TY - JOUR

T1 - H2O diffusion in peralkaline to peraluminous rhyolitic melts

AU - Behrens, Harald

AU - Zhang, Youxue

N1 - Funding Information: Acknowledgments We thank O. Dietrich for preparation of sections for experiments and IR. Inspiring reviews from B. Schmidt and a second anonymous reviewer are acknowledged. This research was supported by the German Science Foundation (DFG Be1720/7 and Be1720/11) and the US National Science Foundation (EAR-0537598, EAR-0711050).

PY - 2008/11/28

Y1 - 2008/11/28

N2 - The diffusion of water in a peralkaline and a peraluminous rhyolitic melt was investigated at temperatures of 714-1,493 K and pressures of 100 and 500 MPa. At temperatures below 923 K dehydration experiments were performed on glasses containing about 2 wt% H2 Ot in cold seal pressure vessels. At high temperatures diffusion couples of water-poor (<0.5 wt% H2Ot) and water-rich (∼2 wt% H2Ot) melts were run in an internally heated gas pressure vessel. Argon was the pressure medium in both cases. Concentration profiles of hydrous species (OH groups and H2O molecules) were measured along the diffusion direction using near-infrared (NIR) microspectroscopy. The bulk water diffusivity (DH2Ot) was derived from profiles of total water (CH2Ot) using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between DH2Ot and CH2Ot Both methods consistently indicate that DH2Ot is proportional to CH2Ot in this range of water contents for both bulk compositions, in agreement with previous work on metaluminous rhyolite. The water diffusivity in the peraluminous melts agrees very well with data for metaluminous rhyolites implying that an excess of Al2O3 with respect to alkalis does not affect water diffusion. On the other hand, water diffusion is faster by roughly a factor of two in the peralkaline melt compared to the metaluminous melt. The following expression for the water diffusivity in the peralkaline rhyolite as a function of temperature and pressure was obtained by least-squares fitting: log DH2Ot1wt% = (-7.09 ± 0.15) - (4,788 ± 166) + (0.56 ± 0.21) × P/T where DH2Ot1wt% is the water diffusivity at 1 wt% H2Ot in m2/s, T is the temperature in K and P is the pressure in MPa. The above equation reproduces the experimental data (14 runs in total) with a standard fit error of 0.15 log units. It can be employed to model degassing of peralkaline melts at water contents up to 2 wt%.

AB - The diffusion of water in a peralkaline and a peraluminous rhyolitic melt was investigated at temperatures of 714-1,493 K and pressures of 100 and 500 MPa. At temperatures below 923 K dehydration experiments were performed on glasses containing about 2 wt% H2 Ot in cold seal pressure vessels. At high temperatures diffusion couples of water-poor (<0.5 wt% H2Ot) and water-rich (∼2 wt% H2Ot) melts were run in an internally heated gas pressure vessel. Argon was the pressure medium in both cases. Concentration profiles of hydrous species (OH groups and H2O molecules) were measured along the diffusion direction using near-infrared (NIR) microspectroscopy. The bulk water diffusivity (DH2Ot) was derived from profiles of total water (CH2Ot) using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between DH2Ot and CH2Ot Both methods consistently indicate that DH2Ot is proportional to CH2Ot in this range of water contents for both bulk compositions, in agreement with previous work on metaluminous rhyolite. The water diffusivity in the peraluminous melts agrees very well with data for metaluminous rhyolites implying that an excess of Al2O3 with respect to alkalis does not affect water diffusion. On the other hand, water diffusion is faster by roughly a factor of two in the peralkaline melt compared to the metaluminous melt. The following expression for the water diffusivity in the peralkaline rhyolite as a function of temperature and pressure was obtained by least-squares fitting: log DH2Ot1wt% = (-7.09 ± 0.15) - (4,788 ± 166) + (0.56 ± 0.21) × P/T where DH2Ot1wt% is the water diffusivity at 1 wt% H2Ot in m2/s, T is the temperature in K and P is the pressure in MPa. The above equation reproduces the experimental data (14 runs in total) with a standard fit error of 0.15 log units. It can be employed to model degassing of peralkaline melts at water contents up to 2 wt%.

KW - HO diffusion

KW - IR spectroscopy

KW - Magma degassing

KW - Peralkaline

KW - Peraluminous

KW - Rhyolite

UR - http://www.scopus.com/inward/record.url?scp=65049086333&partnerID=8YFLogxK

U2 - 10.1007/s00410-008-0363-4

DO - 10.1007/s00410-008-0363-4

M3 - Article

AN - SCOPUS:65049086333

VL - 157

SP - 765

EP - 780

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 6

ER -