Water-Fluxed Fractional Melting: a new, Efficient Mechanism of Garnet Enrichment During Mid- and Lower-Crustal Anatexis

Garnet porphyroblasts of impressive size and/or abundance spatially associated with trondhjemitic leucosomes are known from a variety of migmatites worldwide [e.g. 1-4], and have frequently been interpreted as peritectic products of dehydration melting due to P-T variations controlled by regional-scale tectonothermal processes. However, to produce the observed abundance of garnet isochemically, such melting mechanisms would require significant overstepping of the solidus. They cannot, therefore, satisfactorily explain why partial melting often is triggered only locally and generates leucosome + abundant garnet in well delimited veins, pods and patches, leaving large volumes of paleosome essentially unaltered and garnet-free. Indeed, textural evidence suggests in many cases that partial melting proceeded in an open system, implying channelized volatile and/or melt influx and, possibly, simultaneous melt removal. To test this alternative mechanism for producing leucosome + garnet assemblages, we used the program Adiabat_1ph [5], a flexible front-end to the MELTS thermodynamic model [6] that facilitates such open-system calculations. We simulated the influx of water into a series of originally vapor-free model tonalites and diorites and tracked the effect of melt fractionation on the abundance of formed garnet. Calculations ran at constant P-T conditions close to the original solidus of the system by progressive addition of small amounts of water and continuous removal of generated melts. Our results show that water-fluxed fractional melting generates mainly trondhjemitic liquids and produces 3-5 times more garnet than melting in closed systems in the presence of similar amounts of water. This reaction may be written in a general form as Plg + Amph + Water ± Qtz ± Bt ± Cpx → Grt + Melt ± Cpx. Simulated water-fluxed fractional melting produces nearly homogeneous garnets in diorites, whereas garnets formed in tonalites are zoned due to larger compositional changes in the residual solids of these systems during progressive melting; growth profiles of such garnets will mimic those known from typical prograde P-T paths. Melt removal from the system implies that the volume fraction of leucosomes in the outcrop only provides a minimum estimate of the extent of partial melting. Once accepted and further explored, this melting mechanism may help to refine our view on the evolution of many migmatitic terrains. In particular, it implies that, where abundant garnet is found, the system must have been permeable to fluid flow at peak metamorphic conditions close to the solidus. [1] Daczko et al 2001, JMG 19 and references therein; [2] Pickett & Saleeby 1993, JGR 98; [3] Powell & Downes 1990, in 'High-temperature metamorphism and crustal anatexis' and references therein; Williams et al 1995, JGR 100; [5] Smith & Asimow 2005, G3 6; [6] Ghiorso & Sack 1995, CMP 119.