Experimental constraints on magnesium isotope fractionation during abiogenic calcite precipitation at room temperature

Abstract

Magnesium (Mg) isotopes in carbonate minerals are a useful proxy for paleoclimate studies, but interpretations are often limited by an inadequate understanding of the various factors controlling Mg isotopic fractionation during carbonate formation. Previous work has studied a number of parameters including aqueous chemistry, mineralogy, temperature, and precipitation rate. However, little is known about the impact of solid/solution ratio, calcite growth mechanism, and crystal morphology on isotope fractionation. In this work, two groups of seeded chemo-stat calcite precipitation experiments were conducted at 25 C to explore the potential impact of crystal growth and morphology on the fractionation of Mg isotopes. Group-1 experiments (G1) contained nine individual runs that were performed under identical physicochemical conditions, except for solid/solution ratio and the length of an experiment. The isotope fractionation between precipitated calcite and aqueous solution is limited, with D26Mgcal-sol ranging from 2.58 to 2.40‰ and an average of 2.49 ± 0.12‰ (2SD, n = 9). The Group-2 experiments (G2) contained 3 paired runs with solution Mg/Ca molar ratios of 0.5, 2.0, and 5.0, and yielded D26Mgcal-sol values that ranged from 2.69 to 2.36‰ with an average of 2.62 ± 0.25‰ (2SD, n = 6). The average D26Mgcal-sol value for both sets of experiments is 2.54 ± 0.22‰ (2SD, n = 15), and it is independent of precipitation rate, solution Mg/Ca molar ratio, solid/solution ratio, amount of overgrowth, and mol% Mg content in overgrowth. The form and texture of the calcite overgrowths in our experiments range from {104} rhombohedra with smooth crystal faces containing few macrosteps to {104} rhombohedra containing extensive evidence for 2-D nucleation on crystal faces, to more steeply sided rhombohedra {0kl}. While significant changes in crystal morphology are related to solid/solution ratio and solution composition in the G1 and G2 experiments, respectively, there was no difference in Mg isotope systematics, suggesting that crystal morphology does not affect the Mg isotopic composition of calcite within the range of features investigated and 2-D nucleation may be less affected by calcite growth kinetics than a spiral growth mechanism. Integrating our results with previous published values, an equilibrium isotopic fractionation factor of 2.47 ± 0.09‰ (weighted average ± weighted 2SD, n = 70) between calcite and aqueous solutions is derived at room temperature.