Systematic analysis of K-feldspar 40Ar/39Ar step heating results II: relevance of laboratory argon diffusion properties to nature
Introduction
Because tectonic processes characteristically alter the distribution of heat within the crust, the ability to determine time-dependent paleotemperature variations by means of mineral thermochronometry has increasingly been viewed as fundamental data for deciphering the evolution of basement rocks. Thermochronometry via the 40Ar/39Ar method involves modeling of the inhomogeneous internal distributions of radiogenic 40Ar (40Ar∗) produced by diffusion in silicate minerals (McDougall and Harrison, 1999). Such modeling invariably requires experimental calibration of mineral diffusion properties before quantitative thermal history estimates can be produced. Because these calibrations are necessarily performed at higher temperatures and shorter timescales than those relevant to diffusion in nature, the most fundamental issue in thermochronometry is the relevance of laboratory-determined diffusion properties to argon transport within minerals under crustal conditions.
The most promising opportunity to test this fundamental assumption is provided by step-heating experiments performed with K-feldspar. Currently, there are essentially three ways to gather relevant information about argon diffusion in silicate minerals. These include incremental extraction of 40Ar from a bulk sample in 40Ar/39Ar step-heating experiments (Berger and York, 1981), direct mapping of 40Ar/39Ar gradients via laser ablation from an array of regularly spaced melt pits (Onstott et al., 1991), and laser depth profiling (Arnaud and Kelley, 1997). Laser spot mapping permits ∼20-μm scale imaging of isotope distributions along grain surfaces but does not permit study of smaller features that may serve as important diffusion boundaries. Although laser depth profiling (Kelley et al., 1994) can improve the spatial resolution in the near-surface region by two orders of magnitude (i.e., 0.1 μm), analysis of potential diffusion boundaries is limited to original grain boundaries, cleavages, and similar features of large lateral extent exposed at the surface of grains. Step-heating methods therefore represent the only approach capable of providing volume information from the material. Although isotope distributions are not directly imaged, the natural loss mechanism is potentially reproduced.
Alkali feldspar is the most prevalent potassium-rich silicate of wide crustal occurrence that is sufficiently stable during in vacuo heating to provide a reasonable expectation that natural 40Ar∗ diffusion properties could be reproduced in laboratory step-heating experiments (McDougall and Harrison, 1999). Although isothermal, hydrothermal treatment of hydrous phases is possible, our ability to interpret such experiments is limited to bulk fusion and laser spot analysis. Moreover, the database of argon diffusion measurements in hydrous phases is relatively small Foland 1974, Giletti 1974, Harrison and McDougall 1981, Harrison et al 1985, Baldwin et al 1990, Grove and Harrison 1996. In contrast, many hundreds of diffusion experiments have been performed with K-feldspars sampled from a wide range of geologic environments (Lovera et al., 1997). The microstructural and overall stability of K-feldspar to 1100°C during in vacuo step-heating has been well documented (see Lovera et al., 1993).
The multidiffusion domain (MDD) model has proven capable of describing the full range of laboratory argon diffusion behavior exhibited by basement K-feldspars (Lovera et al., 1997). The leading alternative, the multipath model (Lee, 1995), can only approximate the observed diffusion behavior when its fundamental characteristic (i.e., interaction between diffusion domains) is minimized to such an extent that it becomes indistinguishable from the MDD approach (Arnaud and Kelley, 1997). The MDD model represents argon transport in K-feldspars as being controlled by volume diffusion processes acting on a distribution of length scales. It has been extensively tested and developed since its inception more than a decade ago Lovera et al 1989, Lovera et al 1991, Lovera et al 1993, Lovera et al 1997, Richter et al 1991, Harrison et al 1993, Harrison et al 1994, Quidelleur et al 1997. Although the ability of the MDD model to represent laboratory diffusion properties has become firmly established, its ability to successfully recover basement thermal histories has been repeatedly questioned and remains controversial Parsons et al 1988, Parsons et al 1999, Burgess et al 1992, Foland 1994, Arnaud and Kelley 1997.
Our principal goals in this article are as follows: (1) to identify problematic behavior in K-feldspar 40Ar/39Ar age spectra for thermal history recovery; (2) to examine our database of K-feldspar step-heating experiments to determine the frequency with which misbehaving samples occur; and (3) to quantitatively evaluate the compatibility of 40Ar/39Ar age and laboratory diffusion data from samples within this database to further determine the proportion of basement K-feldspar samples that are likely to be amenable to thermal history analysis. In discussing recent criticisms of the MDD model we also illustrate, by way of numerical simulation, how low-temperature recrystallization or an error in our assumption of the diffusion mechanism might impact calculated thermal histories. Combined with the results from our database samples, these calculations demonstrate the viability of recovering crustal thermal histories through application of the MDD model provided that appropriate samples are analyzed.
Section snippets
Expanded database of K-feldspar 40Ar/39Ar step-heating results
In this section, we examine K-feldspar results obtained between 1994–2000 (Table 1; additional data available from http://oro.ess.ucla.edu/data_repository.html). This database is expanded over that presented in LOVERA et al., (1997), which focused primarily on the ability of the MDD model to represent laboratory argon diffusion properties of K-feldspar. Our primary goal here is to evaluate the self-consistency of natural and laboratory argon diffusion in K-feldspar.
Simple volume diffusion
Correlation between age and laboratory diffusion spectra
The preceding qualitative analysis of our database of step-heating results from basement K-feldspars provides only a general impression of the frequency at which samples yield age spectra compatible with simple volume diffusion theory. We now seek to take our analysis of database samples a step further. Under conditions of monotonic cooling, the MDD model predicts a high degree of correlation between the age spectrum (which typically represents millions to hundreds of millions of years of 40Ar∗
Discussion
Because argon transport in nature cannot be directly observed, speculation regarding what exactly controls the mobility of this trace constituent in geologic materials is necessarily based on circumstantial evidence. Different approaches made on the basis of examination of microscopic and macroscopic properties have been employed to ascertain the validity of extrapolating laboratory properties to natural behavior. We favor a macroscopic approach made on the basis of examination of the
Concluding remarks
The high degree of correlation exhibited between measured age and log(r/ro) spectra determined for the majority of K-feldspars we have analyzed validates extrapolation of experimentally determined diffusion properties to conditions attending natural argon loss within the crust. Our experimental results and numerical analysis indicate that the MDD model, although potentially not a sensitive probe of K-feldspar microstructure, can be a robust tool to recover thermal histories. Despite this, a
Acknowledgements
This research was conducted under the auspices of Department of Energy grant DE-FG-03 to 89ER14049. We thank the numerous analysts involved in generating the 40Ar/39Ar data reported in this article and the assistance of Nyssa Roeth, Ainslie Harrison, and Natalı́ Degrati for their help in compiling the database.
Associate editor: F. J. Ryerson
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