Limits on the precision of geobarometry at low grossular and anorthite content

Abstract

In many cases the grossular content of garnet and/or the anorthite content of the plagioclase used in thermobarometry are very low, leading to a large pressure uncertainty. The dependence of this uncertainty on mole fractions of grossular and anorthite is evaluated by propagating uncertainties in composition and activity terms into the GASP geobarometer for a series of hypothetical rocks equilibrated at 550 ЊC and 6 kbar. Results are Ϯ 0.65 kbar (1 standard deviation) at high mole fractions (X grs ϭ 0.15, X an ϭ 0.98), increasing to Ϯ 1.55 kbar at low mole fractions (X grs ϭ 0.03, X an ϭ 0.133). Specific results vary depending on errors chosen, P-T conditions, thermodynamic database, and activity models used, but an overall trend of increasing uncertainty with decreasing mole fraction is robust. These theoretical conclusions are supported by a data set of 42 amphibolite facies metapelitic samples for which pressure and temperature were determined with and without grossular-anorthite-bearing equilibria. If grossular and anorthite mole fractions are large the difference in P determination is low (Ͻ0.5 kbar), but if these mole fractions are Ͻ0.10 and 0.30, respectively, the difference in P determination is variable and can be high (Ͼ3 kbar). A general guideline is that if the product of grossular and anorthite mole fraction is Ͻ0.05, then grossular-anorthite-bearing equilibria should be used only with great caution.

Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphism in the northern Canadian Cordillera: Recording foreland-directed migration of an orogenic front

“…Absolute errors of thermometric data are considered to be approximately ±50 °C (Berman, 1991), with appreciably smaller errors associated with relative differences between samples. Pressures based on garnet-plagiolcase equilibria were not calculated because of the low anorthite content (X An , = <10%), which reduces the reliability of the geobarometric estimates (Ashworth and Evirgen, 1985;Todd, 1998).…”

Age trends in garnet-hosted monazite inclusions from upper amphibolite facies schist in the northern Grouse Creek Mountains, Utah

“…Numerical simulations of the growth of two garnet grains using the Gibbs method based on Duhem's theorem yielded rim-minus-core changes of +35°C and +0.9 kb, with growth beginning at $600°C (garnet-biotite thermometry) at an assumed pressure of 3.7 kb (Hoisch et al, 2002). Because the albitic compositions of the plagioclase (An 4-7 ) and the Ca-poor compositions of the garnet (X gr < 0.049) yield unacceptably large propagated uncertainties for GASP geobarometry (Todd, 1998), the occurrence of sillimanite provides the most reliable pressure constraint ($3-5 kbar).…”

VEIN AND SKARN FORMATION AT THE CANNINGTON Ag Pb Zn DEPOSIT, NORTHEASTERN AUSTRALIA

“…5d) Coexistence of D 2 -related garnet, albite and sillimanite may be associated with the breakdown of anorthite to give grossular + sillimanite + quartz, but calculation of pressure using the GASP geobarometer is not possible since anorthite and grossular contents are too low (Todd 1998, Holdaway 2001. Nevertheless, albite rim-ming grossular-poor garnet may indicate a lowering of metamorphic pressure during D 2 .…”