Textural and microanalysis of igneous rocks: Tools for understanding igneous processes

Mantle characterization is vital for understanding magmatism. The notion that source characteristics are preserved transparently in primitive magmas from mantle to eruption can be misleading. The crust acts a cool density filter leading primitive magmas to pool, partially crystallize, and thus evolve. The details of this evolution are seldom completely born out by whole-rock geochemistry. During crustal processing of magmas, crystal populations may be recycled between geochemical reservoirs such as end-members involved in magma mixing, assimilated country rock, or from variably evolved zones of a solidifying magma body. Crystals act as physical vessels to carry compositional and temporal information about magma evolution beyond whole-rock compositions. Textural and microanalytical approaches differ from whole-rock approaches, because they provide a way to dissect crystal populations to reveal their chemical evolution as well as physical details of their nucleation and growth. Retrieval of these types of information are vital for understanding crustal magma evolution. The overarching goal of this work is to better understand the physical manner in which magmas solidify, and hence evolve, remains a fundamental problem in igneous petrology. I use crystal size distributions to identify related crystal populations. I use EPMA, LA-ICP-MS, and a microdrilling Sr isotope method to understand the provenance of select plagioclase crystals. This work is focused upon understanding the petrogenesis of basaltic magmas. The Ontong Java Plateau is the size of Greenland yet basalt compositions across the plateau vary little. I propose a thick complex magma chamber system acted to buffer magma compositions, but at the plateau edge magmas were subjected to less density filtration and thus show more diversity. Highly depleted Detroit Seamount basalts formed during late Cretaceous interaction of the Hawaiian hotspot with a spreading center. Some of these basalts contain evidence of magma mixing between unique melts from the hotspot source. I see no role for OIB-MORB magma mixing. Plagioclase dominated crystallization pre-dates peak crustal assimilation at Elan Ban, Kerguelen Plateau. Magma chamber systems beneath each setting studied are complex and multichambered. Future microanalytical work should focus on use of other isotope systems and use of laser ablation methods.