Potassium and boron co-depletion in Canadian Shield brines: evidence for diagenetic interactions between marine brines and basin sediments

Hypersaline Ca/Cl brines of marine origin deep within the crystalline rocks of the Canadian Shield are typically highly depleted in potassium and boron compared to their concentrated seawater parent brine. While this depletion in both solutes is not accompanied by a major alteration in their original marine B/K ratios, their initial marine δ11B has been lowered to 20–30‰, values that are intermediate between that of present day seawater (39‰) and typical crustal values (−3±5‰). Together, these observations eliminate 3 of 4 potential K and B removal mechanisms that may have operated at various times over the long history of shield brines: (1) co-precipitation in late stage evaporative salts in the marine basin where the brine was formed, (2) incorporation in secondary feldspars formed either within basin sediments or the host rocks, and (3) in situ adsorption by secondary clay minerals on fracture surfaces. Rather the δ11B values, together with the consistency in the depleted but similar K/Br and B/Br ratios, suggest that K and B were removed from shield brines by fixation in illitic clay minerals. This likely occurred during the diagenetic illitization of smectite in argillaceous marine sediments through which the brine infiltrated prior to penetrating the underlying crystalline basement rocks.