Pumices, ashes, and tuffs from Mt. St. Helens and from Novarupta contain two principal forms
of titanomagnetite: homogeneous grains with Curie temperatures in the range 350–500 C and oxyexsolved
grains with similar bulk composition, containing ilmenite lamellae and having Curie temperatures above
500 C. Thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic
settings and thermal models show that emplacement temperatures and cooling history may have affected
the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on
the Curie temperature of the homogeneous phase. The exsolved grains are most common where emplacement
temperatures exceeded 600 C, and in laboratory experiments, heating to over 600 C in air causes the
homogeneous titanomagnetites to oxyexsolve rapidly. Where emplacement temperatures were lower, Curie
temperatures of the homogeneous grains are systematically related to overburden thickness and cooling
timescales, and thermomagnetic curves are generally irreversible, with lower Curie temperatures measured
during cooling, but little or no change is observed in room temperature susceptibility. We interpret this irreversible
behavior as reflecting variations in the degree of cation ordering in the titanomagnetites, although
we cannot conclusively rule out an alternative interpretation involving fine-scale subsolvus unmixing. Shortrange
ordering within the octahedral sites may play a key role in the observed phenomena. Changes in the
Curie temperature have important implications for the acquisition, stabilization, and retention of natural
remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates
of volcanic units containing homogeneous titanomagnetites
Jackson, Mike; Bowles, Julie.
Curie temperatures of titanomagnetite in ignimbrites: Effects of emplacement temperatures, cooling rates, exsolution, and cation ordering.
American Geophysical Union.
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.