Other
Scientific paper
Nov 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998e%26psl.163..221k&link_type=abstract
Earth and Planetary Science Letters, Volume 163, Issue 1-4, p. 221-234.
Other
4
Scientific paper
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 (<8 cm), where hematite is abundant. Maximum laboratory unblocking temperatures (Tbmax) for the magnetization are higher than predicted by a thermal cooling model at distances greater than 40 cm but the Tbmax 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 < 25 cm from the dike also suggest an increase in multidomain magnetite. Low temperature experiments indicate the presence of abundant superparamagnetic magnetite. Changes in hysteresis 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 approximately one dike-width suggests that chemical processes contributed to the remanence at temperatures as low as 220°C. Petrographic and geochemical studies suggest that the contact zone was not pervasively altered by hydrothermal fluids. Utilizing the dike as an analog for thermal processes that occur during burial, the results of this study suggest that moderately elevated burial temperatures are sufficient to cause magneto-chemical changes.
Douglas Elmore R.
Engel Michael H.
Katz Boaz
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