Date of Award

7-2010

Document Type

Thesis

Degree Name

Master of Science (MS)

College/School

College of Science and Mathematics

Department/Program

Earth and Environmental Studies

Thesis Sponsor/Dissertation Chair/Project Chair

Stefanie Brachfeld

Committee Member

Sandra Passchier

Committee Member

Matthew L. Gorring

Abstract

This thesis examines potential carriers of magnetic anomalies found in remote areas where direct sampling is impossible. The areas examined are the southern hemisphere of Mars and the East Antarctic Margin. We examine the rock magnetic and remanence properties of synthetic Mars basalts and Antarctic dropstones, to predict the type and intensity of anomaly they would produce.

The anomalies measured within the Martian crust are entirely remanent magnetization and are remarkably orders of magnitude stronger than the strongest terrestrial anomaly. Two basalt compositions denoted M-type and T-type, deemed relevant to the crust of Mars, were synthesized to examine contrasts in rock magnetic and remanence properties following identical thermal histories and oxygen fugacity conditions. We examine the magnetic mineral assemblages produced and assess if they are efficient remanence carriers. The composition denoted T-type is rich in Al and poor in Fe, reflecting constraints provided by thermal emission spectroscopy that the Martian crust is somewhat terrestrial in character. The M-type composition is poor in Al and rich in Fe, reflecting the composition of basaltic liquid in equilibrium with Martian meteorite phase assemblages. The two compositions are identical with respect to MgO, SÌO2 , and TÌO2 . Batches of each composition were cooled from above 1200 °C to 1070 °C at 4 °C/h and annealed at 1070°C for 100 h, then quenched. Samples were then held at 650°C for periods ranging from 21 to 158 days under quartz-fayalite-magnetite (QFM)/02 buffer conditions, then quenched. The experimental conditions are germane to shallow igneous intrusions, which might be a significant volumetric fraction of the Martian crust and potential carriers of crustal magnetic anomalies, and provide an important contrast to a previous set of fast-cooled (3-230 °C/h) basalts our group performed on the same two compositions.

M-type samples contain Fe-Ti-Al-Mg oxide grains 40-50 pm in diameter with skeletal and equant euhedral morphologies. T-type samples contain equant euhedral Fe- Ti-Al-Mg oxides with grain diameters ranging from 15-30 pm as well as elongated anhedral ilmenite grains. For M-type samples both the starting material and the samples annealed at 650 °C have narrow multidomain hysteresis loops. T-type starting materials and samples annealed at 650 °C have pseudo single domain (PSD) hysteresis loops, but the annealed samples plot lower and to the right within the PSD field on a Day plot, indicating coarser magnetic grains. Alternating field demagnetization of anhysteretic remanent magnetization (ARM) shows median destructive fields < 10 mT. M-type samples exhibited higher magnetic susceptibility and intensity of remanence than T-type samples. Both M-type and T-type samples carry an intense natural remanent magnetization (NRM). The NRM is inferred to be a thermoremanent magnetization (TRM) acquired during quenching and air-cooling after the 650 °C anneal. NRM values range from 0.03 to 170 mAm2/kg for M-type samples and 0.005 to 47 mAm2/kg for Ttype samples, values comparable to those observed in rapidly cooled synthetic basalts of the same chemical composition. However, the slow- cooled samples have a much “softer” coercivity spectrum. The multi-domain magnetic mineral assemblage suggests that while intrusions generated by slow-cooled basaltic melts are capable of carrying intense TRMs they may be less stable over geologic time.

The second portion of this study presents a catalog of the magnetic and petrographic properties of dropstones collected by United States Antarctic Program cruise NBP01-01. These dropstones provide direct samples of the subglacial geology and provide a range of susceptibility values, intensity of natural remanent magnetization, and description of the carriers of magnetization, which can help in the interpretation of magnetic anomaly surveys. Samples were collect in three main regions along the East Antarctic Margin, including the George V Coast, Prydz Bay, and MacRobertson Land. The samples are examined and classified optically through thin section analysis, and characterized magnetically through magnetic susceptibility, natural remanent magnetization, magnetic hysteresis, scanning electron microscopy and thermomagnetic measurements. The potential for each sample to generate a dominant induced or remnant magnetization was calculated using the Koenigsberger ratio, Q; where Q = Total Remnant Magnetization / Total Induced Magnetization. The majority of magnetic minerals are found in low abundances (<1%) in granitic and metamorphic rocks and displayed Q values less than 1, suggesting they would produce induced dominated anomalies. The exception to this are two mafic igneous rocks, vesicular basalt and dolerite, that possess single domain magnetic minerals and recorded remanences several orders of magnitude higher than other east Antarctic rock types.

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