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 microstructural 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 ~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.
Bibliographical noteFunding Information:
K.F. Fornash acknowledges support from a U.S. National Science Foundation Graduate Research Fellowship (NSF GRFP) and Graduate Research Opportunities Worldwide grant, and from the University of Minnesota for a Doctoral Dissertation Fellowship and Thesis Research Grant. D.L. Whitney acknowledges funding from the College of Science and Engineering at the University of Minnesota. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the Materials Research Science and Engineering Center program. The authors thank S.C. Kruckenberg for EBSD analysis of lawsonite to help with inter-lab verification of lawsonite indexing, S. Penniston-Dorland and C. Spandler for their thorough and helpful reviews, and G. Bebout for patient editorial handling.
© 2019 The Authors.