ABSOLUTE PALEOINTENSITY AND MAGNETOMINERALOGICAL CHANGES

Determinations of absolute paleointensity are often hampered by magnet o-mineralogical changes produced within samples of lava flows during s uccessive heatings. Such changes which directly affect the capacity fo r a sample to acquire a partial thermoremanent magnetization (PTRM) ar e reflected by negative pTRM checks, a deviation between the original pTRM and a repeated measurement after heating at a higher temperature. Because the deviations indicated by the PTRM checks can provide a dir ect estimate of the effects to the pTRMs, we suggest that they can be used to correct pTRMs. The correction method we present requires addit ional measurements, in particular pTRM checks performed after each pTR M and specific diagrams to detect acquisition of chemical remanence. P erforming multiple successive pTRM checks can also provide information on the blocking temperatures of the alteration product. These experim ents conducted on lava flows from different localities show that the m agnetomineralogical changes mainly involved grains with blocking tempe ratures lower than the last heating step. The corrections have been te sted on recent and historic lava flows from several localities. We hav e also compared results on samples from the same lava flows that were heated in air and in vacuum. The mean paleointensity values were found to lie within less than 10% from the expected field intensity at each site. The corrections resulted in the recovery of reliable paleointen sity determinations for about twice as many specimens heated in vacuum and 65% additional data for specimens heated in air. Successive heati ngs at the same temperature in air are often accompanied by a time-dep endent oxidation of the natural remanent magnetization which is respon sible for paleointensity values lower than the expected field, a probl em that is not related to the corrections. We conclude that correction s can be used with fairly high confidence to the pTRMs performed in va cuum and in air.