The signature of devolatisation: excess 40Ar in high pressure rocks

Smye, Andrew; Warren, Clare J.; Bickle, Mike J. and Holland, Tim (2012). The signature of devolatisation: excess 40Ar in high pressure rocks. In: AGU Annual Meeting, 3-7 Dec 2012, San Francisco, CA, USA.



The presence of excess $^{40}$Ar in (ultra-)high pressure ((U)HP) metamorphic rocks shows that the assumption of open system argon exchange between the source mineral and the grain boundary is not valid. Typically, phengites from (U)HP metasediments have apparent $^{40}$Ar/$^{39}$Ar ages $<50\%$ in excess of the age of peak (U)HP conditions, whereas cogenetic mafic eclogites yield ages up to 700$\%$ older, despite lower bulk-rock K$_2$O concentrations. Given the highly incompatible behaviour of argon, the fraction of protolith-derived $^{40}$Ar that is retained by mica on equilibration under (U)HP conditions reflects the extent to which the rock has behaved as an open or closed system to volatile-loss during subduction. This has not previously been quantified for (U)HP rocks

A model is developed that calculates excess argon age fractions as a function of variable mica—fluid K$_D$, bulk K$_2$O and porosity under closed system conditions. Using two recently-published single-grain $^{40}$Ar/$^{39}$Ar datasets from the Tauern Window [1] and Oman [2] HP terranes, measured excess argon concentrations in mafic eclogites are reproduced only when porosities are $<10^{-4}$ volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are $>10^{-3}$, reflecting loss of significant volumes of lattice-bound H$_2$O relative to mafic rocks during subduction. These results are supported by phase equilibria calculations of H$_2$O loss during progade metamorphism of a MORB and pelitic rock composition: pelites lose H$_2$O continuously along subduction geotherms, whereas MORB compositions require hydration, or, liberate small quantities of structurally bound H$_2$O. Violation of the model assumptions by loss of argon, or transiently higher porosities will lower the excess argon age. Accordingly, the porosity estimates provide a limiting case to examine the effects of fluid availability, permeability and argon diffusivity on the accumulation of excess argon under (U)HP conditions. The model highlights how argon can be used as a tracer for the time-integrated effects of metamorphic devolatisation and as a means to understand the mechanisms by which volatiles are transported to mantle-depths.

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