Evaluating chemical equilibrium in metamorphic rocks using major element and Sm–Nd isotopic age zoning in garnet, Townshend Dam, Vermont, USA

Abstract

A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order) throughout a ca. 40cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogeneity limits the Sm–Nd age precision required to evaluate more subtle age differences within individual garnet crystals. Central wafers from 1–3cm diameter garnet grains within a 1.21×104cm3 block of pelitic schist were used to document concentric growth zoning of major elements, with decreasing Mn and Ca and increasing Fe and Mg from cores to rims. Garnets also preserve growth zoning patterns for HREE and MREE and show evidence for resorption and partial recrystallization of the outermost rims. Similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium P–T–X conditions for major elements over the sample volume.Comparison of 28 rock Sm–Nd isotope values from the sample indicates substantial cm-scale heterogeneity, which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation. Therefore, Sm–Nd isotopic compositions of thirty-eight concentric core to rim garnet segments from ten large (1–3cm) garnets and two small (1–4mm) bulk garnets, with narrow ranges of Mn content, are paired with sixteen matrix/whole-rock Sm–Nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383.1±6.8Ma to 324.5±3.3Ma. Four of the garnets have anomalously young rims that likely result from post-growth alteration. Chlorite, quartz, and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post-growth resorption/recrystallization effects. Excluding these young rims yields a range of ages from 383.1±6.8 (oldest core) to 374.9±1.8Ma (youngest rim). Sm–Nd age precisions >1.5m.y. (and high MSWD) result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix. However, garnet cores with high Mn (n=7), mantles with intermediate Mn (n=14), and rims with low Mn (n=8; including the 2 smaller bulk garnet analyses), define three distinct multi-grain isochrons of 380.3±2.0Ma (n=23, MSWD=14), 377.3±1.4Ma (n=30, MSWD=18), and 376.5±1.0Ma (n=24, MSWD=18), respectively, yielding an average garnet growth duration of 3.8±2.2m.y. These three composite Mn-age zones define a Mn vs. age relationship that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained (to a first order) throughout the ca. 4m.y. duration of garnet growth.