Publication Date

2010

Document Type

Honors Thesis

Department

Geosciences

Keywords

Syenite, Forsterite, Anorthosite, Albite, Intrusions (Geology), Petrology, Rocks, Igneous, Experimental petrology, Kiglapait intrusion, Syenites

Abstract

This study looked at three components of a typical syenite composition: forsterite (Mg2SiO4), anorthite (CaAl2SiO8), and albite (NaAlSi3O8). Various proportions of forsterite and either a pure albite, An48, or An66 feldspar composition were mixed together to simulate a number of possible parent magma compositions. The compositional evolution of these parent magmas was experimentally investigated using a piston-cylinder apparatus in order to simulate the conditions at 5 kbar pressure and across a range of possible temperatures. After holding the sample for 2-4 hours at the desired temperature and pressure in the piston-cylinder, each sample composition was polished, carbon coated, and analyzed using a scanning electron microscope. Glass and mineral compositions were then plotted on a Fo-An-Ab ternary diagram to better constrain the cotectic at 5 kbar. One of the main purposes of this study was to constrain the influence of Mg-rich mafic minerals (such as forsterite, the Mg end-member of olivine) on feldspar compositions at 5 kb pressure. Plotting the compositions of each sample mixture resulted in some interesting trends. Feldspar-rich samples that qualitatively appear to be 100% glass do not plot on the expected bulk compositions. Instead, there is a distinct Na-rich nature to these glasses. As expected, any feldspar crystals that formed had a composition richer in Ca than that of the melt, resulting in glass compositions that are richer in Na and plot to the left of expected bulk compositions for that glass. Not all crystals that were seen compositionally in this way were physically evident on the SEM as Ca-rich crystals settled to the bottom of the less dense, Mg-rich magma. As these crystals were separated iii by only a few millimeters from the top of the graphite chamber, the bulk composition of the system as a whole remained constant. Differences exist between this study's Fo-An-Ab ternary diagram at 5 kbar with the Fa-An-Ab system at the same pressure by Morse et al. (2007). The addition of a more Mg-rich mafic mineral (forsterite) in place of fayalite pushed the cotectic closer to the feldspar join. Consequentially, the forsterite-saturated plagioclase compositions can exist across a wider temperature range (approximately 250°C) than fayalite-saturated plagioclase (27°C) (Morse et al., 2007).

Language

English

Comments

vii, 76 p. : ill. (some col.) Honors Project-Smith College, Northampton, Mass., 2010. Includes bibliographical references (p. 44-45)

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