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Discrepancies of mineral volumes predicted by thermodynamic databases
Abstract
Thermodynamic databases are an essential tool to predict complex
equilibrium mineral assemblages and mineral properties like mineral
volumes. They consist of numerous thermodynamic data of various
minerals, extracted from experiments. Each database follows its own
methodology in calculating chemical and physical properties. Therefore a
direct comparision between different database predictions was avoided,
due to the contrasting methodolgies and philosophy. Here, we present a
direct comparison between the databases of Berman [1] and Holland &
Powell [2][3], focusing on mineral volumes [4]. For this propose, a
reevaluation of the equation of states was nesscary. In this context, we
identify an error also implemented in common thermodynamic softwares,
concerning the calculation of excess volume. Even after treating the
excess energy correctly, volumes show significant discrepancies between
the different database predictions. These discrepancies impact
geodynamic interpretations and geothermobarometrical estimations, due to
the fact that the Gibbs free energy and rock density depends on mineral
volumes. The imagination that pressure can vary by 4 kbar, temperature
by 150°C or rock-density up to 30 %, by changing the thermodynamic
database is dramatic. These enormous differences must be considered
keeping in mind that calculations were done for well studied minerals
(e.g. quartz and forsterite). The results play an important role for
studies of geodynamic interpretations extracted from thermobarometric
software packages like Perple_X, Theriak-Domino or Thermocalc. It is
important to estimate the influence of the thermodynamic database on
Gibbs free energy, volume and rock density. Summarizing, more
experimental data will lead to a better comprehension of these
discrepancies. [1] Berman (1988). Journal of Petrology 29, 445-522.
[2] Holland & Powell (1998) Journal of Metamorphic Geology 16,
309-343. [3] Holland & Powell (2011) Journal of Metamorphic Geology
29, 333-383. [4] Duesterhoeft, Zaehle, De Capitani, Oberhänsli
& Bousquet (2012, in prep.) Submitted to Contributions to Mineralogy
and Petrology.
... To model the order-disorder transitions of quartz, BE88 uses the Lambda transition approach , and HP98 (as well as HP11) the Landau transition approach (Holland & Powell, 1996a. The Landau transition was incorrectly implemented in HP98 and resulted in large discrepancies between measured and predicted volumes, which have implications for determining rock density (Duesterhoeft et al., 2012b). However, the effects on phase diagrams calculated for crustal conditions were minor, so this mistake was corrected only in the subsequent dataset HP11. ...
The astonishing progress of personal computer technology in the past 30 years as well as the availability of thermodynamic data and modeling programs have revolutionized our ability to investigate and quantify metamorphic processes. Equilibrium thermodynamics has played a central role in this revolution, providing simultaneously a physico-chemical framework and efficient modeling strategies to calculate mineral stability relations in the Earth’s lithosphere (and beyond) as well as thermobarometric results. This Perspectives contribution provides a review of the ingredients and recipes required for constructing models. A fundamental requirement to perform thermodynamic modeling is an internally consistent database containing standard state properties and activity–composition models of pure minerals, solid solutions, and fluids. We demonstrate how important internal consistency is to this database, and show some of the advantages and pitfalls of the two main modeling strategies (inverse and forward modeling). Both techniques are commonly applied to obtain thermobarometric estimates; that is, to derive P–T (pressure–temperature) information to quantify the conditions of metamorphism. In the last section, we describe a new modeling strategy based on iterative thermodynamic models, integrated with quantitative compositional mapping. This technique provides a powerful alternative to traditional modeling tools and permits use of local bulk compositions for testing the assumption of local equilibrium in rocks that were not fully re-equilibrated during their metamorphic history. We argue that this is the case for most natural samples, even at high-temperature conditions, and that this natural complexity must be taken into consideration when applying equilibrium models.
... In contrast to JUN92.bs, the tcdb55 database offers an approach to model the effects of melt production but is based on an invalid Landau free energy term ∆G Land published by Holland and Powell (1998). This term significantly affects the volume calculation (and thus the density calculation) of some mineral phases, e.g., alpha-quartz (e.g., Duesterhoeft et al., 2012b), leading to a false standard volume. For example, the alpha-quartz volume under room conditions clearly appears to be too high (V 0 =22.8945 ...
Subduction is primarily driven by the densification of the downgoing
oceanic slab, due to dynamic P-T-fields in subduction zones. It is
crucial to unravel slab densification induced by metamorphic reactions
to understand the influence on plate dynamics. By analyzing the density
and metamorphic structure of subduction zones, we may gain knowledge
about the driving, metamorphic processes in a subduction zone like the
eclogitization (i.e., the transformation of a MORB to an eclogite), the
breakdown of hydrous minerals and the release of fluid or the generation
of partial melts. We have therefore developed a 2D subduction zone
model down to 250 km that is based on thermodynamic equilibrium
assemblage computations. Our model computes the "metamorphic density" of
rocks as a function of pressure, temperature and chemical composition
using the Theriak-Domino software package at different time stages. We
have used this model to investigate how the hydration, dehydration,
partial melting and fractionation processes of rocks all influence the
metamorphic density and greatly depend on the temperature field within
subduction systems. These processes are commonly neglected by other
approaches (e.g., gravitational or thermomechanical in nature)
reproducing the density distribution within this tectonic setting. The
process of eclogitization is assumed as being important to subduction
dynamics, based on the very high density (3.6 g/cm3) of eclogitic rocks.
The eclogitization in a MORB-type crust is possible only if the rock
reaches the garnet phase stability field. This process is primarily
temperature driven. Our model demonstrates that the initiation of
eclogitization of the slab is not the only significant process that
makes the descending slab denser and is responsible for the slab pull
force. Indeed, our results show that the densification of the downgoing
lithospheric mantle (due to an increase of pressure) starts in the early
subduction stage and makes a significant contribution to the slab pull,
where eclogitization does not occur. Thus, the lithospheric mantle acts
as additional ballast below the sinking slab shortly after the
initiation of subduction. Our calculation shows that the dogma of
eclogitized basaltic, oceanic crust as the driving force of slab pull is
overestimated during the early stage of subduction. These results
improve our understanding of the force budget for slab pull during the
intial and early stage of subduction. Therefore, the complex metamorphic
structure of a slab and mantle wedge has an important impact on the
development and dynamics of subduction zones. Further Reading:
Duesterhoeft, Oberhänsli & Bousquet (2013), submitted to Earth
and Planetary Science Letters
[1] This study presents the theory, applicability, and merits of the new THERIAK_D add-on for the open source Theriak/Domino software package. The add-on works as an interface between Theriak and user-generated scripts, providing the opportunity to process phase equilibrium computation parameters in a programming environment (e.g., C or MATLAB®). THERIAK_D supports a wide range of features such as calculating the solid rock density or testing the stability of mineral phases along any pressure-temperature (P-T) path and P-T grid. To demonstrate applicability, an example is given in which the solid rock density of a 2-D-temperature-pressure field is calculated, portraying a simplified subduction zone. Consequently, the add-on effectively combines thermodynamics and geodynamic modeling. The carefully documented examples could be easily adapted for a broad range of applications. THERIAK_D is free, and the program, user manual, and source codes may be downloaded from http://www.min.uni-kiel.de/∼ed/theriakd/.
[1] This study presents the theory, applicability, and merits of the new THERIAK_D add-on for the open source Theriak/Domino software package. The add-on works as an interface between Theriak and user-generated scripts, providing the opportunity to process phase equilibrium computation parameters in a programming environment (e.g., C or MATLAB®). THERIAK_D supports a wide range of features such as calculating the solid rock density or testing the stability of mineral phases along any pressure-temperature (P-T) path and P-T grid. To demonstrate applicability, an example is given in which the solid rock density of a 2-D-temperature-pressure field is calculated, portraying a simplified subduction zone. Consequently, the add-on effectively combines thermodynamics and geodynamic modeling. The carefully documented examples could be easily adapted for a broad range of applications. THERIAK_D is free, and the program, user manual, and source codes may be downloaded from http://www.min.uni-kiel.de/∼ed/theriakd/.
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