Eclogite-facies sea-floor hydrothermally-altered rocks: calculated phase equilibria for an example from the western Alps at Servette.

In the St. Marcel Valley at Servette in the Western Alps metabasic rocks crop out that were variably altered in a sea-floor hydrothermal system. Successively, they were metamorphosed to eclogite facies. This was used as an opportunity to explore the range of eclogite-facies mineral assemblages that can develop in such altered rocks. The continuum is represented qualitatively by a series of rock-types: eclogite, glaucophanite, talc schist and chlorite schist, with increasing alteration, supposed to all have involved eclogite-facies mineral assemblages. Later, they were variably retrogressed under greenschist-facies conditions. The eclogite-facies mineral assemblages developed from the broad alteration trend represented by the four rock-types have been forward-modelled in terms of three composition vectors. P-T pseudo-sections for the compositions used to represent the rock-types suggest that a P-T of about 22 kbar and 600°C is consistent for the conditions of formation of the rock-types. T-X pseudo sections along the composition vectors are used to represent the alteration continuum showing the way the mineral assemblages are affected by the degree of alteration