Isua

The silicate facies is of a black finely laminated type, consisting mainly of magnetite with up to 15 vol. % grunerite and up to 5% carbonaceous matter. The facies is found in layers up to a few dm thick. Common to the silicate facies is the scarcity of quartz and the complete absence of carbonates. The sulphide facies of Isua consists of up to 60 vol. % sulphides (pyrite, pyrrhotite) together with grunerite or actinolite and magnetite (Appel, 1980) 118). 

Models for the formation of Precambrian sediments suggest that the chemical sediments (such as cherts) of the Isua supracrustal belt have formed as shallow water deposits. This is in agreement with structures locally preserved in the metacherts of the sequence. After deposition, the supracrustals were folded and metamorphosed. Finally, the metamorphism reached lower amphibolite facies and in consequence, most of the primary minerals became recrystallized. As a result all chert now appears as quartzite. But apparently metacherts, magnetite iron formation and quartz carbonate rocks have retained their major element chemistry largely unaltered during metamorphism (Nutman et al., 1984) 119). 

Several features indicate that the silica spheres are original constituents of the rocks, Isuaspheres have been found perforated by needles of actinolite (Fig. 36). It is obvious from this observation that the spheres must be older than the metamorphic event that produced the needle. In some specimens, large authigenic crystals of siderite or ankerite, enclose the silica spheres (compare Fig. 34). The rhombohedral crystal faces of siderite often extend considerably, making 

In weathered samples of the Isua quartzite, most carbonaceous material has been leached out along with the associated carbonates, leaving spherical cavities in the place of the Isuaspheres. These have been mistaken for fluid inclusions by 

Bridgwater et al. (1981)121) whose erroneous interpretation was subsequently corrected by Roedder (1981) 122), (comp. Figs. 35 b, c, d). As can be shown, the cavities form mainly by dissolution of carbonaceous matter and the ferruginous dolomite. So they represent merely an intermediate stage in the destruction of former microspheres (Fig. 16, p. 24). 

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A 13C/12C ratio about 3% above that of a standard value (the so-called VPDB standard) had been found in western Australian rock samples from the Pilbara Formation similar values were found for the Isua rock. However, since this no longer had its original morphology, the yeast-like relicts found by Pflug (1978) may not be real, and doubts have been cast, in particular by the American scientists J. W. Schopf andE. Roedder (Breuer, 1981,1982). Neither the Isua microfossils nor their 13C/12C isotope ratio could convince Bill Schopf that 3.8-billion-year-old samples were really involved. The Isua graphite flakes could also be a charred residuum from the... 

Among the oldest rocks on Earth are those on Isua, an island off the coast of Greenland they are 3.8 Gyr old, formed some 0.7 Gyr after accretion of the Earth. The rocks mark the beginning of the Archean period of geological time. The Isua rocks suggest that there was an extensive hydrosphere at this time, with erosion, transportation and deposition of minerals from water solution. The oldest lunar rocks, however, record an earlier high-temperature event - the Earth-Moon capture event. 

Caro G, Bourdon B, Birck JL, Moorbath S (2003) Sm- Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth s mantle. Nature 423 428-432 Chen JH, Wasserburg GJ (1981) The isotopic composition of uranium and lead in Allende inclusions and meteoritic phosphates. Earth Planet Sci Lett 52 1-15... 

Fig. 14a-dL UV-visible absorption spectra of fossil organic particles after demineralization a) Alginite from Permian Boghead coal, b) Huronispora (Gun-flint), c) Isuasphaera (Isua), d) Graphite (Pflug, 1985) 18>... 

Fig. 16. Post depositional alteration of Isuaspheres included in the Isua quartzite, i. Before recrystallization of the chert matrix 2, 3. After recrystallization, 4. Higher magnification of one single Isuasphere that remained as a cavity in the quartz grain after leaching of the carbonaceous matter through the fissure fp = imprint of the sheathal structure in the wall of the cavity, c = carbonate. Bar for Figs. 1-3 see left of Fig. 1, for Fig. 4 see above the Fig. (Pflug, 1985)18>...

Fig. 19 a and b. Laser mass spectra (positive ions) of spherical organic microstructures included a) in Gunflint chert (Huronispora), b) in Isua quartzite (Isuasphaera). Wall of the spheres is focussed. Field of measurement 1 pm diameter (Pflug, 1982)161 (compare Figs. 6a-f). 

Fig. 38 a and b. Microstructures resembling iron bacteria contained in the Isua banded iron formation. Fig. b) shows a portion of Fig. a) in higher magnification. Electron micrographs from demineralized rock section (Pflug, 1984b)12+1... 

A study of the Isua organic matter is still in progress (McKirdy Hahn, 1982)s>. Volatile organics have been found to occur chiefly in quartzites and in silicate-rich BIF. Alkane distributions show great variability, even over distances of a... 

In summary, it can be stated that there is nothing markedly unusual about the Isua supracrustal facies and their organic contents. They can be compared closely with younger Precambrian, and in some cases Phanerozoic lithologies and successions. Depositional mechanisms and hydrosphere — atmosphere chemical equilibria appear to have been within the range of more recent times (Nutman et al., 1984)119). It seems from all indication that the advent of photoautrophy preceeded Isua times. It was probably the most crucial single event to make an impact on the evolution of the terrestrial atmosphere. 

There exists a continuous record of organic microstructures in ancient minerals and sediments from the 3.8 Ga old metasediments of the Isua Supracrustal Belt (Fig. 40). It has been pointed out that variations in the organic carbon content of... 

When did life start This question was discussed in detail at a recent symposium (Moorbath, 1983) 125). Nobody seriously considered a date younger than 4000 MY to be realistic. The consensus was based on a cautious interpretation of the evidence from Isua. Others prefered to base their opinion on the general evidence for the cessation of extraterrestrial bombardment at around 4000-3900 MY ago. 

The age of meteorites tells us that the solar system - and therefore the Earth - was born roughly 4.6 billion years ago. The oldest terrestrial rocks are zircone crystals (zirconium silicates) which are 4.2 billion years old, but these stones do not tell us much apart from their age, because they are igneous, or magmatic, rocks whose melting processes have erased any trace of history. Much more interesting are the sedimentary rocks, because these were formed by materials that sank to the bottom of ancient seas, and may still contain remnants of the past. The oldest sediments have been found at Isua, in Greenland, and are 3.8 billion years old, which means that there were immense streches of water on our planet at that time, and that the first oceans had originated many millions of years earlier. 

The Isua sedimentary beds, on the other hand, contain iron compounds that could have formed only in the absence of oxygen, which means that the Earth did not have an oxidizing atmosphere for hundreds of millions of years. Those same sediments, however, contain also many types of carbon compounds, and this shows that, by Isua s times, there were in the atmosphere substantial quantities of carbon dioxide (C02), and probably nitrogen (N2), two gases which are neither reducing nor oxidizing. 

High-quality terrestrial data now have been generated for the Sm- " Nd (half-life = 106 Myr) chronometer (Goldstein and Galer, 1992 Harper and Jacobsen, 1992 McCulloch and Bennett, 1993 Sharma et al., 1996). Differences in " Nd/ " Nd in early Archean rocks would indicate that the development of a crust on Earth was an early process and that subsequent recycling had failed to eradicate these effects. For many years, only one sample provided a hint of such an effect (Harper and Jacobsen, 1992) although these data have been questioned (Sharma et al., 1996). Recently very high precision measurements of Isua sediments have resolved a 15 4 ppm effect (Caro et al., 2003). 

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