EARTH ANALOG FOR MARTIAN MAGNETIC ANOMALIES: A STUDY OF THE REARING POND GABBRO INTRUSION, NORTHERN WISCONSIN

The analysis of strongly magnetized ancient rocks can be used for the study of magnetic anomalies on Mars. Magnetization directions and rock magnetic properties of Keweenawan rocks from the Mid-Continent Rift system (~1.1 billion years old) can give insight into remanent magnetic anomalies over time on Earth and other planets. The Keweenawan rocks used in this study were sampled near Rearing Pond, Wisconsin for rock magnetic and paleomagnetic studies because of the area's strong positive magnetic anomaly. 148 oriented cores from 17 different sites in the Rearing Pond gabbro intrusion make up the population under study. Magnetic measurements were made using an alternating fields and thermal demagnetization instrument and a cryogenic magnetometer. The cores were demagnetized to 99 mT or to 600°C to investigate remanent directions, from these Zijderveld diagrams were plotted and characteristic directions for each sample determined. Although all samples have strong magnetization, directions are scattered and there is little consistency between sites except that the majority of inclinations are positive. Average natural remanent magnetization (NRM) for each site ranged from 21.2 to 8.9 A/m, much higher than average NRMs of terrestrial basalts of ~ 4 A/m. Average susceptibilities ranged from 1.03E-01 to 6.35E-02 (SI units). Four cores were tested for composition and magnetic susceptibility on a KLY4 Kappabridge instrument, indicating Curie temperatures near 580 C and revealing magnetite as the primary magnetic mineral present. A scanning electron microscope was used in backscatter mode to image the oxide minerals, which were found to be predominantly large low-titanium magnetites. Koenigsberger ratios (remanent magnetization to induced magnetization) were calculated for each site, giving average site ratios ranging from 0.98 to 13.62, where ratios higher than one indicate the predominance of remanent magnetization over induced magnetization. The ratios measured indicate strong remanences are an important contributor in the magnetic anomaly observed in the Rearing Pond intrusion and can be used as an analogy to remanent anomalies on Mars.