The Conglomerate Test As A Magnetic Means Of Placing A Temperature Minimum On The Diatremes At OKA Canada

Abstract

The magnetic conglomerate test was applied to samples from eight Cretaceous diatremes located in and around the Oka Carbonatite Complex near Montreal, Canada. Fifty-four subsamples from the eight diatremes were tested in an attempt to place constraints on their temperatures of emplacement. These eight outcrops were chosen because they appear to petrographically represent different facies of diatreme emplacement. The opaque minerals samples were petrographically analyzed using reflected light and chemically analyzed using an electron microprobe for evidence of major alterat1on. Several discrete subsamples from each sample were cleaned magnetically using A.F. and thermal demagnetizers. The subsamples of each sample were found to have similar high temperature primary thermoremanent magnetization. The alpha-95 numbers, a measure of the precision of the calculated mean plot, ranged from 10 ° to 25 ° . These small cone angles equate with a high degree of non-randomness. This concordancy of thermoremanent magnetizations (TRM's) suggests that the magnetic minerals of the diatremes were locked in place as constituents of a solidified magma before the temperature of the minerals dropped below the 555°C average blocking temperature (calculated using Vegardian assumptions) of the magnetic minerals in the diatremes. The temperature of 555°C determined by this project, allows heat budget modelling taking into account the Joule-Thomson effect, addition of relatively cold xenoliths, the heat of crystallization of magma, and the original heat of the magma. A maximum of 2.4 x 1019 calories (including 10km^3 of1200°C magma and an endothermic Joule-Thomson effect) and minimum of 4.2 x 1015 calories (including 0.1 km^3 of 720° C magma and an exothermic Joule-­Thomson effect) could be calculated as the heat given off from an entire diatreme. The endothermic Joule-Thomson effect could not have been important in the Oka diatreme outcrops because CO2 is not endothermic until a depth of 2.5 km. , and the erosion level at Oka is at least 2.4 km deep. Relative to the magnetic blocking temperature of 555° C from this study, if Oka diatremes were CO2-rich then the J-T cooling effect would only have been involved after the magnetic imprinting. The exothermic Joule-Thomson effect was relatively insignificant in magnitude, and had little effect on the overall heat budget of the diatremes.