Radiogenic isotope geochemistry and the evolution of the Earth's surface and interior

The radiogenic isotopic composition of Earth materials can be used to elucidate the nature of geologic processes operating over a wide range of timescales, lengthscales, and in different environments. As a means of understanding the evolution of the Earth's sediment-covered surface, the uranium-series (U-series) comminution age method was applied to detrital sediments. The U-series comminution age method is a way of using sediment (234U/ 238U) activity ratios to obtain the elapsed time since a detrital sediment particle has been reduced below a threshold grain diameter. To determine the method's applicability to terrestrial sediments, comminution ages were determined for sediment core samples from the glacigenic alluvial Kings River Fan (California). This case study showed that the measured (234U/ 238U) activity ratios have behavior that is consistent with the comminution age model based on alpha recoil loss of 234U. Comminution ages calculated from the measured U isotopes are similar to the available independent age constraints on the core samples. Acid-leaching pretreatment procedures for the comminution age method were also investigated to determine an appropriate way of removing unwanted nondetrital phases. A minimum in the (234 U/238U) activity ratio of the leached residue as a function of leaching strength was used a criterion for identifying an optimal leaching procedure. Based on this criterion, a procedure closely following the classic sequential extraction protocol of Tessier et al. (1979) was the most favorable pretreatment procedure of the methods tested.;The Earth's subsurface and deep interior was investigated using modeling of neodymium isotopes and trace elements to address the longstanding conundrum of whole-mantle versus layered-mantle convection. Accounting for the presence of trace amounts of water in the source region for mid-ocean ridge basalts (MORBs) can resolve this conundrum, since wet partial melting affects the composition of the melt residues. Over time, subduction of these wet melting residues with whole-mantle convection dynamics can produce a compositionally-layered mantle with chemical characteristics consistent with the observed composition of erupted oceanic basalts.