Document Type
Thesis
Abstract
Hydrothermal experiments were conducted on high-silica (73-75 wt% SiO2), fluid-saturated melts at 844-862°C and ca. 50 MPa using crushed glass of the Los Posos rhyolite. Water and salts including NaCl, KCl, Ca(OH)2, and CaHPO4 and HCl were added proportionally to the experiments to restrict the variability of the aluminosity of the melt. The Durango apatite, which contains 3.53 wt% F and 0.41% Cl, was added as “seeds”<5µm in diameter to stimulate apatite growth during the experiments. Samples were loaded into gold capsules and run in cold-seal pressure vessels for durations of 286- 1008 hours. Temperature was cycled at ±20°C to promote apatite crystallization. Electron microprobe analyses of run-product glasses and embedded apatite grains support calculation of a range of partition coefficients (Dapat/mt Cled mt — wt% Cl in apatite/wt% Cl in melt) of 4.7 to 15.9. The molar ratio of Cl in experimental apatites, or XCl, ranges from 0.19 to 0.56, while XF ranges from 0.08 to 0.63. The computed values for XOH range from 0.24 to 0.38. We find that normalizing XCI to XOH of apatites dramatically improves the precision when using apatite compositions to model Cl contents of melts. We compare our Los Posos rhyolite experiments with published data on 50 MPa thyodacite experiments and find that Cl partitioning is significantly different in each system. Given the importance of chlorine in fluid equilibria, ore transport, and magma evolution, applications of apatite as a proxy for Cl contents in melts are unbounded. It is found that in order to accurately use the volatile composition of natural and synthetic apatites to calculate the volatile composition of melts in felsic systems, several chemical factors, including wt% SiO2 and the aluminosity/alkalinity of melts, should be incorporated as parameters to enhance relevant modeling. This allows geochemists to place better constraints on processes associated with crystallizing Cl-bearing magmatic systems.
Date Awarded
2016
Department
Geology
Advisor 1
Brent E. Owens
Recommended Citation
Flesch, Ryan, "Using Apatite to Model Chlorine Contents of High SiO2 Magmas: An Enhanced Methodological Approach" (2016). Geology Senior Theses. William & Mary. Paper 361.
https://scholarworks.wm.edu/geologyseniors/361
