Prograde P-T path of lawsonite-bearing blueschists: insights from Ile de Groix and implications for fluid content and rheology of subducted oceanic crust

The presence or absence of fluids strongly affects rock rheology. Lawsonite is a very hydrous mineral (~12 wt.% H₂O), characteristic of cold subduction zones. Its destabilization may generate fluid overpressure, reduce effective stress, and trigger brittle failure through dehydration embrittlement. On the other hand, its H₂O-consuming growth may deplete available fluids from the matrix and drive the rock dry. The quantity of lawsonite, the locus of maximum dehydration, and the amount of fluids produced/consumed depend on the pressure-temperature (P-T) path of the subducted crust. An accurate interpretation of P-T paths of natural blueschists is therefore crucial.

At Ile de Groix (Armorican Massif, France), garnet-bearing blueschists display cm-sized lawsonite pseudomorphs smoothly wrapped by an epidote- and glaucophane-bearing foliated matrix. Both garnet and pseudomorphed lawsonite porphyroblasts contain sigmoidal inclusion trails of fine-grained oriented epidote, glaucophane and titanite, continuous with the matrix schistosity. Garnet is zoned (rimward decrease of Mn and increase of Mg) and locally included in pseudomorphed lawsonite. Lawsonite pseudomorphs comprise coarse unoriented epidote, paragonite and chlorite. Textural analysis therefore suggests a prograde synkinematic growth of garnet and lawsonite in an epidote-bearing matrix. In the light of calculated phase diagrams, this points to a prograde P-T path dominated by a near-isothermal compression from LT epidote-blueschist facies toward peak pressure conditions in the epidote + lawsonite stability field, at ~19 kbar and ~550°C, consistent with garnet rim composition and modal proportions of major phases. 

Thermodynamic modeling further indicates that lawsonite growth in an epidote-bearing blueschist leads to the complete consumption of free fluid, resulting in a dry, fluid-absent rock near peak pressure conditions. However, dry rocks are commonly stronger than their wet equivalents. Our results thus suggest that, contrary to common expectations, hydration reactions may locally induce an increase in rock strength, as exemplified by lawsonite crystallization during the prograde transition from epidote- to lawsonite-blueschist subfacies. Such reactions could provide an explanation for earthquakes occurring within the lawsonite stability field, well prior to its destabilization.