Studies in crystallization and differentiation in magmatic systems. Part A: Crystal size distributions and growth and nucleation rates in a Reunion Island sill. Part B: The petrogenesis of rhyolite on Rapa Nui (Easter Island).

摘要

Unraveling the origin of the proto-continental crust is central to understanding Earth's evolution. The crust clearly arose by differentiation of mantle-derived basalt, but how? I approach this problem by studying differentiation processes that occur within purely basaltic settings beneath present-day ocean islands. This work consists of two independent investigations of magmatic processes in igneous systems: Part B is a study of the petrogenesis of rhyolite on Rapa Nui (Easter Island), and Part A is a study of crystal sized distributions and growth and nucleation rates in a Reunion Island sill.; The Part B is primarily a petrologic study that addresses the issue of how and at what rate early continental crustal material could have formed. Chemical modeling, petrography, magma physics constraints, and previous studies of sills and lava lakes show that solidification front instability is the dominant mechanism of differentiation on Rapa Nui. Furthermore, this study suggests that thickened (greater than 15 km), relatively evolved regions (i.e., ocean islands that have erupted rhyolite) of the oceanic crust can develop on the order of less than seven million years, an incredibly short period of time when compared to the age of Earth (4.55 billion years).; Part A, a crystal kinetics study, is of a narrower scope. The effects of various magmatic processes, such as flow-differentiation and texture maturation, are characterized by crystal size distribution measurements of samples throughout an eight meter-thick sill on Reunion Island. In the central region of the body, growth rates for plagioclase and Fe-Ti oxides range from {dollar}1times10sp{lcub}-9{rcub}{dollar} to {dollar}4times10sp{lcub}-9{rcub}{dollar} cm/s and {dollar}1times10sp{lcub}-9{rcub}{dollar} to {dollar}2times10sp{lcub}-9{rcub}{dollar} cm/s, respectively, while nucleation rates range from {dollar}8.3times10sp{lcub}-4{rcub}{dollar} to {dollar}2.3times10sp{lcub}-3{rcub}{dollar} nuclei/cm{dollar}sp3{dollar}-s and {dollar}5.6times10sp{lcub}-3{rcub}{dollar} to {dollar}1.4times10sp{lcub}-2{rcub}{dollar} nuclei/cm{dollar}sp3{dollar}-s, respectively.