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 metadiabase 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 tau Ti-Mo, which defines the loss of Mo relative to the relatively immobile element Ti. The saprolites are also isotopically light: delta Mo-98 values range from -0.89 parts per thousand (relative to NIST 3134) to -0.05 parts per thousand, mean delta Mo-98 = -0.40 parts per thousand, compared to +0.55 parts per thousand (NIST3134) for the underlying unweathered bedrock. By contrast, the ferruginous bauxites generally record addition of Mo relative to the fresh bedrock (zero to positive tau Ti-Mo) and generally have higher delta Mo-98 values than the parental basalts: delta Mo-98 of the bauxites range from -0.14 parts per thousand to +0.38 parts per thousand compared to -0.33 parts per thousand and +0.02 parts per thousand for the unweathered parental basalt. Low delta Mo-98 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 delta Mo-98 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 tau Ti-Mo and delta Mo-98, 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.

Automated summary

Study of molybdenum isotopes in three deep weathering profiles reveals that saprolites generally lose molybdenum and have lighter isotopic values, while ferruginous bauxites tend to gain molybdenum and have heavier isotopic values in continental weathering processes.