Geochemistry of molybdenum in the continental crust

Abstract

The 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 µg/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.

Ultra-trace Element Characterization of the Central Ottawa River Basin using a Rapid, Flexible, and Low-volume ICP-MS Method

“…Prior to the oceans, decoupling of Mo-W can be traced across estuaries where Mo behaves more conservatively than W (van der Sloot et al 1985, Bauer et al 2018 as well as into terrestrial environments as indicated by river waters with Mo/W ratios ranging from 0.3-68 (Mohajerin et al 2016, Bauer et al 2018. Terrestrial fractionation appears to begin in weathering profiles, but relative Mo-W mobility is probably controlled initially by protolith mineralogy (Greaney et al 2018) as evident by preferential W>Mo release recorded in a basaltic saprolite . The details of pedogenic Mo-W fractionation are poorly understood.…”

Molybdenum and Boron Isotopic Compositions of Porphyry Cu Mineralization‐Related Adakitic Rocks in Central‐Eastern China: New Insights Into their Petrogenesis and Crust‐Mantle Interaction

“…In contrast, a lack of Mo isotope fractionation was observed in the dry Hekla volcanic rocks and less differentiated MORB (Bezard et al, 2016; Yang et al, 2015). Molybdenum is reported to be mainly hosted in rutile, titanite, iron oxides, amphibole, and biotite, occupying the octahedral site by replacing Ti 4+ , Fe 3+ , Nb 5+ , and Ta 5+ (Adam & Green, 2006; Bali et al, 2012; Greaney et al, 2018). Conversely, Mo coordination in silicate melts is predominantly tetrahedral, indicating that silicate minerals should favor the lighter Mo isotopes (Willbold & Elliott, 2017).…”

“…However, some researchers envision only a minor influence of mantle (SCLM) sulfides on the bulk Mo budget due to the relatively low Mo contents and small proportion of sulfides (Wang & Becker, 2018). Nevertheless, the Mo contents in sulfides or sulfidic liquid can be quite heterogeneous (0.05–250 ppm; Greaney et al, 2018; Voegelin et al, 2012). The proportion of sulfides in the mantle may also be heterogeneous, because they are likely to aggregate or pond in the form of monosulfide solid solution or sulfide liquid (Liang et al, 2017).…”

“…Journal of Geophysical Research: Solid Earth (0.05-250 ppm;Greaney et al, 2018;Voegelin et al, 2012). The proportion of sulfides in the mantle may also be heterogeneous, because they are likely to aggregate or pond in the form of monosulfide solid solution or sulfide liquid (Liang et al, 2017).…”

Mineralogy and Leachability of Natural Rocks–A Comparison to Electric Arc Furnace Slags

“…A variety of rare accessory minerals including molybdenite and wulfenite has also been identified in a separate thesis [35]. It is suggested that molybdenite mineralization in this sample occurred due to the hydrothermal input of Mo and S. EPMA of the remaining phases indicate that molybdenite is the main Mo-bearing phase in this sample, but feldspars and muscovite contain MoO 3 concentrations in the range of the detection limit (0.06 wt %) which is 300 times more than the 0.2 mg/kg given by [36]. Interestingly, in another study of the same material wulfenite (PbMoO 4 ) was also found [35].…”