Molybdenum isotope behaviour in aqueous systems

• Main Author: Neely, Rebecca Anna
• Other Authors: Sigurður R. Gíslason, Kevin W. Burton, Jarðvísindadeild (HÍ), Faculty of Earth Sciences (UI), Verkfræði- og náttúruvísindasvið (HÍ), School of Engineering and Natural Sciences (UI), Háskólí Íslands, University of Iceland
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Iceland, School of Engineering and Natural Sciences, Faculty of Earth Sciences 2017
• Subjects: Molybdenum; Isotopes; Groundwater; Geothermal; Frumefni; Samsætur; Grunnvatn; Jarðhiti; Doktorsritgerðir; Mývatn; Iceland
• Online Access: https://hdl.handle.net/20.500.11815/345

Molybdenum isotopes are used to quantify changes in Earth’s paleoredox conditions but their application relies upon a simplified model in which rivers dominate the ocean input with minor contributions from hydrothermal fluids. The effect of groundwater discharge is rarely considered. This study finds that cold groundwaters (δ98MoGROUNDWATER -0.1‰) are compositionally similar to their host rocks (δ98MoBASALT -0.15‰) whilst hydrothermal waters are enriched in heavy isotopes (δ98MoHYDROTHERMAL +0.2‰ to +1.8‰). Using flux estimates from the literature, the inclusion of these data results in the revaluation of the Mo ocean input from +0.5‰ (just rivers) to +0.35‰ (combined), in the modern day. As a bioessential element, Mo is important in many biogeochemical cycles: especially, as a cofactor in nitrogenase, the most common nitrogen fixing enzyme. Biological fractionations of some 1.5‰ are observed, with light Mo removed from Lake Mývatn by cyanobacterial uptake during an algal bloom. If preserved, these biological fractionations may need to be considered in the interpretation of the sedimentary record. Despite the growing evidence that the vapour-phase - formed through magma degassing and fluid boiling - can selectively concentrate and transport metals, the effects on metal stable isotopes remain poorly understood. For example, Mo isotopes show great variability in ore deposits, some of which is attributed to vapour-phase transport. Here we examine the vapour-phase in four geothermal systems in Iceland; the vapour-phase is always lighter than the brine with enrichment factors of some εV-L -2.9‰. This is an important first step towards understanding the mechanisms behind vapour transport and isotopic effects. Mólýbden (Mo) samsætur eru oft notaðar til að meta oxunarstig við yfirborð jarðar á ýmsum skeiðum jarðsögunnar. Aðferðin, sem beitt er, byggist á einföldu sjávarlíkani, þar sem styrkur og samsætur Mo í sjó stjórnast fyrst og fremst af árvatni, en jarðhiti á úthafshryggjum hefur til þessa verið talinn hafa lítil ...