VOLUME CHANGE DURING PARTIAL MELTING REACTIONS: IMPLICATIONS FOR MELT EXTRACTION, MELT GEOCHEMISTRY AND CRUSTAL RHEOLOGY

Abstract

The volume change associated with dehydration melting has been investigated experimentally in muscovite and biotite-bearing assemblages because it is a possible driving force for melt segregation during orogenesis. Experiments have been performed on cores of a muscovite+biotite-bearing pelite and on a biotite+plagioclase+quartz gneiss. The muscovite+biotite-bearing pelite produced a similar set of melt-filled cracks to that observed in muscovite-bearing quartzite under partial melting conditions of 700 MPa, and 850 and 900 °C. However, no cracking was observed in the biotite gneiss under a range of temperature conditions (700 MPa, 800-900 °C). The textures of the partially melted rock samples suggest the volume change and associated dilational strain accompanying melting in assemblages with only biotite is insignificant or negative. This is confirmed by calculations of the dilational strain in the biotite gneiss experiments and other experiments in the literature. For example, the dilational strain associated with partial melting of biotite-bearing metagreywacke assemblages ranges from +1.90% to -12.24% (given 30% modal biotite, calculated on a 1-oxygen basis), becoming negative when garnet is produced at higher pressure. In contrast, the dilational strain associated with melt-induced cracking in a muscovite-bearing metapelite is higher, +6.76%, for the same modal abundance. These results suggest that volume change alone is not an important driving force for melt segregation in biotite-only-bearing assemblages, and external deformation at higher melt fractions may be required to segregate melt from the lower crust during partial melting. Reaction-controlled segregation is possible in muscovite-bearing rocks and melt will be more easily expelled in the upper to mid levels of the crust because of rapid pore pressure development during early stages of melting. Major element chemistry of melt in the two-mica assemblage is dominated by muscovite melting, even when assemblage contains reacting biotite. Some implications of these results are that: (1) the melt that escapes at low melt volumes from the mid-crust is likely to have a muscovite-melting chemical signature; and (2) in the lower portions of the crust where melting is controlled by biotite stability, melt may become trapped within and along grains and remain distributed, pervasively, at the grain scale until greater melt fractions are generated. Recent modeling of orogenic belts shows that the evolution of collisional belts likely involves the prolonged presence of a weak crustal layer. Melt trapped along grain boundaries from low dilational strain melting reactions may be a mechanism for keeping melt in the crust and weakening it during active orogenesis.