Th and U partitioning between monazite and felsic melt

Abstract

We present experimental measurements of monazite/melt partition coefficients (Kmonz/melt) for thorium (Th) and uranium (U) in haplogranitic Na2O–Al2O3–SiO2 melts (with 6–10wt.% H2O) at 900°C–1100°C and 1GPa pressure. Our results show the following trends: (1) KTh and KU in felsic melts are strongly dependent on melt Al/Na. Both partition coefficients reach maxima in mildly peraluminous region and decrease toward strongly peraluminous (Al/Na>1) and peralkaline (Al/Na<1) melt compositions. KTh and KU vary by 2 orders of magnitude for Al/Na in the range of 0.4 to ~2. (2) For identical melt compositions, the effect of temperature on KU is pronounced: A 200°C increase in temperature results in a 6-fold decrease in KU. In contrast, the same temperature change results in only 50% decrease of the less temperature-sensitive KTh. (3) One experiment with 10wt.% H2O added does not change K, which may suggest little dependence of K upon water content from 6 to 10wt.%. (4) KTh is much larger than 1 at all natural magmatic temperature, so the removal of monazite during fractionation crystallization or its retention as a residual phase during partial melting has a significant effect on the content of Th in the coexisting melt phase. (5) U is much less compatible than Th, and could even behave incompatibly at high temperatures (e.g., at 1000°C, KU is 0.01 for melts containing 75wt.% SiO2 and Al/Na~0.4).