Magnesium Isotope Study On The Laacher See Carbonatite-Syenite Series

Liquid immiscibility in magmas is one of the most important processes to form igneous carbonatite,which can occur widely in differentiation of magmas generated from carbonated mantle domains.Therefore,it is necessary to find out the fractionation behavior of Mg isotopes in carbonatite-silicate liquid immiscibility,when we try to use Mg isotopes to trace the deep carbon cycling.To this end,a detailed Mg isotope study is conducted on a set of homogenous carbonatitesilicate xenoliths collected from the Laacher See Tephra,Eifel,Germany.The CO2 contents of the xenoliths range from 0.47 wt%to 28.82 wt%and the SiO2 contents from 57.61 wt%to 19.51 wt%,representing a series of rocks that are undergoing magma differentiation and carbonatite-silicate liquid immiscibility.In addition,this study also analyzed a carbonititic syenite from the neighboring Rieden volcano,which shown a lower degree of evolution and therefore could represent a more primitive magma in the volcanic field.The result shown that the δ26Mg of the Rieden carbonititic syenite is-0.205±0.052‰,consistent with the mantle Mg isotopic value within error range.However,whole-rock Mg isotopic fractionations of the Laacher See carbonatitesilicate xenoliths are significant,δ26Mg ranging from-1.55±0.05‰ to 0.56±0.03‰.Acid leaching experiments shown that there is a significant Mg isotopic fractionation between 0.04N HCl leachate and residue,and the Δ26MgCal-Res(δ26MgCal-δ26MgRes)vary from-1.92‰ to-0.36‰,suggesting a similar significant Mg isotope fractionation between calcite and residue(silicate minerals+magnetite).δ26MgCal of calcites range from-1.87±0.19‰～-0.77±0.23‰,and δ26MgRes of residues range from-1.14±0.20‰～0.51±0.16‰.There is a negative correlation betweenΔ26MgCal-Res and δ26MgRes,indicating that the Mg isotopic fractionation between leachate and residue is mainly controlled by δ26MgRes.According to the previous electron probe microanalyses data and the amount of minerals present in the samples obtained,the percentage of MgO can be calculated in each Mg-bearing mineral(calcite,albite,sanidine,nosean,biotite,pyroxene,magnetite and glass).Both the Δ26MgCal-Res and δ26MgRes are controlled by percentages of MgO in two types of minerals classified:1)one including calcite,albite,sanidine and nosean,which tend to be enriched in light Mg isotopes(L),and 2)one including biotite,pyroxene,magnetite and glass,which tend to be enriched in heavy Mg isotopes instead(H).The whole rock δ26Mg are related to the relative Mg proportions between the two mineral types,indicating a causal effect of it.Local accumulation of typical minerals during magma differentiation could lead to the observed significant whole-rock Mg isotope fractionation.The whole rock δ26Mg has no obvious correlation with CO2,indicating that carbonatite-silicate liquid immiscibility and migration and accumulation of carbonatite melts did not significantly affect the Mg isotopic composition of the system.That may be related to the extremely low MgO content of the carbonatite magma.