WHY LAWSONITE IS ONE OF THE MOST IMPORTANT MINERALS IN THE PLANET AND WHY IT IS RARELY OCCURS IN THE GEOLOGIC RECORD

Lawsonite is an abundant hydrous mineral in oceanic crust and associated sediments at depths of ~45 to 300 km in most subduction zones. It contains ~12 wt% H₂O and is a major host for U, Th, Pb, Sr, rare-earth, and other elements. Lawsonite is therefore important in water and element cycling between the Earth’s surface and interior via subduction zones, and rocks containing lawsonite can be investigated to reconstruct fluid-rock reactions and conditions during subduction. However, lawsonite rarely survives to be preserved in the geologic record because it easily transforms to epidote and other minerals during decompression or heating, and so lawsonite-bearing rocks are somewhat rare. Worldwide, there are nine known localities in which fresh lawsonite occurs in the matrix of eclogite (high-pressure metabasalt), and an additional seven sites in which lawsonite occurs only as inclusions in garnet in eclogites that lack matrix lawsonite. Lawsonite blueschist is much more common than lawsonite eclogite; in some cases, blueschist is associated with eclogite (lawsonite-bearing, epidote-bearing), and in others, blueschist is not associated with (exposed) eclogite. We have analyzed the major- and trace-element (including REE) composition and zoning of lawsonite from most of the world’s lawsonite eclogite localities and a representative suite of blueschists, and have determined the oxygen isotope composition of lawsonite in some of these as a further monitor of fluid-rock reactions. Our dataset, which includes lawsonite-bearing metabasaltic rocks, silica- and carbonate-rich metasedimentary rocks, metasomatic rocks, and lawsonite-rich veins, shows that lawsonite composition and zoning are sensitive indicators of reaction history in subduction zones, and specifically of fluid-rock reactions that drive element cycling. In addition, because most exhumed lawsonite eclogites record pressure-temperature conditions that correspond to the depth at which the slab-mantle interface transitions from decoupled to coupled, lawsonite-bearing rocks provide insights into the thermal history and dynamics of subduction zones.