A laser microprobe, mass spectrometric study of Ar, Kr, K, Cl and Br in an “unconformity garnet”, associated fluid inclusions, staurolite and micas from Vermont, U.S.A.

Abstract

Garnet prophyroblasts in paragonite schist of the Pinney Hollow Fm. on the west limb of the Athens mantled gneiss dome in Vermont exhibit apparent angular unconformities. Cores of garnets contain inclusions of chloritoid, rutile and ilmenite that define a foliation which is truncated at a high angle by the foliation in the rims. Electron microprobe traverses across a garnet indicate that the core-rim boundary is the beginning of a steep compositional gradient in Ca, Mg, Mn and Fe that characterizes the rim. Following electron microprobe study, a sample was irradiated in a nuclear pile and then studied by laser microprobe noble gas mass spectrometry. Ar isotopes were measured in garnet and in adjacent micas and staurolite, and Ar plus Kr isotopes were measured in fluid inclusions (FI) in quartz. In garnet and in FI, trapped 40 Ar 36 Ar varies between ∼ 300 and 2000, with no simple correlations indicated between trapped Ar isotope ratios and K, Cl and 84Kr or petrographic features. Similar trapped Ar occurs in the core and rim of garnet and in FI in quartz. Most FI indicate 40 Ar e Cl > 10 −3 and Cl 36 Ar < 10 6 (atom ratios). Br abundances in FI are usually too low to be measured, indicating Br Cl < ∼ 2 · 10 −3 (atom ratio). 84 Kr 36 Ar in FI varies between ∼ 0.02 and ∼ 0.08. Gases extracted from laser craters in white micas in schist outside the garnet indicate mean 40 Ar 39 Ar ages of 362 ± 5 Ma and in biotites 332 ± 7 Ma, with K-bearing inclusions in the garnet rim yielding ages close to 360 Ma. In the garnet core, interpretation of 40 Ar 39 Ar ages is ambiguous due to the combination of low concentrations of in situ radiogenic 40Ar and comparatively large amounts of trapped 40Ar e poorly correlated with other parameters measured.