Mechanisms of metasomatic reactions

Abstract

Fluid-aided mass transfer and subsequent mineralre-equilibrationarethetwodefiningfeaturesofmetasomatismand must be present in order for metamorphism to occur.Taking into account geological time scales, the amount offluidrequiredneednotbegreat(considerablylessthan0.01%of the total volume) nor even in continuous flow but the fluidmust be chemically reactive with respect to the minerals itencounters as well as promote efficient mass transfer.Evidence for the passage of fluids through rock can take theformofalteredmineraltraceelementchemistry,partialtototalre-equilibration of all mineral phases, as well as reactiontextures along mineral grain boundaries. In addition, bothreactiveandnon-reactivefluidscanandoftendoleavebehindtraces of their passage in the form of fluid inclusions.Coupled with igneous and tectonic processes, metaso-matism has played a major role in the formation of theEarth’s continental and oceanic crust as well as in theirevolution and subsequent stabilization. Metasomatic pro-cesses can include ore mineralization (Wilkinson andKesler 2007); metasomatically induced alteration of oceaniclithosphere (Hopson et al. 2008); mass transport in andalteration of subducted oceanic crust and overlying mantlewedge (Zack and John 2007), which has subsequentimplications regarding mass transport, fluid flow, andvolatile storage in the lithospheric mantle overall (Mukasaet al. 2007), as well as both regional (Ordonez et al. 2008)and localized crustal metamorphism (Harlov et al. 2006).Metasomatic alteration of accessory minerals such as zircon(Geisler et al. 2007) or monazite (Dumond et al. 2008) canallow for the dating of metasomatic events as well as giveadditional information regarding the chemistry of the fluidsresponsible.Papers submitted to this volume cover a wide diversityof topics with regard to metasomatism. These range fromlow- to high-grade processes and from upper-crustal shearzones and veins to high and ultra-high pressure metamor-phic rocks as well as rocks from the lithospheric mantle. Inthe first paper, Glodny and Grauert investigate lowtemperature (100–200°C) elemental (K, Cl, Na, Ca) andRb–Sr isotopic changes associated with the formation ofzoned alteration halos along post orogenic hydrothermalveins in a metaquartzdioritic Variscan gneiss from theArtenberg quarry near Steinach, Kinzigtal, Schwarzwald,SW Germany. Sengupta et al. next describe a series ofgranulite-facies (750°C; 900 MPa) skarns associated withmarbles and calc–silicate rocks in migmatitic metapeliteslocated near Sittampundi, south India. Here, aqueous fluids,derived from prograde metamorphism of the metapelites,are hypothesized to be the likely source for the metasomaticfluids responsible for the formation of the skarns. In arelated paper, Hansen and Harlov describe granulite-faciesorthopyroxene-bearing leucosomes with subordinate biotitein finer grained hornblende–biotite–pyroxene-bearing mig-matites from Cone Peak, California, USA (700–750°C;750MPa)andtheSatnur–Sangamroad,SouthernKarnataka,India (700–800°C; 500–600 MPa). Compositional differ-ences between the paleosome and neosomes at Cone Peakare best explained by metasomatic interaction withconcentrated brines while elevated Cl concentrations influorapatites in the leucosome suggest interaction with a