Komiya et al. (1999) demonstrated that within the accretionary complex of the Early Archean ISB, basaltic lavas overlying hyaloclastite beds and chert/BIF sediments were deposited in an open sea. Accretionary geology enables us to estimate the tectonic setting of the Isua mafic volcanism. There are three distinctive mafic igneous suites in the ISB (Komiya et al. 1999). The first consists of pillowed and massive lava flows with subordinate amounts of related basaltic dykes and hyaloclastite layers that are overlain by thick-bedded chert layers with no terrigenous material (Fig. 16ab). This evidence indicates that the mafic rocks were formed remote from a continental margin or island arc, and that the mafic volcanism preceded deposition of the pelagic sediment, suggesting that the mafic volcanism occurred at a mid-oceanic ridge (Isua MORB; Fig. 16). The second suite includes abundant doleritic dykes and sills that intrude pillowed lava flows, lavas interlayered with chert, and bimodal felsic and mafic volcaniclastic rocks in Duplex II, but they do not intrude clastic sediments (Fig. 17). They were formed between an oceanic ridge and a subduction zone, suggesting derivation from an oceanic island or oceanic plateau (Isua OIB). The third suite includes abundant doleritic dykes, which intruded all of the above units, including the clastic sediments; they may be related to post-accretionary volcanism (Fig. 13). Isua OIB have lower Al₂O₃ and higher FeO*, TiO₂, Y, Zr and Nb contents than Isua MORB at a given MgO content. Isua MORB show LREE-depleted REE patterns, whereas Isua OIB have flat REE patterns (Fig. 18, Komiya et al., 2004). The differences of field occurrence are well correlated with the compositional differences, indicating mantle heterogeneities in the Early Archean. Isua MORB have higher Al₂O₃ and lower TiO₂, CaO, Na₂O, Zr and Y contents than modern MORB at any given MgO content. They also have higher FeO* at high MgO contents (>10 wt%). Major element chemistry of Isua MORB indicates that the source mantle of Isua MORB had approximately 10 wt% FeO (Fig. 18). The higher content of FeO in the Early Archean mantle suggests that upper mantle FeO content has decreased over time. The estimated potential temperature of the source mantle of Isua MORB is approximately 1480 ℃, about 150 ℃ higher than the modern equivalent (Fig. 19).

Figure 16a: The bottom of the oceanic plate stratigraphy: basaltic pillow lava, hyaloclastite, and bedded white chert in stratigraphically ascending order, from right to left sides. The basalt originates from 3.8 Ga MORB, overlain by thick-bedded chert.

Figure 16b:Isua MORB, comprising pillow lava and hyaloclastite. The basalt is overlain by well-bedded white chert.

Figure 17: Isua OIB, interlayered with chert beds, or intruding the chert layer as a sill.

Figure 18a:MgO variation diagrams of major elements for mid-oceanic ridge basalt (MORB) and oceanic island basalt (OIB) in the Isua Supracrustal Belt along with modern MORB and OIB. Compositional differences between Isua MORB and OIB are present in TiO₂, FeO and CaO contents. Primitive Isua MORB has a higher FeO content than primitive modern MORB

Figure 18b:MgO variation diagrams of trace elements for Isua mid-oceanic ridge basalt (MORB) and oceanic island basalt (OIB) along with modern MORB and OIB. Compositional differences between Isua MORB and OIB are present in Zr, Y and Nb contents.

Figure 18c:Chondrite-normalized rare earth element (REE) patterns of Isua mid-oceanic ridge basalt (IMORB) and oceanic island basalt (IOIB). Isua MORB shows a light REE-depleted pattern, whereas OIB shows a flat REE pattern.

Figure 19:Secular changes of potential mantle temperature (green line) and FeO content (blue line) of the upper mantle, along with potential mantle temperature of a plume source (red line, Takahashi, 1990; Nisbet et al., 1993; Sakurai et al., 2001). The left vertical axis represents the potential temperature of the mantle, whereas the right vertical axis represents the FeO content of the upper mantle.