Crust early earth

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). 

Reductants for chemoautotrophs are generally deep in the Earth s crust. Vent fluids are produced in magma chambers connected to the Atheno-sphere. As such, the supply of vent fluids is virtually unlimited. While the chemical disequili-bria between vent fluids and bulk seawater provides a sufficient thermodynamic gradient to continuously support chemoautotrophic metabolism in the contemporary ocean, in the early Earth the oceans would not have had a sufficiently large thermodynamic energy potential to support a pandemic outbreak of chemoautotrophy. 

The crust, hydrosphere and atmosphere formed mainly by release of materials from within the upper mantle of the early Earth. Today, ocean crust forms at midocean ridges, accompanied by the release of gases and small amounts of water. Similar processes probably accounted for crustal production on the early Earth, forming a shell of rock less than 0.0001% of the volume of the whole planet (Fig. 1.2). The composition of this shell, which makes up the continents and ocean crust, has evolved over time, essentially distilling elements from the mantle by partial melting at about 100 km depth. The average chemical composition of the present crust (Fig. 1.3) shows that oxygen is the most abundant element, combined in various ways with silicon, aluminium (Al) and other elements to form silicate minerals. 

A study of the radiogenic isotope memory of the Earth s mantle clearly shows that the mantle is not an independent part of the Earth system, nor has it been for a long time. But rather, it records a history of the extraction and recycling of both basaltic and continental crust. Because of the relatively slow mixing rates and rates of diffusion, compositional heterogeneities within the mantle produced by these processes may be preserved for >1.0 Ga so that significant parts of the Earth s prehistory can be seen in recent mantle melts. Mantle melts from early Earth history (basalts and komatiites), therefore, have the potential to record very early mantle heterogeneities. 

Alternative views of early Archaean mantle evolution require that mantle depletion started as early as ca. 4.5 Ga (see compilation in Rollinson, 1993). These models imply significant mantle Sm-Nd fractionation in the very early Archaean and have major implications for the differentiation of the early Earth. One such study is that of Bennett et al. (1993) who measured very high eNd values (+3.5 to +4.5) in 3.81 Ga Amitsoq gneiss samples. Collerson et al. (1991) also calculated an isochron eNd value of +3.0 for 3.8 Ga-old peridotites from northern Labrador. The extreme deviation from CHUR early in Earth history (Fig. 3.27) was interpreted by Bennett et al. (1993) as evidence for an extreme and very early fractionation of the Earth s mantle relative to CHUR. Such an event implies the formation of extensive continental crust prior to 3.8 Ga, for which there is no independent geological evidence. This apparent paradox and the claim for very early extensive mantle differentiation led to a detailed reexamination of the Bennett... 

A number of models for the very early Earth propose the existence of a Hadean (ca. 4.5 Ga) basaltic crust. The evidence for such a model comes from the study of 142Nd and is consistent with the observed extreme fractionation of 207Pb (Kamber et al., 2003), and with Hf-isotope studies on zircons (Bizzarro et al., 2003). For this reason it has been proposed that an early, trace element enriched mafic crust, created perhaps as a lid to a Hadean magma ocean, was subducted and buried deep in the mantle, where it is thought to be still stored (Galer Goldstein, 1988, 1991). Hence, this material is a candidate for a lower mantle layer and is another possible explanation for a layered mantle created very early in Earth history. 

A potentially significant reservoir, and one which a number of authors have suggested is important in the context of continent formation, is the subcontinental lithospheric mantle (SCLM). Kramers (1987, 1988), suggested that the TTG magmas of the Archaean crust formed in an open-system magma layer in the early Earth, the cumulates from which are now preserved as the SCLM. More recently Abbott et al. (2000) proposed a model of continental growth founded upon the premise that the continental crust was extracted from the SCLM. 

If the Zahnle and Sleep (2002) model for COa-drawdown is correct, and C02 was principally stored in the oceanic crust during the Archaean, this does not necessarily negate the calculations of Kramers (2002), but it does shift the time of C02-drawdown back to perhaps the mid-Archaean. However, a problem with the Zahnle and Sleep (2002) model is that, unlike the Urey cycle, the Archaean oceanic weathering cycle has no inbuilt temperature feedback and needs a "thermostat" to maintain equable surface temperatures on the early Earth. This problem could be solved (just) if there were very high levels of COa in the atmosphere, but the better solution is to include methane as a significant component of the Archaean atmosphere. 

We also know that the Earth reservoirs have changed in composition over time. Such changes have been documented in this book. See for example - the isotopic evolution of the mantle (Chapter 3, Section 3.2.3), the secular evolution of the continental crust (Chapter 5, Section 5.3), the evolution of the composition of the atmosphere (Chapter 5, Section 5.3) and oceans (Table 5.5). Secular change in the biosphere, a process which we otherwise call evolution, is discussed in Chapter 6. Charting these changes and identifying the precise character of the systems of the early Earth is a task which is well underway. 

Nakamura, K. and Kato, Y., 2004. Carbonatisation of oceanic crust by the seafloor hydrothermal activity and its significance as a C02 sink in the early Earth. Geochim. Cosmochim. Acta, 68, 4595-618. 

Peck, W.H., Valley, J.W., Wilde, S.A., and Graham, C., 2001. Oxygen isotope ratios and rare earth elements in 3.3 to 4.4 Ga zircons ion microprobe evidence for high SlsO continental crust and oceans in the early Earth. Geochim. Cosmochim. Acta, 65, 4215-29. 

---