The geobarometric significance of plagioclase in mantle peridotites: A link between nature and experiments

Few previous studies on orogenic plagioclase peridotites, and recent low-P subsolidus experiments on lherzolites (Borghini et al., 2010; J.Petrology 51, 229–254) have revealed that the composition of plagioclase is a marker of varying P conditions in mantle peridotites. Here we confirm this inference, by comparing microstructural–chemical features observed in natural plagioclase-facies recrystallized lherzolites from the External Liguride ophiolitic Unit (EL, Northern Apennine, Italy) with the results of previous (Borghini et al., 2010) and new (this study) experiments on fertile lherzolite with the same bulk composition. Detailed microanalytical work and textural observations on the EL lherzolites revealed systematic chemical zoning in texturally associated plagioclase and pyroxene neoblasts. Anorthite-reverse zoning in plagioclase is coupled to Al, Na and Ca core-rim variations in granoblastic ortho- and clinopyroxenes, documenting two major stages of equilibration within the plagioclase-facies stability field. Consistent chemical variations in minerals are found in subsolidus experiments on fertile lherzolite in the pressure range 0.25–0.8 GPa. Results of experiments were fitted by multiple least-squares regression analysis to derive an expression for anorthite isopleths as a function of P and T in fertile lherzolites. Application of this equation to the EL lherzolites provided equilibrium pressure of 0.7 and 0.3 GPa for the two recrystallization stages, indicating exhumation from about 22 to 9 km depth. Results of combined experimental work and microstructural–chemical data on natural plagioclase lherzolites, highlight that the composition of plagioclase varies significantly in a rather narrow pressure range, and the shallow (< 1 GPa) P-T evolution of mantle peridotites can be therefore successfully traced by combining chemical zoning in plagioclase and geothermometric investigations on texturally associated pyroxenes. Experimentally-derived Na-Ca partitioning between plagioclase and clinopyroxene can be used to test the attainment of chemical equilibrium in neoblastic plagioclase-bearing mantle assemblages, thus enlarging the applicability of this approach in equilibrated plagioclase peridotites of different origins.

► We document that the plagioclase composition is a marker of P conditions in mantle peridotites.
► A detailed micro-analytical work on ophiolitic plagioclase-bearing lherzolites is reported.
► We implement an experimental dataset on a mantle peridotite composition.
► We compare experimental and natural data to trace the shallow P-T evolution of the mantle.
► We discuss the applicability of our approach in the context of other plagioclase peridotites.