Lawsonite Composition and Zoning as an Archive of Metamorphic and Deformation Processes in Subduction Zones

Abstract:

Lawsonite [CaAl₂Si₂O₇(OH)₂·(H₂O)] is of critical importance to studies of element and water cycling in subduction zones because it is stable at high pressures (and may be the main hydrous phase at P > 2.5 GPa) and is a significant reservoir for trace elements (REE, Sr, Pb, Th, U). It can also be used to date subduction metamorphism and to interpret the deformation and rheologic behavior of subducted oceanic crust and associated sediments. Yet, despite its great potential for providing information about subduction processes, little is known about the composition and zoning patterns of lawsonite, in part because lawsonite is rarely preserved unaltered in exhumed subduction complexes.

Fresh, unaltered lawsonite is preserved in blueschist and eclogite facies metabasaltic and metasedimentary rocks of the Sivrihisar Massif, Turkey. In this HP/LT complex, lawsonite was stable along the prograde path (inclusions in garnet), at the metamorphic peak (matrix grains), and along the retrograde path (lawsonite-rich veins), allowing for investigation of changes in lawsonite composition and microstructures as a function of P-T-fluid-deformation conditions.

Results from studies of lawsonite major and trace element compositions and zoning patterns in a variety of rock types and textural and structural positions show that lawsonite displays a variety of REE patterns (flat REE or LREE-, MREE-, or HREE-enriched) as well as zoning patterns (core-to-rim, sector, oscillatory), particularly in the transition metals Fe, Ti, and Cr. Variations in REE patterns are strongly influenced by the coexisting mineral assemblage (presence or absence of garnet and other REE-rich phases during lawsonite growth), whereas variations in zoning patterns likely reflect both crystallographic and environmental controls. Lawsonite composition may also be affected by deformation: EBSD analysis shows that subgrain development is associated with compositional changes (e.g., Ti).