AUTHIGENESIS OF MAGNETITE IN ORGANIC-RICH SEDIMENT NEXT TO A DIKE - IMPLICATIONS FOR THERMOVISCOUS AND CHEMICAL REMAGNETIZATIONS

Rock magnetic results from organic-rich sediment around a Tertiary dik e in Scotland indicate changes in magnetic mineralogy and magnetite co ntent as well as grain size in the contact zone. Sediment specimens wi thin 85 cm of a 90 cm thick dike contain a Tertiary magnetization that is equivalent in direction to the thermal remanent magnetization in t he dike whereas the sediment at greater distances does not carry a sta ble remanence. Based on acquisition of isothermal remanent magnetizati on (IRM), thermal decay of triaxial IRMs, and S-ratio determinations, magnetite is the dominant magnetic phase in the sediment except near t he dike (<8 cm), where hematite is abundant. Maximum laboratory unbloc king temperatures (Tb-max) for the magnetization are higher than predi cted by a thermal cooling model at distances greater than 40 cm but th e Tb-max values are not corrected for gradual cooling of-the dike and are, therefore, overestimates. Trends in rock magnetic parameters such as IRM at 300 mT, and anhysteretic remanence (ARM) are consistent wit h an increasing contribution of single domain/pseudosingle domain magn etite towards the dike. An increase in saturation IRM (SIRM) along wit h a decrease in ARM/SIRM values < 25 cm from the dike also suggest an increase in multidomain magnetite. Low temperature experiments indicat e the presence of abundant superparamagnetic magnetite. Changes in hys teresis parameters are consistent with changes in the relative amounts of the different magnetite size fractions towards the dike. The onset of changes in the magnetite grain size budget starting at 85 cm or ap proximately one dike-width suggests that chemical processes contribute d to the remanence at temperatures as low as 220 degrees C. Petrograph ic and geochemical studies suggest that the contact zone was not perva sively altered by hydrothermal fluids. Utilizing the dike as an analog for thermal processes that occur during burial, the results of this s tudy suggest that moderately elevated burial temperatures are sufficie nt to cause magneto-chemical changes. (C) 1998 Elsevier Science B.V. A ll rights reserved.