VSM Paleointensity Determination

An example of a paleointensity determination using the MicroMag 3900 VSM. In this example, a sample (a 57 mg chip off a basalt cube taken from a core collected on Ocean Drilling Program Leg 200) was given a laboratory TRM of known size at 600°C.

This particular basalt had been used in prior experiments in which it altered at a temperature above 400°C during initial heating up to 600°C but it did not alter noticeably on subsequent reheating. For this very simple and rapid test, the sample was heated to 600°C from room temperature (this takes about 10 minutes), a field of 83 microTeslas was applied and the sample was held at 600°C for ~5 min before being cooled to room temperature. The field was then set to zero and the laboratory TRM (considered to be the NRM for this test) was then measured as the sample was heated progressively in 10°C steps up to 600°C. Each step takes ~60 seconds. In other experiments we tried heating every 10°C in intervals as short as 30 seconds, but the sample failed to equilibrate to the set temperature in such short times. Also in other tests, we did more conventional paleointensity determinations with heating, acquisition of pTRMs, and pTRM checks done in progressive steps rather than one step. Such checks are critical for normal paleointensity determinations but are unnecessary for this laboratory test case in which we already know that further sample alteration does not occur. After the decay of the NRM was determined, the field was set to 52 microTeslas with the sample at 600°C for ~5 min and then cooled to room temperature. The decay of this TRM was then monitored as the sample was heated to 600°C. An Arai plot giving the acquisition of the TRM vs the decay of the artificial NRM gives a slope of 1.57, which results in a paleointensity estimate of 82 microTeslas. Again, we could have done multiple steps and pTRM checks rather than heating directly to 600°C, but the results would not change. This entire experiment took about 2.5 hours. To give an idea of the sensitivity of the instrument, the moment of the sample varied from 1E-03 emu to 2E-06 emu (1E-06 to 2E-09 Am^2). The quoted sensitivity for the VSM is 1 microemu (1E-01 A m^2) for a 1 second measurement. The field control appears to be good to about plus or minus 1 microTesla.