  Tiny basaltic samples containing finely grained titanomagnetite with Curie
temperature less than 100 C were heated in air in weak field (25 to 100microT)
at temperatures between 400 and 560 C for times as long as 32 hours.
Oxyexsolution of titanomagnetite resulted in the crystallization of interacting
single domain particles with Curie point close to 540 C and the concomitant
development of one of two types chemical remanence, depending upon thermal
treatment: isothermal chemical remanence (CRM) or thermochemical remanence
(TCRM), the latter acquired under the combined effects of chemical change and
temperature decrease. CRM and TCRM acquired under various conditions were
subjected to Thellier-type experiments. The CRM/TRM ratio is found to be less
than one and increases rapidly with acquisition temperature (0.35 at 400 C,
0.65 at 450 C, and 0.90 at 500 C). Thus, very large underestimate of
geomagnetic field paleostrength can occur when a natural CRM is not recognized
as such and is believed to be a natural TRM. In an attempt to mimic deuteric
oxyexsolution in cooling magma, TCRM was imparted from 560 to 400 C at a
cooling rate of 0.1 C/mn. The TCRM/TRM is found to slightly exceed one (1.10),
in agreement with the theoretically predicted effect of the difference in
cooling rate.
