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Friend C.R.L.,45 Stanway Road | Friend C.R.L.,Chinese Academy of Geological Sciences | Nutman A.P.,University of Wollongong | Nutman A.P.,Chinese Academy of Geological Sciences
Geology | Year: 2011

The chemistry of surviving pieces of Eoarchaean mantle together with related crust helps us determine early crust-forming mechanisms. Two lenses of high-Mg, low-Al dunite within a ca. 3720 Ma part of the Isua supracrustal belt in Greenland are interpreted as relicts of Eoarchaean mantle with minimal crustal disturbance. The lenses are within altered, higher Al, Ca ultramafic schists and are intercalated with amphibolitized pillow basalts and gabbros with island arc chemical signatures, all intruded by 3715-3710 Ma tonalites. One variety of dunite is dominated by forsterite (Fo90-92) olivine with accessory chromite and rare clinopyroxene, which does not show high field strength element (HFSE) anomalies. Another variety contains olivine (Fo96-98), some intergrown with Ti-humite group minerals with strong positive HFSE anomalies that are complementary to the negative HFSE anomalies of the adjacent amphibolites. We propose that these dunites are tectonic slivers of ca. 3720 Ma subarc mantle that preserve evidence for varying interaction with mafic magmas in a ≥850 °C, 1.7-2.0 GPa subcrustal environment. These are by far the oldest direct geochemical link between coeval mantle and crustal rocks, and are new evidence for subduction zone-like environments on the early Earth. © 2011 Geological Society of America.

Nutman A.P.,University of Wollongong | Nutman A.P.,Chinese Academy of Geological Sciences | Friend C.R.L.,45 Stanway Road | Bennett V.C.,Australian National University | And 2 more authors.
Precambrian Research | Year: 2010

Chemical (meta)sedimentary rocks in the amphibolite facies ≥3700Ma Isua supracrustal belt (W. Greenland) are mostly strongly deformed, so there is only a small chance of the survival of features such as stromatolites or microfossils that would be direct proof of a ≥3700Ma biosphere. Therefore the search for evidence of ≥3700Ma life in Isua rocks has focused on chemical signatures, particularly C-isotopes. The new approach presented here is based on whole rock chemistry rather than isotopic signatures. Isua chemical sedimentary rocks have Ca-Mg-Fe bulk compositions that coincide with ferroan dolomite - siderite/Fe-oxide mixtures. Most have low Al2O3, TiO2 contents (<0.5 and <0.05wt% respectively) showing minimal contamination from terriginous materials. Identical seawater-like REE+Y shale-normalised trace element signatures with La, Ce, Eu and Y positive anomalies are found in magnetite-rich banded iron formation (BIF - such as the geochemical standard IF-G), dolomite-rich rocks and quartz-carbonate-calc-silicate rocks. Additionally from a rare, small area of low deformation in Isua, there are ~3700Ma pillow lava interstices consisting of quartz+tremolite+calcite derived from pre-metamorphic dolomite+silica. Thus the dolomite in the chemical sediments and the pillow interstice was part of the pre-metamorphic assemblage, and was deposited from seawater and/or low-temperature groundwater (as shown by the REE+Y chemistry). Therefore, at least some Isua carbonate rocks are sedimentary or diagenetic in origin rather than being formed by metasomatism at 600-500°C as proposed by Rose et al. (1996. American Journal of Science 296, 1004-1044).Low-temperature dolomite formation in modern sediments (sabkha to deep ocean) and its deposition from low-temperature groundwater within basalts has only been directly observed in the field and replicated in laboratory experiments through anaerobic microbial mediation. Therefore, microbial mediation appears to be essential for the formation of low-temperature dolomite. From this, we propose that the evidence for the formation of low-temperature pre-metamorphic dolomite in Isua prior to metamorphism provides a new, simple, and relatively direct line of evidence for ≥3700 Ma life. © 2010 Elsevier B.V.

Nutman A.P.,University of Wollongong | Nutman A.P.,Chinese Academy of Geological Sciences | Bennett V.C.,Australian National University | Friend C.R.L.,45 Stanway Road
Australian Journal of Earth Sciences | Year: 2012

Chemical sedimentary rocks such as banded iron formation (BIF) and pillow basalts are persistent features of the oldest volcanic and sedimentary record by 3.8-3.7 Ga, and are direct evidence for oceans by the start of the Archean. However, their presence does not dictate an equitable 3.8-3.7 Ga terrestrial climate. This is because they could have formed in oceans below global pack ice on a frigid Earth. The oldest known depositional structures occur as locally preserved features in ca 3.7 Ga deformed, amphibolite facies rocks of the Isua supracrustal belt (Greenland). These include units up to ~1 m thick, in which there are stacked or jumbled clasts of chert. Detailed structural analysis shows that these rocks are not tectonic breccias. Also, their reported location in chemical sedimentary units capping slightly older volcanic rocks shows they are unlikely to be mass-flow deposits in a deep basin. In both composition and structure, these units resemble edgewise breccias observed on later Precambrian and Phanerozoic chemical sediment platforms, which formed when laminated sediments are disrupted by storm waves. Hence, the wave origin shows oceans were not ice covered, because inthat case atmospheric storms would not generate waves. The Isua supracrustal belt also contains 3.72-3.70 Ga felsic and pelitic sedimentary rocks, derived from juvenile volcanic arc sources. These sedimentary rocks have chemical weathering indices that deviate from those of both fresh Eoarchean and modern igneous rocks. Furthermore, their weathering indices are congruent with rare examples of weathered (not hydrothermally altered) Isua volcanic rocks we have identified. Although no doubt the chemistry of Eoarchean weathering processes was different from those now, this nonetheless shows that these sedimentary rocks contain large contributions from highly weathered source materials. Rapid advanced weathering (these rocks consist of materials shed from an arc) is most feasible with anequitable to hot climate, rather than a frigid one, because higher temperatures enhance chemical reactions between rocks, atmospheric gases and precipitation. Wave-generated structures in chemical sedimentary rocks and advanced weathering at ca 3.7 Ga point to an equitable terrestrial climate, with at least partially ice-free oceans. With the faint early sun at ca 3.7 Ga, proposed mechanisms to facilitate this are either a stronger greenhouse atmosphere (e.g. more CO 2), or that early Earth had a lower albedo because it was essentially oceanic, without exposed continental crust and ice caps. © 2012 Copyright Taylor and Francis Group, LLC.

Horie K.,Japan National Institute of Polar Research | Horie K.,Hiroshima University | Nutman A.P.,University of Wollongong | Nutman A.P.,Australian National University | And 2 more authors.
Precambrian Research | Year: 2010

Field studies integrated with cathodoluminescence petrography and SHRIMP U-Pb dating of zircons from >150 orthogneisses and metatonalites from the Eoarchaean Itsaq Gneiss Complex (southern West Greenland) shows that only a minority contain ≥3840. Ma zircons, whereas the majority carry only younger ones. Rocks containing ≥3840. Ma zircons vary from very rare single-phase metatonalites to more common complexly banded tonalitic migmatites. The former metatonalites have simple oscillatory-zoned ≥3840. Ma zircon with limited recrystallisation and overgrowth, whereas the more common migmatites have much more complicated zircon populations with both ≥3840. Ma and 3650-3600. Ma oscillatory-zoned zircon, more extensive recrystallisation and widespread complex core-rim multiple growth relationships.With only 100-160. ppm Zr in the tonalites and likely melt generation temperatures of >1000 °C, the experimentally determined zircon solubility-melt composition relationships established by other workers shows that the precursor melts to the Itsaq Complex tonalites were strongly undersaturated in zircon, thus any entrained xenocrystic zircon would have been rapidly dissolved. Therefore, the ≥3840. Ma oscillatory-zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur.With an established igneous age of ≥3840. Ma established from such relationships, we interpret the correlated variation between the field nature of these rocks and their zircon petrography/age structure as due to superimposition onto ≥3840. Ma tonalite protoliths of variable amounts of heterogeneous strain, heterogeneous distribution of melt patches formed during in situ anatexis at up to ~800 °C, plus granitic veining. This explains why geologically simple metatonalites have simple zircon populations, whereas complex orthogneisses have complex zircons. The large amount of integrated field, geochemical and zircon data rule out an alternative interpretation, that the ≥3840. Ma zircons represent an igneous xenocrystic component, present in younger rocks to varying degrees. If this were true, then the structurally simple (less reworked) rocks should still display complex zircon populations.Gneisses with ≥3840. Ma zircon are commonest on Akilia and neighbouring islands, in Itilleq fjord (~65. km east Akilia) and on the north of Ivisaartoq (~150. km northeast of Akilia). These include from Itilleq a 3891 ± 6. Ma gneissic tonalite (with minor neosome)-which is currently the oldest rock recognised in the Itsaq Gneiss Complex. Overall, the ≥3840. Ma tonalites are a widespread and unevenly distributed in the Itsaq Gneiss Complex, and they are a volumetrically minor component compared with ~3800, 3750 and 3700. Ma tonalite generations.Using the subset of our data covering Itilleq and the neighbouring fjords, migmatite samples with ≥3800. Ma igneous zircon are mutually exclusive from migmatite samples with ~3700. Ma igneous zircon. This suggests that prior to an amalgamation event followed by 3660-3600. Ma high-grade metamorphism, ≥3840. Ma tonalites might have resided in a terrane discrete from ~3700. Ma tonalites. This is in accord with interpretation of the non-migmatised part of the Complex in the Isua area, where a terrane of ~3800. Ma tonalites with a minor associated ≥3840. Ma component and a terrane with ~3700. Ma tonalites were tectonically juxtaposed at ~3660. Ma. © 2010 Elsevier B.V.

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