A thermodynamic model for F-Cl-OH partitioning between silicate melts and apatite including non-ideal mixing with application to constraining melt volatile budgets

• Main Authors: Li, Weiran, Costa, Fidel
• Other Authors: Asian School of the Environment
• Format: Article
• Language: English
• Published: 2020
• Subjects: Engineering::Environmental engineering; Apatite; Eruption
• Online Access: https://hdl.handle.net/10356/143573
• Institution: Nanyang Technological University

The abundance and composition of volatiles in subvolcanic melts play a key role in controlling eruptive styles of volcanoes,but they are difficult to determine directly due to volatile loss during magma transport to the surface and eruption. Most con-straints on volatile abundances are obtained by studying melt inclusions in minerals, but not all samples contain suitableinclusions, and they can be modified by a range of post-entrapment and re-equilibration processes. Apatite incorporates sev-eral volatile elements such as F, Cl, H, C and S into its structure, and thus has been proposed as an alternative tool for track-ing the melt volatile contents. However, application of the apatite approach replies on the partitioning of volatiles betweenapatite and silicate melts, which has been found to show non-Nernstian behaviour but yet to be quantified. Here we propose athermodynamic model that considers the non-ideal mixing in apatite solution, and includes the interaction parameters (WG)and Gibbs free energy properties calculated by regressing experimental data from the literature. We find thatWGfor the Cl-Fbinary join is larger than those for the Cl-OH, and F-OH joins, indicating a stronger non-ideality. We propose two equationsfor calculating the exchange coefficients (KD) between apatite (Ap) and silicate melts as:lnKDAp meltOH F ¼ 1RT f94;600 5600ðÞ 40ð 0:1Þ T 1000 7 4ðÞ XApF XApOH 11 7ðÞ XApCl gandlnKDAp meltOH Cl ¼ 1RT f72;900 2900ðÞ 34ð 0:3Þ T 1000 ½5 2ðÞ XApCl XApOH 10ð 8Þ XApF gwhere temperature (T) is in kelvins, apatite compositions are expressed in mole fractions (XApi), and R is the universal gasconstant. With the two equations above, we established a calculation procedure for estimating the water concentrations in themelt, and have developed it into an online calculator (https://apthermo.wovodat.org/). Application of this method to volcanicapatite from the literature (e.g. from Pinatubo, Campi Flegrei, Santiaguito and Augustine) gives melt water concentrationsthat are equivalent or higher than those measured from melt inclusions. Our new calibrations of the exchange coefficientsallow us to obtain more robust estimates of melt volatile budgets, which provide more insights into the effects of volatileson a variety of volcanic and plutonic phenomena.