MAJOR- AND TRACE-ELEMENT ZONING AS A FUNCTION OF GARNET CRYSTALLIZATION TEMPERATURE

Some trace elements diffuse more slowly in garnet than major elements and thus potentially can preserve aspects of the geological history that have been obliterated in the major-element record. To investigate this phenomenon, major- and trace-element zonations were imaged and measured on carefully located central sections for two sets of garnets of varying sizes from the Llano Uplift in Texas that reached different peak metamorphic temperatures. The higher-temperature garnets (peak metamorphic temperature 750 ± 50°C) reveal homogenization of the major elements Mg, Ca, Fe, and Mn, while typical growth zoning patterns with steep gradients are preserved in the lower-temperature garnets (peak metamorphic temperature 585 ± 50°C). These lower-temperature garnets also maintain zoning in the trace elements Y, Ti, Ge, and Yb. However, in the higher-temperature garnets, striking growth-zoning rings occur in Y and Yb and subtler zonation in Ti is preserved, despite the homogenization of major elements, illustrating that some trace elements do preserve a history undetectable solely from major elements.

Subtle features of the zoning suggest that the garnet crystals may not have reached chemical equilibrium for all elements during growth. The lower-temperature garnets exhibit euhedral Ca lows at the same relative location in three variably sized crystals that have different MnO, FeO, and MgO weight percents at the site of the Ca low. In the higher-temperature crystals, the Y and Yb growth-zoning rings only occur in the medium-sized garnet and not the largest or smallest garnets. These observations are inconsistent with the assumption of rock-wide equilibration for Ca, Y, and Yb during garnet growth.