Petrology and Geochemistry of MORB and OIB in the Mid-Archean North Pole Region, Pilbara Craton, Western Australia: Implications for the Composition and Temperature of the Upper Mantle at 3.5 Ga

Abstract

It is necessary and important to determine the tectonic setting of mafic volcanism, because compositions of magma are strongly related to tectonic setting. Our recent geological investigation of the North Pole (NP) region (3.5 Ga), Pilbara, showed that the greenstone belt was an accretionary complex, defined by oceanic-plate stratigraphy and duplex structure. Moreover, application of accretionary geology reveals that there are two types of tholeiitic basalts from different tectonic settings: midocean ridge (MORB) and oceanic island (OIB). This work presents geochemistry of major, trace, and rare-earth elements (REE) of greenstones originating as MORB and OIB in the mid-Archean. Relict igneous clinopyroxenes (Cpx) in NP MORB and OIB have compositional variations equivalent to chemical variations of Cpx in modern MORB and OIB. In addition, TiO2 content and Mg# = (MgO/(MgO+FeO*)) in the host magma calculated from those in Cpx are consistent with whole-rock compositions, indicating preservation of original compositions in terms of TiO2, FeO, and MgO contents. The Cpx in NP MORB and OIB show LREE-depleted and slightly LREE-depleted REE patterns, respectively. Calculated REE patterns of host magmas also are in agreement with the wholerock compositions. Analyses of 64 alteration-free greenstones selected from 2662 samples indicate that NP MORB is enriched in Al2O3 and depleted in TiO2, FeO*, Y, and Zr at any MgO content, compared with NP OIB. In addition, NP MORB show a slightly LREE-depleted, whereas NP OIB forms a slightly LREE-enriched REE patterns. Such differences suggest that the Archean mantle already was differentiated. However, the compositional difference between the source mantles is relatively small, and differentiation into MORB and OIB source mantles was in the process of development. NP MORB are enriched in FeO, compared with modern MORB. Comparison of NP MORB with recent melting experiments shows that the source mantle of the NP MORB had Mg# = 85-87 and was enriched in FeO relative to the modern upper mantle. A possible mechanism to decrease the FeO content in the mantle is precipitation of metallic iron at the point of slab penetration into the lower mantle through time. The potential temperature of the source mantle of NP MORB is estimated to be ~1500 °C, indicating that even the mid-Archean mantle was hotter by at most ~150-200°C than the modern mantle, in contrast to the estimate from peridotitic komatiites.