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TitleGeochemistry of molybdenum in the continental crust
AuthorGreaney, A T; Rudnick, R L; Gaschnig, R M; Whalen, J B; Luais, B; Clemens, J D
SourceGeochimica et Cosmochimica Acta vol. 238, 2018 p. 36-54, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20190049
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html
Subjectsgeochemistry; mineralogy; igneous and metamorphic petrology; tectonics; crustal studies; continental crust; molybdenum geochemistry; paleoenvironment; tectonic setting; subduction; rifts; accretion; orogenies; greenstone belts; intrusions; magmatism; bedrock geology; lithology; granitic rocks; igneous rocks; intrusive rocks; basalts; granites; tonalites; trondhjemites; granodiorites; volcanic rocks; lavas; titanium geochemistry; whole rock geochemistry; trace element geochemistry; magmas; fluid dynamics; phase relations; partial melting; fractional crystallization; cesium geochemistry; niobium geochemistry; lanthanum geochemistry; mineralogical analyses; sulphides; silicates; Archean; Barberton Greenstone Terrane; Canadian Shield; Superior Province; North Caribou Terrane; English Lake Plutonic Suite; Central Wabigoon Subprovince; Zimbabwe Craton; Qikiqtarjuaq Plutonic Suite; Kilauea Iki Lava lake; Phanerozoic; Precambrian; Proterozoic
Illustrationsphotomicrographs; tables; plots; pie charts
ProgramGEM2: Geo-mapping for Energy and Minerals Western Cordillera, Regional porphyry transitions
Released2018 07 10
AbstractThe use of molybdenum as a quantitative paleo-atmosphere redox sensor is predicated on the assumption that Mo is hosted in sulfides in the upper continental crust (UCC). This assumption is tested here by determining the mineralogical hosts of Mo in typical Archean, Proterozoic, and Phanerozoic upper crustal igneous rocks, spanning a compositional range from basalt to granite. Common igneous sulfides such as pyrite and chalcopyrite contain very little Mo (commonly below detection limits of around 10 ng/g) and are not a significant crustal Mo host. By contrast, volcanic glass and Ti-bearing phases such as titanite, ilmenite, magnetite, and rutile contain significantly higher Mo concentrations (e.g., up to 40 micrograms/g in titanite), and can account for the whole-rock Mo budget in most rocks. However, mass balance between whole-rock and mineral data is not achieved in 4 out of 10 granites analyzed with in-situ methods, where Mo may be hosted in undetected trace molybdenite. Significant Mo depletion (i.e., UCC-normalized Mo/Ce < 1) occurs in nearly every granitic rock analyzed here, but not in oceanic basalts or their differentiates (Greaney et al., 2017; Jenner and O'Neill, 2012). On average, granites are missing ~60% of their expected Mo contents. There are two possible reasons for this: (1) Mo partitions into an aqueous magmatic vapor/fluid phase that is expelled from cooling plutons, and/or (2) Mo is partitioned into titaniferous phases during partial melting and fractional crystallization of an evolving magma. The first scenario is likely given the high solubility of oxidized Mo. However, correlations between Mo/Ce and Nb/La in several plutonic suites suggest fractionating phases such as rutile or Fe-Ti oxides may sequester Mo in lower crustal rocks or in subducting slabs in arc settings.
Summary(Plain Language Summary, not published)
This report tests the mineralogical hosting of molybdenum (Mo) of Archean, Proterozoic, and Phanerozoic upper crustal igneous rocks, as means to constrain whether Mo can reliably used as an indicator for changes in atmospheric conditions over time. Actual Mo abundances in granites are typically 40% of their calculated abundance, indicating Mo is lost during geologic processes, with possible mechanisms for this assessed.

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