Authigenesis of magnetite in organic-rich sediment next to a dike: implications for thermoviscous and chemical remagnetizations

Abstract Rock magnetic results from organic-rich sediment around a Tertiary dike in Scotland indicate changes in magnetic mineralogy and magnetite content as well as grain size in the contact zone. Sediment specimens within 85 cm of a 90 cm thick dike contain a Tertiary magnetization that is equivalent in direction to the thermal remanent magnetization in the dike whereas the sediment at greater distances does not carry a stable remanence. Based on acquisition of isothermal remanent magnetization (IRM), thermal decay of triaxial IRMs, and S -ratio determinations, magnetite is the dominant magnetic phase in the sediment except near the dike ( max ) for the magnetization are higher than predicted by a thermal cooling model at distances greater than 40 cm but the 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 with an increasing contribution of single domain/pseudosingle domain magnetite towards the dike. An increase in saturation IRM (SIRM) along with a decrease in ARM/SIRM values