3.7 Ga Isua Supracrustal Belt sediments record formation of fore-arc basin with conditions conducive to proliferation of life

The Isua Supracrustal Belt hosts >3.7 Ga old detrital meta-sediments that consist of turbiditic and pelagic pelites. These meta-sediments contain some of the oldest evidence for life in the form of abundant graphite with low δ¹³C. The continuous occurrence of this graphite throughout these meta-sediments is consistent with their formation in a basin with a continued pelagic biomass productivity, fueled by a consistent source of nutrients. Understanding the environment in which these early life-forms thrived is fundamental to our understanding of which conditions are conducive to life on Earth. In order to trace the tectonic and compositional development of the basin and proximal terranes, we have characterized a ca. 80 m rock core that samples the basaltic basement, iron-rich meta-sediments, detrital meta-sediments and contacts between these lithologies. The basement consists of basalts with major and trace elemental compositions that are similar to boninites and are conformably overlain by iron rich mixed chemical and detrital sediments. Iron concentrations fall gradually upwards in the core, with sporadic reoccurring iron-rich layers occurring in the upper core. The gradual change in iron concentrations shows that the core contains a broadly conformable and coherent stratigraphy that records the formation of the volcanic basement, followed by iron deposition enhanced by post-volcanic hydrothermal circulation. Detrital sediments were being deposited as soon as the basin floor was formed, indicating that proximal erodible terranes were already present. A combination of proxies, consisting of La/Yb, Ti/Zr and petrographic observations show that this detrital component was derived from ca. 60 % tonalities and 40 % non-boninitic basalts. These conditions are best explained by the formation of a volcanic fore-arc basin in front of a pre-existing differentiated terrane formed by melting of hydrated basalts. This sequence of events is consistent with cumulative zircon ages in surrounding gneisses that suggest episodic collisions of primitive arcs, followed by re-initiation of subduction. The active volcanism, tectonism and formation of (semi)restricted basins in this environment likely allowed the accumulation of nutrients required for the proliferation of life.