CONSTRAINING RADIUM AND BARIUM PARTITION COEFFICIENTS OF POTASSIUM FELDSPAR IN YOUNG (<1100 YEARS), ALKALI-RICH MAGMAS ERUPTED AT CHANGBAISHAN VOLCANO, CHINA: A MASS BALANCE APPROACH

Radium isotope dating has become more popular for constraining the ages of young volcanic rocks erupted in the recent past. Usually, feldspar is targeted but Ra isotopes of the melt component can also be used to constrain maximum eruption ages. For such dates, feldspar ages are highly dependent on the partition coefficients chosen, which are normally derived from experimental and theoretical bases. It is very difficult to obtain direct empirical measurements for feldspars because the best estimates originate from zero-aged crystals that are free of melt inclusions or any other materials that might retain Ra (or Ba) at low levels. As such, we estimate Ra and Ba partition coefficients for potassium feldspars from extremely young, alkali-rich trachytes where the melt component (represented by crystal-free pumice) is subtracted from the whole rock component (melt plus crystals) to obtain average D_Ra and D_Ba values of feldspar. These values depend on the volume of crystals present in the whole rocks as determined using concentration differences between whole rocks and crystal-free pumices for potassium feldspar incompatible elements such as Zr and Nb. Age corrections are also required for volcanic rocks with reasonable or known eruption ages. For rocks in which feldspar compositions appear uniform, D_Ra/D_Ba varies between ~0.3-0.6, which is in agreement with experimental and theoretical calculated values for relevant trachyte temperatures (800°-900°C). For rocks in which feldspar compositions vary (anorthoclase-orthoclase compositions), D_Ra/D_Ba values are generally lower (0.1 to < 0.3) at relevant trachyte temperatures (800°-900°C). As such, it initially appears that alkaline rocks reflect similar D_Ra/D_Ba values as expected for non-alkaline compositions at intermediate SiO₂ compositions.