Disequilibrium for Ca during growth of pelitic garnet

Abstract

A B S TR A CT Compositional zoning in hundreds of almandine-rich garnets in amphibolite by facies micaceous quartzites from the Picuris Range, north-central New Mexico USA, indicates that although Mn, Mg and Fe achieve chemical equilibrium at hand-sample scale during garnet growth, Ca does not. Instead, Ca concentrations at the surface of growing garnets appear to depend strongly on kinetic factors that govern the local chemical environment, yielding disequilibrium for Ca at scales larger than the region immediately surrounding an individual porphyroblast.Detailed zoning profiles were obtained for 371 garnet crystals in a small volume of a single sample of garnetiferous quartzite, and core analyses were made of 97 additional crystals. Each analysis was made on a section that passed precisely through the morphological centre of the crystal, located by means of 3-D imagery from computed X-ray tomography. The data reveal strong correlations between crystal size and concentrations of Mn, Mg and Fe (but not Ca) in garnet cores; a relationship between crystal size and isolation; rigorous cross-correlations among concentrations of Mn, Mg and Fe (but not Ca); and systematic variations in Ca concentrations as a function of crystal size and core composition that are anomalous in comparison to the behaviour of the other divalent cations.We interpret these observations as the result of thermally accelerated diÂusion-controlled garnet growth, in circumstances that promoted rapid intergranular diÂusion and thus rock-wide equilibration of Mn, Mg and Fe, but that prevented equilibration at similar scale for Ca because of its more sluggish intergranular diÂusion. The anomalous behaviour of Ca is made evident in these garnets by the presence of sharp spikes in Ca concentration, which are demonstrably not a consequence of any simultaneous rock-wide event, such as a change in pressure, temperature, or some other intensive parameter. Instead, Ca concentrations probably reflect the local extent of reaction in the immediate vicinity of each porphyroblast. To the degree that such kinetic factors introduce departures from chemical equilibrium for Ca, thermobarometric estimates that involve grossular contents of pelitic garnet will be in error.

THERIA_G: a software program to numerically model prograde garnet growth

“…Such a geometric relation was repeatedly observed for MnO, FeO, and MgO in garnet of metapelites (e.g. Kretz 1993;Chernoff and Carlson 1997;Spear and Daniel 1998;Zeh 2006). As described above, THERIA_G considers this relation for the distribution of newly formed Fig.…”

“…In distributing the newly formed garnet it is assumed that the radial growth rate during a specific growth increment is constant for all garnet porphyroblasts. This assumption is motivated by theoretical considerations (Kretz 1974) and by empirical evidence (Kretz 1993;Chernoff and Carlson 1997;Spear and Daniel 1998). Hence, for a given relative CSD of garnet and at the respective point in P-T-t space, THERIA_G distributes the newly formed volume of garnet onto the surfaces of the existing garnet crystals such that the radial increase is the same for each individual.…”

Porphyroblast crystallization: linking processes, kinetics, and microstructures

“…Hirsch et al 2003), but electron backscatter diffraction (EBSD) measurements can provide unambiguous confirmation (or contraindication) of multiple proximal nucleation events, even in rare circumstances in which significant reorientation and/or recrystallization has caused partial amalgamation of multiple Downloaded by [Stanford University Libraries] at 17:37 26 September 2012 originally independent crystals by reducing misorientations among them (Spiess et al 2001;Dobbs et al 2003). For garnet porphyroblasts, polycrystalline aggregates resulting from impingement during growth are common and well-documented, for example by Chernoff and Carlson (1997), Hirsch et al (2003), Meth and Carlson (2005), Whitney et al (2008), and others. For QTA, correction techniques have been implemented, based on observability criteria that account for the likelihood of failing to recognize polycrystalline aggregates as separate crystals; these improve assessments of nucleation suppression and growth suppression in such instances (Ketcham et al 2005).…”

“…Chernoff and Carlson 1997) or the centre of helical symmetry in crystals with spiral internal fabrics (Robyr et al 2009). But asymmetric growth, produced for example by inhibition of growth against nutrient-poor layers or veinlets, can lead to crystals whose morphological centre does not coincide with the centre of concentric patterns of compositional zoning: an example would be the 'partial' crystals described in Meth and Carlson (2005).…”

“…Meth and Carlson 2005), as well as the alternative case in which nucleation is a protracted process that spans nearly the entire crystallization interval, with new crystals appearing even in the final stages of reaction (e.g. Chernoff and Carlson 1997;). …”