Abstract
The occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict that the P–T locus of the equilibrium reaction pargasite + forsterite = Na-phlogopite + 2 diopside + spinel, over the range 1.3–3.0 GPa/540–1500 K, yields a negative Clapeyron slope of -0.003 GPa K–1 (on average). The intersection of the P–T locus of supposed equilibrium with the new mantle geotherm calculated in this work allowed us to state that the Mount Leura xenoliths achieved equilibrium at 2.3 GPa /1190 K, that represents a plausible depth of ~ 70 km. Metasomatic K-Na-OH rich fluids stabilize hydrous phases. This has been modelled by the following equilibrium equation: 2 (K,Na)-phlogopite + forsterite = 7/2 enstatite + spinel + fluid (components: Na2O,K2O,H2O). Using quantum-mechanics, semi-empirical potentials, lattice dynamics and observed thermo-elastic data, we concluded that K-Na-OH rich fluids are not effective metasomatic agents to convey alkali species across the upper mantle, as the fluids are highly reactive with the ultramafic system and favour the rapid formation of phlogopite and amphibole. In addition, oxygen fugacity estimates of the Mount Leura mantle xenoliths [Δ(FMQ) = –1.97 ± 0.35; –1.83 ± 0.36] indicate a more reducing mantle environment than what is expected from the occurrence of phlogopite and amphibole in spinel-bearing peridotites. This is accounted for by our model of full molecular dissociation of the fluid and incorporation of the O-H-K-Na species into (OH)-K-Na-bearing mineral phases (phlogopite and amphibole), that leads to a peridotite metasomatized ambient characterized by reduced oxygen fugacity.
Highlights
The occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle
This mineral is frequently observed in cratonic mantle xenoliths hosted in rocks of the diatremic association, such as kimberlites, and occurs mainly along with amphibole, another hydrous mineral, in ultramafic rocks formed by cumulate processes[5,6], in peridotite massifs[7] and in off-craton mantle spinel-bearing x enoliths[8,9,10]
Modal and chemical compositions of the two samples agree with harzburgite residues’, as proven by 22–25% of anhydrous, or hydrous (1 wt% H2O), melting of fertile lherzolites (PM-DMM-like) at ~ 2 GPa in various experimental studies[42,43,44,45,46]
Summary
The occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. The. presence of phlogopite and/or amphibole in an anhydrous-dominated ultramafic system is widely accepted to mark the occurrence of “modal mantle metasomatism”[12], as phlogopite and amphibole crystallization is mainly due to the interaction between metasomatic K(Na)-OH rich “fluids/melts” (e.g., alkaline mafic melts) and variously depleted peridotites, in a porous flow r egime[13,14,15]. Presence of phlogopite and/or amphibole in an anhydrous-dominated ultramafic system is widely accepted to mark the occurrence of “modal mantle metasomatism”[12], as phlogopite and amphibole crystallization is mainly due to the interaction between metasomatic K(Na)-OH rich “fluids/melts” (e.g., alkaline mafic melts) and variously depleted peridotites, in a porous flow r egime[13,14,15] Such metasomatic reactions have extensively modified the sub-continental lithospheric mantle (SCLM) since the A rchean[16,17,18,19,20,21,22]. The preservation of phlogopite and amphibole during low degree partial melting induces significant depletion of potassium in the derived m elts[28]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
