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 Δ26Mgcal-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 Δ26Mgcal-sol values that ranged from −2.69 to −2.36‰ with an average of −2.62 ± 0.25‰ (2SD, n = 6). The average Δ26Mgcal-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.

中文翻译：

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室温下非生物方解石沉淀过程中镁同位素分馏的实验限制

摘要 碳酸盐矿物中的镁 (Mg) 同位素是古气候研究的有用代表，但由于对碳酸盐形成过程中控制 Mg 同位素分馏的各种因素了解不足，解释往往受到限制。以前的工作研究了许多参数，包括水化学、矿物学、温度和沉淀速率。然而，关于固溶比、方解石生长机制和晶体形态对同位素分馏的影响知之甚少。在这项工作中，在 25 °C 下进行了两组晶种恒化方解石沉淀实验，以探索晶体生长和形态对 Mg 同位素分馏的潜在影响。第 1 组实验 (G1) 包含 9 个在相同物理化学条件下进行的单独运行，除了固溶比和实验长度。沉淀方解石和水溶液之间的同位素分馏是有限的，Δ26Mgcal-sol 范围从 -2.58 到 -2.40‰，平均值为 -2.49 ± 0.12‰（2SD，n = 9）。第 2 组实验 (G2) 包含 3 个配对运行，溶液 Mg/Ca 摩尔比为 0.5、2.0 和 5.0，产生的 Δ26Mgcal-sol 值范围从 -2.69 到 -2.36‰，平均值为 -2.62 ± 0.25 ‰（2SD，n = 6）。两组实验的平均 Δ26Mgcal-sol 值为 -2.54 ± 0.22‰ (2SD, n = 15)，它与沉淀速率、溶液 Mg/Ca 摩尔比、固/溶液比、过度生长量和mol% 过度生长中的 Mg 含量。在我们的实验中，方解石过度生长的形式和质地范围从 {104} 菱面体（具有包含很少宏观台阶的光滑晶面）到 {104} 菱面体（包含大量晶面二维成核证据），再到更陡峭的菱面体 {0kl}。虽然在 G1 和 G2 实验中晶体形貌的显着变化分别与固/溶液比和溶液组成有关，但 Mg 同位素系统学没有差异，表明晶体形貌不影响该范围内方解石的 Mg 同位素组成研究的特征和二维成核可能比螺旋生长机制受方解石生长动力学的影响更小。将我们的结果与之前公布的值相结合，平衡同位素分馏因子为 -2.47 ± 0。