Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmgp11b0080b&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #GP11B-0080
Other
1519 Magnetic Mineralogy And Petrology, 1540 Rock And Mineral Magnetism, 1595 Planetary Magnetism: All Frequencies And Wavelengths
Scientific paper
Anorthosites, rocks composed of 90% plagioclase feldspar, are not uncommon on Earth, predominant on the Moon, and suspected units on both Mars and Mercury. Due to the minor amount of oxide minerals in most anorthosites, they have long been considered weakly-magnetic. We studied four related but distinct anorthosite units (Egersund-Ogna, Haland-Helleren, Ana-Sira and Garsaknatt) in the Rogaland Igneous Complex in Southern Norway, emplaced into Sveconorwegian basement around 930 Ma. Aeromagnetic maps of the region show moderate positive to large negative anomalies associated with the anorthosites. Measurements on 43 sites (279 samples) of susceptibility, natural remanent magnetization (NRM) and hysteresis properties provide a startling picture of the magnetic behavior of these rocks. Mean NRM values on each anorthosite range from a low of 0.6 A/m on the Egersund-Ogna body to 5.9 A/m on the Haland-Helleren, placing these rocks in similar NRM range to young basalts. Susceptibility varies widely from body to body, with a low of 4.88 x 10-4 SI on the Egersund-Ogna to 2.40 x 10-2 SI on the Haland-Helleren. All units have average Koenigsberger Ratio (Q) values greater than 1, ranging from 8 for the Garsaknatt to 61 for the Egersund-Ogna. With the exception of a few samples in the Garsaknatt, mean destructive fields for alternating field demagnetization for all bodies are greater than 40 mT. Most samples show considerable intensity remaining after thermal demagnetization to 560C and appreciable amounts above 580C. Hysteresis properties from the anorthosites show a wide range of Mr/Ms and Hcr/Hc values. Optical investigation of polished sections reveals the presence hemo-ilmenite in Ana Sira, Egersund-Ogna and Haland-Helleren anorthosites. Minor amounts of magnetite are restricted to the Garsaknatt and parts of the Haland-Helleren anorthosites. Although these four anorthosites have a wide range of magnetic properties, they all have appreciable remanent magnetization and all are capable of producing moderate to strong remanent-dominated anomalies. Because anorthosites are common on the Moon and suggested to exist on other planets, these rocks should be considered as possible sources for planetary paleomagnetism and/or magnetic anomalies.
Brown Laurel
McEnroe Suzanne A.
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