Lithium isotope analysis of olivine by SIMS: Calibration of a matrix effect and application to magmatic phenocrysts |
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| Authors: | David R. Bell Richard L. Hervig Peter R. Buseck Sonja Aulbach |
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| Affiliation: | 1. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;1. State Key Laboratory of Geological Processes and Mineral Resources, Center for Global Tectonics, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. Three Gorges Research Center for Geo-hazards, China University of Geosciences, Wuhan 430074, China;3. Department of Earth and Environmental Sciences, University of Windsor, Windsor, ON N9B 3P4, Canada;1. Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, United Kingdom;2. School of Earth, Atmospheric and Environmental Sciences, Williamson Building, University of Manchester, Manchester M13 9PL, United Kingdom;3. German Research Centre for Geosciences (GFZ), Section 3.3 Chemistry and Physics of Earth Materials, Telegrafenberg, 14473 Potsdam, Germany;4. Bayerisches Geoinstitut, University of Bayreuth, 95447 Bayreuth, Germany |
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| Abstract: | High-spatial resolution analysis of light element isotope variations by secondary-ion mass spectrometry (SIMS) has numerous applications in geochemistry and cosmochemistry. Recent attention has focused on 7Li/6Li variations in magmatic phenocrysts to infer the volatile degassing history of their parent magmas, and on minerals from mantle samples to determine source-region processes and the recycling history of mantle reservoirs. In these studies the effect of mineral composition on the 7Li/6Li ratio measured by SIMS has been considered secondary, and generally disregarded. We show, using a suite of nine olivines analyzed by MC-ICP-MS or TIMS, that there is a substantial effect of composition on the 7Li/6Li ratio of olivine measured by SIMS. For magnesian olivine (74 < Fo < 94) the effect is a linear function of composition, with δ7Li increasing by 1.3‰ for each mole percent decrease in forsterite component. At higher Fe contents, the relationship ceases to be linear. The composition range over which linear behavior is exhibited appears to depend on instrumental conditions. A calibration of this matrix effect over the linear range is presented, assuming the measurement of 7Li/6Li relative to an olivine standard of known composition. Application of this calibration to a suite of olivines separated from basaltic lavas from Ko'olau, Hawai'i demonstrates that the matrix effect is responsible for a geologically spurious correlation between δ7Li and Mg#. However, after correction, the olivines retain evidence of Li isotope heterogeneity, the degree and nature of which differs in each of the four separates studied. These results emphasize the importance of compositional correction for SIMS measurement of δ7Li in olivine, particularly in zoned crystals, and support previous conclusions that Li isotope variability in igneous materials is subject to late-stage disturbance. The significant matrix effect demonstrated for olivine suggests that matrix effects in other minerals require further evaluation. |
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