Scales of disequilibrium and rates of equilibration during metamorphism*

Mounting evidence suggests that partial disequilibrium—meaning disequilibrium for some elements, but not for others—may be a common but rarely detected phenomenon during metamorphic mineral growth, even in ordinary prograde reactions that progress to completion. Detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements, with strong temperature dependence. Under lower greenschist-facies conditions, Fe and Mg may equilibrate at hand-sample scale, whereas Mn and Ca may not equilibrate even at millimeter-scale. Although Mn may achieve hand-samplescale equilibration under upper greenschist-facies conditions, Ca and many trivalent cations (e.g., REEs) may not do so until temperatures exceed those of the middle amphibolite facies. Even in the lower granulite facies, some elements (e.g., Y, Yb) show indications of disequilibrium at sub-centimeter-scales during garnet growth. Analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion, the same mechanism known to govern porphyroblast crystallization in many metamorphic environments. Despite this importance to petrology, few quantitative determinations exist of intergranular diffusion rates under metamorphic conditions. Using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the Llano Uplift, U.S.A., a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of Al in fluid-undersaturated systems. This result and an earlier estimate for fluid-saturated systems provide bracketing values for Al diffusivity during metamorphism. That inference adds to a growing recognition that the kinetics of intergranular diffusion govern rates of metamorphic crystallization and chemical equilibration in many ordinary circumstances. Unfortunately, those rates remain largely unknown. Part II of this address leaves speculation behind and presents a new approach that yields a high-precision, quantitative determination of the intergranular diffusivity of Al—the element that most studies have shown to control rates of recrystallization—by exploiting a natural example of coupled intergranular and intracrystalline diffusion. PART I: SCALES OF DISEQUILIBRIUM Disequilibrium is a familiar state of affairs in metamorphic rocks, but most of the commonly recognized examples of it are instances of incomplete reaction. Coronal microstructures around olivine crystals in metagabbros are exemplary cases of prograde reactions brought to a halt before they reached completion (e.g., Whitney and McLelland 1973; Ashworth and Birdi 1990; Johnson and Carlson 1990); similar textures surrounding resorbed garnets are equally persuasive cases of retrograde reactions that ceased before the reactants were wholly consumed (e.g., Misch and Onyeagocha 1976; Carlson and Johnson 1991). Somewhat more subtle and decidedly more complicated are linked segregations and pseudomorphous growth structures, like * Adapted from the Presidential Address given at the annual meeting of the Mineralogical Society of America, November 14, 2000, in Reno, Nevada. † E-mail: [email protected] CARLSON: SCALES OF DISEQUALIBRIUM AND RATES OF EQUILIBRIUM 186 those described elegantly by Carmichael (1969) and Foster (1986); again, these are systems that display, in obvious textural relationships, reactions arrested while still in progress. All of the foregoing examples have been explained in impressive detail by non-equilibrium thermodynamic models that have at their core the concept of intergranular diffusion as the ratelimiting mechanism during recrystallization. Evidence is also accumulating, however, to demonstrate that intergranular diffusion is rate-limiting in many reactions that have gone to completion (Carlson 1989; Vance and O’Nions 1990; Carlson and Denison 1992; Carlson et al. 1995; Denison and Carlson 1997; Hirsch 2000; Hirsch and Carlson 2001). One wonders if these rocks might not have experienced significant levels of disequilibrium during mineral growth, just like their counterparts in which reactions terminated before completion, considering that their recrystallization was similarly governed by the kinetics of intergranular diffusion. The observations reviewed below suggest that many such rocks do indeed harbor evidence of a failure to equilibrate at thin-section scale during mineral growth, despite their lack of any indicators of incomplete reaction. In fact, certain anomalies of garnet composition in these rocks may constitute a record of systematic variations in the length scales over which different elements can equilibrate at different metamorphic grades.