Mavis Z. Stout

Geobarometry and geothermometry of plagioclase-biotite-garnet-muscovite assemblages

The assemblage, plagioclase-biotite-garnet-muscovite is widespread in garnet to lower sillimanite zone metasedimentary rocks. The equilibria, (1) pyr+gr+mu=3an+phl and (2) alm+gr+mu=3an+ann, involve a change in Al coordination from 6 to 4 and in Mg-Fe coordination from 8 to 6 and should be strongly pressure dependent. Using an ionic solution model we can define solid activity products for (1) and (2). Using a linear least squares analysis of activity products and estimated P-T from natural assemblages we can derive equilibrium constant equations for (1) and (2). Combining either of these equations with that for Mg-Fe exchange for garnet-biotite from Ferry and Spear (1978) allows estimation of P and T of metamorphism. Pressures estimated from these equilibria compare favorably to pressures estimated from garnet-plagioclase-Al2SiO5-quartz equilibria.

TiO2 activity in metamorphosed pelitic and basic rocks: principles and applications to metamorphism in southeastern Canadian Cordillera

AbstractThe activity of TiO2 can be precisely defined as a function of pressure, temperature and activities of other components for common mineral assemblages in metapelites (ilmenite-quartz-garnet-plagioclase-Al2SiO5) and in metabasites (plagioclase-sphene-ilmenite-quartzgarnet). These mineral assemblages can be modelled by the equilibria: 1) 3ilmenite+Al2SiO5+2quartz=almandine+3TiO2 2) anorthite + 2sphene = grossular + 2TiO2 + quartz 3) 3anorthite+3quartz+6ilmenite = grossular+ 6TiO2+2almandine. These mineral assemblages can be used at

Petrologic variations in quaternary volcanic rocks, British Columbia, and the nature of the underlying upper mantle

a_{{\text{Tio}}_{\text{2}} } = 1

Heat effects of assimilation, crystallization, and vesiculation in magmas

(rutile saturation) and a given T to get maximum pressure limits of some metapelites and metabasites. When electron microprobe analyses of mineral grains adjacent to Ti-bearing phases are made, these data give maximum pressure estimates in reasonable agreement with other geobarometers. The activity of TiO2 in many metapelites is very near rutile saturation, but for metabasites the activity of TiO2 in some sillimanite zone rocks is as low as 0.6. The solubility of TiO2 in biotite, hornblende and garnet is a complex function of T, P, the activities of components in coexisting minerals and crystal chemical constraints in these minerals. At a given P and T the solubility of TiO2 in biotite and hornblende does not appear to be strongly dependent upon

Geobarometry of low-temperature eclogites: applications of isothermal pressure-activity calculations

a_{{\text{Tio}}_{\text{2}} } = 1

Garnet-Hornblende Thermometry,

for sphene and ilmenite versus rutile-bearing assemblages.