Different Isotope Fractionation Behaviors Of Magnesium At The Earth's Surface And In The Mantle

Magnesium isotope fractionation have been discussed for both high P-T crust-mantle conditions and low P-T earth's surface environment.Magnesium isotope analysis of laterite nickel deposit from Philippine ultrabasic rocks shows that there is a great negative correlation between Mg isotope and Mg content in the weathering profile.The change of Mg content and isotope from sapropel to laterite is mainly caused by the loss of Mg during weathering,and the main controlling factor for the change of Mg isotope may be the adsorption and desorption effect of clay minerals from upper part of laterite to clay.The distinct positive correlation between light magnesium isotope and magnesium content indicates that light magnesium isotopes have a higher mobility than heavy magnesium isotopes.The second part of this study is about equilibrium fractionation factors of magnesium isotopes between pyrope,majorite,clinopyroxene,orthopyroxene and olivine under high temperature and high pressure using density functional theory calculation.The calculation results are in good agreement with previous study(Schauble,2011)which suggests that our calculation result is reliable.The results show that magnesium isotope fractionation between minerals can be significant due to different coordination numbers of magnesium in minerals even in high P-T environment.Specifically,pyrope(8-coordinated)is more enriched in light magnesiurm isotopes than olivine and pyroxene(6-coordinated).Calculations of magnesium isotope fractionation between clinopyroxene and pyrope coincide with the observations of natural eclogite(Li et al.,2011:Wang et al.,2012);In addition,pressure has significant effects on magnesium isotope fractionation between garnet,pyroxene and olivine.Magnesium isotope equilibrium fractionation between pyrope and pyroxene(and olivine)can be regarded as a novel and independent thermometer with accuracy better than the traditional garnet-clinopyroxene Fe-Mg exchange therm ometer because the fractionation is significantly larger than the current magnesium isotope analysis error(?0.025%amu'1,2sd).This magnesium isotope thermometer can be used in high temperature mafic igneous and metamorphic rocks where garnet coexists with pyroxene or olivine.The formula of equilibrium magnesium isotope fractionation factors between garnet,pyroxene and olivine are also given here.