Electron microscopic and rock magnetic studies of magnetic minerals in mafic and carbonate rocks.

Abstract

Scanning and transmission electron microscopic (SEM/STEM) and rock magnetic studies of magnetic minerals in three types of rocks have been undertaken to identify and characterize the carriers of the natural remanent magnetization (NRM) and to determine the relation of concurrent changes in rock chemistry and magnetizations. Basalt samples from the Atlantic Ocean floor are dredged pillows with ages from 0-1 to 70 Ma. The major magnetic mineral in the youngest samples is titanomagnetite. SEM/STEM observations, Mossbauer, XRD and Curie temperature measurements indicate that grains larger than a few $\mu$m range from nearly unoxidized titanomagnetite to near end-member titanomaghemite. However, these large grains are unlikely to carry much of the NRM. SEM/STEM observations also revealed the presence of fine-grained titanomagnetite (single domain size) in interstitial glass of the younger samples. In older samples the glass and the magnetite are increasingly altered. Rock magnetic data show that the fine-grained titanomagnetite in ocean floor basalts is the main carrier of NRM and that its alteration is a major factor in the decrease of NRM with age. Gabbro samples from the Stillwater Complex contain 3 ancient magnetizations: A, B, and C. A is inferred to be a primary magnetization whereas B and C are thought to be secondary. Samples with the A component contain magnetite needles within plagioclase, whereas in samples with B and C components these needles are absent. In contrast the latter samples have anhedral secondary chromian magnetite, in accord with an origin related to Proterozoic mineralization. Thermal demagnetization experiments on the Leadville dolomite and limestone samples reveal three magnetizations: A, B, and C. On the basis of a comparison between directions of these magnetizations and the apparent polar wander path, they are all inferred to be secondary. SEM/STEM observations and rock magnetic analyses suggest that the carriers of these three magnetizations are pyrrhotite, fine-grained magnetite growing on quartz, albite, and chlorite, and fine-grained magnetite inclusions. All these types of magnetic minerals have formed by secondary process during chemical reactions in Late Cretaceous-Early Tertiary, Jurassic, and Permian times.