【China Molybdenum Industry News Network】
We present the first observations of natural mass-dependent fractionation of the isotopic composition of molybdenum (Mo), using multi-collector inductively coupled plasma mass spectrometry. Variations in the isotopic composition of Mo are reported as N 97a95 Mo (= ((97 Mo/ 95 Mo) sample /(97 Mo/ 95 Mo) standard 31)U1000x). External analytical precision of N 97a95 Mo is 6 þ 0.25x (2c) on natural samples.
Our data demonstrate a clear offset of s 1x between sediments deposited under anoxic conditions (N 97a95 Mo = +1.02 to +1.52x relative to our in-house standard) and ferromanganese nodules (N 97a95 Mo = 30.63 to 30.42x). N 97a95 Mo of Pacific Ocean seawater (N 97a95 Mo = +1.48x) lies within the range of values for anoxic sediments, closest to modern Black Sea anoxic sediments. Molybdenites from continental ore deposits have intermediate N 97a95 Mo ranging from 30.26 to +0.09x. Variations in the abundances of 92 Mo, 95 Mo, 96 Mo, 97 Mo and 98 Mo are consistent with mass-dependent fractionation. A sporadic unidentified interference occurs at mass 94 and 100 Mo is not measured.
We hypothesize that the N 97a95 Mo offset between anoxic sediments and ferromanganese nodules results from Mo isotope fractionation during inefficient scavenging of Mo from seawater by Mn oxides under oxic conditions. The similarity in N 97a95 Mo of anoxic sediments and seawater is consistent with the very efficient removal of Mo from seawater under anoxic conditions in the presence of H 2 S. 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 N 97a95 Mo 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 N 97a95 Mo of anoxic sediments. Hence, the Mo isotope system may be useful in paleoredox investigations. ß 2001 Elsevier Science B.V. All rights reserved.
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