Early Archean

Schopf JW (1993) Microfossils of the early Archean Apex Chert new evidence of the antiquity of life. Science 260 640-646... 

Schopf JW, Packer BM (1987) Early Archean (3.3-billion to 3.5-billion-year-old) microorganisms from the Warrawoona Group, Australia. Science 237 70-73... 

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). 

Chase C. G. and Patchett P. J. (1988) Stored mafic/ultramafic crust and Early Archean mantle depletion. Earth Planet. Sci. 

Goldstein S. L. and Galer S. J. G. (1992) On the trail of early mantle differentiation ratios of early Archean... 

Peck W. H., Valley J. W., Wilde S. A., and Graham C. M. (2001) Oxygen isotope ratios and rare earth elements in 3.3. to 4.4 Ga zircons ion microprobe evidence for high continental crust and oceans in the Early Archean. Geochim Cosmochim. Acta 65, 4215-4229. 

Although there is some debate as to whether initial e d values were as positive as - -3 to -f 4, in the early Earth, it is quite clear that e d values of at least - -2 at 3.8 Ga were prevalent in the Early Archean upper mantle. Confirmation of very positive (e d > 4) values in rocks older than... 

Figure 2 The relationship between /, the chondrite normalized Stn/ Nd, eNj evolution, and time. The curves represent the rate of change of SNd for different / values in the Early Archean mantle. Atime is the difference between the age of the Earth ( 4.5 Ga) and the sample age. The shaded area shows the range of initial cnj compositions that have been proposed for 3.8 Ga mantle based on data from Early Archean gneisses. Depending on the age of the mantle fractionation event, the Early Archean upper mantle may have been equally, or even more TREE depleted than the modern N-MORB source mantle.

Ga would be extremely important, as this would require extreme early differentiation of the Earth s mantle within the first 100-400Myr of Earth history. As shown in Figure 2, to generate SNd of - -2 to - -4 at 3.8 Ga requires ratios in the pre-3.8 Ga upper mantle similar to those in the present-day depleted mantle, yet the modem mantle records the effects of extraction of the whole of the continental cmst. Thus, neodymium isotopic compositions of the early preserved continental cmst provide strong evidence that portions of the Earth s upper mantle in the Early Archean were significantly lithophile-element-depleted requiring very early (>4.0 Ga) differentiation. 

Figure 3 Mantle evolution defined by the initial snf of juvenile granites and their zircon separates. The snf compositions (closed symbols) have been recalculated using the most recently proposed Lu decay constant of Bizzarro et al. (2003). For comparison are shown the initial values (open symbols) calculated using the Blichert-Toft and Albarede (1997) decay constant. Note that the change of decay constant results in an increase in calculated initial SHf compositions of some Early Archean samples. The current database suggests rapid Lu/Hf fractionation in the early Earth and the possibility of episodic changes in mantle evolution similar to that seen in SNd evolution (sources Vervoort and Blichert-Toft, 1999 Amelin et al., 1999, 2000 ...

Figure 4 Initial snf values of >3.9 Ga detrital zircons from the Jack Hills, Western Australia (Amelin et al., 1999) and Early Archean Itsaq complex gneisses (Vervoort and Blichert-Toft, 1999). The same data have been recalculated using two recently proposed revisions to the Lu decay constant the closed symbols use the decay constant proposed by Bizzarro et al. (2003). The open samples are calculated using the 6% lower decay constant proposed by Scherer et al. (2001). The present-day chondrite parameters used are Hf/ Hf = 0.282772 and Lu/ Hf = 0.0332 (Blichert-Toft and Albarede, 1997).

The oldest samples that provide direct constraints on the osmium isotopic composition of the upper mantle are rare spinel peridotites contained within the Early Archean Itsaq gneiss complex of southwest Greenland that are interpreted to be —3.81 Ga abyssal peridotites (Friend et al., 2002). The measured and initial compositions determined from low-Re/Os spinel and olivine mineral separates from these peridotites are the most primitive, in the sense of closest to solar-system initial compositions, Os isotopic compositions yet obtained on any terrestrial material (Bennett et al., 2002). This shows that at least some, if not all, of the Early Archean upper mantle was characterized by chondritic Os/ Os isotopic compositions. Osmium isotopic constraints from this time period (—3.8-3.9 Ga) are of particular interest as they provide a rough constraint on the timing of the addition of the late veneer of... 

Common to all crustal growth models is that simple cmstal extraction using the known amount of Early Archean cmst cannot readily account for the degree of depletion evident in some interpretations of the initial and enr compositions of the Archean mantle (Section 2.13.2). In the next sections, the effects of processes other than progressive growth of the continental cmst from a depleting mantle are considered. 

Collerson K. D., Campbell L. M., Weaver B. L., and Palacz Z. A. (1991) Evidence for extreme mantle fractionation in early Archean ultramafic rocks from northern Labrador. Nature 349, 209-214. 

Horstwood M. S. A., Nesbitt R. W., Noble S. A., and Wilson J. F. (1999) U-Pb zircon evidence fo an extesive early Archean craton in Zimbabwe a reassessment of the timing of craton formation, stabilization, and growth. Geology 27, 707-710. 

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