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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Oxygen isotope variations in primitive tholeiites of Iceland: evidence from a SIMS study of glass inclusions, olivine phenocrysts and pillow rim glasses
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Oxygen isotope variations in primitive tholeiites of Iceland: evidence from a SIMS study of glass inclusions, olivine phenocrysts and pillow rim glasses

机译：冰岛原始tholeiite中的氧同位素变化：来自SIMS对玻璃夹杂物，橄榄石表晶和枕边玻璃的研究证据

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摘要

Oxygen-isotope compositions are reported for glass inclusions, their host olivines (Fo_(87-90)) and tholeiitic pillow rim glasses from the neovolcanic rift zone of Iceland. The inclusion compositions range from enriched ([La/Sm]_n = 1.4 - 4.8) to very depleted ([La/Sm]_n = 0.08 - 0.35). The whole rocks and matrix glasses display little variation ([La/Sm]_n = 0.41 - 0.51). Glass inclusions vary somewhat stronger in ~(18)O/~(16)O ratios (δ~(18)O = 4.0 - 6.2 ± 0.6 ‰) than the matrix glasses (δ~(18)O = 4.6 -5.6 ± 0.6‰). The range of olivine phenocryst composition is even wider (δ~(18)O = 2.4 - 6.9 ± 0.5 ‰), and in some cases the variations within individual crystals reach 1.0 - 2.5‰. Most lass inclusions are in O-isotopic equilibrium with the surrounding host olivine but not with the central parts of the host crystals; similarly, most olivines are out of equilibrium with their host matrix glasses. This implies that the glass inclusions and olivine phenocrysts can preserve O-isotopic compositions inherited from contrasting parental mantle-derived magmas, which might have mixed in a common vlocanic system. The isotopic heterogeneity of individual olivine crystals allows one to estimate their residence times in the magma reservoir using the rate of ~(18)O diffusion in olivine. The time required to achieve isotopic homogeneity over 90 - 520 μm ranges from 25 to 1030 yr. The δ~(18)O values found in depleted glass inclusions and their host olivines are unlikely to have resulted from magma contamination by upper crustal rocks. We suggest two possibilities, implying either an O-isotope heterogeneity of the Icelandic mantle or a mixing of two end-members: a refractory component with 'normal-mantle' or somewhat elevated δ~(18)O values, high Ca/Na and strong light rare earth element depletion mixed with a component having lower δ~(18) and Ca/Na but also depleted in incompatible elements. Based on δ~(18)O-La/Sm and δ~(18)O-δ~(11)B relationships, the variations of δ~(18)O between 4.6 and 6.2 ‰ in enriched and depleted glass inclusions are interpreted to reflect those of primary magmas and, consequently, of their source region.
机译：氧同位素组合物用于玻璃包裹体，其宿主橄榄石（Fo_（87-90））和来自冰岛新火山裂谷区的可塑枕缘玻璃。夹杂物的组成从富集（[La / Sm] _n = 1.4-4.8）到非常贫乏（[La / Sm] _n = 0.08-0.35）。整个岩石和基体玻璃几乎没有变化（[La / Sm] _n = 0.41-0.51）。玻璃夹杂物的〜（18）O /〜（16）O比率（δ〜（18）O = 4.0-6.2±0.6‰）比基质玻璃（δ〜（18）O = 4.6 -5.6±0.6 ‰）。橄榄石表晶的组成范围甚至更大（δ〜（18）O = 2.4-6.9±0.5‰），在某些情况下，单个晶体内的变化达到1.0-2.5‰。大多数的夹杂物与周围的宿主橄榄石处于O同位素平衡，但与宿主晶体的中心部分不处于O同位素平衡。同样，大多数橄榄石与其宿主基质玻璃不平衡。这意味着玻璃包裹体和橄榄石的隐晶石可以保留由相反的父母地幔衍生的岩浆继承而来的O同位素组成，这些岩浆可能混合在一个普通的火山岩系统中。单个橄榄石晶体的同位素异质性使人们可以利用橄榄石中〜（18）O的扩散速率来估计它们在岩浆储层中的停留时间。在90至520μm范围内实现同位素同质化所需的时间为25至1030年。贫玻璃夹杂物及其宿主橄榄石中的δ〜（18）O值不太可能是由上地壳岩石的岩浆污染造成的。我们提出了两种可能性，这暗示着冰岛地幔的O同位素异质性或两个末端成员的混合：具有“正常地幔”的耐火成分或较高的δ〜（18）O值，高的Ca / Na和混合了具有较低δ〜（18）和Ca / Na的组分的强光稀土元素贫化，但还贫化了不相容的元素。基于δ〜（18）O-La / Sm和δ〜（18）O-δ〜（11）B的关系，解释了富集和贫化玻璃夹杂物中δ〜（18）O在4.6到6.2‰之间的变化。反映原始岩浆的反射，并因此反映其源区。

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