COMPOSITIONAL CONSEQUENCES OF LAWSONITE BREAKDOWN IN REGIONS OF WARM SUBDUCTION

The significance of pressure-dependent decomposition of hydrous phases such as amphibole in creating volcanic arcs has been recognized for many years. We propose that the water content and chemical composition of lavas erupted at volcanic arcs are consequences not only of the pressure-dependent decomposition of amphibole, but also of the temperature-dependent decomposition of lawsonite. Although stable to high pressures (>10 GPa), lawsonite decomposes at relatively low temperatures (~500°C at 2.0 GPa). In addition, lawsonite has a strong affinity for Sr, while the minerals formed as a result of lawsonite breakdown (garnet and zoisite) prefer rare earth elements (REE). Consequently, the decomposition of the extremely hydrous mineral lawsonite (~11.5% H₂O) to less hydrous reaction products such as garnet and zoisite (~0-2% H₂O) produces fluids enriched in Sr and depleted in REE. The behavior of lawsonite in subduction zones supports previous observations of high Sr/Y and La/Yb in primitive andesites and dacites (commonly called adakites) erupted at warm subduction zones.

In a relatively warm subduction zone, lawsonite breakdown likely precedes or is simultaneous to that of amphibole. The result is a hydrous volcanic front erupting lavas with compositions distinct from those erupted at a volcanic front formed by the breakdown of amphibole in the presence of residual lawsonite. The decoupling of the pressure-dependent amphibole decomposition reaction and temperature-dependent lawsonite decomposition reaction may explain why the adakite signature is sometimes associated with a volcanic front, as in Mexico and the Cascades, and sometimes associated with more localized tectonic anomalies (i.e. slab tears), as in the Aleutians. A significant consequence of temperature-dependent lawsonite decomposition is that the adakite signature can be produced by wet melting of the mantle in regions of warm subduction without melting of the slab in the traditional sense.