Late Cretaceous exposure and paleoweathering of a basaltic dike, Makhtesh Ramon, Israel: Geochemical and stable isotope studies

Alteration of a basaltic dike exposed in Nahal Ardon, Makhtesh Ramon, was studied. The core of the dike consists of olivine, pyroxene, plagioclase, and ore minerals and is altered from the center towards the margins where these minerals are replaced by secondary phases. The secondary assemblage includes calcite, smectite, kaolinite, alkali feldspars, chalcedony, and iron and titanium oxides. Element mobility and mass movement during alteration were examined by means of mass balance calculations. Element concentrations were also normalized on Ti, which appears to be the least mobile element. Aluminum is similarly immobile during alteration. Calcium concentration increased in the moderately altered rocks and decreased in the highly altered rocks. Potassium was significantly added to the dike during the advanced stages of alteration. Fe, Mg, Na, and Si are generally depleted during alteration. The growth of secondary minerals at low temperatures at the expense of primary magmatic minerals is also reflected in the changes of calcite-free whole-rock δ¹⁸O values, from +8‰ in the almost fresh rock to +19.7‰ in the dike's margin. δ¹³C values of calcites (-9.1 to -4.1‰ PDB) indicate that the alteration occurred in the presence of soil-derived CO₂, obtained by the interaction of meteoric water with marine limestones and dolomites. The age of the dike was determined by ⁴⁰Ar/³⁹Ar dating as 135 Ma (Lower Cretaceous), whereas the alkali feldspar was formed ∼55 Ma years later, in the Lower Campanian uplift and erosion phase. It is proposed that all alteration processes occurred in an open water-rock system. A predominant part of the low-temperature alteration, which includes the precipitation of secondary alkali feldspars, kaolinite, calcite, and Ti-rich ore minerals, took place in the Upper Cretaceous, long after the dike emplacement.