The procedure for thermal demagnetization involves heating a specimen to an elevated temperature (Tdemag) below the Curie temperature of the constituent ferromagnetic minerals and then cooling to room temperature in zero magnetic field. This causes all grains with blocking temperature (TB) < Tdemag to acquire a “thermoremanent magnetization” in H = 0, thereby erasing the NRM carried by these grains. In other words, the magnetization of all grains for which TB < Tdemag is randomized, as with low hc grains during AF demagnetization. SD grains with short relaxation time also have low TB and can more easily acquire secondary NRM, whereas SD grains with long relaxation time are stable against acquisition of secondary NRM. Thus, thermal demagnetizers are effective in selectively erasing secondary NRM when Tdemag > TB of grains carrying secondary NRM, leaving unaffected the ChRM carried by grains with longer relaxation time (= higher TB).
Schonstedt TSD-1 & Magnon TD-700 (Thermal Demagnetizer)
The TSD-1 and TD-700 is used to provide progressive thermal demagnetization of rock specimens by heating them to any specified temperature up to 800oC and then cooling them in a low magnetic field environment (<10 nT). External fields are attenuated by the shield assembly so that the instrument can be operated in a laboratory environment.
The instruments consist of two separate chambers: one for heating and one for cooling. They are arranged coaxially so that as soon as specimens in the furnace chamber reach thermal equilibrium, the specimen holder can be pushed directly into the cooling chamber. Complete chamber isolation and individual controls allow one specimen batch to be blower cooled while another batch is being heated. An outstanding feature of the instrument is its processing speed. The low thermal mass of both the furnace and sample holder promotes rapid heating and cooling of specimens. The short cycle time helps to minimize the amount of chemical change in specimens by limiting their exposure to high temperature.
The basic sample holder is an open assembly with two inconel tubes on which lies a quartz tray for placing specimens. The quartz tray can hold up to 10 specimens, each 2.5 cm in diameter x 2.5 cm in length. Scientists on board the JOIDES Resolution often find it convenient to heat/cool a batch of specimens in the TSD-1 while measuring a second batch on the cryogenic magnetometer and vice versa, thereby speeding up the processing of data on thermally demagnetized samples.