MAGNETIC DISCRIMINATION OF PYRRHOTITE-BEARING AND GREIGITE-BEARING SEDIMENT SAMPLES

By using bulk samples, rock magnetic measurements were performed to di scriminate between pyrrhotite- and greigite-bearing shallow marine sed iments that are now uplifted above sea level in southwestern Taiwan. T hermal demagnetization of a composite isothermal remanent magnetizatio n (TRM) was found to be effective in differentiating between the two t ypes of sediments. To check the thermal instability and estimate the t rue unblocking temperature (T-B) spectra of sediments containing these minerals, saturation IRMs (SIRMs) were imparted at each temperature s tep during demagnetization. While pyrrhotite-bearing samples showed un ambiguous T-B temperature spectra, greigite-bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization. Such thermal instabil ity of greigite is a practical and important clue for its identificati on. Zero-field warming of IRM from 5 to 300 K sensitively indicates th e presence of pyrrhotite and trace magnetite in bulk samples without a ny magnetic separation.