Paper No. 8
Presentation Time: 10:20 AM
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, 690oC and 800oC. 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 690oC the glass composition is roughly homogeneous, close to the 200 MPa H2O haplogranite minimum, and does not change with run time. At 800oC 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 800oC experiment form linear trends in (wt %) normative Qtz-Ab-Or space, with endpoints within the 800oC 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 800oC.