Lawsonite Microstructures and Fabric Development at the Slab-Mantle Interface

Katherine F. Fornash, University of Minnesota-Twin Cities
Donna L. Whitney, University of Minnesota-Twin Cities
Christian Teyssier, University of Minnesota-Twin Cities
Nicholas Seaton, University of Minnesota-Twin Cities

Lawsonite, a Ca-Al rich hydrous mineral, is of critical importance to element and water cycling in subduction zones because it has a high water content (11.5 wt.% H2O), 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 in high-pressure assemblages, particularly the REE, Sr, Pb, Th, and U. In addition, the presence, abundance, and crystallographic orientation of lawsonite can significantly affect the deformation and rheological behavior of subducted oceanic crust and associated sediments, and may influence the seismic properties of subducted slabs. The scarcity of well-preserved lawsonite in subduction-related rocks exhumed to the Earth's surface, particularly in eclogite, however, has prevented a comprehensive understanding of the deformation behavior of lawsonite and the factors controlling the development of fabrics in lawsonite, which are important for understanding the effects of lawsonite on the physical properties of subducted slabs.

One of the few places in the world with fresh, unaltered lawsonite in eclogite and blueschist facies rocks is the Sivrihisar Massif, Turkey, which contains a coherent sequence of lawsonite-bearing metabasaltic and metasedimentary rocks that were metamorphosed and deformed at the slab-mantle interface (45 – 80 km), and therefore provide an opportunity to systematically compare lawsonite fabrics in rocks with different modal amounts of rheologically significant minerals (e.g., glaucophane, omphacite, lawsonite, quartz), integrated with information about lawsonite crystal size (aspect ratio), compositional zoning patterns, shape, and twinning. Studies to date of lawsonite CPO from natural lawsonite-bearing assemblages have resulted in two types of patterns: one type is characterized by a concentration of [001] axes parallel to lineation and the other is characterized by a concentration of [001] axes perpendicular to foliation. We have documented the first type in lawsonite-bearing metabasalt (eclogite, blueschist) and the second in lawsonite-bearing quartzite, but other researchers have documented both types in metabasalt. Regardless of variations in lawsonite CPO or rock type, omphacite and glaucophane CPO remain consistent across the analyzed samples. Quartz c-axis patterns vary in eclogite-facies and blueschist-facies quartzite, although lawsonite CPO in quartzite does not vary. Samples in which lawsonite is polycrystalline or occurs as aggregates, such as from the core of eclogite pods, tend to yield more diffuse patterns. Ongoing research is investigating controls on lawsonite CPO, and their consequences for subduction processes.


Cutting edge research in structural geology geophysics geochemistry and tectonics