Experimental study of monazite solubility in haplogranitic melts: a new model for peraluminous and peralkaline melts

Abstract: Monazite is a common accessory mineral in peraluminous, metaluminous and peralkaline granitic/rhyolitic rocks. Considering the importance of monazite in geochemical and geochronological studies, a monazite solubility model that can be applied to a wide compositional range of magmas is desirable. To accomplish this, monazite solubility experiments were performed at atmospheric (1400 C) and crustal pressures (1–3 kbar, 720–850 C, H2O-saturated), using haplogranitic compositions ranging from peraluminous to peralkaline, doped with synthetic pure LaPO4. The concentrations of La in the melts increase sharply with increasing temperature and peralkalinity of the melt. We combined our new data with those of previous studies to describe the solubility of monazite in peralkaline to peraluminous melts. Our new monazite saturation model, which incorporates temperature, pressure, water content, melt and monazite composition is given by: lnRLREE = 12.77(±0.49) + 1.52(±0.15)M + 22 0.44(±0.10)(H2O)0.5 9934(±632)/T 36.79(±6.15)P/T + lnXLREE mnz where RLREE is the sum of the concentrations of La to Sm in monazite-saturated melt, in ppm, M is a dimensionless compositional parameter (Na + K + 2Ca) · Al1 · (Al + Si)1, similar to the compositional parameter used in an earlier model by Montel; Na, K, Ca, Al, Si are in moles, H2O is water content in weight percent, T is the temperature in K, P is the pressure in kbar, and XLREE mnz is the mole fraction of LREE in monazite LREEmnz/ (LREEmnz + Ymnz + Thmnz + Umnz). This model reproduces 76% and >95% of the data to within uncertainties of ±10% and ±20%, respectively. It may be applied to felsic melts poor in CaO + FeO + MgO (<3 wt%) from peraluminous to peralkaline compositions. Key-words: monazite; solubility; melt composition; agpaitic index; peralkaline; peraluminous; haplogranite; model; experimental petrology.