Quantitative modelling of the morphology of age plateaux in muscovite argon spectra and the implications for thermochronology.

Abstract\nKEY POINTSFor muscovite in ancient Proterozoic and Archean terranes, quantitative modelling of age spectrum morphology shows that a significant plateau segment does not form in an age spectrum unless there was rapid cooling that either began before and continued through, or which initiated at, the time given by the plateau age. Further, the modelling shows that a well-developed plateau is not preserved unless such rapid cooling continues until ambient temperatures are reached that are <∼220–200 °C. It thus follows that the age of a well-developed plateau segment in a muscovite age spectrum is an accurate constraint on when the rock was first exhumed to (and/or last cooled at) shallow crustal levels. In a sample held at a higher ambient temperature, argon continues to be lost from the lattice at a significant rate, well past the time of so-called ‘isotopic closure’ predicted by application of Dodson’s formula. Parametric modelling shows that monotonic cooling paths do have a well-defined point, at which isotopic closure takes place, but to form an age plateau: (1) the rate of cooling must be rapid; and (2) cooling must continue at the same or similar rates, until temperatures are reached that are low enough to inhibit significant further loss of argon to the environment. Quantitative modelling shows how Dodson’s formula begins to fail once ambient temperatures rise above these numerically computed values for isotopic closure. Yet these numerically computed closure temperatures are far below the nominal closure temperature for muscovite, typically quoted as ∼ 350–400 °C. Therefore, these results have significant implications for models that utilise closure temperatures based on estimates using Dodson’s formula, when attempting to invert thermochronological datasets to obtain constraints on the temperature–time path.Quantitative modelling of age spectrum morphology can constrain possible temperature–time (<italic>T–t</italic>) paths because the shape of the age spectrum itself provides considerable information.Well-formed age plateaux developed in argon spectra from muscovite imply very rapid cooling towards ambient temperatures of ∼ 220–200 °C, which are well below the nominal closure temperatures for this mineral, typically quoted as 150–200 °C higher.There are multitudinous circumstances in which Dodson’s formula fail badly, when used to produce an estimate of closure temperature. Successful applications of this formula requires fast cooling to have continued to relatively low ambient temperatures.<italic>T–t</italic> paths should not be constructed by joining the dots provided by age estimates linked to closure temperatures inferred using Dodson’s formula since such <italic>T–t</italic> paths may be counter to the mathematics underlying the concept of isotopic closure. [ABSTRACT FROM AUTHOR]