Lawsonite: A delicate, yet fundamental mineral at HP-LT conditions

High-P–low-T (HP–LT) rocks have long focused the attention of researchers in several branches of the Earth Sciences, from petrology to geochemistry and geophysics. Index minerals such as garnet, omphacite and glaucophane are in the geologist’s mind the essence of HP–LT metamorphism since Eskola’s metamorphic facies classification in 1920. By contrast, for a series of reasons mainly dealing with its less com- mon occurrence at the Earth’s surface, the mineral lawsonite (Ransome, 1895) had initially raised less interest. Lawsonite has long been recognized as an important rock component at sub-greenschist to blueschist facies metamorphic conditions (e.g. Ernst, 1973). On the other hand, at higher pressure condi- tions, lawsonite was believed to be rarer, requiring ‘forbidden zone’ P–T conditions to form, or to be destabilized at the blueschist–eclogite transition. How- ever, in the last decades, a series of works spanning natural observations, experimental petrology and ther- modynamics have shown lawsonite to be stable at very high-P conditions (Pawley, 1994; Kerrick & Connolly, 2001; Usui et al., 2003; Poli et al., 2009). In particular, in more recent years, there has been growing interest on HP and UHP eclogite facies, lawsonite-bearing rocks (see reviews by Tsujimori et al., 2006; Wei & Clarke, 2011). Lawsonite in eclogites in fact defines the colder side of the eclogite metamorphic facies at HP–LT conditions, and may therefore be considered as a marker for petrological and tectonic processes taking place at HP–LT anomalous geothermal regimes occurring at subduction zones (e.g. Philippon et al., 2013). For this reason, lawsonite has become of broad scientific interest and now represents a major point of discussion in the Earth Sciences, including its central role in subduction dynamics.