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The change in shape of inclusion minerals is a time-dependent process that is controlled by grain-boundary diffusion to minimize the interfacial free energy between inclusion and host minerals. We analyzed olivine inclusions in the orthopyroxene matrix in mantle xenoliths erupted by kimberlites in the Kalahari craton, southern Africa. Most fine-grained inclusions are rounded, whereas coarse-grained ones show a polyhedral outline with slightly rounded edges. The critical radius of the largest rounded inclusion is in the size range 240–280 μm in several kimberlite suites in this craton; this corresponds to a minimum annealing time-scale of the order of billions of years, which agrees with the isotopic model ages of the mantle xenoliths. The coincidence of the annealing time-scale of the mantle xenoliths with the major age of the crust formation in this region suggests a coupled link between the formation of the Archean craton and that of the underlying lithospheric mantle.

We performed rare-earth element (REE) geochemistry and U-Pb geochronology on apatites in metasediments from the ~3.8 Ga Isua supracrustal belt (ISB) and Akilia Island, West Greenland, together with stepwise combustion isotopic investigation of carbon and nitrogen for the apatite-bearing quartz-magnetite BIF of uncontested sedimentary origin from northeastern ISB. Ion microprobe analyses reveal that apatites in psammitic schist from the ISB show a U-Pb isochron of 1.5 ± 0.3 Ga. This age is similar to those of Akilia apatite and the Rb-Sr age of 1.6 Ga for the pegmatitic gneiss in the Isukasia area in literature, suggesting a late (~1.5 Ga) metamorphic event (≥400°C). Pb isotopic ratios of apatite in the quartz-magnetite BIF are also affected by the late metamorphic event around 1.5 Ga. Chondrite-normalized REE patterns of apatites in the BIF show flat patterns with a significant positive Eu anomaly, suggesting hydrothermal influence; this is consistent with a primary depositional origin. In contrast with the quartz-magnetite BIF, apatites in the psammitic schist from the ISB and those in the Akilia BIF show different REE patterns, which resemble those of apatites from secondary mafic and felsic rocks, respectively. Carbon isotopic ratios for the quartz-magnetite BIF by stepwise combustion suggest that two components of reduced carbon are present. One is released below 1000°C (mainly 200–400°C; lowtemperature carbon = LTC), and the other above 1000°C (high-temperature carbon = HTC). δ 13 C values of the LTC are about –24‰. The LTC is clearly contaminant incorporated after metamorphism, because such a low-temperature component could not have survived the ≥400°C metamorphic event. On the other hand, δ 13 C values of the HTC are –30‰ for one aliquot and –19‰ for another. The HTC is probably sequestered within magnetite in the BIF, because the decrepitation temperature of magnetite is ~1200°C. The HTC could not exist within quartz and apatite (decrepitation temperatures: 400–600°C and 600–800°C, respectively), or along grain boundaries. Because the magnetite is concordant with bedding surfaces, it is plausible that the HTC was incorporated in the magnetite during diagenesis. Thus, HTC is the most important candidate for primary carbon

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 co...

The Isua supracrustal belt (∼3.8 Ga) constitutes the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of more than 1500 rock samples of the Isua belt have enabled us to estimate the extent of regional metamorphism, the petrotectonic environment, and the subduction-zone geothermal gradient in the Archean. The following line of evidence indicates progressive, prograde metamorphism from greenschist (Zone A) through albite-epidote-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeastern part of the Isua supracrustal belt: (1) the systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of the composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) the absence of relict minerals of high-grade amphibolitic metamorphism even in the lowest metamorphic zone. Metabasites of the Isua belt vary extremely in Mg#, causi...