Partitioning of OH-F-Cl between biotite and silicate melt: Experiments and an empirical model

Abstract

The partitioning of hydroxyl (OH), fluorine (F) and chlorine (Cl) between biotite and silicate melt has been experimentally investigated at 100–500 MPa and 850–975 °C. A variety of starting glasses was used, covering alkali-rich basalt, trachyandesite, trachyte, tephriphonolite and dacite, in order to investigate potential compositional effects of both melt and biotite on the partitioning behaviors. The investigated F contents in biotite and silicate melt are ranging within 0.2–8.2 wt% and 0.1–4.8 wt% respectively, yielding a Nernst-type partition coefficient DFBt-melt within 1.5–10. The investigated Cl contents in both biotite and silicate melt are much lower (≤0.15 wt% and ≤ 1.3 wt% respectively), and DClBt-melt values are < 1 (0.02–0.4). The results are used to estimate the exchange coefficients (ratio of D values) for F-OH and Cl-OH pairs expressed as KdF/OHBt-melt=XFBt/XOHBt/XFmelt/XOHmelt, and KdCl/OHBt-melt=XClBt/XOHBt/XClmelt/XOHmelt, in which XFBt, XClBt and XOHBt are mole fractions of F, Cl and OH in biotite, and XFmelt, XClmelt and XOHmelt are mole fractions of F, Cl and OH in silicate melt, respectively. KdF/OHBt-melt is in the range of 1–50, which is higher than KdCl/OHBt-melt (0.07–5) by about one order of magnitude, implying that the relative incorporation preference of F, OH and Cl between biotite and melt is highest for F, lower for OH and lowest for Cl. Biotite and melt compositions exert important effects on the OH-F-Cl partitioning between biotite and melt. Multiple linear regression of data from literature and this study allows us to propose the following empirical equations:lnKdF/OHBt-melt=0.995-1.619∗DASI+3.613∗XMgBt-17.523∗XTiBt (R2 = 0.83, sd = 0.31),lnKdCl/OHBt-melt=3.380-2.454∗CSI-0.398∗XMgBt-22.377∗XTiBt (R2 = 0.94, sd = 0.26),in which XMgBt = Mg/(Mg + Fe3++Fe2++Mn + Ti + AlVI) of biotite, XTiBt = Ti/(Mg + Fe3++Fe2++Mn + Ti + AlVI) of biotite, DASI (deviation from ASI of 1) = |1–ASI|, with ASI = molar Al2O3/(Na2O + K2O + CaO) of the melt, and CSI (chlorine solubility index) = molar (Al1/2 + Ca1/2 + Mg1/2 + Na)/Si of the melt. The empirical models confirm the so-called Mg-F preference and Mg-Cl avoidance rules in biotite, and also reveal that increasing Ti content in biotite tends to lower KdF/OHBt-melt and KdCl/OHBt-melt. Furthermore, DASI and CSI are effective parameters that reflect the incorporation abilities of F and Cl in melt as a function of melt composition. A wide prediction potential of the OH-F-Cl exchange partition model is expected, because fairly large ranges in temperature (640–975 °C), melt composition (ASI = 0.7–1.4; CSI = 0.6–1.6) and biotite composition (XMgBt = 0–0.88) are covered. Calculated KdF/OHBt-melt and KdCl/OHBt-melt from the models are in good agreement with experimentally determined values, and the deviation from the models are less than 20% relative in both cases. Applying the empirical models, we calculated melt F and Cl concentrations based on biotite compositions from several intrusive-volcanic rocks that are associated with different deposit types. The results show that the magmas associated with Mo and REE deposits are characterized by high F (up to ca. 10,000 ppm) but relatively low Cl. On the contrary, the magmas that have contributed to the formation of porphyry Cu and Au deposits contain much higher Cl (up to ca. 10,000 ppm) than F.