The molybdenum isotopic compositions of I-, S- and A-type granitic suites
The molybdenum isotopic compositions of I-, S- and A-type granitic suites
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DOI：
10.1016/j.gca.2017.01.027
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发表时间：
2017-05
影响因子：
5
通讯作者：
J. Yang;J. Barling;C. Siebert;J. Fietzke;E. Stephens;A. Halliday
中科院分区：
文献类型：
--
作者：
J. Yang;J. Barling;C. Siebert;J. Fietzke;E. Stephens;A. Halliday
本研究报告了52个古生代花岗岩类岩石的钼同位素组成与对比源亲和力（A-，I-和S-型）从拉克伦褶皱带（LFB）和新英格兰岩（NEB），无论是在澳大利亚东南部，和三个组成分区岩体（洛赫杜恩，克里弗尔和舰队）位于苏格兰南部高地。结果表明，火成岩的δ 98 Mo变化较大，变化范围为-1.73 ‰ ~ 0.59‰，不同类型之间存在明显的重叠。未观察到δ 98 Mo与δ 18 O或ASI（氧化铝饱和度指数）之间的关系，表明Mo同位素不能清楚地区分火成岩与沉积物源类型。岩浆作用、物源混合作用、区域地质作用以及热液活动对这些花岗岩的钼同位素组成起着重要的控制作用。结果表明，钼主要赋存于黑云母中，少量赋存于角闪石中。δ 98 Mo与K/Rb和[Fe 2 O3]之间的负相关性反映了富铁矿物和角闪石矿物可能优先结合轻同位素。I型花岗岩的初始87 Sr/86 Sr与δ 98 Mo之间存在正相关关系，反映了不同来源物质的混合作用。苏格兰和澳大利亚的花岗岩尽管区域地质不同，但δ 98 Mo与初始87 Sr/86 Sr的曲线趋势惊人地相似。局部热液作用可导致花岗岩中δ 98 Mo较低，如来自洛赫杜恩和克里弗尔的三个样品中所见，它们的δ 98 Mo非常轻，<-1 ‰。据此估算出δ 98 Mo = 0.14 ± 0.07‰（95% s.e.）为中生代上地壳。这比玄武岩稍重，表明同位素较轻的下地壳和/或同位素较轻的脱水板和/或远洋沉积物随时间的推移而俯冲造成的地壳系统变化。
This study reports Mo isotopic compositions for fifty-two Palaeozoic granitic rocks with contrasting source affinities (A-, I- and S-type) from the Lachlan Fold Belt (LFB) and the New England Batholith (NEB), both in SE Australia, and three compositionally zoned plutons (Loch Doon, Criffell, and Fleet) located in the Southern Uplands of Scotland. The results show relatively large variations in δ98Mo for igneous rocks ranging from −1.73‰ to 0.59‰ with significant overlaps between different types. No relationships between δ98Mo and δ18O or ASI (Alumina Saturation Index) are observed, indicating that Mo isotopes do not clearly distinguish igneous vs. sedimentary source types. Instead, effects of igneous processes, source mixing, regional geology, as well as hydrothermal activity control the Mo isotope compositions in these granites. It is found that Mo is mainly accommodated in biotite and to a lesser extent in hornblende. Hornblende and Fe3+-rich minerals may preferentially incorporate light isotopes, as reflected by negative correlations between δ98Mo and K/Rb and [Fe2O3]. There is a positive correlation between initial87Sr/86Sr and δ98Mo in I-type granitic rocks, reflecting the admixing of material from isotopically distinct sources. Granitic rocks from Scotland and Australia display strikingly similar curvilinear trends in δ98Mo vs. initial87Sr/86Sr despite the differing regional geology. Localized hydrothermal effects can result in low δ98Mo in granite, as seen in three samples from Loch Doon and Criffell which have anomalously light δ98Mo of <−1‰. Based on this study, an estimate of δ98Mo = 0.14 ± 0.07‰ (95% s.e.) for the Phanerozoic upper crust is proposed. This is slightly heavier than basalts indicating an isotopically light lower crust and/or a systematic change to the crust resulting from subduction of isotopically light dehydrated slab and/or pelagic sediment over time.