Lawsonite composition and zoning as an archive of metamorphic processes in subduction zones

The hydrous, high-pressure mineral lawsonite is important in volatile and element cycling between the crust and mantle in subduction zones and may also influence the rheology and deformation behavior of the subducted crust and associated sediments. However, despite its potential geochemical and geodynamic significance, little is known about the trace element affinity and the types and origins of zoning patterns in lawsonite. To evaluate the significance of trace element variations and zoning in lawsonite, we conducted a geochemical and microstructurel study of lawsonite in a suite of different rock types from the Sivrihisar Massif, Turkey, one of the few places in the world where pristine lawsonite has survived in eclogite during exhumation from depths of similar to 75-80 km. Lawsonite in metamafic, metasedimentary (impure quartzite and quartz-rich schists), and metasomatic chlorite-rich rocks contains Fe, Ti, and/or Cr as major constituents (substituting for Al) and commonly displays zoning in these elements. Intragrain variations (up to two orders of magnitude) in rare earth elements and other trace elements are also common and in some cases correlate with transition-metal zoning. For some elements (e.g., Ti), uptake was crystallographically controlled, whereas for others, compositional variations may reflect changes in the local metamorphic environment, such as the growth or breakdown of other mineral phases that compete for trace elements (garnet, titanite, epidote-group minerals, apatite) or shifts in the bulk-rock composition during subduction. Deformation may have assisted the mobilization of some elements during and after crystal growth, including relatively immobile elements such as Ti. Intersample variations in lawsonite composition likely reflect variations inherited from the protolith. Lawsonite from Sivrihisar metamafic rocks has high Sr/Pb, whereas lawsonite from quartz-rich metasediments yielded lower Sr/Pb, with a few exceptions that may indicate interactions between oceanic crust and sediments during metamorphism. This study shows that lawsonite composition, zoning, and microstructure can be used to track processes during subduction metamorphism and deformation and can potentially be used to document fluid-rock interaction within and between different lithologic layers.