Molybdenum partitioning behavior and content in the depleted mantle: Insights from Balmuccia and Baldissero mantle tectonites (Ivrea Zone, Italian Alps)

Abstract

Constant concentration ratios of trace elements of similar incompatibility in oceanic basalts have been used to determine the abundances and ratios of incompatible elements in mantle sources. However, the Mo content in the depleted mantle estimated from Mo/Ce of oceanic basalts is much lower than the estimates based on Mo contents of mantle peridotites (25 ± 7 ng/g versus 113–180 ng/g). This discrepancy partly reflects uncertainties about the geochemical behavior of Mo in the upper mantle. New Mo concentration data obtained on unserpentinized spinel-facies lherzolites, harzburgites, dunites and pyroxenites (n = 47) from the Balmuccia and Baldissero mantle tectonites (Ivrea-Verbano Zone, Italian Alps) provide new insights into the magmatic behavior of Mo and its content in the depleted mantle.The peridotites and pyroxenites display variable and very low Mo contents (4–16 and 3–11 ng/g, respectively). A slightly serpentinized peridotite shows the highest Mo content of 37 ng/g. The Mo contents show no systematic variation with other incompatible or compatible elements. Chromium spinel in the mantle rocks likely does not control the behavior of Mo because complete or incomplete digestion of spinel did not lead to a noticeable change in Mo contents. The data indicate incorporation of little Mo into olivine, pyroxene and spinel, consistent with the predominant occurrence of Mo6+ and the experimentally-determined very low partition coefficients at typical upper mantle conditions. Bulk rock Mo contents from these mantle rocks, upper limits of Mo contents in sulfides and the lack of variations of Mo concentrations with varying sulfide fractions indicate a minor influence of mantle sulfides on the bulk Mo budget. The incorporation of little Mo into pyroxenes of the mantle rocks contrasts with Ce which is mainly controlled by clinopyroxene. This contrasting behavior suggests a higher incompatibility of Mo than Ce during mantle melting. Modelling polybaric melting of mantle peridotites indicates that melts formed at low degrees of partial melting (e.g., <2–3%) would not retain the Mo/Ce of the mantle sources. In contrast, >5% melting, as in tholeiites and komatiites, leads to extraction of nearly the complete inventory of Mo and Ce into the melts and thus Mo/Ce of the melts should be representative of mean mantle source compositions, even if Mo and Ce show different bulk partition coefficients. Molybdenum contents of mantle rocks from the Ivrea Zone and the data on oceanic basalts and komatiites indicate that the depleted upper mantle has a low Mo content of <30 ng/g.