Paleomagnetic dating of the smectite‐to‐illite conversion: Testing the hypothesis in Jurassic sedimentary rocks, Skye, Scotland

Abstract

A hypothesized connection between clay diagenesis and magnetite authigenesis is supported by the results of paleomagnetic, rock magnetic, geochemical, and petrographic studies on Jurassic sedimentary rock of Skye, Scotland. The results have implications for understanding remagnetization mechanisms and for the development of a paleomagnetic method to date clay diagenesis. Previous diagenetic studies indicate that rock in north Skye contains abundant detrital smectite, whereas the clays in the same age rock in south Skye have altered to illite because of Tertiary igneous activity. Geochemical (87Sr/86Sr, δ13C, δ18O) studies confirm that sedimentary rocks in south Skye are altered. The magnetization in the rocks in north Skye is weak and unstable. In contrast, the rocks in south Skye contain a multicomponent magnetization. At intermediate temperatures (225°–450°C) a magnetization with southerly declinations and negative inclinations or its antipodal equivalent is present. At higher temperatures (450°–580°C) a magnetization with northerly declinations and positive inclinations or its antipodal equivalent is removed. Both magnetizations have directions similar to Tertiary igneous rocks on Skye. Rock magnetic studies indicate the presence of pyrrhotite and magnetite. The intermediate‐temperature component resides in pyrrhotite and/or magnetite and is interpreted as a thermoviscous remanent magnetization or a thermochemical remanent magnetization related to the heat and/or hydrothermal activity associated with the igneous activity. The high‐temperature component resides in magnetite and is a chemical remanent magnetization (CRM). The presence‐absence test and the timing of acquisition for this CRM suggest that magnetite authigenesis is related to the smectite‐to‐illite conversion and that clay diagenesis is a viable remagnetization mechanism.