US EPA

4842620 

Journal Article 

The molybdenum isotopic compositions of I-, S-and A-type granitic suites 

Yang, Jie; Barling, J; Siebert, C; Fietzke, Jan; Stephens, Ed; Halliday, AN 

2017 

Yes 

Geochimica et Cosmochimica Acta
ISSN: 0016-7037 

205 

168-186 

10.1016/j.gca.2017.01.027 

WOS:000402487700010 

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 delta Mo-98 for igneous rocks ranging from -1.73% to 0.59% with significant overlaps between different types. No relationships between delta Mo-98 and delta O-18 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 delta(98) Mo and K/Rb and [Fe2O3]. There is a positive correlation between initial 87 Sr/86 Sr and d 98 Mo 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 d 98 Mo vs. initial Sr-87/Sr-86 despite the differing regional geology. Localized hydrothermal effects can result in low delta Mo-98 in granite, as seen in three samples from Loch Doon and Criffell which have anomalously light delta(98) Mo of < - 1%. Based on this study, an estimate of delta(98) Mo = 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. (C) 2017 The Authors. Published by Elsevier Ltd. 

Molybdenum isotopes; Granitic rock; Granite; A-type; I-type; S-type; Lachlan Fold Belt; Loch Doon; Criffell; Fleet; The upper crust