Argon isotopic zoning in mantle phlogopite
Publication Year
1988
Type
Journal Article
Abstract
Incremental-heating and laser-probe 40 Ar/ 39 Ar analyses were performed on phlogopite extracted from a garnet-lherzolite mantle nodule entrained by the Precambrian (1200 Ma) Premier kimberlite, South Africa. The spatial resolution of the laser probe has enabled the characterization of argon isotopic zoning in a single phlogopite grain. An apparent age contour map records lower ages (1.2 Ga) along grain margins and high apparent ages up to 2.4 Ga) at the core. The latter ages are caused by excess argon contamination and subsequent partial diffusive loss, and have no age significance. Comparison with step-heating results indicates that argon spatial distributions inferred from in-vacuo step-heating experiments are, at best, grossly approximate. Variations in the laser-probe apparent ages were observed only laterally across the phlogopite cleavage surface, indicating that argon transport occurs preferentially along phlogopite cleavage planes. Age profiles, when modeled using one-dimensional radial geometry (cylindrical coordinates), do not conform to classical Fick s law diffusion, suggesting that the characteristic dimension of diffusion for argon in phlogopite may be highly variable within individual grains. © 1988 Geological Society of America.
Keywords
Isotopes,
Mica,
Probes,
Zoning,
Cleavage plane,
Cleavage surfaces,
Cylindrical coordinates,
Garnet-lherzolite,
Heating experiment,
Individual grains,
Radial geometries,
spatial resolution,
Argon lasers,
argon isotope,
argon-argon dating,
cleavage,
garnet,
grain size,
heating,
kimberlite,
lherzolite,
mantle,
phlogopite,
Precambrian,
spatial resolution,
zoning,
Kimberley [Northern Cape],
Northern Cape,
South Africa
Journal
Geology
Volume
16
Pages
542-546