PACROFI VI - Electronic Program

Microthermometry of volatile-rich silicate melt inclusions in granitic rocks

Rainer Thomas, Dieter Rhede, and Robert B. Trumbull

GeoForschungsZentrum Potsdam D-14473 Potsdam (Germany)

Melt inclusions provide compositional information about granitic melts which cannot be obtained by analysis of bulk rocks. This is particularly true for the volatiles, which are depleted in rocks, even some vitrophyres, because of loss by degassing. Most natural melt inclusions are too small for direct in-situ chemical analysis by FTIR or SIMS techniques, and it is difficult or impossible to find enough large inclusions for a statistically significant number of measurements in a sample.

We discuss a simple method of estimating total volatile contents in granitic melts from microthermometric measurement of melt inclusions as small as 5 microns, which eliminates the size problem and thereby permits collection of large numbers of data. The method works in analogy to fluid inclusion microthermometry; however it takes into account the rate of melting reactions. We determine the beginning of melting temperature (solidus TS) of inclusions by a step-heating and quenching technique.

Homogenization in silicate melt inclusions is a diffusion-controlled process and this is the basis for determining volatile contents and viscosity. All heating runs are held for a constant duration; thus we can estimate the diffusion rate by measuring the size of the smallest inclusion which has homogenized in a given run. Differences in diffusion rate, at constant T, P and bulk composition, are a measure of total "volatility" of the melt (the most important contributors being H2O, CO2, F, P, Li). This "total volatility" in analogy to "total salinity" of fluid inclusions, is expressed by the equivalent concentration of the most abundant volatile species, H2O and written "CW as eq.-wt% H2O". As a by-product of the method, estimates of melt viscosity can also be made.

We present values for volatile content (as eq.-wt.% H2O), solidus temperatures and the viscosity of melt inclusions in quartz and topaz from granitic and pegmatitic rocks of the Erzgebirge, Germany, the Slavkovsky les (Czech Republic) and the Massif Central, France. The volatile concentrations range from 5 to 12 eq.-wt.% H2O. Microprobe analyses show that fluorine and phosphorus are the main volatiles present apart from H2O. Viscosity values derived from the data show Arrhenian behaviour in the temperature range of 500 - 1000oC. Some of the most F-rich melts possess very low viscosities on the order of 101 to 103 Pa.s at 800oC, which have important implications for the separation of residual melts in silicate crystal - melt - (vapour) systems.