This research began with a simple question: Is molybdenum a chalcophile element – i.e., is it hosted in sulfides? I began to answer this question with three granites at the University of Maryland, then followed Mo through the Archean continental crust, Hawaiian lavas, subducted oceanic crust, the lithospheric mantle, glacial sediments, and ultimately weathering profiles at the University of California Santa Barbara. Each measurement prompted new questions about the geochemical behavior of Mo in different reservoirs. Along the way, we explored other nominally chalcophile elements like Ga, Ge, Mo, Ag, Cd, In, Sn, Sb, W, Tl, and Bi. Partitioning and isotope fractionation studies of several of these elements (e.g., Cd, Sn, Tl, W) are some of the newest frontiers in geochemistry, as their behavior has the potential to inform us of geological processes that shape our crust and mantle.
Nevertheless, molybdenum remained the central theme of these research projects. Using a combination of laser ablation ICP-MS, standard addition single collector ICP-MS and isotope dilution multi collector ICP-MS, I present six studies on the geochemistry of Mo. In the first two chapters we establish that Mo is not hosted in sulfides in the granitic upper crust and in basaltic to intermediate magmas of Kilauea Iki lava lake in Hawaii. We also find that Mo is systematically depleted from each granite analyzed, relative to the LREE, which led to the establishment of three hypotheses: 1) Mo is fractionated during igneous differentiation, 2) Mo is retained in the slab during subduction, or 3) Mo is removed from cooling granite plutons during fluid exsolution to eventually precipitate as molybdenite (MoS2) in epithermal veins. The first hypothesis was the basis for an undergraduate research project that refuted the idea (Appendix G), the second hypothesis formed Chapter III, and the results of these studies ultimately support hypothesis 3, suggesting that, on average, 60% of the Mo delivered to the upper crust in plutons forms MoS2.
Chapters III and VI follow Mo into the lithospheric mantle and lower crust where we find that Mo isotopes are fractionated during subduction, as isotopically light Mo is incorporated into rutile and isotopically heavy Mo is transported into the mantle wedge. This may partially explain why the continental crust is isotopically heavier than peridotites and basalts. Additionally, we find that Mo is added via metasomatic fluids or melts to residual peridotites where it is ubiquitous along grain boundaries. Molybdenum is enriched in metasomatically deposited sulfides, however its abundance in “primary” mantle sulfides is similar to that measured in crustal sulfides. Mo and W-rich fluids derived from subducted slabs may be the metasomatic agent that infiltrated these peridotites.
Finally, Chapters IV and V explore “low-temperature” isotopic fractionation of Mo during continental weathering at the Earth’s surface. We find that Mo isotopes in glacial diamictites that record weathering of the crust over the last three billion years become progressively lighter with time. We hypothesize that this is due to the onset of oxidative weathering at the Great Oxidation Event (2.4 Ga) and test this hypothesis with studies of modern weathering profiles. Data from saprolites and bauxites confirm that when Mo is lost during continental weathering, it is fractionated, and the light Mo isotopes are retained in the regolith, as observed in the diamictites. We conclude that Mo isotopes record the onset of oxidative weathering at ~ 2.4 Ga, and therefore the rise of oxygen in our atmosphere.