Chalcophile Element Geochemistry

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

The geochemical behavior of the economically important chalcophile (sulfur-loving) elements is relatively poorly understood due to historical difficulties in analyzing these elements at the low concentrations in which they occur in common rocks. Recent advances in analytical methods now provides the ability to analyze these elements at relatively high precision at very low (e.g., nanogram per gram) concentration levels. This team aims to use the concentrations of nominally chalcophile elements (specifically Cu, Ga, Ge, As, Mo, Ag, Cd, In, Sn, Sb, W, Tl, Pb, and Bi) to address three fundamental questions in Earth science: a) how has the continental crust formed and evolved over time?, b) how much oxygen was in the atmosphere following the "Great Oxidation Event" (GOE) that occurred at 2.3-2.4 billion years ago?, and c) what is Earth's volatile element inventory? Researchers will investigate these questions through three complementary projects. The first seeks to understand why molybdenum is systematically depleted in granites relative to rare earth elements by analyzing its concentrations in rocks (eclogites) that represent the residues of ancient subducted oceanic crust, and by determining the behavior of Mo during magmatic differentiation that produces granites. Through these studies, they will evaluate whether the upper crustal Mo depletion is due to its retention in subducted slabs, its retention in lower continental crustal cumulates, or its partitioning into a magmatic vapor phase and precipitation in molybdenum sulfide (molybdenite) in hydrothermal veins. The results will shed light on the main processes responsible for generation of the upper continental crust (UCC) and will place constraints on total amount of molybdenite in the UCC, allowing for more robust calculation of pO2 in the ancient atmosphere. The second project seeks to determine the cause of the change in the molybdenum isotope composition of the upper continental crust over geologic history, as observed in glacial deposits (diamictites). The team will analyze Mo isotopes in several modern weathering profiles to determine whether the onset of weathering in the presence of oxygen can explain the diamictite data. If they find that Mo isotopes fractionate during weathering in today's oxygenated atmosphere, it may allow them to use the diamictite data to place constraints on the amount of atmospheric oxygen that was present in the past. Finally, they will determine the distribution of the full suite of chalcophile elements within rocks of the upper mantle that are now exposed in the Pyrenees Mountains. Unlike similar rocks that are rapidly transported to Earth's surface in magmas, these mantle outcrops have retained sulfides, and thus will allow researchers to determine how these elements behave during mantle melting, and also allow for an independent estimate of Earth's abundance of moderately to highly volatile chalcophile elements such as As, Cd, Ga, In, Sn and Tl. Determining the volatile element abundances of Earth provides insights into how our planet formed.