HYDROGEN IN PYROPE GARNETS FROM NEW YORK STATE KIMBERLITES

All mantle-derived magmas contain a significant fraction of volatile components (e.g. CO₂, H₂O, SO₂). How these volatile components are stored in the upper mantle, which is composed primarily of nominally anhydrous phases, is a question of fundamental importance to understanding basic petrologic processes and mantle dynamics. Recent studies indicate that nominally anhydrous silicate minerals in the lower crust and upper mantle may actually be significant reservoirs of hydrogen available for metamorphic and igneous processes. In the eastern United States, the only samples of the underlying mantle available for direct observation are xenoliths and xenocrysts in Mesozoic kimberlites that intruded along the western flank of the Appalachian Mountains from Quebec to Tennessee.

Over 90 small (<1.5m wide) kimberlitic dikes intruded the Paleozoic sedimentary sequence of central and western New York State. While variable in texture, bulk composition, and mineralogy, some contain a macrocryst assemblage characteristic of disaggregated upper mantle peridotite xenoliths: olivine (Fo_87-91, usually serpentinized), diopside (up to 1.6 wt % Cr₂O₃), and pyrope garnet (up to 5.8 wt.% Cr₂O₃). Garnets for this study were extracted from a dike exposed along Taughannock Creek in Tompkins County, a dike that contains at least three compositional varieties of garnet: Cr-pyrope, pyrope, and magnesian almandine (Lupulescu and Bailey, unpub. data). Five garnet fragments ranging in size from 0.2 to 2.0 mm were hand picked from crushed samples of this dike.

Measurements of hydrogen concentrations in these crystals were done using a Fourier Transform Infrared (FTIR) microscope. With the FTIR microscope, an infrared source beam is aimed through a doubly polished mineral sample. Atomic vibrations in the crystal lattice result in specific absorption peaks, (particularly those centered around 3500 cm^-1), that reflect the presence of structural hydrogen. Of the five samples examined, one yielded unusable data, two had negligible hydrogen concentration, and two had unusually high hydrogen concentrations (1900 ppm ± 200 and 450 ppm ± 60, respectively). Additional microprobe and FTIR analyses are planned to confirm these preliminary results and to see if there is any correlation between bulk composition and hydrogen content of the pyrope macrocrysts.