Experimental constraints on the partitioning behaviour of uranium (VI) in dolomite

Uranium occurs as a trace element in carbonate minerals, which facilitates the geochemical evaluation of carbonate rocks throughout the geological record. Recent advancements in the δ 238 U paleoredox proxy and U-Pb geochronology, for example, have elucidated the timing and conditions under which carbonate minerals formed. However, carbonate rocks are highly subject to diagenesis (e.g., neomorphism, replacement, recrystallization) and the geochemical behaviour of U(VI) during these reactions is unconstrained. This is particularly true in the case of dolomite [CaMg(CO 3 ) 2 ], a carbonate mineral that often forms by the replacement of CaCO 3 . Here we show, using high temperature (150–200°C) dolomitization experiments, that U(VI) is highly rock-buffered in dolomite (i.e., a significant proportion of U(VI) is inherited from the precursor CaCO 3 ). Thus, we propose a novel method to calculate a partition coefficient ( D* ) for dolomite, which accounts for the total quantity of a given trace element that can be partitioned between the CaCO 3 reactant, the diagenetic fluid, and the CaMg(CO 3 ) 2 product; modulated by the fluid-to-rock ratio. Our results indicate that the D* for U(VI) is an order of magnitude higher for the dolomitization of aragonite (3.6–7.4) than for the dolomitization of calcite (0.7–2.5). Dolomite stoichiometry (mol%CaCO 3 ), cation ordering, and unit cell volume exert a first-order control on the amount of U(VI) that can fit within its crystal structure, which in turn dictates the D* . As a result, the enlarged unit cell of ‘protodolomite’ or ‘very high magnesium calcite’ (326–332 Å 3 ) gives rise to a higher D* than for stoichiometric, ordered dolomite. We also demonstrate that dolomite can undergo later mineralogical stabilization and recrystallization, which results in a systematic decrease in D* as the unit cell volume decreases (321–324 Å 3 ). The results of this experimental study provide novel insights into the role of diagenesis in shaping the geological record, with clear stipulations of the mineralogical and petrological constraints that should be required to interpret the geochemical signature of ancient carbonate rocks.