The (chemical) potential for understanding overstepped garnet nucleation and growth

Abstract

Overstepping of porphyroblast-forming reactions has been shown to occur in regional, contact, and subduction zone metamorphism, calling into question the paradigm that metamorphic mineral reactions occur at or very close to thermodynamic equilibrium. These overstepped reactions result from the fact that nucleation and growth of new phases requires a thermodynamic driving force, or a departure from equilibrium. We use phase equilibria modeling to elucidate the energetic consequences of overstepped garnet nucleation and growth by comparing the chemical potentials of garnet-forming oxide components (MnO, CaO, FeO, MgO, Al 2 O 3) in two sets of calculations: one in which Gibbs free energy is minimized and one in which the minimization proceeds under identical conditions but in the forced absence of garnet. We focus on twelve examples from the literature which have previously described garnet nucleation as minimally overstepped (garnet nucleation at the P-T of initial garnet stability) or garnet nucleation as more substantially overstepped (garnet nucleation at P-T conditions greater than initial garnet stability). For a small P-T interval above nominal garnet-in reactions, differences in the chemical potentials between the two calculations are commonly minimal. In all This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Thermal pulse induced by emplacement of Ramba leucogranites in southern Tibet

“…This nucleation‐related temperature overstep is moderate and is within the range of other studies of low‐ to mid‐pressure pelitic systems (30–50°C; e.g. Gorce et al, 2021; Nagurney et al, 2021; Pattison & Tinkham, 2009; Waters & Lovegrove, 2002; Wolfe & Spear, 2018), although some studies document minor (<10°C, e.g. George & Gaidies, 2017; Moynihan & Pattison, 2013; Nabelek & Chen, 2014) or larger discrepancies (>100°C; e.g.…”

“…This coincidence might suggest transient cycles of overstepping and relaxation (Spear, 2017; Spear & Wolfe, 2019), which might be sensitive to the chemical‐potential gradient of a specific component (e.g. Nagurney et al, 2021).…”

Testing the equilibrium model: An example from the Caledonian Kalak Nappe Complex (Finnmark, Arctic Norway)

“…The influence of energy barriers on the conditions of mineral reactions during petrogenesis is the subject of ongoing research as it may affect the tectonic and geodynamic interpretations of the petrogenetic record contained in metamorphic rocks. It has been suggested that the departure from stable equilibrium required to overcome the energy barriers for mineral reactions in some metamorphic rocks may be significantly larger than the uncertainties inherent in the analytical and theoretical methodologies used to investigate them, resulting in metastable states with characteristic disequilibrium assemblages, disequilibrium mineral compositions, and disequilibrium microstructures (e.g., Carlson, 2011; Carlson et al, 2015; Chakraborty et al, 2007; Gaidies, 2021; Gaidies et al, 2011; Kelly et al, 2013; Nagurney et al, 2021; Pattison & Tinkham, 2009; Pattison et al, 2011; Spear & Wolfe, 2019; Spear & Pattison, 2017; Spear et al, 2014; Waters & Lovegrove, 2002; Zeh & Holness, 2003). However, quantitative knowledge of the energy barriers associated with some of the fundamental subprocesses of metamorphic mineral reactions, such as nucleation and growth, is still lacking.…”

Defining the Timing, Extent, and Conditions of Paleozoic Metamorphism in the Southern Appalachian Blue Ridge Terranes of Tennessee, North Carolina, and Northern Georgia

“…Garnet cores equilibrated at ∼530°C in PGAP and within the staurolite field at ∼600°C in 12NC03 (Figures 9a and 9d), in both cases significantly above the lower limit of garnet stability. These results may reflect thermodynamic or kinetic controls (e.g., Nagurney et al., 2021; Pattison & Tinkham, 2009; Wolfe & Spear, 2018), inadequate selection of the chemical core of garnet, or diffusion of major elements during or after garnet growth. The pressure‐temperature path predicted from the garnet core composition and the predicted minerals and observed peak mineral assemblage for PGAP‐1 extends from ∼525°C to ∼635°C at ∼5 kbar with no required change in pressure.…”