Background
The natural remanent magnetization of the old rocks, e.g. pre-Cambrian rocks and chondrite meteorites, provide us infomation about paleo-magnetic enviroment of ancient earth and early solar system.
However, such ancient rocks have been suffered from chemical and thermal alteration during their long histories.
On the other hand, some part of magnetic materials may have survived the alteration processes.
Recent study suggest that magnetic inclusions (magnetites, kamacite) in sillicates (plagioclace, olivine) are very stable against the thermochemical alterations. Therefore, we should focus on such sub-mm sized grains to get robast paleomagnetic information from the ancient rocks.
To decide the carriers of primary NRM we need to compare with the results of SEM and EPMA analyses. Moreover, we should check the magnetic behavior of them during AF and thermal demagnetization experiments. However, the ordinary bulk measurement can only give us the averages of NRM of both the primary and secondary processes in origin.
One possible solution is picking up grains from rock samples with keeping their orientation, but it is very difficult. Also, such a sub-mm grains is difficult to make a thin section for SEM observation!
Previously, the scanning SQUID microscopy produce good results on the Martian meteorite. Although its spatial and magnetic resolution is exellent, the initial and operational costs are really expencive. The MFM, which often used for rock magnetic study, cannot measure the intensity of the magnetic field yet, and the scanning area is quite small (only 10~100 um^2!). Other magnetic probe microscopes, MR, GMR, and Hall probe microscopes, have not achieved sufficient magnetic sensitivities to observe the rock samples.
The MI (Magneto-Impedance) magnetic sensor has a good magnetic sensitivity that next to the SQUID sensors. The SQUID must be cooled by expensive liquid He, but the MI sensor does not need any cooling systems that are required for the SQUIDs. Moreover the driving circuit of the MI sensor is very simple. Thus, a magnetic microscope utilizing the MI sensor can be a low cost, room-temperature operable, high sensitive magneic microscope that is able to observe the ancient magnetic records of meteorites and Pre-Camblian rocks.
Targets
So, the targets of this microscope are magnetic minerals in ancient rocks, typically kamacite inclusions within olivines in primitive chondrites.
Of cource, this equipment can observe any kind of magnetic infomation if the intensity is enough strong.
Our preliminary measurements have imaged the surface magnetic field of
- LL3 ordinary chondrite (plate)
- CR2 carbonaceous chondrite (plate)
- CV3 Allende carbonaceous chondrite (plate, thin section)
- L6 Tenham shocked ordinary chondrite (plate, thin section?)
- gabbro (plate)
- shocked granite (plate, thin section)
- 1$ bill
- credit card and magnetic cards.