Geochemical characterisation of gases along the Dead Sea Rift: Evidences of mantle-CO2 degassing

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Method to determine Zr, Hf and REE in Dead Sea Fault and Pantelleria waters

The samples collected to determine REE, Zr and Hf in Dead Sea Fault and Pantelleria waters were treated in laboratory following the method described by Raso et alii (2013). In each water sample (1 liter) 1 mg of Fe was added and subsequently ammonia ultrapure solution to attain a pH between 8.0 and 8.5, in order to precipitate REE onto solid Fe(OH)3. The treated solution was agitated for 3 hours and after 48 hours the solution was filtered onto Millipore membranes with 0.45 μm porosity to collect the solid precipitated. The next step was to dissolve the Fe(OH)3 onto the filter in 5 ml of 6M HCl ultrapure solution. The last step was to dilute the solution 1:5 with ultrapure water to allow the introduction of the sample in ICP-MS. The iron concentration was analysed by ICP-OES for each solution to check the recovery of the added iron.

Speciation calculations and saturation indexes

The Saturation Indexes (SI) and the aqueous speciation of elements were calculated using PHREEQC software package (version 3.0.6; Parkhurst and Appelo, 2010). The simulations were carried out using the database LLNL.

Equation to determine anomalies of REE

Anomalies of Cerium and Europium in waters were calculated with respect to the neighboring elements normalized to the reference matherial, using the equation proposed by Alibo and Nozaki (1999): REEn/REEn*=2×(REE)n/[(REE)n-1+(REE)n+1] (REE)n is the concentration of the element chosen to calculate the anomaly, while (REE)n-1 and (REE)n+1 represent the previous and the subsequent element along the REE series, respectively.

Y/Ho and Zr/Hf fractionation in spring and wells

The investigated waters should have Y/Ho and Zr/Hf molar ratios close to the hosting rocks, 49 and 80 respectively (White et al., 2009). The Y/Ho and Zr/Hf ratios found in the investigated springs and wells show values far from the local rock, suggesting the occurrence of processes partitioning these couples of elements (Fig. 5.11). Assuming that these elements are released into waters with Y/Ho and Zr/Hf ratios close to the source rocks, secondary processes (as scavenging into and/or onto the secondary minerals) would explain the fractionation of the twin pairs in the investigated waters. The processes responsible of fractionating Zr-Hf pairs are not well known and up to this day no experiment was carried out in laboratory. On the contrary, when compared to the Y and Ho, Zr and Hf are not complexed by carbonate species. Byrne (2002) shows that Zr and Hf are mainly complexed by hydroxyl groups in the pH range between 6 and 8: Zr, Hf(OH)4 and Zr, Zr-Hf and REE signatures discriminating the effect of atmospheric fallout from the hydrothermal input in volcanic lake waters Hf(OH)5 -. Qu and co-authors (2009) have recognized the fractionation of Y and Ho during the precipitation of calcite and aragonite with enhanced removal of Ho respect to Y, due to the different electronic configuration of these elements. Moreover, several studies have shown the ability of Fe- Mn oxyhydroxides precipitation to fractionate Y-Ho and Zr-Hf. Inguaggiato et alii (2015) investigated Y/Ho and Zr/Hf ratios in Nevado del Ruiz volcanic waters (Colombia), showing superchondritic values in waters with pH from 6 to 8.8, where the precipitation of Fe and Al oxyhydroxides occurs. Schmidt and co-authors (2014) investigated Zr/Hf ratios in seawater hydrogenetic ferromanganese crusts, showing strong enrichment compared to the average crust and highlighting an enhanced Hf removal respect to Zr. Moreover, the preferential removal of Hf with respect to Zr in SiO2 solid phase was found (Firdaus et al., 2011 and references therein). Particularly, Censi and co-authors (2015) investigated a microsystem occurring in the south-western
sector of “Specchio di Venere” lake, identifying higher surface-reactivity of Hf than Zr in siliceous stromatolies and microbial mats.

The source of REE, Zr and Hf in “Specchio di Venere”

The main problem concerning the anomalous behaviour of REE, Zr and Hf in water lake is to understand which is the main process controlling the geochemistry of REE, Zr and Hf in Specchio di Venere water lake.
The geochemistry of REE, Zr and Hf in alkaline lakes is poorly documented. Johannesson and Lyons (1994) investigated the Mono lake water recognizing a pattern increasing from La to Lu, highlighting the importance of carbonate complexes for the REE distribution.
The atmospheric fallout delivered by Sahara Desert (North Africa) inevitably involves Pantelleria Island, located 70 km at east from the Tunisian coast. The open water body of “Specchio di Venere” lake shows MREE enrichment not recognized in the other waters collected from springs and wells Zr-Hf and REE signatures discriminating the effect of atmospheric fallout from the hydrothermal input in volcanic lake waters in Pantelleria Island (Fig. 5.6). In particular, the lake water body has REE amounts higher compared to the other waters, including Polla 3 thermal spring feeding the water lake along the shoreline. This evidence suggests an external process adding REE to the lake water, increasing the relative abundance of MREE with respect to LREE and HREE.

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Yttrium and Holmium

Differently from the Zr and Hf behaviour during dissolved complexation, the Y and Ho dissolved species always show the same ionic charge if formed with the same ligand (i.e. [(Y,Ho)CO3]+, [(Y,Ho)(CO3)2]-, [(Y,Ho)Cl]2+, [(Y,Ho)F]2+ (Fig. 6.5). Therefore, the Y–Ho decoupling observed in almost all of the Group 1 and 2 waters (Fig. 6.4) cannot be driven by electrostatic considerations, probably depending from the different covalent character of the dissolved Y and Ho complexes (Bau, 1996). This suggestion is confirmed by the preferential Ho scavenging onto Fe oxyhydroxide relative to Y (Bau, 1999). At the same time, laboratory experiments on CaCO3 crystallisation (both calcite and aragonite) indicate the preferential incorporation of Ho into CaCO3 relative to Y (Qu et al., 2009). These results were confirmed by Tanaka et al. (2004, 2008) recognizing a preferential Y enrichment relative to Ho in the dissolved phase during calcite crystallisation that was interpreted as a Ho–CO3 and Y–CO3 bonding difference in carbonate minerals.

Table of contents :

ABSTRACT
RIASSUNTO
RÉSUMÉ
ACKNOWLEDGEMENTS
CONTENTS
LIST OF FIGURES
PREFACE
CHAPTER 1 Introduction
1.1 General aspects and aim of the work
1.2 The aqueous geochemistry of REE
1.3 The normalization of REE
1.4 The aqueous geochemistry of zirconium and hafnium
CHAPTER 2 Investigated areas and background information
2.1 Nevado del Ruiz
2.2 Pantelleria Island
2.3 Dead Sea Fault area
CHAPTER 3 Materials and methods
3.1 Sampling and analytical methods
3.2 Method to determine Zr, Hf and REE in Dead Sea Fault and Pantelleria waters
3.3 Speciation calculations and saturation indexes
3.4 Equation to determine anomalies of REE
CHAPTER 4 Geochemistry of Zr, Hf and REE in a wide range of pH and water composition: The Nevado del Ruiz volcano-hydrothermal system (Colombia)
4.1 RESULTS
4.1.1 General aspects
4.1.2 REE, Zr and Hf
4.2 DISCUSSION
4.2.1 REE behaviour
4.2.2 The behaviour of twin pairs (Y-Ho; Zr-Hf)
4.3 CONCLUDING REMARKS
CHAPTER 5 Zr-Hf and REE signatures discriminating the effect of atmospheric fallout from the hydrothermal input in volcanic lake waters
5.1 RESULTS
5.1.1 General aspects
5.1.2 REE, Zr and Hf
5.2 Discussions
5.2.1 Aqueous speciation
5.2.2 REE behaviour in springs and wells
5.2.3 Ce anomaly
5.2.4 Y/Ho and Zr/Hf fractionation in spring and wells
5.2.5 The source of REE, Zr and Hf in “Specchio di Venere”
5.3 Concluding remarks
CHAPTER 6 Geochemistry of Zr, Hf and REE in a wide spectrum of Eh and water composition: The case of the Dead Sea Fault system (Israel)
6.1 RESULTS
6.1.1 General aspects
6.1.2 REE, Zr and Hf
6.2 DISCUSSION
6.2.1 Zirconium and hafnium
6.2.2 Yttrium and Holmium
6.2.3 REE distribution
6.3 CONCLUDING REMARKS
CHAPTER 7 General conclusions
CHAPTER 8 Geochemical characterisation of gases along the Dead Sea Rift: Evidences of mantle-CO2 degassing
8.1 INTRODUCTION
8.2 MATERIALS AND METHODS
8.3 RESULTS AND DISCUSSION
8.3.1 Mantle derived helium along Dead Sea Fault
8.3.2 Origin of CO2
8.4 IMPLICATIONS
8.5 CONCLUNDING REMARKS
CHAPTER 9 References
APPENDIX

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