this trend through time suggests that northern Iceland has been underlain by a more depleted mantle source than southern Iceland throughout the island’s history.

中文翻译：

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冰岛硅质岩浆的时空成因：锆石和全岩中保存的同位素记录

冰岛证明了在没有成熟岛弧或先前存在的大陆地壳的情况下产生丰富的硅质岩浆的潜力。锆石年龄（U-Th 和 U-Pb）和同位素组成（Hf 和 O），结合全岩同位素数据（Nd、Hf、Pb），可以深入了解这些硅质岩浆的岩石成因和地幔遗产。锆石和全岩样本代表了冰岛过去 15 Ma 的地质演化、地理范围（边缘峡湾到新火山区）和现代构造环境（裂谷内、裂谷传播、裂谷外）。在冰岛的整个历史中，冰岛硅质岩浆的产生一直受到热液蚀变地壳的影响，通过同化和/或厌氧消化。锆石（δ18O 中值 +3.1‰；低于 +5.3‰>98%）和硅质岩石中持续耗尽的 O 同位素表明了这一点。锆石 δ18O 值似乎从大约 20 年以来变得更低且更加多样化。0.7 Ma（中位数+1.9‰）。这种减少可能反映了更新世期间参与热液蚀变的大气水的 δ18O 较低和/或来自低 δ18O 蚀变地壳的更大体积贡献。历史上活火山的锆石 O 成分证实，蚀变地壳在裂谷内的作用比裂谷外更大；在传播裂谷环境中，火山中 δ18O 的多样性表明来自改变的地壳的贡献变化很大。硅质记录（全岩和锆石）显示了地理位置和同位素组成之间的相关性，这似乎与当地的构造环境无关。在 65° N 以上收集的所有年龄的硅质样品具有更多的放射成因全岩 Hf 和 Nd 同位素组成，与在冰岛南部收集的样本相比，放射性铅更少；已发表的玄武岩同位素数据表明了类似的与时间无关的纬度趋势。随着时间的推移这种趋势的持续存在表明，在整个岛屿的历史上，冰岛北部的地幔源比冰岛南部更贫乏。