Disentangling the compositional variations of lawsonite in blueschist-facies metasediments (Schistes Lustrés, W. Alps)

Lawsonite is important as a carrier of H₂O in subduction zones and as a petrological tracer. The trace-element content of lawsonite in mafic rocks has been used as a record of fluid–rock interactions but has received less attention in metamorphosed oceanic sediments. This study documents the major, and trace-element composition, together with ⁸⁷Sr/⁸⁶Sr isotopic ratios, of the different lawsonite types identified in the upper units of the Schistes Lustrés complex of the Western Alps, a paleo-accretionary prism of the Liguro-Piemont slow-spreading ocean subducted up to 40 km. The lawsonite-rich upper units of the Schistes Lustrés complex are principally composed of metamorphosed pelitic sediments and carbonates. Lawsonite content reaches 40 vol.% in the rock matrix and in veins. All lawsonite types originate from prograde metamorphic reactions which occurred up to peak metamorphism. Lawsonite compositions have been measured in situ with electron microscopy, microprobe, and laser-ablation mass spectrometry. Lawsonite separates have been measured for Sr content and ⁸⁷Sr/⁸⁶Sr isotopic ratios using TIMS. Bulk rock compositions have been measured with ICP-OES and ICP-MS. Analysis contamination by minute retrograde white mica inclusions in lawsonite crystals precluded using univalent cations for petrological interpretations. For other trace elements, the variability of lawsonite appears extremely high, with crystal compositions varying between samples and between crystals in individual samples. However, clear patterns emerge between lawsonite types, reflecting equilibrium and out-of-equilibrium processes unrelated to pressure–temperature conditions. At crystal scale, textural hourglass zoning predominates in lawsonite from the schist, while growth zoning is found in both schists and veins. The combination of both mechanisms results in spectacular zoning in Ti and in rare-earth element contents spanning four orders of magnitude. Over time, the La/Yb ratio decreases strongly (from ~100 to ~1), as La appears much more sensitive to surface effects leading to textural hourglass zoning. Interface-coupled dissolution–precipitation also contributes to decoupling of less mobile elements in the schist. This is best observed for Ti, but rare-earth element and chromium contents are affected too. These processes are considered to occur near closed-system conditions. Late lawsonite overgrowths with higher Sr contents are interpreted as reflecting system opening in the veins, yet sometimes observed in the schists. Strontium isotopic ratios measured on separated lawsonite crystals and on bulk rocks also show very large spreads. This indicates that the system did not equilibrate during blueschist-facies metamorphism, due to bulk rock heterogeneities during deposition and limited mineral reactivity at the local scale. Strontium isotopic ratios do not correlate with Sr content which rules out limestone devolatilization as the main driver of lawsonite overgrowths. In places, lawsonite is far more radiogenic than the host rocks, indicating either the destabilization of local Rb-rich phases (such as mica, illite, and smectite) or the infiltration of externally derived fluids. The overall compositional evolution of lawsonite records incremental system opening. However, lawsonite offers no evidence of large-scale mass transfer and external fluid influx in the Schistes Lustrés complex, even though the vein network is abundant and extends over time.