Natural mass-dependent variations in the isotopic composition of molybdenum

We present the firs! observations of natural mass-dependent fractionation of the isotopic composition of molybdenum (Me), using mufti-collector inductively coupled plasma mass spectrometry. Variations in the isotopic composition of Mo are reported as delta Mo-97/95 (=((Mo-97/Mo-95)(sample)/(Mo-97/Mo-95)(standard)-1)x100 parts per thousand). External analytical precision of delta (97)/Mo-95 is < +/- 0.25 parts per thousand (2 sigma) on natural samples. Our data demonstrate a clear offset of > 1 parts per thousand between sediments deposited under anoxic conditions (delta Mo-97/95=+1.02 to +1.52 parts per thousand relative to our in-house standard) and ferromanganese nodules (delta Mo-97/95 = -0.63 to -0.42 parts per thousand). delta Mo-97/95 of Pacific Ocean seawater (delta Mo-97/95 =+1.48 parts per thousand) lies within the range of values for anoxic sediments, closest to modern Black Sea anoxic sediments. Molybdenites from continental ore deposits have intermediate delta Mo-97/95 ranging from -0.26 to +0.09 parts per thousand. Variations in the abundances of Mo-92, Mo-95, Mo-96, Mo-97 and Mo-98 are consistent with mass-dependent fractionation. A sporadic unidentified interference occurs at mass 94 and Mo-100 is not measured. We hypothesize that the delta Mo-97/95 offset between anoxic sediments and ferromanganese nodules results froth Mo isotope fractionation during inefficient scavenging of Mo from seawater by Mn oxides under oxic conditions. The similarity in delta Mo-97/95 of anoxic sediments and seawater is consistent with the very efficient removal of Mo from seawater under anoxic conditions in the presence of H2S. The data can be interpreted in terms of a steady-state mass balance between the Mo flux into the oceans from the continents and the Mo flux out of the oceans into oxic and anoxic sediments. Such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of Mo to anoxic and oxic sediments. These findings suggest that delta Mo-97/95 in seawater may co-vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans, and that this variation may be recorded in delta Mo-97/95 of anoxic sediments. Hence, the Mo isotope system may be useful in paleoredox investigations. (C) 2001 Elsevier Science B.V. All rights reserved.