Probing chemical transport through a rock using a porphyroblast population: Insights from the distributions of major, minor, and trace elements in garnet from the Danba dome (SW China)

Abstract

A population of garnet porphyroblasts, spaced apart between c. 6 mm and 40 mm, in a metapelite of the kyanite zone from the Danba dome was investigated to assess the equilibration length scales of major and trace elements in the intergranular medium during garnet crystallization. Garnet crystallization occurred from ∼525 °C / ∼6.2 kbar to peak conditions of ∼635 °C / ∼6.2–7.7 kbar. Enrichments in the concentrations of Fe, Nb, Cr, and V, as well as depletions in the contents of Ca, Li, Na, Y, and heavy rare earth elements (HREE) in the early-grown garnet portions indicate that garnet initially pseudomorphed biotite. Results of the statistical analysis of the garnet 3D distribution suggest clustering nucleation mechanisms, providing textural evidence of spatial variations in the distributions of energetically favourable nucleation sites early in the garnet crystallization history. With the exception of the core domains of two early-grown crystals (SC1 and SC8), the distributions of Mg, Ca, Mn, and Fe reflect growth zoning of a garnet population, overprinted by intracrystalline diffusion. The concentrations of Li, Na, Sc, Y, Dy, and Ho in simultaneously grown segments of all garnet crystals are quantitatively comparable, suggesting that the intergranular medium can be considered compositionally homogeneous with regards to these elements, indicative of rapid intergranular chemical transport over a distance of at least c. 40 mm. Differences in the concentrations of Er, Tm, Yb, and Lu are only observed in crystals more than c. 12 mm apart, with the concentrations of these elements decreasing as their atomic masses increase. This systematic trend is interpreted to reflect the possible influence of the relative atomic masses of these elements on the length scales of their diffusion through the intergranular medium during garnet growth. Elements that were heterogeneously distributed across the intergranular medium at the mm-scale during garnet growth include Cr and Zr, reflecting the compositional heterogeneity of the protolith, preserved due to the negligible transport of these elements during the metamorphism. This study emphasizes that the equilibration length scales of trace elements during metamorphism may be larger than previously assumed, presumably depending on the duration of the metamorphism.