Geochemical investigations of the oldest (greater than 3800 Ma) abyssal peridotites: implications for early differentiation Earth processes

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Scientific paper

Peridotites from southern West Greenland are currently the best characterised "sample" of the early Archean upper mantle. As such they are the focus of integrated geochemical, geochronological, field investigations and are providing unique insights into early earth processes including the role of impacts, early Earth environments, mantle evolution and mechanisms of crust formation. These rocks are located in the northern part of the early Archean Itsaq Gneiss Complex and occur as <500m long enclaves of variably altered ultramafics containing rare relics of unaltered dunite-harzburgite. The ultramafic enclaves are associated with mafic supracrustal and plutonic rocks, and siliceous metasediments. SHRIMP U/Pb geochronology on igneous zircons from components of the surrounding and intruding orthogneisses, indicate an age for the ultramafic rocks of >3800 Ma (1). The diverse ultramafic and mafic rocks are interpreted as first being tectonically intercalated, and then enclosed in much more voluminous tonalitic rocks during the development of a 3790-3810 Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. Although these samples represent early Archean lithospheric mantle that was trapped within ancient sialic crust during its formation, their chemical affinities (e.g. Si-Al-Mg proportions, ol/opx ratio and mineral compositions) are much closer to modern abyssal peridotites than to cratonic Archean lithospheric mantle as sampled by peridotite xenoliths from southern Africa and Siberia. These samples provide the first direct constraints on mantle redox conditions in the pre-3800 mantle. Their V, MgO, and Ni systematics in comparison with younger mantle suites argue for a mantle environment equal to or even more oxidising than the present upper mantle. The antiquity and chemical integrity of some of these samples is demonstrated by preservation of extremely unradiogenic 187Os/188Os isotopic compositions (2). The age of the peridotites overlaps with estimates for the timing of the lunar terminal cataclysm (3.8-4.0 Ga). The similarity of the highly siderophile element characteristics of these c.3800 Ma samples to modern mantle peridotites, argues for a limited contribution from late impacts, such as those that formed the major lunar basins, to the siderophile element inventory of the Earth's upper mantle. This suggests a benign environment for the development of early terrestrial life. References: 1. Friend C, Bennett V., and Nutman A. (2002) Contrib. Min. Petrol., 143: 71-92. 2. Bennett V., Nutman A., and Esat T. (2002) Geochim. Cosmochim. Acta 66: 2615-2630.

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