Oscar M. Lovera

Tibetan tectonics from 40Ar/39Ar analysis of a single K-feldspar sample

*°Ar/39Ar data on an alkali feldspar sample from the Quxu pluton, Gangdese batholith, southern Tibet, allow a detailed assessment of unroofing and uplift history between 35 and 18 Ma. The 39Ar Arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter, log(D/r2), and reciprocal temperature, which we interpret to be the result of a distribution of distinct diffusion-domain sizes. We use an alternative way of plotting the Arrhenius data that exhibits domain size versus cumulative % 39Ar released during step heating. The 4°Ar/39Ar age spectrum of the sample has features, such as local age plateaus, that are most easily explained in terms of the distinctive closure age of particular domains. The fact that the same distribution of diffusion-domain sizes explains both the Arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment. Modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling-history segment rather than a single time temperature datum. We demonstrate the robustness of the cooling-curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history. The best-fit cooling curve for the Quxu sample shows a decreasing rate of cooling in the time interval 35-18 Ma, followed by a very rapid cooling beginning at about 18 Ma. We have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the Quxu cooling curve, and find that in the 35-20 Ma time interval, the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing (unroofing rates of approximately 0.05 mm/yr) followed by a brief period ( < 5 Ma) of very rapid unroofing with rates of order 2 mm/yr.

40Ar39Ar results for alkali feldspars containing diffusion domains with differing activation energy

Abstract Extension of the single diffusion domain/activation energy closure model of Dodson to apply to minerals with a discrete distribution of domain sizes appears to reconcile 40 Ar 39 Ar age spectra of alkali feldspars with their associated Arrhenius plots. However, small remaining discrepancies, particularly apparent in log ( r r 0 ) plots, suggest that some, and perhaps most, alkali feldspars contain diffusion domains with activation energies that may vary by as much as 8 kcal/mol. An important consequence of even relatively small variations in activation energy between domains is that the shape of an age spectrum can change dramatically by varying the laboratory heating schedule. We find that Arrhenius and log (r r 0 ) plots have the potential to reveal even small differences in activation energy (~2 kcal/mol) between domains, at least in cases where the domains are well separated in size. Variations in activation energy of ~5 kcal/mol can result in differences in calculated closure temperature of up to 30°C from that obtained assuming equal activation energies for all domains. Overestimates of apparent activation energy and other inconsistencies resulting from reversed heating experiments may reflect annealing of subgrain features which define the smallest diffusion domain size. Use of the diffusion compensation relationship may provide a way to assess the relative distribution of diffusion domain sizes in a sample containing domains with multiple activation energies.