Armouring effect on Sr-Nd isotopes during disequilibrium crustal melting: the case study of frozen migmatites from El Hoyazo and Mazarron, SE Spain

Crustal melting is responsible for the production of large volumes of rhyolitic melt and therefore is central to understand the rheology of the crust and the mechanisms of crustal differentiation. The attainment of isotopic equilibrium during melting of crustal rocks is implicitly assumed in most isotopic dating and tracing studies. This assumption considers the melting event as an instantaneous process and does not take into account the duration of anatexis. To assess the critical role of the timescale of crustal melting, we have studied the unique occurrence of erupted migmatites enclosed as xenoliths in the El Hoyazo and Mazarron dacites of the Neogene Volcanic Province of SE Spain. These xenoliths represent the residue after some 30-60 % rhyolitic melt extraction at P-T conditions of 5-7 kbar and similar to 850 degrees C, and consist of biotite, plagioclase, sillimanite, garnet, cordierite, graphite and abundant glass inclusions (i.e., not extracted rhyolitic melt) within each mineral phase. The timescale of melt extraction was similar to 3 Myr and < 0.8 Myr at El Hoyazo and Mazarron, respectively, resembling the duration of melting events during rapid anatexis caused by basalt underplating and crustal assimilation processes. In both localities, the minerals and glass inclusions of erupted migmatites preserve a significant Sr and minor Nd isotope disequilibrium. At Mazarron the isotopic disequilibrium is most marked owing to the shorter residence time of the melt within the source. The isotopic disequilibrium is not caused by the major xenolith-forming minerals but rather by the accessory phosphate inclusions (apatite +/- monazite +/- xenotime) hosted in garnet and biotite. The preservation of isotopic disequilibrium in these accessory phases has been facilitated by both their intrinsically low Sr and Nd diffusion coefficients and the armouring effect caused by their occurrence within biotite and garnet crystals, which acted as chemical barriers to Sr and Nd diffusion. This result implies that modelling of radiogenic isotope equilibration in natural systems should consider elemental diffusion in a composite medium with a resistance at the interface, i.e. different partition coefficients between adjacent mineral phases.