Open AccessDiffusion of Ca and F in haplogranitic melt from dissolving fluorite crystals at 900°–1000°C and 100 MPa
Author(s) -
Gabitov R. I.,
Price J. D.,
Watson E. B.
Publication year - 2005
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2004gc000832
Subject(s) - fluorite , electron microprobe , dissolution , diffusion , analytical chemistry (journal) , mineralogy , thermal diffusivity , materials science , fluorine , crystal (programming language) , microprobe , geology , thermodynamics , chemistry , metallurgy , chromatography , physics , computer science , programming language
The diffusion rates of Ca and F into haplogranitic melt were determined by concentration profiles in glasses produced by dissolving fluorite crystals into haplogranitic melt at 900°–1000°C and 75–100 MPa. Starting products were synthesized from oxides of haplogranitic composition at two aluminosities ([Na 2 O + K 2 O]/Al 2 O 3 = 1.05 and 1.68). Fluorite crystals or crystal powders were coupled with powders or solid cylinders of haplogranitic glass. Water was added to the runs, resulting in two runs at H 2 O = 1.20 and 1.40 wt% and three runs thought to be water saturated. The materials were then subjected to the conditions of interest in either a Stellite cold seal apparatus or an internally heated pressure vessel. Electron microprobe traverses of the quenched glasses produced in these experiments revealed negative concentration gradients in Ca and F away from the fluorite‐glass boundary. These gradients are comparable to an error function solution (in the first approximation) for specific Ca and F diffusivities (D Ca and D F , respectively). The Ca diffusion coefficient increased from 3.36 ± 0.19 · 10 −9 to 2.16 ± 0.28 · 10 −8 cm 2 /s and F diffusivity increased from 1.16 ± 0.09 · 10 −8 to 6.59 ± 0.62 · 10 −8 cm 2 /s as temperature increased from 900° to 1000°C. These diffusivities are comparable to other published melt diffusion data for the dissolution of multicomponent minerals where the diffusants move independently. Calcium and fluorine diffusivities increase from 2.77 ± 0.09 · 10 −9 to 2.16 ± 0.28 · 10 −8 cm 2 /s and from 2.27 ± 0.10 · 10 −8 to 6.59 ± 0.62 · 10 −8 cm 2 /s, respectively, with increasing water content at 1000°C, likely due to melt depolimerization. The large difference in the concentrations of Ca ( C 0 Ca ) and F ( C 0 F ) at the fluorite‐glass interface suggests that these two elements diffuse independently in the melt. In contrast, the concentration gradient for Na parallels that of F, suggesting complexing between these two elements.