Experimental CRM production in a basaltic rock; evidence for stable, intermediate palaeomagnetic directions

Abstract

Remagnetization experiments involving secondary thermoremanent and chemical remanent magnetization (TRM‐CRM) have been carried out on samples from a basaltic lava flow. The rock carries a stable NRM residing in deuterically oxidized (class II‐III) titanomagnetite grains, and some haematite formed by low‐temperature oxidation. Heating to 525d̀C in a controlled field of 52 μT for periods ranging from 1 to 400 hr, resulted in oxidation to haematite of a parent phase close to magnetite in composition. The direction of the resulting CRM is controlled both by the remagnetizing field and the primary remanence (NRM), giving rise to stable high‐blocking remanence components with intermediate directions, and precluding identification of the primary NRM component. Control experiments carried out on chemically stable specimens gave a partial TRM (PTRM) parallel to the remagnetizing field, with blocking temperatures in agreement with single‐domain theory. After removal of this secondary PTRM, the primary direction in these specimens was regained. The results imply that caution is advisable in analysing the palaeomagnetic directions of any rock which has undergone a thermochemical event. Copyright © 1991, Wiley Blackwell. All rights reserved