Chemical Composition of Pre-Cambrian Slates with Notes on the Geochemical Evolution of Lutites

The average (unweighted) chemical composition of 33 pre-Cambrian slates is (in per cent): SiO₂-56.30; TiO₂-0.77; Al₂O₃-17.24; Fe₂O₃-3.83; FeO-5.09; MnO-0.10; MgO-2.54; CaO-1.00; Na₂O-1.23; K₂O-3.79; P₂O₅-0.14; combined water-3.31; moisture-0.38; CO₂-0.84; C-1.18; SO₃-0.28; FeS₂-1.98. The average comprises 9 analyses by the author and 24 (21 published, 3 unpublished) by others. The geographic distribution of the samples is as follows: 4 from Ontario; 18 from Michigan and Wisconsin; and 11 from Minnesota. The stratigraphic distribution is: 5 from the Archean, 25 from the Huronian, and 3 from the Keweenawan. Comparison of the pre-Cambrian average with averages of Paleozoic and Mesozoic-Cenozoic shales (Clarke) reveals that progressively younger lutites contain successively less Al₂O₃, FeO, total iron, K₂O, and C, and successively more CaO, P₂O₅, moisture, CO₂, and SO₃. Substitution of a pre-Cambrian average weighted for sediment types changes none of the trends except that of carbon. The validity of the Paleozoic average (51 samples) is attested by correspondence with the average of 36 American slates (Eckel), 27 of which are Paleozoic. The Paleozoic shale and slate averages also indicate that chemical changes attending metamorphism of shale to slate are restricted to partial reduction of iron and loss of moisture and cannot account for the differences observed between the averages of the three ages. The greater abundance of CaO, CO₂, P₂O₅, and SO₃ in the averages of younger rocks may be related to the development of life, and the other consistent variations noted may be due to progressively coarser textures in the younger samples.