Experimental constraints on magnetic stability of chondrules and the paleomagnetic significance of dusty olivines
Résumé
Dynamic crystallization experiments are conducted under a magnetic field to determine both magnetic and mineralogical properties of chondrules. The experiment reproduced synthetic dusty olivine samples that were formed by a high temperature reduction of an initially fayalitic olivine. Backscattered-electron microscopy observations confirmed that synthetic dusty olivine contains abundant fine, submicron-sized Ni-poor Fe inclusions in the cores of MgO-rich olivine grains, similar to that in natural chondrules. Alternating field demagnetization experiments of dusty olivine samples indicate mean destructive fields of up to 80 mT, suggesting the submicron-sized Fe inclusions are a carrier of stable remanence. In natural chondrules, fine Fe inclusions in the dusty olivine may have been armored against chemical alteration by surrounding host olivine crystals. Since the fine Fe inclusions were probably heated above the Curie temperature during the last chondrule forming events, the fine Fe inclusions in dusty olivine can acquire thermal remanent magnetization during the chondrule formation event. Theoretical time–temperature relation of such fine-grained Fe (kamacite) grains suggested that a paleomagnetic data observed above 490 °C in thermal demagnetization experiments of dusty olivines is reliable despite the low-grade metamorphism of unequilibrated ordinary chondrites (e.g., LL3.0). Therefore, the presence of fine Fe inclusions in dusty olivine in unequilibrated ordinary chondrites constrains that such dusty olivine in chondrules is a good candidate as an un-altered and stable magnetic recorder of the early solar magnetic field.