Date of Graduation

8-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Space & Planetary Sciences (PhD)

Degree Level

Graduate

Department

Graduate School

Advisor

Vincent Chevrier

Committee Member

John Dixon

Second Committee Member

Jason Tullis

Third Committee Member

Bill Durham

Fourth Committee Member

Larry Roe

Keywords

Pure sciences; Earth sciences; Mars; Phyllosilicates; Shock-induced alteration; Thermal alteration

Abstract

This study investigates the stability of phyllosilicates on the surface of Mars through laboratory experiments and analysis of terrestrial analogs. Phyllosilicates are mostly found in the oldest Noachian terrains on Mars and hence hold clues to the planet's earliest aqueous and geologic history. Phyllosilicates relevant to Mars were heated up to ~1100°C for up to 24 hours and impacted with projectile velocities up to ~4.5 km/s. Heated samples were analyzed using x-ray diffraction (XRD) and Fourier Transform Infrared (FT-IR) spectroscopy in the near- (NIR, 1.0-2.5 µm) and mid-infrared (MIR, 5.0-15.0 µm) ranges. Impacted samples were also analyzed using Raman spectroscopy and cathodoluminescence (CL). Because of the precise evolution with temperature, NIR spectra can be used to estimate the temperature to which a sample was heated. At higher temperatures (> 700°C), MIR spectra are better for identifying secondary phase formation. All shocked samples, except prehnite, showed evidence of destruction of their mineralogical structure. NIR spectra of shocked clays were not strongly affected but their MIR spectra changed significantly. This could explain some discrepancies between NIR and thermal IR spectra of some phyllosilicates found in association with impact craters on Mars. The IR spectra can help determine formation processes by enabling a distinction between clays that were pre-existing and altered by impacts and those that were formed by impact-induced hydrothermal processes. Shocked serpentine partially transformed into magnesite, indicating shock-induced carbonation of serpentine which has never been shown before. These processes could explain the close association between serpentine and magnesite around impact craters on Mars. To better understand phyllosilicate formation and alteration on Mars, terrestrial analogs were also investigated. Samples from the intrabasaltic bole beds from the Deccan Volcanic Province, India, were analyzed. Red layers contained hematite and montmorillonite; yellow layers contained vermiculite and montmorillonite; green layers contained celadonite and nontronite. While the Deccan samples are all mineralogically different, they are chemically similar to each other and to the underlying original basalt, suggesting transformation from one mineral to the next without ion transfer or loss. In fact, celadonite transforms into smectites often with a vermiculite-intermediate step. This may help explain the stratigraphy of Mawrth Vallis, suggesting an evolution of the alteration process from deuteric alteration to low-temperature weathering in a closed system.

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