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The fluid evolution of the Mount Mica and Irish Pit pegmatites, Maine| Evidence from stable isotopes

The nature of the lithospheric mantle beneath orogenic belts is incompletely understood due to the paucity of mantle xenolith-bearing basaltic magmas in such regions. One such place where we are afforded the opportunity to study the deep lithosphere beneath an orogenic belt is in central Texas, United States. Mantle xenoliths occur in Late Cretaceous alkali magmas erupted through the remnants of the Appalachian-Ouachita structural belt. Here, we show that geochemical signatures in the form of enrichments in fluid-mobile trace elements (e.g., La) relative to fluid-immobile trace elements (e.g., Nb) are preserved in these xenoliths. We interpret these signatures to represent metasomatism by subduction-related fluids, which implies that the mantle xenoliths represent fragments of continental lithospheric mantle that served as the upper plate during a convergent episode. These observations suggest that some of the original continental lithosphere was preserved beneath the orogenic belt during collision and did not undergo wholesale delamination.

Sub-Milankovitch climate oscillations are well documented phenomena in the Gulf of Mexico during Marine Isotope Stage (MIS) 3 and Termination I, however very little is known about equivalent events during older time intervals. Basin 4 is located on the northwest slope of the Gulf of Mexico and has provided a detailed record of late MIS 6 and Termination II. The results of this study show that the delta18O record of planktonic foraminifer G. ruber contains millennial-scale climate oscillations during MIS 6, a series of meltwater spikes, and a climate reversal during Termination II. Paired delta18O -- Mg/Ca data across these events reveal that the unusually large amplitudes in the delta 18O record cannot be explained by sea surface temperature (SST) or ice volume, but rather are a response to isotopically light glacial meltwater from the paleo-Mississippi river. This conclusion supports the studies of similar oscillations during Termination I and MIS 3.

Thesis (M.S.)--Lehigh University, 2009. / Adviser: Gray E. Bebout.

This thesis consists of three main chapters preceded by an introduction that discusses the importance of diffusion in minerals to constrain the geochemistry of various magmatic processes. The first chapter deals with the experimental technique and measurement of tracer element diffusion data in garnet and clinopyroxene. Self-diffusion coefficients of selected REE have been measured as a function of temperature (770°C-1050°) at 1 bar and oxygen fugacity (fO₂) corresponding to that defined by the iron-wustite buffer. The experimental results indicate small variations of diffusivity for REE in both garnet and clinopyroxene and an activation energy which is similar to the activation energy for diffusion of major components. In the second chapter the atomistic mechanism of Nd diffusion in garnet is investigated by molecular dynamics (MD) simulation. An optimization procedure based on genetic algorithm provides the semi-empirical coefficients that are used to reproduce the repulsive forces between atoms. Results from MD simulations at high pressure and temperature show that Schottky defect is the most favorable mechanism for vacancy formation in the intrinsic region. The preferred reaction to incorporate neodymium in the dodecahedral site involves transferring an iron atom to the octahedral site after removing the aluminum atom from the lattice site. A model of diffusion in the extrinsic region with a prescribed vacancy defect fraction in the garnet (10⁻⁴) also provides an acceptable result. The third chapter considers some of the potential applications of the REE diffusion data in garnet and clinopyroxene to magmatic processes. REE patterns obtained from the solution of a moving boundary problem shows that incompatible elements are more sensitive to disequilibration controlled by diffusion. Melt generated by disequilibrium melting is less enriched in incompatible elements than melt produced by an equilibrium melting process. Solution of a multiphase flow model, including the chemical transport equations with diffusion in a solid phase, permits a more realistic investigation of the disequilibrium melting process. During the ridge evolution the model predicts negligible effect of solid state diffusion on the geochemical evolution of the partial melt and the residual solid.