Hydrogen-alkali exchange between silicate melts and two-phase aqueous mixtures: an experimental investigation |
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| Authors: | Thomas J Williams Philip A Candela Philip M Piccoli |
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| Institution: | (1) Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland at College Park, College Park, MD 20742-4211, USA, US |
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| Abstract: | Experiments were performed in the three-phase system high-silica rhyolite melt + low-salinity aqueous vapor + hydrosaline
brine, to investigate the exchange equilibria for hydrogen, potassium, and sodium in magmatic-hydrothermal systems at 800
°C and 100 MPa, and 850 °C and 50 MPa. The K
aqm/melt
H,Na and K
aqm/melt
H,K for hydrogen-sodium exchange between a vapor + brine mixture and a silicate melt are inversely proportional to the total
chloride concentration (ΣCl) in the vapor + brine mixture indicating that HCl/NaCl and HCl/KCl are higher in the low-salinity
aqueous vapor relative to high-salinity brine. The equilibrium constants for vapor/melt and brine/melt exchange were extracted
from regressions of K
a
q
m
/
m
e
l
t
H
,
N
a
and K
a
q
m
/
m
e
l
t
H
,
K
versus the proportion of aqueous vapor relative to brine in the aqueous mixture (Faqv) at P and T, expressed as a function of ΣCl. No significant pressure effect on the empirically determined exchange constants was observed
for the range of pressures investigated. Model equilibrium constants are: K
aqv/melt
H,Na(vapor/melt)=26(±1.3) at 100 MPa (800 °C), and 19( ± 7.0) at 50 MPa (850 °C); K
aqv/melt
H,K=14(±1.1) at 100 MPa (800 °C), and 24(±12) at 50 MPa (850 °C); K
aqb/melt
H,b(brine/melt)= 1.6(±0.7) at 100 MPa (800 °C), and 3.9(±2.3) at 50 MPa (850 °C); and K
aqb/melt
H,K=2.7(±1.2) at 100 MPa (800 °C) and 3.8(±2.3) at 50 MPa (850 °C). Values for K
aqv/melt
H,K and K
aqb/melt
H,K were used to calculate KCl/HCl in the aqueous vapor and brine as a function of melt aluminum saturation index (ASI: molar
Al2O3/(K2O+Na2O+CaO) and pressure. The model log KCl/HCl values show that a change in melt ASI from peraluminous (ASI = 1.04) to moderately
metaluminous (ASI = 1.01) shifts the cooling pathway (in temperature-log KCl/HCl space) of the aqueous vapor toward the andalusite+muscovite+K-feldspar
reaction point.
Received: 22 August 1996 / Accepted: 5 February 1997 |
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| Keywords: | |
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