The Stability of Plagioclase in the Upper Mantle: Subsolidus Experiments on Fertile and Depleted Lherzolite

Abstract

Plagioclase peridotites are important markers of processes that characterize the petrological and tectonic evolution of the lithospheric mantle in extensional tectonic settings. Studies on equilibrated plagioclase peridotites have documented continuous chemical changes in mantle minerals in response to plagioclase crystallization, potentially tracing the re-equilibration of mantle peridotites up to very low pressure. This experimental study provides new constraints on the stability of plagioclase in mantle peridotites as a function of bulk composition, and the compositional and modal changes in minerals occurring within the plagioclase stability field as a function of P–T–bulk composition. Subsolidus experiments have been performed at pressures ranging from 0·25 to 1·0 GPa, and temperatures ranging from 900 to 1200°C on fertile and depleted anhydrous lherzolites modelled in the system TiO₂–Cr₂O₃–Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂ (Ti,Cr-NCFMAS). In the fertile lherzolite (Na₂O/CaO = 0·08; X_Cr = 0·07) a plagioclase-bearing assemblage is stable up to 0·7 GPa, 1000°C and 0·8 GPa, 1100°C, whereas in the depleted lherzolite (Na₂O/CaO = 0·09; X_Cr = 0·10) the upper limit of plagioclase stability is shifted to lower pressure. The boundary between plagioclase lherzolite and spinel lherzolite has a positive slope in P–T space. In a complex chemical system, the plagioclase-out boundary is multivariant and sensitive to the X_Cr value [X_Cr = Cr/(Cr + Al)] of spinel. This latter is controlled by the reaction MgCr₂O₄ + CaAl₂Si₂O₈ = MgCrAlSiO₆ + CaCrAlSiO₆, which is a function of the Cr–Al partitioning between spinel and pyroxenes, and varies with the X_Cr value and chromite/anorthite normative ratio of the bulk composition. Within the plagioclase stability field, the Al content of pyroxenes decreases, coupled with an increase in the anorthite content in plagioclase, and Ti and X_Cr in spinel with decreasing pressure; these chemical variations are combined with systematic changes in modal mineralogy governed by a continuous reaction involving both plagioclase and spinel. As a consequence, the composition of plagioclase varies significantly over a rather narrow pressure range and is similar at the same P–T conditions in the investigated bulk-rocks. This suggests the potential application of plagioclase composition as a geobarometer for plagioclase peridotites.