HR: 0800h
AN: GP11D-0870    [Abstracts]
TI: Magnetic Properties and Paleointensity of a Mid-Miocene Gabbro from the Costa Rica Accretionary Wedge, ODP Leg 170
AU: * Hawkins, L K
EM: lisa.hawkins@gmail.com
AF: Western Washington University, Department of Geology 516 High Street, Bellingham, WA 98225 United States
AU: Burmester, R F
EM: russb@cc.wwu.edu
AF: Western Washington University, Department of Geology 516 High Street, Bellingham, WA 98225 United States
AU: Housen, B A
EM: bernieh@cc.wwu.edu
AF: Western Washington University, Department of Geology 516 High Street, Bellingham, WA 98225 United States
AB: Samples of gabbro from cores recovered by the Ocean Drilling Program Leg 170 have been analyzed for magnetic mineralogy and paleointensity using a Thellier-Thellier method. The cores are from the Costa Rica Accretionary Wedge, Site 1039, Hole C at a minimum depth of 423 meters below the sea floor. This gabbroic unit is younger than 16.49 Ma based on nannofossils in the intruded oozes. This makes it ideal for a paleointensity study because few paleointensity data exist for this time period in this region. Rock magnetic investigations included continuous low field thermomagnetic analysis, alternating field (AF) demagnetization, optical microscopy, scanning electron microscopy (SEM), and element analysis. AF demagnetization paths are straight to the origin on orthogonal vector endpoint diagrams suggesting that the magnetization is single component. Optical microscopy and SEM observations identified titanomagnetites as the most abundant magnetic mineral in the samples; occurring as either coarse- or fine-grains with euhedral shape. Titanomagnetite composition, estimated from weight percents provided by element maps from the backscattered electrons, are both low and high titanium, TM22 and TM74. Reversible susceptibility vs. temperature curves from low field thermomagnetic analysis to 710$\deg$C suggests that the minerals do not alter in an argon atmosphere. Curie temperatures below 200$\deg$C and above 400$\deg$C calculated from these curves are consistent with the SEM results. The modified Thellier technique used a 0.4 uT field and an argon atmosphere. Low temperature demagnetization before and after some temperature steps was used to look for different behavior of the large and small magnetite grains. Based on Thellier and thermal experiments, most of the samples have about equal contributions from low and high Curie temperature titanomagnetite. Although alternating field demagnetization produced straight line paths on demagnetization diagrams, thermal results show that the two magnetites carry different components. We are currently exploring the reasons for the difference, e.g., the contribution of drill string remanence, and whether either magnetite can be used to determine the paleomagnetic field intensity.
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting