DENSITY-MEASUREMENTS ON MELTS AT HIGH-PRESSURE USING THE SINK FLOAT METHOD - LIMITATIONS AND POSSIBILITIES/

We investigated the use of the sink/float method to determine the dens ity of silicate melts with a range of compositions intended to provide a general understanding of density changes as functions of compositio n and pressure. For each composition, estimates of both the melting te mperature and the melt viscosity (which determines the required experi mental run time) at the pressures of interest are required prior to th e density experiments. We constrained the melting relationships for tw o sodium silicate melts and a melt of albite composition and were able to calculate the maximum possible viscosities of the sodium silicate melts, The sink/float method is limited by the absence of high pressur e-high temperature density data for minerals that may be used as buoya ncy markers in the experiments. These data need to be sufficiently pre cise to justify their use in the determination of melt densities at hi gh pressure. At present, high-quality data are available for only a fe w minerals, such as spinel, forsterite, quartz and some garnets. Anoth er limitation is that the markers should not react with the melt. Reac tion between the buoyancy markers, melt and capsule material changes t he density of both melt and markers. Errors on the order of greater th an or equal to 2% on the determined densities may be introduced by the changing density of the mineral markers through reaction with the mel t. Although the sink/float method is very elegant, it has significant limitations that prohibit the precise determination of the density of an arbitrary silicate melt at high pressure; in most cases, only loose constraints on the density and compressibility of melts can be obtain ed as a function of pressure.