The lawsonite paradox: a comparison of field evidence and mineral equilibria modelling

Abstract

Lawsonite equilibria are predicted to occur over a broad P-T spectrum developed during subduction, yet lawsonite-bearing assemblages are rare. In the context of mafic mineral equilibria modelled for the range of common crustal metamorphism (4-23 kbar, 400-750°C) using the system Na 2 O-CaO-K 2 OFeO-MgO-Al 2 O 3 -SiO 2 -H 2 O and the software THERMOCALC THERMOCALC, unusually high water contents are demanded by lawsonite assemblages. As a consequence, lawsonite assemblages are predicted to have difficulty forming and lawsonite equilibria to be uncommon. Metabasalt undergoing cooler subduction may experience substantial periods involving the metastable persistence of mineral assemblages because of water under-saturation with non-occurrence of recrystallization. If formed, lawsonite-bearing assemblages are observed to be highly unstable; their preservation requires that exhumation be accompanied by substantial cooling. The amount of structurally bound H 2 O in minerals plays a critical role in the formation and preservation of mineral assemblages, controlling key changes in rocks undergoing subduction.

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

“…Alternatively, quantitative mapping of local domains in a thin section can be undertaken, negating the need to combine maps with separately measured mineral compositions (e.g. Clarke et al, 2001Clarke et al, , 2006Marmo et al, 2002;Fitzherbert et al, 2005;Kelsey et al, 2005;Tinkham and Ghent, 2005;Halpin et al, 2007b;Brovarone and Agard, 2013).…”

Lawsonite-bearing chloritoid–glaucophane schist from SW Tianshan, China: Phase equilibria and P–T path

“…Lawsonite is an important metamorphic mineral formed at high-pressure (HP) and low-temperature (LT) conditions in oceanic subduction zones with a wide P-T range of stability as suggested by both experimental work (Grevel et al, 2001;Liou, 1971;Okamoto and Maruyama, 1999;Pawley, 1994;Schmidt, 1995;Schmidt andPoli, 1994, 1998) and thermodynamic modeling (Clarke et al, 2006;Wei and Song, 2008;Wei and Zhang, 2008;Will et al, 1998). However, natural lawsonite-bearing assemblages are rarely preserved (Maruyama et al, 1996;Tsujimori et al, 2006;Zhang et al, 2007a,b;Zhang et al, 2009).…”

“…Lawsonite is difficult to form and preserve (Clarke et al, 2006), because it requires water to form during subduction and its preservation requires a special retrograde P-T path during exhumation.…”

The Narooma Terrane offshore: a new model for the southeastern Lachlan Orogen using data from rocks dredged from the New South Wales continental slope

“…(a) Pressure and temperature of metamorphism for site C1/83 9 amphibolites determined using the plagioclaseÀhornblende geothermobarometer of Plyushina (1982). (b) Metamorphic pressure and temperature conditions determined from mineral assemblages developed in the metamafic rocks from the continental slope (Hubble, 1989) and mica cell parameters from Narooma (Offler et al, 1998) and Batemans Bay (Prendergast et al, 2011). assemblages determined by Clarke, Powell and Fitzherbert (2006) in their study of the metamorphism of metabasites. The absence of chlorite and diopside in these rocks places lower (»460 C) and upper (»570 C) bounds on the peak temperature while the coexistence of hornblende ÀactinoliteÀplagioclase rather than hornblende C plagioclase places a similar upper bound on the temperature of metamorphism.…”