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Controls on mineral-fluid oxygen isotope fractionations in geologic systems

English title Controls on mineral-fluid oxygen and hydrogen isotope fractionations in specific geologic systems
Applicant Vennemann Torsten
Number 111757
Funding scheme Project funding (Div. I-III)
Research institution Institut de Minéralogie et Géochimie Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geochemistry
Start/End 01.04.2006 - 31.03.2008
Approved amount 117'879.00
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Keywords (9)

Isotope geochemistry; oxygen; quartz; kinetic and equilibrium fractionations; isotope; fractionation; sector zoning; trace elements; Alpine

Lay Summary (English)

Lead
Lay summary
The proposal has aimed at improving our knowledge on low temperature processes governing oxygen isotope fractionations between minerals and fluid in natural systems through detailed studies of the growth mechanisms and textural as well as chemical and isotopic zonations of quartz crystals. The approach evaluated the growth and sector zoning of oxygen isotope and trace element compositions, the reasons for such zonations and its implications for the fluid-mineral interactions in the context of the Alpine metamorphism. Quartz samples with clearly different crystal habitus have been selected from localities where the conditions of crystal growth are known on the basis of their fluid and mineral inclusions and where the latter can be compared to those occurring in the adjacent wall rocks. The results indicate that the samples are clearly zoned when viewed with cathodoluminescence and that the different zones correspond to different Al, Li, and hydroxyl contents. In-situ methods of analyses (e.g., electron microprobe, laser-ablation ICP-MS, and ion-probe) demonstrate that Al concentrations in some growth zones and even in distinct sectors can reach up to 1800 ppm, most prominently during the last phase of crystal growth in all localities chosen. Good correlation between Al and Li as well as hydroxyl content are observed, supporting coupled substitution of Si. Oxygen isotope compositions can also differ by several permil and this is also the case between different sectors of the same growth zone, but only for sectors with high Al content. Differences in both trace element and oxygen isotope composition have been measured for different crystallographic faces, with prismatic faces incorporating lower amounts of trace elements compared to rhombohedral r and z faces. In some cases r and z faces may also be different. Kinetic as well as growth mechanism effects are considered as reasons for these differences, while oxygen isotope differences may be related to growth mechanism effects or disequilibrium fractionations. Fine-scale variations within individual localities support variable fluid infiltration events during the early stages of crystal formation. The fluids have had isotopic compositions that differ by several permil, requiring different extents of fluid-rock interactions with fluids being buffered by different types of rocks too. A late-stage, regionally infiltrating meteoric fluid is responsible for the outer Al-rich zones and late needle quartz crystals. It is clear that the Alpine vein quartz crystals have recorded a complex fluid history. Such information is lost if bulk crystals are being analyzed.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
126973 Rheology and Permeability of Extensional Detachments 01.11.2009 Project funding (Div. I-III)
124787 Faults and fluids: deep penetration of meteoric waters into the Alps during their exhumation 01.05.2009 Project funding (Div. I-III)
102014 Controls on mineral-fluid oxygen and hydrogen isotope fractionations in specific geologic systems 01.10.2003 Project funding (Div. I-III)
143885 Origin and growth of Alpine fissure minerals: fluid-mineral interaction and implications for the Alpine metamorphism 01.10.2012 Project funding (Div. I-III)

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