Precipitation kinetics and C isotope fractionation of rhodochrosite at 298.15 K

Abstract

Before the rise of atmospheric oxygen, the release and transport of soluble manganese ( ) represented the entry point of the earliest Mn cycle. The Mn cycle on early Earth is thought to have resembled that of Fe due to their geochemical similarities. However, kinetic data pertaining to mineralisation are lacking, and thus we lack a complete understanding of the fate of in aqueous systems on early Earth. This study investigates Mn mineralisation and precipitation kinetics through three processes at room temperature: (1) the homogeneous nucleation of rhodochrosite (MnCO3) from oversaturated solutions, (2) seeded rhodochrosite crystal growth under varying solution chemistry, and (3) the competing homogeneous nucleation between rhodochrosite and -silicates in silica-rich solutions. These experimental data show that homogeneous nucleation of rhodochrosite only takes place above a significantly elevated solution saturation ( Ω ≳ 380). The rate of nucleation ( r n u c l ) can be described with a generalised rate law: r n u c l ( m o l k g − 1 s − 1 ) = 0 . 147 exp − 83 . 974 ( log Ω a v g ) 2 . Once nucleated, the crystal growth rate ( r g r w ; BET-surface area normalised) can be delineated as a function of solution chemistry (1 < Ω < 380), following a rate law that reflects a mixed diffusion- (1st order) and surface reaction-controlled (2nd order) growth mechanism: r g r w ( m o l k g − 1 s − 1 ) = 1 0 − 11 . 684 ( Ω a v g − 1 ) 1 . 420 , with R 2 = 0.91. At comparable over-saturations, the growth rate of rhodochrosite is about 10 times faster than that of siderite (FeCO3), but 6 orders of magnitude slower than that of calcite (CaCO3). Despite their similar growth kinetics, far-from-equilibrium rhodochrosite growth results in a negligible kinetic C isotope effect ( Δ 13 C less than − 1 . 27 ‰) in contrast to siderite, which has been shown to exhibit significant kinetic isotope fractionation. This discrepancy is attributed to extensive dissolution and re-precipitation during rhodochrosite growth, which homogenises the isotopic composition of the precipitate and that of the parent solution. Competing nucleation experiments between -carbonate and -silicate in silica-rich solution ([SiO2(aq)] = 1.25 mmolkg-1) demonstrated that, contrary to the Fe(II)-silicate system, -silicates do not precipitate under conditions relevant to surface waters on early Earth. Together, these laboratory observations imply that Mn(II)-bearing solid phases were unlikely to limit aqueous Mn(II) concentrations in anoxic aquatic systems on early Earth, and suggest that the most important removal pathways involved redox transformations and/or incorporation into Ca-bearing carbonate minerals.