Constraining absolute deformation ages: the relationship between deformation mechanisms and isotope systematics

Abstract

Abstract-Isotope diffusion in a mineral is strongly temperature dependent but is also a function of grain size. Deformation must therefore be an important consideration in the interpretation of isotopic data because it provides a means of modifying grain size and shape.We illustrate the range of different deformation mechanisms common in micas and use simple models to investigate the relationship between these and isotope diffusion. We consider three different thermal scenarios with deformation taking place during a) the prograde heating path, b) at the closure temperature of the deforming mineral and c) at temperatures significantly below the closure temperature. We have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages. This modelling illustrates that obtaining deformation ages is critically dependent on an understanding and recognition of the different deformation mechanisms that have affected the sample. In the cases where deformation causes a change in grain size, it is important to characterise both the temperature at which deformation takes place and the closure temperature of grains formed during the deformation. The development of grains with Tc greater than the deformation temperature may record a deformation age. Examples of this condition include i) neocrystallisation; ii) grain size reduction occurring at temperatures below Tc (of the reduced grain size) where the deformation mechanism has reset the grains; and iii) deformationinduced grain coarsening.

10 km Minimum throw along the West Bohemian shear zone: Evidence for dramatic crustal thickening and high topography in the Bohemian Massif (European Variscides)

“…450°C. This high closure temperature has been proposed for those cases where cooling occurred under nearly static conditions without fluid-assisted recrystallisation (Villa 1998; see also Reddy and Potts 1999, and references therein).…”

Thermochronology in Orogenic Systems

“…Many deformed rocks display multiple generations of tectonite fabrics, and techniques such as laser microprobe 40 Ar/ 39 Ar dating of polished thin sections permit the dating of specific minerals in structural context and, thus, determination of the absolute ages of different fabric-forming events. Although care must be taken to ensure that the dates for older grains are not compromised by deformation-induced loss of radiogenic 40 Ar (Dunlap and Kronenberg, 2001;Kramar et al, 2001;Mulch et al, 2002;Reddy and Potts, 1999), some successful studies have been carried out (e.g., Chan et al, 2000;Di Vincenzo et al, 2001;Hames and Cheney, 1997;Mulch and Cosca, 2004;Mulch et al, 2005b). This approach ultimately may prove to be particularly valuable for the study of the duration of and interval between fabric-forming events during orogenesis.…”

Syn-collisional transform faulting of the Tan-Lu fault zone, East China

“…3) were separated by means of standard separation techniques for 40 Ar/ 39 Ar dating. Size fractions of >100 lm were chosen to obtain the separates due to difficulty in separating minerals smaller than 100 lm that also may have low closure temperatures (Reddy and Potts 1999) and are thus not suitable for analysis. On the basis of microscopic observations the following samples were selected for analysis: bulk separates from the eastern margin of the DOB are porphyroclastic phengite, samples X15, X57-2, and X57-5 from the western margin of the SOB are mixtures of porphyroclastic and matrix phengite, sample X15 is porphyroclasts, and samples X19, X42, and X57-4 from ultramylonite are matrix phengite, in which phengite occurs only as a synkinematic mineral in matrix and has grain sizes of 20-150 lm.…”