Magnesium Isotope Geochemistry

Abstract

Magnesium (Mg) has an atomic number of 12 and belongs to the alkaline earth element (Group II) of the Periodic Table. The pure Mg is a silvery white metal and has a melting point of 650 °C and boiling point of 1090 °C at 1 standard atmosphere (Lide 1993–1994). The electronic configuration of Mg is [Ne]3s2, with low ionization energies, which makes Mg ionic in character with a common valance state of 2+ and a typical ionic radius of 0.72 A (Shannon 1976). Magnesium is a major element and widely distributed in the silicate Earth, hydrosphere and biosphere (Fig. 1a). It is the fourth most abundant element in the Earth (after O, Fe and Si, MgO = 25.5 wt%) (McDonough and Sun 1995), the fifth most abundant element in the bulk continental crust (MgO = 4.66 wt%) (Rudnick and Gao 2003) and the second most abundant cation in seawater (after Na, Mg = 0.128 wt%) (Pilson 2013). Nonetheless, the mantle has > 99.9% of Mg in the Earth because of its high MgO content (37.8 wt%, McDonough and Sun 1995) and mass fraction. The high abundance of Mg in the silicate Earth makes it a major constituent of minerals (e.g., olivine, pyroxene, garnet, amphibole, mica, spinel, carbonate, sulfate, and clay minerals) in igneous, metamorphic and sedimentary rocks. Magnesium has three stable isotopes, with mass numbers of 24, 25 and 26, and typical abundances of 78.99%, 10.00% and 11.01%, respectively (Berglund and Wieser 2011) (Fig. 1b), and a standard atomic weight of 24.305 (CIAAW 2015). Because of the limitations in the mass spectrometry, many previous Mg isotopic studies have concentrated on either mass independent isotope anomalies to look for the radiogenic 26Mg produced by the decay of short-lived 26Al (Gray and Compston 1974; Lee and …