The effect of composition on chlorine solubility and behavior in silicate melts.

We have performed experiments at 1.5 GPa and 1400 °C on 25 different bulk compositions to determine the effects of major element compositions on the Cl contents of silicate melts at known fugacities of Cl₂ and O₂. The experimental method involved mixing a "sliding" Cl buffer, a mixture of AgCl, AgI, and Ag with the silicate bulk composition and performing the experiment in a graphite capsule together with a source of CO₂ (AgCO₃). The graphite capsules were sealed inside welded Pt tubes to maintain a CO₂-CO atmosphere with oxygen fugacity fixed at the C-CO-CO₂ (CCO) buffer. During the experiment, the Cl buffer segregates leaving a Cl-bearing melt, which quenches to a glass. We used the results to define chloride capacity C_Cl for each melt at the pressure and temperature of the experiment: C C l = C l (w t f C l 2 × f O 2 4 Chloride capacity was found to correlate positively with optical basicity and NBO/T and negatively with ionic porosity and the Larsen index. We combined our new data with the results of Thomas and Wood (2021) to derive an equation describing the composition, pressure and temperature dependence of the chloride capacity: log C C l = 1.601 + 4470 X C a − 3430 X S i + 2592 X F e − 4092 X K − 894 P / T. In this equation, X_Ca, X_Si, and so on refer to the oxide mole fractions on a single-cation basis, P is in GPa and T in K. The equation reproduces 58 data points with an r² of 0.96 and a standard error of 0.089. The addition of literature data on hydrous experiments indicates that the effects of <4.3 wt% H₂O are small enough to be ignored. We also performed experiments aimed at determining the conditions of NaCl saturation in melts. When combined with literature data we obtained: log C l − = log a N a C l + 0.06 − 2431 X C a + 3430 X S i − 2592 X F e + 3484 X N a + 4092 X K − 2417 / T where (Cl^–) is the Cl content of the melt in wt% a_NaCl is the activity of NaCl (liquid) and the other symbols are the same as before. The results indicate that basalt dissolves ~8 times more Cl than rhyolite at a given NaCl activity i.e., Cl is ~8 times more soluble in basalt than in rhyolite. [ABSTRACT FROM AUTHOR]