HR: 0830h
AN: PP11A-0207    [PDF]
TI: Mg Isotopic Compositions of Modern Marine Carbonates
AU: * Krogstad, E
EM: ejk@dlc.ku.dk
AF: Danish Lithosphere Centre, Ostervoldgade 10 L, Copenhagen K, DK-1350 Denmark
AU: Bizzarro, M
AF: Danish Lithosphere Centre, Ostervoldgade 10 L, Copenhagen K, DK-1350 Denmark
AU: Hemming, N
AF: School of Earth and Environmental Sci., Queens College, Flushing, NY 11367 United States
AB: We have used a MC-ICP-MS to measure the isotopic composition of magnesium in a number of samples of modern marine carbonate. Due to the large mass difference between 26Mg and 24Mg (similar to that between 13C and 12C), there is potential for mass fractionation during geologic and biologic processes that may make this isotope system useful for geochemical studies. These samples are from the study of Hemming and Hanson (1992, GCA 56: 537-543). The carbonate minerals analyzed include aragonite, low-Mg calcite, and high-Mg calcite. The samples include corals, echinoderms, ooids, etc., from subtropical to Antarctic settings. Mg purification was accomplished by ion-exchange chromatography, using Bio-Rad AG50W-X12 resin on which greater than 99 percent recovery of Mg is achieved. Samples were introduced into the MC-ICP-MS (VG Axiom) using a Cetac MCN-6000 nebuliser. We use a standard-sample-standard bracketing technique, and samples are analysed at least three times. For lab standards we find that the reproducibility on the 26Mg/24Mg to be about ñ 0.12 permil (2 s.d.). We monitored our separated samples for Na and Ca, as we have found that high Ca/Mg and Na/Mg produce variable magnesium isotopic fractionation during mass spectrometry due to as yet unclear matrix effects. We have normalized our results to our measured values for seawater. We observed a d26Mg(s.w.) range of -1.4 to -2.4 permil in our modern carbonate samples relative to present day seawater. Due to the long residence time of Mg in the oceans (ca. 50 my), this must be due to kinetic or biologic effects. Our d25Mg(s.w.) variations as a function of d26Mg(s.w.) plot along the terrestrial fractionation trend. With an average d26Mg(s.w.) of ca. +0.5 permil in all samples of mantle lithologies and mantle-derived igneous rocks (Bizzarro et al., Goldschmidt abs., 2003), we can assume that the Mg isotopic composition of Earth's river water lies between ca. -2.4 and +0.5 permil (relative to seawater). The actual value will vary according to the d26Mg of provenance carbonates and silicic crust, and the relative rates of chemical weathering and physical denudation. It is unlikely therefore that the Mg isotopic composition of seawater has remained constant through time.
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1094 Instruments and techniques
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting