SPATIAL AND TEMPORAL DISTRIBUTION OF TRACE ELEMENTS WITHIN METAMORPHIC GARNET FROM TOWNSHEND DAM, VERMONT

Garnet has the potential to record detailed histories of growth and crystallization because it is stable over a wide range of pressures and temperatures. In addition, it is a useful tool for dating metamorphism using trace-element chronometers. Our results suggest that rock-wide equilibrium (on a scale of several centimeters) for trace elements may not be achieved for some amphibolite-facies schists. A garnet schist from the well-studied locality at Townshend Dam, VT was the subject of a crystallization kinetics study that coupled Sm-Nd isotopic ages with Mn composition to use Mn as a proxy for age of crystallization. This study builds upon those results to understand trace-element behaviors through space and time within a volume of this metamorphic rock.

From a 15cm x 15cm x 9cm subvolume, 24 garnets (previously disaggregated for kinetics study) were selected for their variety of size, shape, location and nearest-neighbor distance. Laser Ablation ICP-MS spots located along core-rim traverses of central sections on all samples provided data for major, minor and trace elements.

Heavy rare earth elements (HREE) are greatly enriched in all garnets compared to the light rare earth elements, which, along with Nd, are weakly- to un-zoned within all garnets. Mn progressively decreases in concentration from the core to rim of all garnets, and Mn concentration serves as a proxy for time. Comparing all trace elements to Mn, the data show that V, Ga, Ge, Nb, Sn, Lu and Yb correlate; Co, Zn, Y, Sm, Eu, Gd, Tb, Er, Tm, Dy and Ho correlate inversely; and Cr, Li, Zr and Nd are uncorrelated.

Spidergrams showing REE at varying Mn concentrations reveal differing HREE trends among the 24 garnets. At 6 wt% Mn, garnets segregate into two different HREE concentrations. At 1.5 wt% Mn, HREE concentrations spread out among the garnets from 10 to 1,000 ppm. At 0.2 wt% Mn, most of the garnets’ HREE fall within 100 -1,000 ppm, but HREE in three garnets are an order of magnitude lower. The differences described here among garnets may relate to the competition for nutrients among neighboring garnets. Therefore, garnets that are close to one another will compete more for compatible elements, and those that grow without nearby neighbors will have higher concentrations of HREE.