No Water, No Cyanobacteria—No Calc-alkaline Magmas: Progressive Oxidation of the Early Oceans May Have Contributed to Modernize Island-Arc Magmatism

Abstract

A remarkable change in Earth history took place in Late Archean time. The time-span 3.0-2.5 Ga marks the progressive end of the low-P, high-T subduction regime and komatiitic magmatism. This coincides with a period of rapid crustal growth, when up to 70% of the present continental masses formed. Most authors would agree that such change indeed took place, although the why and how remain a controversial issue. By the end of the Archean, calc-alkaline magmatism was firmly established, and a key point relates to the nature of these arc-related igneous rocks. These magmas are not only characterized by silica enrichment, but are also more oxidized than their primitive counterparts (e.g., komatiites, MORBs). Thus, an increase in the oxygen fugacity of the upper mantle is here regarded as the turning point that allowed formation of calc-alkaline complexes during Archean time. In this respect we suggest that the appearance of cyanobacteria more than 3.7 billion years ago, may have played a decisive role in changing the old Archean tectonomagmatic scenario, by increasing the oxidizing potential of the oceans, and therefore inducing iron oxide precipitation onto the ocean floors in the form of FeO(OH)-like minerals. If subduction processes, whatever the type, were already active by Early—Meso Archean time (as seems to be the case), it follows that a massive amount of iron oxides must have been introduced into the mantle wedge. The melting of these chemical sediments would have resulted in liberation of oxygen, thus contributing to the oxidation of this realm, and therefore, triggering the onset of "modern" calc-alkaline, magmatism.