Non-traditional stable isotope fractionation during magmatic differentiation: In situ Fe-Mg-isotope analyses of phenocrysts by femtosecond laser ablation MC-ICP-MS
Mineralogy, Petrology and Geochemistry
Final Report Abstract
In this project, a successful method was developed to perform in situ high-precision Fe and Mg isotope analyses of glass standards and magmatic crystals (olivine, cpx) with femtosecond LA-MC-ICP-MS. This method was applied to analyze the Fe and Mg isotope composition of important geological glass standard reference materials, including the “MPI-DING” and the USGS reference glasses (BIR-1G, BCR-2G, BHVO-2G). The method was then applied on a variety of magmatic olivine crystals and basaltic glass, including drill core samples from the two DSDP Holes at the Mid-Atlantic Ridge (MAR), from the ODP Hole 896A at the Costa Rica Rift (CRR), as well as fresh olivines and microcrystalline matrix from the Shatsky Rise oceanic plateau (Site U1349, Ori massif). Significant Fe and Mg isotopic zoning was observed in these samples, whereas Fe and Mg isotopes were commonly negatively correlated to each other and the width of isotopic zoning was usually identical to the width of chemical zoning (Mg#), indicating that isotopic zoning was essentially driven by Fe-Mg exchange diffusion. Based on these findings, five scenarios of crystal growth, crystal dissolution with subsequent or parallel diffusion have been developed that would result in characteristic coupling of Fe-, Mg-isotopic and Mg# zoning. A major outcome of this study is thus that Fe-, Mg-isotopic and Mg# zoning combined, not only provide clear evidence for the diffusion origin of a chemical profile in a magmatic crystal, but also helps unraveling complex magma evolution histories. For the investigated MOR samples, modeling of the combined chemical and isotopic diffusion profiles revealed short magma residence times of usually less than a few years. Several related studies have been carried out already in the meantime, using the here developed methods, including one on the continental arc volcano Irazu (Costa Rica) and one on Martian shergottites. Finally, the diffusive fractionation effect of Fe and Mg isotopes in olivine was also experimentally calibrated as a function of crystallographic orientation providing additional constraints for the modelling of natural Fe and Mg isotope profiles to obtain time scales.
Publications
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(2014) High-Precision Fe and Mg Isotope Ratios of Silicate Reference Glasses Determined In Situ by Femtosecond LA-MC-ICP-MS and by Solution Nebulisation MC-ICP-MS. Geostand. Geoanalytical Res. 38, 311–328
Oeser M., Weyer S., Horn I. and Schuth S.
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(2015) Constraints on the Evolution of Magmas from Diffusion-driven Chemical and Fe-Mg Isotopic Zoning in Natural Olivines analyzed by Femtosecond-LA-ICP-MS. Dissertation, TIB/UB Hannover, 172 Seiten
Oeser-Rabe M.
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(2015) Processes and time scales of magmatic evolution as revealed by Fe–Mg chemical and isotopic zoning in natural olivines. Geochim. Cosmochim. Acta 154, 130–150
Oeser M., Dohmen R., Horn I., Schuth S. and Weyer S.
