Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle

Abstract

Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnetperidotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine^garnetwebsterite, garnet-websterite, orthopyroxene^phlogopite^garnetwebsterite, coarse-grained phlogopite^garnet-websterite, phlogopiteĝ arnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite.The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO 2 and Al 2 O 3 . Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni

Pseudomorphic replacement of diopside during interaction with (Ni,Mg)Cl2 aqueous solutions: Implications for the Ni-enrichment mechanism in talc- and serpentine-type phases

“…These ultramafic rocks, although initially mainly composed of olivine and pyroxenes, are in most cases texturally and compositionally altered under a wide range of metamorphic conditions (e.g. Burkhard and O'Neil, 1988;Rampone et al, 1993;Vrijmoed et al, 2013). The critical role of fluid during these alteration processes has a special place among fluid-rock interaction as it not only changes rock properties such as density (reduced from 3.3 to 2.5 g/cm 3 ) (e.g.…”

Thermodynamic equilibrium at heterogeneous pressure

“…The currently presented Gibbs energy method delivers the same result, thereby increasing the confidence in the geobarometer of Tajčmanová et al (2014). Both equilibrium methods (barometry and Gibbs minimization) are used to deduce pressure variations in Opx of the Svartberget UHP garnet-peridotite from the WGR, Norway (Vrijmoed et al 2006(Vrijmoed et al , 2013. Again, both methods consistently predict opposite results compared to what is expected from a conventional phase diagram or garnetorthopyroxene barometry.…”

“…Recent investigations show the Opx zoning is more complex and does not fit diffusional profiles but is possibly linked to metasomatic processes(Fig. 11dinVrijmoed et al 2013). Diffusion of Al in Opx is usually assumed slow, mostly based on Al diffusion experiments in Cpx(Smith and Barron 1991;McFarlane et al 2003)…”

New Occurrence of Pyroxenites in the Veria-Naousa Ophiolite (North Greece): Implications on Their Origin and Petrogenetic Evolution

“…Natural and experimental data reveal that Si-rich melts [56,57], or aqueous [58] or supercritical fluids [59] are released from the underlying subducted oceanic slab into the sub-arc region driving the dissolution of mantle olivine and the neoformation of pyroxenes and Al-phases in a range of pressures between 0.8 and 3.4 GPa [60][61][62][63][64][65][66][67][68]. Experimental results of Grant et al [69] at 800 MPa suggest that neoblasts of orthopyroxene and clinopyroxene form at a relatively narrow temperature range of around 800 to 900 • C (with few exceptions up to 1100 • C).…”