Experimental Insights into the Stability and Composition of Hydrous Phases in the Metasomatized Mantle Lithosphere

Abstract:

Mantle xenoliths and exhumed peridotite bodies from all tectonic settings record pervasive metasomatism of the mantle lithosphere by volatile-rich fluids. These fluids commonly enrich the lithospheric mantle not only in H₂O but also K₂O and Na₂O as well as other minor elements. As a result, alkali-bearing hydrous phases (amphibole and biotite) become an important part of the mineralogy of mantle lithosphere. We have performed high-pressure-temperature experiments on alkali-enriched fertile and depleted mantle with low bulk H₂O (0.65 wt.%) from 950 – 1150°C and 2 – 4 GPa to determine the stability fields and composition of these hydrous phases in the mantle as a function of temperature, pressure and bulk composition. We find that our stability field for amphibole at 0.65 wt.% H₂O is similar to that of (1) and (2). This stability for amphibole extends to higher pressures and temperatures than that found by (3), indicating that bulk H₂O content exercises an important control on hydrous mineral phase stability (4). We also find that metasomatic compositional changes induce other mineralogical changes. The stability and composition of other phases, particularly the typical aluminous phases spinel and garnet, are significantly affected by the formation of Al-bearing amphibole. We combine our experimental results with those of other experimental studies to more rigorously assess and quantify the influence of compositional changes in the lithospheric mantle on hydrous and anhydrous phase stability. (1) Niida & Green (1999) CMP 135, 18-40; (2) Fumagalli et al. (2009) CMP 158, 723-737; (3) Grove et al. (2006) EPSL 249, 74-89; (4) Green et al. (2010) Nature 448-451.