Molybdenum isotope fractionation in hydrothermal and igneous systems

Molybdenum isotopes have been applied to track the paleoredox conditions of the oceans (e.g., Barling et al., 2001; Siebert et al., 2003), pathways of Mo scavenging from ocean water (Nägler et al., 2011) as well as to study Mo enrichment processes in hydrothermal systems; i.e. moylbdenite mineralization and ore genesis. (e.g. Hannah et al., 2007; Greber et al. 2011). Earlier work about the Mo isotope composition of molybdenites from various different deposits found a wide range of Mo isotopes ratios (e.g. Hannah et al., 2007; Malinovsky et al., 2007). Greber et al., (2011) showed that the previously described wide isotope pattern in molybdenites also occur within a single deposit and suggested that Mo isotope fractionation is suspected to occur in both, the molybdenite precipitation processes as well as in the magmatic source of the hydrothermal fluids.

To investigate this latter explanation more closely, molybdenites from the Questa porphyry-type ore deposit (USA; NM) are studied. First results, in combination with an other thesis about the Kos island arc volcanic system, show significant and resolvable Mo isotope fractionation at temperatures of between ca. 700 and 350°C.

References:

Barling J., Arnold G.L. and Anbar A.D. (2001); Natural mass-dependent variations in the isotopic composition of molybdenum. Earth and Planetary Science Letters, 193, 447-457.

Greber N.D., Hofmann B.A., Voegelin A.R., Villa I.M. and Nägler T.F. (2011); Mo isotope composition in Mo-rich high- and low-T hydrothermal systems from the Swiss Alps. Geochimica et Cosmochimica Acta, 75, 6600-6609.

Hannah J.L., Stein H.J., Wieser M.E., de Laeter J.R. and Varner M.D. (2007); Molybdenum isotope variations in molybdenite: Vapor transport and Rayleigh fractionation of Mo. Geology, 35, 703-706.

Malinovsky D., Hammarlund D., Ilyashuk B., Martinsson O. and Gelting J. (2007); Variations in the isotopic composition of molybdenum in freshwater lake systems. Chemical Geology, 236, 181-198.

Nägler T.F., Neubert N., Böttcher M.E., Dellwig O. and Schnetger B. (2011); Molybdenum isotope fractionation in pelagic euxinia: Evidence from the modern Black and Baltic Seas. Chemical Geology, doi:10.1016/j.chemgeo.2011.07.001.

Siebert C., Nägler T.F., von Blanckenburg F. and Kramers J.D. (2003); Molybdenum isotope records as a potential new proxy for paleoceanography. Earth and Planetary Science Letters, 211, 159-171.