What is Magnetic in the Lower Crust? Studies from the Athabasca Granulite Terrane, Canada

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

The International Decade of Geopotential Research, a global program concentrated on gravity and magnetic data from satellite observations, is drawing to a close with its most ambitious program. SWARM, a trio of satellites to measure magnetic fields, is due to launch in 2011 and will collect an unprecedented amount of field data. One of the goals of the mission is to investigate the magnetism of the Earth's crust, providing a bridge between models for the lower crust from satellite missions to our knowledge from aeromagnetic surveys. Previous studies of satellite data on crustal magnetism have led to models requiring high susceptibility and thick continental crust with strong magnetizations to duplicate the observed fields. Investigations of lower crustal rocks, now uplifted and exposed at the surface due to deformation, do not reveal similarly high susceptibilities, causing us to question magnetic processes in lower crustal rocks. One of the world's largest exposed sections of lower crustal rocks, the Athabasca Granulite Terrain in northern Canada, provides us an excellent place to study first hand the magnetic character of these deep-seated rocks. Aeromagnetic anomaly maps of the region reveal a complex and varied pattern with numerous positive and negative areas; the collection of rock samples from the area will allow us to study the magnetic nature and history of the materials. Results will aid in investigating the history of the Earth's magnetic field, and in unraveling the geologic history of our planet. The nature of the magnetic properties of lower crustal rocks, now exposed at the surface in the Athabasca Granulite Terrain, northern Canada, will aide in answering the question "What is magnetic in the lower crust" Earlier studies of lower crustal rocks exposed at the surface have failed to provide sufficient magnetization to model observed magnetic anomalies from satellite data. Our goal is to provide a detailed interpretation of lower crustal rocks in an area where aeromagnetic studies indicate there are numerous distinct anomalies and link this to satellite observations and models of the lower crust. It is hypothesized that remanent magnetization, possibly due to lamellar magnetization found in hemo-ilmenite rocks, must be a component of the observed magnetizations. Methods will include ground magnetic and susceptibility measurements in situ, collections of rocks from key field locations identified using aeromagnetic anomalies, and laboratory measurements of numerous magnetic properties. Measurements of hysteresis, susceptibility, and magnetization behavior at high and low temperatures along with microscopy studies, both optical and SEM, of well-identified lower crustal rocks will provide detailed information on the magnetic signature, behavior, and mineralogy of the materials responsible for the observed anomalies. All magnetic studies will be keyed to on-going geologic and metamorphic research in the terrain, providing additional constraints, information, and interpretations for aid in answering this question.