Carbon isotope effects in carbonate systems

Abstract

Global carbon cycle models require a complete understanding of the δ13C variability of the Earth’s C reservoirs as well as the C isotope effects in the transfer of the element among them. An assessment of δ13C changes during CO2 loss from degassing magmas requires knowledge of the melt-CO2 carbon isotope fractionation. In order to examine the potential size of this effect for silicate melts of varying composition, 13C reduced partition functions were computed in the temperature range 275 to 4000 K for carbonates of varying bond strengths (Mg, Fe, Mn, Sr, Ba, Pb, Zn, Cd, Li, and Na) and the polymorphs of calcite. For a given cation and a given pressure the 13C content increases with the density of the carbonate structure. For a given structure the tendency to concentrate 13C increases with pressure. The effect of pressure (‰/10 kbar) on the size of the reduced partition function of aragonite varies with temperature; in the pressure range 1 to 105 bars the change is given by: (1)Δ13Cpaverage=−0.01796+0.06635∗103T+0.006875∗106T2 For calcite III the pressure effect is on average 1.4× larger than that for aragonite at all temperatures. The nature of the cation in a given structure type has a significant effect on the carbon isotope fractionation properties. The tendency to concentrate 13C declines in the series magnesite, aragonite, dolomite, strontianite, siderite, calcite, smithonite, witherite, rhodochrosite, otavite, cerrusite. For divalent cations a general expression for an estimation of the reduced partition function (β) from the reduced mass (μ = [MCation × MCarbonate]/[MCation + MCarbonate]) is: (2)1000lnβ=(0.032367−0.072563∗103T−0.01073∗106T2)∗μ−14.003+29.953∗103T+9.4610∗106T2 For Mg-calcite the 13C content varies with the Mg concentration. The fractionation between Mg-calcite (X = mole fraction of MgCO3) and calcite is given by: (3)1000ln(αMgCalite−Calcite)=[0.013702−0.10957×103T+1.35940×106T2−0.329124×109T3+0.0304160×1012T4]×X1.5 The results of the computations were used together with previously published experimental vaporous CO2-silicate melt fractionations to determine, at 1200°C, a relationship between melt-CO213C fractionation and melt composition, expressed as molecular proportions of the cations Mg, Fe, Mn, Ca, Na, K and Si and Al: (4)1000lnαMelt−CO2=5.14×Mg+Fe+Mn+Ca+Na+KSi+Al+0.86 A conceptual model to understand this relationship was developed. The results of the computations approximate closely the experimentally determined vaporous CO2-CaCO3 fractionations at high temperatures. Empirically derived dolomite-calcite and calcite-graphite 13C isotope geothermometers agree with results of the present work.