Assessing molybdenum isotope fractionation during continental weathering as recorded by weathering profiles in saprolites and bauxites

Molybdenum isotopes in three deep and well-characterized weathering profiles – a saprolite formed on meta-diabase from South Carolina, USA, and two ferruginous bauxites formed on Columbia River Basalts in Oregon and Washington, USA – elucidate Mo isotope behavior during continental weathering. The saprolite records an overall loss of Mo relative to the fresh bedrock, as indicated by negative τ^Mo_Ti, which defines the loss of Mo relative to the relatively immobile element Ti. The saprolites are also isotopically light: δ⁹⁸Mo values range from −0.89‰ (relative to NIST 3134) to −0.05‰, mean δ⁹⁸Mo = −0.40‰, compared to +0.55‰ _NIST3134 for the underlying unweathered bedrock. By contrast, the ferruginous bauxites generally record addition of Mo relative to the fresh bedrock (zero to positive τ^Mo_Ti) and generally have higher δ⁹⁸Mo values than the parental basalts: δ⁹⁸Mo of the bauxites range from −0.14‰ to +0.38‰ compared to −0.33‰ and +0.02‰ for the unweathered parental basalt. Low δ⁹⁸Mo values in the saprolites likely reflect preferential retention of isotopically light Mo adsorbed onto accessory Fe-oxy-hydroxides and clays during weathering, whereas the high δ⁹⁸Mo values in the bauxites reflect the addition of isotopically heavy Mo from groundwater. When the three profiles are combined, there is a positive correlation between τ^Mo_Ti and δ⁹⁸Mo, suggesting that when Mo is lost during continental weathering, the resulting regolith is isotopically light, whereas groundwater addition can shift the regolith to heavier values. Because saprolites are a more common weathering product than bauxites, we conclude that, in general, continental weathering fractionates Mo isotopes such that the weathered upper crust retains isotopically light Mo. In contrast, the groundwater that leaches Mo from the weathered crust is isotopically heavy. Thus, chemical weathering of continents generates the isotopically heavy riverine signature observed globally, and partially contributes to the isotopically heavy seawater signature. Finally, these data, in conjunction with previously published data for glacial diamictites, can be used to assess changes in the crustal Mo isotope signature over the last 2.9 Ga.