Constraint on the time scale of biotite-grade metamorphism during Acadian Orogeny from a natural garnet-garnet diffusion couple

Abstract

We have determined the concentration profiles across the interface of a natural garnetgarnet couple in a biotite-grade rock from eastern Vermont. The couple consists of a grossular-spessartine garnet that had formed during regional metamorphism associated with the Acadian orogeny on an almandine core, which had crystallized during an earlier episode of metamorphism related to the intrusion of Fairlee granite at 411 :t 5 Ma. The concentration profiles were measured by both electron microprobe and analytical transmission electron microscope, and they were modeled to retrieve the value of JD(t)dt through the time that diffusion was effective but without recourse to any diffusion data. The length of the concentration profiles measured in microprobe is barely resolvable from that resulting from a convolution effect from the spatial averaging in the spot analyses. Deconvolution of the microprobe profiles yields a value of JD(t)dt = 0-3.4 x 1O-11cm2, suggesting very little or no diffusion. TEM analyses of the concentration profiles, which are not subject to any significant convolution effect, show a very small but definitive diffusion zone across the interface of the garnet-garnet couple, which yields a value of JD(t)dt = 7.6 x 10-12 cm2. Because D is a function of time through its dependence on temperature, this value of JD(t)dt provides an important constraint on the thermal history during the regional metamorphism. As an example, we used it in conjunction with the available diffusion data for garnet to derive -40-50 Ma as the probable time scale for the biotite-grade metamorphism, taking into account the effects of the off-diagonal terms and thermodynamic nonideality on the diffusion process. INTRODUCTION ing biotite-grade regional metamorphism associated with the Acadian orogeny. From V-Pb data of zircon grains Overgrowth ofa mineral on itself is a well-documented abraded from the intrusion, the Fairlee granite has been petrographic feature in rock samples. Specifically for gardated to have intruded at 411 :t 5 Ma (Moench and nets, there are many reported instances in which a second Aleinikoff 1991). Rumble and Finnerty (1974) presented metamorphic event led to overgrowth of garnet of differpreliminary data on the concentration change of divalent ent composition on another garnet crystal that had formed cations, as determined by step scanning in an electron during an earlier metamorphic episode (e.g., Albee 1968; microprobe, across the interface of one of these natural Brown 1969; Rosenfeld 1970; Edmunds and Atherton garnet-garnet diffusion couples. Although not of the qual1971; Rumble and Finnerty 1974; Erembert and Ausity required for quantitative modeling, the scale of these trheim 1993). Although little work has been conducted reported concentration changes was sufficiently long, in in this area, the diffusion-induced compositional zoning comparison with the spatial averaging effect of the probe across the interface of an overgrowth of a mineral on beam (Ganguly et al. 1988), to make us believe that a itself can be used to extract important constraints on the more careful determination of the compositional zoning time scale of geologic or planetary processes (e.g., Ganby electron microprobe would yield unambiguous data guly et al. 1994). for modeling the time scale of its development. Thus, we Rumble and Finnerty (1974) described an overgrowth selected a pair of rock samples from the same thin-section ofgrossular-spessartine garnet on an almandine core from block used by Rumble and Finnerty (1974) and prepared eastern Vermont. From field and petrographic relations, polished thin sections for electron microprobe analysis of they concluded that the almandine core had crystallized concentration profiles. However, our expectation was not during contact metamorphism caused by the intrusion of fulfilled because the spatial extent of compositional change Fairlee granite, whereas the overgrowth had formed duracross the interface was much smaller than we had ex0003-004X/96/0910-1208$05.0