Using fluid buffering equilibria to determine values of H₂O activity: an example from the amphibole-bearing spinel lherzolites of Caussou, French Pyrenees

Abstract

Samples were collected from the Caussou peridotite in the French Pyrenees. This peridotite represents a portion of the Earth's upper mantle. These samples contain the minerals olivine, clinopyroxene, orthopyroxene, and amphibole. Therefore, the H₂O-buffering equilibria: 2Tremolite + 2Forsterite [-] 4Diopside + 5Enstatite + 2H₂O 2Ca₂Mg₅Si₈O₂₂(OH)₂ + 2Mg₂SiO₄ [] 4CaMgSi₂O₆ + 5Mg₂Si₂O₆ + 2H₂O can be applied to constrain values of a[H]₂[O] in these mantle samples. The samples collected from the Caussou peridotite contain grains with different sizes. Included are large grains, porphyroclasts, that range in size from 2 to 4 mm; an intermediate grain size, medium-sized grains, that range in size from 0.6 to 1.0 mm; and finally a very small grain size, neoblasts, that range in size from 0.01 to 0.10 mm. Amphibole grains are texturally associated with the medium-sized and neoblast grains, indicating that the amphibole grains are in equilibrium with these grain sizes. Consequently, the calculation of a[H]₂[O] using the above equilibria has been based upon the compositions of grains with these sizes. Use of the above equilibria requires independent determination of temperature values. Thus, geothermometry was used to determine temperature estimates. This geothermometry suggests temperatures ranging from 720 to 810 ⁰C for the medium-sized grains, and 640 to 740 ⁰C for the neoblast grains. Pressure estimates of 8 to 10 kilobars are based upon previous work in this peridotite. These values of temperature and pressure, in conjunction with the above equilibria, yield values for a[H]₂[O] that are []0.10.