EarthRef.org Digital Archive (ERDA) -- Cooling curve for the Grenville Province based on 40Ar/39Ar analyses

Description
This cooling curve for the Haliburton Highlands in the Grenville structural province in Ontario is derived by determining a combined date and closure temperature from the same 40Ar/39Ar incremental heating experiment on minerals. Closure temperatures for the minerals were calculated with a presumed cooling rate of 5°C/Ma and must at least have five points that fall on a straight line in a Arrhenius plot. From these plots the closure temperatures can be calculated if one knows the rate parameters for diffusion of Argon, which include the appropriate diffusivity coefficients, the geometry of the grains, the effective radius or grain size, and a cooling rate representative of the terrane in which the rocks and minerals were formed. Typically the closure temperatures decrease sequentially from hornblende, biotite, K-feldspar to plagioclase. The results in this diagram show that at 1000 Ma the geothermal gradient was high at about 33°C/Ma, whereas this gradient has declined sharply to 2°C/Ma at 925 Ma and even 0.3°C/Ma at 700 Ma. Modified after Berger & York 1981.

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