Experimental partitioning of Rb and K between phlogopite and a concentrated (K,Rb)Cl brine: implications for the role of concentrated KCl brines in the depletion of Rb in phlogopite and the stability of phlogopite during charnockite genesis

In this study, phlogopite synthesis experiments were carried out at 800 °C and at 1000 and 2000 MPa for low concentrations of Rb (XRb=Rb/(Rb+K)<0.1) in a concentrated (K,Rb)Cl brine+(Rb,K)Cl salt (XH2Obulk=0.3). For each experiment, solid run products consisted of approximately 70–80% phlogopite (XRb=0.03–0.12) with both a minor celadonite component (≈10%) as well as a Cl component in the order of 5–12 M% relative to OH. The remaining 20–30% consisted of approximately equal amounts of newly grown sanidine and quartz. There is no physical evidence that any of these phases are metastable. Rb fractionated strongly into the phlogopite, whereas for the K-feldspar, Rb was almost equally distributed between the feldspar and fluid+salt. At 800 °C, the derived distribution coefficients KDPhlg–fluid (Rb–K) and KDSnd–fluid (Rb–K) for Rb (Henryʹs law) are 1.88±0.12 and 0.86±0.14, respectively, at 2000 MPa and 1.52±0.05 and 0.6±0.4, respectively, at 1000 MPa. Several conclusions may be drawn from these experimental results in conjunction with observations in the field. The first is that phlogopite, with a minor celadonite component and 5–12 M% Cl relative to OH, can form in a relatively dry system consisting of a concentrated (Rb,K)Cl brine+(K,Rb)Cl salt (XH2Obulk=0.3) at high temperature and pressures, i.e., 800 °C and 1000–2000 MPa. The presence of celadonite and Cl− apparently stabilises the phlogopite in the presence of quartz at low H2O activities as compared to end-member OH-phlogopite which breaks down to enstatite+K-feldspar in the presence of quartz. A second conclusion is that in concentrated KCl brines, Rb is preferentially partitioned into the phlogopite under these P–T and αH2O conditions. This agrees with similar studies involving 1 and 2 m (K,Rb)Cl salt solutions as well as a simple K–Rb exchange experiments between end-member OH-phlogopite and a concentrated (K,Rb)Cl brine (XH2Ofluid=0.6, XRb=0.05) at 800 °C and 1000 MPa.