HR: 14:10h
AN: GP23B-03 INVITED     [Abstracts]
TI: The carriers of magnetic fabric in volcanic rocks
AU: * Kr\'asa, D
EM: krasa@lmu.de
AF: Department of Earth and Environmental Sciences, Geophysics Section, University Munich Theresienstr. 41, Munich, 80333 Germany
AU: Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: University of Hawaii at Manoa, Hawaii Institute of Geophysics and Planetology, 1680 East-West Road, Honolulu, HI 96822 United States
AB: Studies of the magnetic fabric of volcanic rocks have become an important tool to characterize flow directions in lava bodies, and to investigate dike emplacement and the plumbing of volcanic systems in general. However, depending on factors like the composition of the primary magma, the cooling history and the alteration regime, the magnetic fabric information can be carried by a large variety of ferri- and paramagnetic minerals. The composition and genesis of these minerals influence the ability of volcanic rocks to acquire information about the above mentioned dynamic processes. This presentation focuses on the relation between magnetic mineralogy, magnetic fabric and the techniques to determine the latter. Previous work on the topic will be reviewed and a case study will be discussed. We studied samples from the dike complex of the Koolau volcano on Oahu, Hawaii. These dikes are exposed as a result of the Nuuanu giant landslide. This landslide removed approximately the upper 3000 m of the volcanic edifice of the Koolau volcano. The dikes were thus emplaced deeply inside the Koolau volcano and were subjected to mild metamorphism. AMS measurements show different types of magnetic fabric for the 8 studied dikes. Rock magnetic and microscopic investigations as well as microprobe analyses indicate that the dikes can be subdivided into two groups with distinct magnetic mineralogies: 1. Samples containing almost unaltered coarse grained Ti-poor titanomagnetite and 2. highly altered samples where the primary titanomagnetite is replaced by its alteration products, maghemite and hematite. In both AF and thermal demagnetisation experiments a reversed ChRM for both groups of samples can be isolated. The strongly altered samples (second group) display an antiparallel overprint which is removed between 200 and 300$^{\circ}$C. Strong field thermomagnetic measurements show that above $400^{\circ}$C, the maghemite phase of the second group completely inverts to hematite. AMS measurements after heating indicate that the alteration processes triggered by heat treatment of the samples enhance the magnetic fabric.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting