Davies, G.R. [Promotor]Allan, N.L. [Copromotor]Westrenen, W. van [Copromotor

Sijl, J. van

van Sijl, J.

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VU Research PortalTrace elements in high pressure and temperature fluids in subduction zonesvan Sijl, J.2011document versionPublisher's PDF, also known as Version of recordLink to publication in VU Research Portalcitation for published version (APA)van Sijl, J. (2011). Trace elements in high pressure and temperature fluids in subduction zones.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.E-mail address:vuresearchportal.ub@vu.nlDownload date: 23. May. 2021ISBN: 978-90-8659-547-1Trace elements in high pressure and temperature fluids in subduction zones    Jelle van SijlTrace elements in high pressure and temperature fluids in subduction zonesJelle van Sijlwater

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Trace elements in high pressure and temperature fluids in subduction zones

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147
Conclusions and Outlook
Chapter 7
148
Trace elements in high pressure and temperature fluids in subduction zones
Conclusions and outlook
 In this thesis, I combined computer models of trace element complexation in subduction 
zone fluids with high-pressure, high-temperature mineral-fluid partitioning experiments to shed 
new light on the behaviour of key trace elements in subduction zone fluids.
 The thermodynamic cycle that was developed in Chapter 1 provides insight into the 
relative hydration Gibbs free energies of rare earth element (REE)-water clusters embedded in 
a solvation model computed from a first principles quantum mechanical point of view. All REE 
hydration free energies computed using density functional theory (DFT) are negative at both 
ambient pressure-temperature conditions and the high pressures and temperatures characteristics 
of subduction zones, indicating thermodynamically favourable solvation by water. The hydration 
free energy is more negative for smaller complexes (heavy lanthanides) relative to larger complexes 
(light lanthanides) by ~361 kJ mol-1. Aqueous complexation of REE with fluoride is energetically 
more favourable than with chloride by ~206 kJ mol-1. REE-Cl and REE-F complexes are both 
energetically favoured with respect to REE-water clusters. The results indicate that fluid REE 
contents can be affected by the presence of fluoride or chloride, but that changes in REE-ratios 
are unlikely to occur.
 The latter is consistent with experimental results (Chapter 6) that show  heavy REE / 
light REE diopside-fluid partition coefficient ratios to be unaffected by the presence of 10 wt.% 
fluoride and chloride solutions. Embedded cluster calculation results suggest absolute REE 
concentrations are likely to be increased in solution due to the presence of complexing ligands. 
This is not observed in our experiments, which leads us to conclude that it is likely that for 
diopside-fluid REE partitioning, diopside exerts a dominating control on REE behaviour. The 
thermodynamic cycle and embedded cluster calculations allow for pressure and temperature 
variations to be included indirectly. Parameterised changes in temperature (e.g. dielectric 
constant) and density do not significantly affect REE-ratios.
 Due to the nature of these static simulations, geometric parameters are optimised 
assuming a certain coordination environment (the local environment of the solvated ion) at 
the given PT conditions. Coordination number variations due to pressure and temperature are 
therefore only implicitly included. It is concluded that a better understanding of the detailed 
structure and dynamics of hydration shells and the effect of ions on the hydrogen bond network 
of liquid water under supercritical conditions is of critical importance.
 This is the main objective of Chapter 2 and 3, where we report changes in coordination 
environment of the lightest high field strength element (HFSE) titanium in pure water as a 
function of temperature and water density relevant to subduction zone settings using first principles 
molecular dynamics (FPMD). Equilibrating the ambient condition initial geometry of the 
simulations by quenching from higher temperatures may drive the system into a metastable 
state, where the dynamics do not evolve fast enough to escape the local minimum without 
considerable computational effort. Using metadynamics to overcome this obstacle, we observe 
an unanticipated five-fold coordination of Ti(IV) is favoured under ambient conditions, with 
rapid interconversions between square pyramidal and trigonal bipyramidal structures. This is 
in marked contrast to the long-held view that Ti is six-fold coordinated, and demonstrates the 
149
Chapter 7
added value of (accelerated) dynamic over static simulations.
 At supercritical conditions the Ti coordination number increases from five to six, 
adopting both octahedral and trigonal prismatic geometries, with Ti-O bond lengths in excellent 
agreement with static DFT molecular cluster calculations. At 1000 K, a water density increase 
from 0.9 – 1.3 g cm-3 (corresponding to a pressure increase from approximately 0.9-3.6 GPa) 
increases the number of first hydration shell water molecules from less than five to six. Thus 
for tetravalent titanium in solution, the effect of higher pressure, favouring larger coordination 
numbers, dominates the effect of an increase in temperature, which favours lower coordination.
 Studies simulating ion hydration under comparable PT-conditions are rare. Nevertheless, 
we note strikingly similar coordination changes with density for titanium when compared to 
lithium, despite the very different Li-O and Ti-O bond strengths. Future simulations on the 
supercritical solvation behaviour of other tetravalent ions are required to assess whether the 
pattern that we observe for titanium and lithium is unique to these elements.
 In addition to geometry changes, titanium-water complexes display marked 
modifications of their electronic structure with temperature and density (Chapter 3). The main 
two observations related to these modifications are the marked increase in first hydration shell 
water molecule dipole moments at ambient and supercritical conditions relative to the bulk water 
dipole moments. Also, at ambient conditions there is still a discernable effect of the central Ti ion 
on the dipole moments beyond the first hydration shell whereas this is absent at supercriticality.
 At 1000 K and water densities from 0.6 – 1.5 g cm-3, corresponding to pressures of 
approximately 0.3 – 8 GPa, fluoride and chloride form aqueous complexes with titanium, i.e. 
hydrated TiF and TiCl (Chapter 4). Hydration enthalpy calculations show that complexation 
reactions of the type observed here will increase the solubility of Ti, qualitatively consistent with 
recent experimental data showing enhanced rutile solubility in the presence of Cl- and F- (e.g. 
Rapp et al. 2010). Although HFSE solvation is anticipated to be enhanced in the presence of 
these complexing agents, there may be a large dependence on the absolute halogen abundances 
and their preference for other cations in solution. The relation between halogen availability and 
HFSE mobility is sometimes linked to the relative stability of fluorapatite. Future studies that 
focus on the connection between (fluoro-)phosphate stability (e.g. apatite, monazite, xenotime) 
and nominally insoluble element mobility may provide further clues to this relation.
 Despite the observed TiF and TiCl complex formation in our simulations, our 
experiments indicate the concentration of Ti in the fluid is not enhanced in either chloride 
or fluoride-rich aqueous solutions in the system diopside-fluid (Chapter 6). Observations also 
indicate that Zr and Hf are enriched in pure water and F-bearing solutions relative to diopside 
whereas complexation with chloride is much less favourable. We conclude from these experiments 
that complexing halogens in solution have the potential to significantly alter the relative 
compatibility of trace elements, and hence partition coefficient ratios can vary significantly 
depending on crystal- and fluid chemistry. 
 We also conclude that there appears to be a disagreement between the experimentally 
observed influence of halogens on diopside-fluid partitioning of some HFSE and energetically 
favourable formation of titanium complexation from first principles molecular dynamics. 
Resolution of this apparent disagreement requires additional experimentation as well as FPMD 
simulations on other HFSE in more complex, multi-component solutions. Also, simulations 
150
Trace elements in high pressure and temperature fluids in subduction zones
should account for the competing effects between fluid and mineral. As an illustration of such 
more advanced atomic-scale computer models, we establish a fully quantum mechanical picture 
of rutile-melt and rutile-aqueous fluid HFSE partitioning relevant in particular to subduction 
zone settings (Chapter 5). Assuming similar hydration behaviour of HFSE to titanium, we 
compare Nb and Ta incorporation in rutile relative to a melt or aqueous fluid. We observe 
that the energetic differences between Nb and Ta are larger in the case of element exchange 
between rutile and aqueous fluids relative to melts. This is consistent with the notion that trace 
element partitioning behaviour strongly depends on local liquid environments as well as crystal 
chemistry.
 Making use of Bader partitioning analysis of the total electronic charge density, we 
show that charges and radii of elements in their bulk oxide phases are different compared to 
values for elements as trace element defect in rutile. Even for isovalent element substitution 
reactions, the higher charge of the substituting element relative to Ti requires some form of 
charge compensation mechanism. In rutile, a mechanism involving polarisation of the octahedral 
oxygen atoms in combination with charge reduction of non-nearest neighbour titanium atoms 
accounts for charge differences. This demonstrates the power of quantum chemical over classical 
simulations to rationalise trace element partitioning data, and provides a first step towards truly 
atomic-scale models of mineral-liquid partitioning.


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Trace elements in high pressure and temperature fluids in subduction zones

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