COMPOSITION AND DISTRIBUTION OF MELT DURING PARTIAL MELTING OF A LEUCOGRANITE AT 200 MPA H2O

One of the hypotheses for the origin of pegmatite magmas is the H₂O-saturated low-degree melting of quartzo-feldspathic protoliths. To test this idea, and to examine diffusional properties pertinent to the anatectic generation and homogenization of granitic liquids, we experimentally investigated the kinetics of melting of a leucogranite at 200 MPa H₂O, 690^oC and 800^oC. The starting material were cylinders 3.5 mm (diameter) x 7 mm (length) of a fine-grained (0.3 mm) leucogranitic aplite (Qtz + sodic Pl + Kfs + traces of Bt) from the Little Three pegmatite, California. Cores were run in cold-seal vessels for 6 to 2880 hours. Back scattered electron imaging of run products shows that silicate glass (melt at run conditions) occurs as a pervasive intergranular film mostly along quartz/feldspars and quartz/biotite boundaries and quartz/plagioclase/K-feldspar triple junctions, and less commonly along plagioclase/K-feldspar boundaries and within fractured quartz and feldspar grains. In contrast with previous studies we found that (1) melt films occupy close to 100% of grain boundary surfaces; and (2) melt film thickness is roughly constant throughout a given experimental charge, increasing with melt H₂O concentration, experimental temperature, and run duration. At 690^oC the glass composition is roughly homogeneous, close to the 200 MPa H₂O haplogranite minimum, and does not change with run time. At 800^oC glass is heterogeneous, with composition reflecting adjacent mineralogy in most cases. The molar ratios ASI (=mol. Al₂O₃/CaO+Na₂O+K₂O) and Al/Na are constant throughout the entire charge. Molar Al/Ca is greater close to feldspars (either Pl or Kfd) and lower close to Qtz, whereas Al/K shows the opposite behavior. Glasses within any given 800^oC experiment form linear trends in (wt %) normative Qtz-Ab-Or space, with endpoints within the 800^oC isotherms, nearly constant Ab/Or ratio, and variable Qtz/Fsp ratio. The linear trends do not pass through the eutectic composition, and with increasing run time their Ab/Or and Qtz/Fsp ratios move toward those of the starting composition. These results are in accordance with recent investigations on the diffusive transport properties of granitic melts at 200 MPa H₂O and 800^oC.