Nd-, O-, and H-isotopic evidence for complex, closed-system fluid evolution of the peralkaline Ilı́maussaq intrusion, south Greenland
Nd-, O-, and H-isotopic evidence for complex, closed-system fluid evolution of the peralkaline Ilı́maussaq intrusion, south Greenland
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格陵兰岛南部过碱性 Ilı́maussaq 侵入体复杂、封闭系统流体演化的 Nd、O 和 H 同位素证据
DOI：
10.1016/j.gca.2003.12.008
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发表时间：
2004
影响因子：
5
通讯作者：
G. Markl
中科院分区：
文献类型：
--
作者：
M. Marks;T. Vennemann;W. Siebel;G. Markl
南格陵兰岛碱性-泥质侵入岩中角闪洞、斜辉石和橄榄石分离物的氧同位素组成（相对于VSMOW为+5.2 ~ +5.7‰）和钕同位素组成(原生岩浆矿物εNd（T)= - 0.9 ~ - 1.8，晚期伟晶岩和热液脉矿物分离物εNd(T)= - 0.1 ~ - 0.5）相对均匀，表明该火成岩杂岩为封闭体系演化，支持地幔成因岩浆。与均匀的氧和钕同位素数据相比，手工挑选的角闪岩分离物的δD值在- 92和- 232‰之间变化，是已知火成岩角闪岩中最贫氘的值之一。与这些角闪石共存的流体相氧同位素组成均匀，在岩浆水的正常范围内，但极不均匀且D/H比低，表明氧-氢同位素系统存在解耦。在解释角闪岩中异常低的δD值的几种可能性中(例如，与大气水的晚期热液交换，广泛的岩浆脱气，有机质污染，和/或铁含量和压力对角闪石-水分馏的影响)，对δD值范围的最可能解释是角闪石受到次级相互作用的影响，并与通过岩浆晚期氧化内部生成的ch4和/或H2获得的贫d流体重新平衡。这一解释与已知的岩浆岩中存在丰富的岩浆ch4以及先前对岩石和流体同位素组成的研究相一致。
Relatively homogeneous oxygen isotope compositions of amphibole, clinopyroxene, and olivine separates (+5.2 to +5.7‰ relative to VSMOW) and neodymium isotope compositions (εNd(T)= −0.9 to −1.8 for primary magmatic minerals and εNd(T)= −0.1 and −0.5 for mineral separates from late-stage pegmatites and hydrothermal veins) from the alkaline to agpaitic Ilı́maussaq intrusion, South Greenland, indicate a closed system evolution of this igneous complex and support a mantle derivation of the magma. In contrast to the homogeneous oxygen and neodymium isotopic data, δD values for hand-picked amphibole separates vary between −92 and −232‰ and are among the most deuterium-depleted values known from igneous amphiboles. The calculated fluid phase coexisting with these amphiboles has a homogeneous oxygen isotopic composition within the normal range of magmatic waters, but extremely heterogeneous and low D/H ratios, implying a decoupling of the oxygen- and hydrogen isotope systems. Of the several possibilities that can account for such unusually low δD values in amphiboles (e.g., late-stage hydrothermal exchange with meteoric water, extensive magmatic degassing, contamination with organic matter, and/or effects of Fe-content and pressure on amphibole-water fractionation) the most likely explanation for the range in δD values is that the amphiboles have been influenced by secondary interaction and reequilibration with D-depleted fluids obtained through late-magmatic oxidation of internally generated CH4and/or H2. This interpretation is consistent with the known occurrence of abundant magmatic CH4in the Ilı́maussaq rocks and with previous studies on the isotopic compositions of the rocks and fluids.