A refined dissolution method for rare earth element studies of bulk carbonate rocks

Abstract

Extraction of primary geochemical signals from bulk carbonate rocks is a key task in palaeoenvironmental studies. REE behaviour is investigated during stepwise dissolution of carbonate rocks. The experiment is designed to achieve total carbonate dissolution in ten steps, aiming for approximately 10% of the total carbonate to be dissolved in each step using acetic acid. An additional eleventh step using excess acid completed each experiment. Results show that calcite is preferentially dissolved before dolomite though no significant effect on the REE pattern of the rock is observed. Secondary carbonate phases and adsorbates are likely to be dissolved at the beginning of the dissolution process but this does not increase REE concentration though the REE pattern may be altered e.g. lessening of negative Ce anomaly, altering of Eu anomaly, Y/Ho ratio and relative fractionation between LREE and HREE. Non-carbonate minerals e.g. terrestrial particulate matter, Fe–Mn oxyhydroxides and phosphates, are likely to be dissolved towards the end of the total dissolution, especially in the final excess acid step. This should increase REE concentrations and alter REE distribution patterns greatly. In contrast, solutions from intermediate steps are less contaminated. Further, compared with using 10% v/v acetic acid, the solutions obtained using 5%v/v acid return more pristine REE information from the carbonate.A proposed method for REE studies of bulk carbonate rocks requires an initial dissolution of 30%–40% followed by the sampling dissolution of 30%–40% of total carbonate using 5%v/v acetic acid to produce REE information considered to best represent that of the carbonate source water. Following these steps 20%–30% of the carbonate should remain undissolved, thus minimising contamination from non-carbonate minerals.