2 research outputs found
Dissolution of quartz, albite, and orthoclase in H2O-saturated haplogranitic melt at 800C and 200 MPa: diffusive transport properties of granitic melts at crustal anatectic conditions.
24 pages, 14 figures, 6 tables.We have conducted experiments on dissolution of quartz, albite, orthoclase, and corundum into H2O-saturated haplogranite melt at
800 C and 200 MPa over a duration of 120–1488 h with the aim of ascertaining the diffusive transport properties of granitic melts at crustal anatectic temperatures. Cylinders of anhydrous starting glass and a single mineral phase (quartz or feldspar) were juxtaposed
along flat and polished surfaces inside gold or platinum capsules with 10 wt % added H2O. Concentration profiles in glass (quenched melt) perpendicular to the mineral–glass interfaces and comparison with relevant phase diagrams suggest that melts at the interface are saturated in the dissolving phases after 384 h, and with longer durations the concentration profiles are controlled only by
diffusion of components in the melt. The evolution of the concentration profiles with time indicates that uncoupled diffusion in the melt takes place along the following four linearly independent directions in oxide composition space: SiO2, Na2O, and K2O axes (Si-, Na-, and K-eigenvectors, respectively), and a direction between the Al2O3, Na2O, and K2O axes (Al-eigenvector), such that the Al/Na molar ratio is equal to that of the bulk melt and the Al/(Na þ K) molar ratio is equal to the equilibrium ASI (¼ mol. Al2O3/[Na2O þ K2O]) of the melt. Experiments in which a glass cylinder was sandwiched between two mineral cylinders—quartz and albite, quartz and K-feldspar, or albite and corundum—tested the validity of the inferred directions of uncoupled diffusion and explored longrange chemical communication in the melt via chemical potential gradients. The application of available solutions to the diffusion equations for the experimental quartz and feldspar dissolution data
provides diffusivities along the directions of the Si-eigenvector and Al-eigenvector of (2 0–2 8) · 10 15 m2/s and (0 6–2 4) ·
10 14 m2/s, respectively. Minimum diffusivities of alkalis
[ (3–9) · 10 11 m2/s] are orders of magnitude greater than
the tetrahedral components of the melt. The information provided here
determines the rate at which crustal anatexis can occur when sufficient
heat is supplied and diffusion is the only mass transport (mixing) process in the melt. The calculated diffusivities imply that a quartzo-feldspathic source rock with initial grain size of 2–3 mm undergoing hydrostatic, H2O-saturated melting at 800 C (infinite heat supply) could produce 20–30 vol. % of
homogeneous melt in less than 1–10 years. Slower diffusion in H2O-undersaturated melts will increase this time frame.Support for this research was provided by National Science Foundation grants EAR-990165, INT-9603199, EAR-9618867, EAR-9625517, EAR-9404658, and a postdoctoral grant to A.A.V. from the Universidad de
Granada, Spain. The Electron Microprobe Laboratory at the University of Oklahoma was created with US DOE grant DE-FG22-87FE1146 and upgraded with NSF grant EAR-9404658 and support from the University of Oklahoma Office of Research Administration. We thank
Don Baker, Alberto PatinËœo Douce, Dennis Geist and an anonymous referee for thorough reviews that improved greatly the consistency and clarity of the manuscript.Peer reviewe
Dissolution of quartz, albite, and orthoclase in H2O-saturated haplogranitic melt at 800C and 200 MPa: diffusive transport properties of granitic melts at crustal anatectic conditions.
24 pages, 14 figures, 6 tables.We have conducted experiments on dissolution of quartz, albite, orthoclase, and corundum into H2O-saturated haplogranite melt at
800 C and 200 MPa over a duration of 120–1488 h with the aim of ascertaining the diffusive transport properties of granitic melts at crustal anatectic temperatures. Cylinders of anhydrous starting glass and a single mineral phase (quartz or feldspar) were juxtaposed
along flat and polished surfaces inside gold or platinum capsules with 10 wt % added H2O. Concentration profiles in glass (quenched melt) perpendicular to the mineral–glass interfaces and comparison with relevant phase diagrams suggest that melts at the interface are saturated in the dissolving phases after 384 h, and with longer durations the concentration profiles are controlled only by
diffusion of components in the melt. The evolution of the concentration profiles with time indicates that uncoupled diffusion in the melt takes place along the following four linearly independent directions in oxide composition space: SiO2, Na2O, and K2O axes (Si-, Na-, and K-eigenvectors, respectively), and a direction between the Al2O3, Na2O, and K2O axes (Al-eigenvector), such that the Al/Na molar ratio is equal to that of the bulk melt and the Al/(Na þ K) molar ratio is equal to the equilibrium ASI (¼ mol. Al2O3/[Na2O þ K2O]) of the melt. Experiments in which a glass cylinder was sandwiched between two mineral cylinders—quartz and albite, quartz and K-feldspar, or albite and corundum—tested the validity of the inferred directions of uncoupled diffusion and explored longrange chemical communication in the melt via chemical potential gradients. The application of available solutions to the diffusion equations for the experimental quartz and feldspar dissolution data
provides diffusivities along the directions of the Si-eigenvector and Al-eigenvector of (2 0–2 8) · 10 15 m2/s and (0 6–2 4) ·
10 14 m2/s, respectively. Minimum diffusivities of alkalis
[ (3–9) · 10 11 m2/s] are orders of magnitude greater than
the tetrahedral components of the melt. The information provided here
determines the rate at which crustal anatexis can occur when sufficient
heat is supplied and diffusion is the only mass transport (mixing) process in the melt. The calculated diffusivities imply that a quartzo-feldspathic source rock with initial grain size of 2–3 mm undergoing hydrostatic, H2O-saturated melting at 800 C (infinite heat supply) could produce 20–30 vol. % of
homogeneous melt in less than 1–10 years. Slower diffusion in H2O-undersaturated melts will increase this time frame.Support for this research was provided by National Science Foundation grants EAR-990165, INT-9603199, EAR-9618867, EAR-9625517, EAR-9404658, and a postdoctoral grant to A.A.V. from the Universidad de
Granada, Spain. The Electron Microprobe Laboratory at the University of Oklahoma was created with US DOE grant DE-FG22-87FE1146 and upgraded with NSF grant EAR-9404658 and support from the University of Oklahoma Office of Research Administration. We thank
Don Baker, Alberto PatinËœo Douce, Dennis Geist and an anonymous referee for thorough reviews that improved greatly the consistency and clarity of the manuscript.Peer reviewe