In contrast to Li/Ca and Mg/Ca, Li/Mg is strongly anticorrelated with temperature in aragonites precipitated by the benthic foraminifer Hoeglundina elegans and a wide range of scleractinian coral taxa. We propose a simple conceptual model of biomineralization that explains this pattern and is consistent with available abiotic aragonite partition coefficients. Under this model the organism actively modifies seawater within its calcification pool by raising its [Ca2+], using a pump that strongly discriminates against both Li+ and Mg2+. Rayleigh fractionation during calcification effectively reverses this process, removing Ca2+ while leaving most Li+ and Mg2+ behind in the calcifying fluid. The net effect of these two processes is that Li/Mg in the calcifying fluid remains very close to the seawater value, and temperature-dependent abiotic partition coefficients are expressed in the biogenic aragonite Li/Mg ratio. We further show that coral Sr/Ca is consistent with this model if the Ca2+ pump barely discriminates against Sr2+. In H. elegans the covariation of Sr/Ca and Mg/Ca requires either that the pump more strongly discriminates against Sr2+, or that cation incorporation is affected by aragonite precipitation rate via the mechanism of surface entrapment. In either case Li/Mg is minimally affected by such ‘vital effects’ which plague other elemental ratio paleotemperature proxies.