Metastable freezing: A new method for the estimation of salinity in aqueous fluid inclusions
File(s)Wilkinson_2016_Final_Preprint.pdf (2.43 MB)
Accepted version
Author(s)
Wilkinson, JJ
Type
Journal Article
Abstract
On cooling during microthermometry, fluid inclusions invariably supercool before freezing
under disequilibrium (metastable) conditions to form ice and hydrates. Measurements of fluid
inclusions from the Irish Zn-Pb hydrothermal system reveal a strong linear correlation (R2 =
0.968) between final ice melting temperature (TmI) and metastable freezing temperature (Tmf)
of the form:
TmI = 0.563 Tmf + 22.7 (+1.5/-3.5)
The relationship is shown to be independent of heating-freezing stage model, host mineral, and
largely of inclusion size, but is affected by the presence of CO2 and by cooling rate. The
correlation shows that metastable freezing is predictable and in fact, in small droplets of pure
solution, occurs at a well-defined, salinity-dependent temperature referred to as the
homogeneous freezing point. This relationship allows salinity to be estimated in fluid inclusions
where the optical recognition of final ice melting is not possible due to small inclusion size or
cloudy samples, or where inclusions go into a metastable, vapor-absent, state because of
collapse of the bubble on freezing. Using a cooling rate of ~50°C/min, inclusion salinity is given
by:
Salinity (wt% NaCl equivalent) = -69.7 - 2.617Tmf - 0.02603Tmf2 - 0.0000994Tmf3
The homogeneous freezing point is controlled by an equilibrium thermodynamic property
related to the activity of water. In small droplets of pure solution, as approximated by fluid
inclusions, freezing will occur when the water activity is 0.305 above that of the stable ice
melting condition at the same temperature, independent of solute type. “Early” metastable
freezing, at a temperature above the homogeneous freezing point may occur in very large
inclusions, or those containing “seed” particles or CO2. In such cases, the salinity will be
underestimated by the equation above.
under disequilibrium (metastable) conditions to form ice and hydrates. Measurements of fluid
inclusions from the Irish Zn-Pb hydrothermal system reveal a strong linear correlation (R2 =
0.968) between final ice melting temperature (TmI) and metastable freezing temperature (Tmf)
of the form:
TmI = 0.563 Tmf + 22.7 (+1.5/-3.5)
The relationship is shown to be independent of heating-freezing stage model, host mineral, and
largely of inclusion size, but is affected by the presence of CO2 and by cooling rate. The
correlation shows that metastable freezing is predictable and in fact, in small droplets of pure
solution, occurs at a well-defined, salinity-dependent temperature referred to as the
homogeneous freezing point. This relationship allows salinity to be estimated in fluid inclusions
where the optical recognition of final ice melting is not possible due to small inclusion size or
cloudy samples, or where inclusions go into a metastable, vapor-absent, state because of
collapse of the bubble on freezing. Using a cooling rate of ~50°C/min, inclusion salinity is given
by:
Salinity (wt% NaCl equivalent) = -69.7 - 2.617Tmf - 0.02603Tmf2 - 0.0000994Tmf3
The homogeneous freezing point is controlled by an equilibrium thermodynamic property
related to the activity of water. In small droplets of pure solution, as approximated by fluid
inclusions, freezing will occur when the water activity is 0.305 above that of the stable ice
melting condition at the same temperature, independent of solute type. “Early” metastable
freezing, at a temperature above the homogeneous freezing point may occur in very large
inclusions, or those containing “seed” particles or CO2. In such cases, the salinity will be
underestimated by the equation above.
Date Issued
2017-02-01
Date Acceptance
2016-10-04
Citation
Economic Geology, 2017, 112 (1), pp.185-193
ISSN
0013-0109
Publisher
Society of Economic Geologists
Start Page
185
End Page
193
Journal / Book Title
Economic Geology
Volume
112
Issue
1
Copyright Statement
© 2017 by the Society of Economic Geologists
Subjects
Science & Technology
Physical Sciences
Geochemistry & Geophysics
HOMOGENEOUS ICE NUCLEATION
HYDROTHERMAL ORE-DEPOSITS
ZN-PB DEPOSITS
GENESIS
MINERALIZATION
TEMPERATURE
CIRCULATION
IRELAND
BRINES
SYSTEM
Geology
0403 Geology
0402 Geochemistry
Publication Status
Published