Experimental determination of Ra mineral/melt partitioning for feldspars and 226Ra-disequilibrium crystallization ages of plagioclase and alkali-feldspar

Radium and other trace element partition coefficients have been experimentally determined for plagioclase and alkali-feldspar in equilibrium with silicate melts. In anorthitic plagioclase, alkalis (e.g. Na, K, Rb, Cs) and the heavy earth alkalis (e.g. Ba, Ra) are incompatibles, in albitic plagioclase Na is compatible, whereas Ca and Sr are always compatible in plagioclase. DRaplagioclase/melt was determined to be 0.017 ± 0.006 (An91), 0.025 ± 0.009 (An81) and 0.47 ± 0.08 (An34) for three different An contents, the ratio DRa/DBa increasing with decreasing An component. In alkali-feldspar, K, Rb, Ba, Ra, and Sr are compatible, whereas Na, Cs, and Ca are incompatible. DRaalkali-feldspar/melt was determined to 2.54 ± 0.04 for a Or75Ab21An4 composition, the ratio DRa/DBa is 0.55 ± 0.01. For plagioclase, DRa and XAn are related as RT lnDRa [kJ/mol] = - 52.54(± 5.75)XAn - 4.25(± 1.51) and DRa/DBa = exp((- 9.47(± 6.21)XAn - 13.66(± 1.52))/RT) [kJ/mol], allowing calculation of DRa from the An content of magmatic plagioclase. For alkali-feldspar we propose the equation DRa/DBa = exp((- 16.06(± 0.58) +15.52(± 0.67) XOr)/RT) [kJ/mol]. The experimentally determined partition coefficients are then used to recalculate Th-Ra-Ba model ages, leading to generally shorter feldspar residence times. The variations encompass 20 to 90% shorter crystallization ages with respect to the originally calculated ones, in two cases feldspar crystallization ages increase slightly. For several cases the measured plagioclase and melt have either never been in equilibrium or the Th-Ra-Ba system has been disturbed after crystallization.