Basalt early basaltic crust

It was in this earliest Earth System that there was the strongest interaction between the different Earth reservoirs. There were intense, dynamic interactions between core, mantle, proto-ocean, and atmosphere. In addition there was probably an early basaltic crust, now long since lost by recycling into the mantle. 

It is likely that there is no primitive, undifferentiated mantle still preserved within the Earth s mantle. Nd-isotopes indicate that the mantle experienced a major differentiation event, perhaps as early as 30 Ma after the formation of the solar system, in which a Fe-rich basaltic crust formed on a magma ocean and was... 

The recognition of a 142Nd anomaly within the mantle implies that the Earth experienced a major, very early differentiation event. The study by Boyet and Carlson (2005) showed that lunar basalts also have elevated 142Nd/144Nd ratios relative to primitive chondrites, implying that the Moon was formed from an Earth that had already experienced major differentiation. This means that the early differentiation of the Earth took place within 30 Ma of the formation of the solar system. Whilst the precise nature of this differentiation event is not known, a favored model is the formation of an Fe- and trace element-enriched basaltic crust, perhaps as an initial crust to a magma ocean. It is postulated that this crust is now isolated from the convecting mantle and is located deep within the lower mantle. 

The earliest terrestrial 187Os/188Os data come from the early Archaean (3.81 Ga) chromitites of the Itsaq Gneisses in west Greenland (Bennett et al., 2002 Rollinson et al., 2002). These samples show a range of compositions but with a mean value which is slightly supra-chondritic (Frei 8k Jensen, 2003). This could indicate that the enriched Os-isotope reservoir discussed above was in existence even in the very early Archaean and indicates the very early recycling of basaltic crust. 

It is clear that the Earth s mantle has at least two Os-isotopic reservoirs - a plume-related isotopically enriched reservoir and a chondritic upper mantle reservoir. Both have long histories (Fig. 3.32). The variations in composition within the upper mantle reservoir reflect Re-depletion and enrichment related to melt extraction. The isotopically enriched plume reservoir represents chemically isolated, rhenium-enriched, recycled oceanic lithosphere. There is some evidence to suggest that this enriched reservoir may have been in existence since the early Archaean (Walker Nisbet, 2002) and was the source of some Archaean komatiites and the 3.81 Ga Itsaq Gneiss chromitites. If this is true, then basaltic crust was being created and recycled even before 4.0 Ga. Estimates of the present size of this high Re/Os basaltic reservoir vary from 5% to >10% of the whole mantle (Bennett et al., 2002 Walker et al., 2002). 

II- Early Archaean mantle differentiation related to the extraction of basaltic crust... 

The link between the creation of oceanic crust and the formation of continental crust is important and needs to be explored. On the one hand there is Os-isotope evidence to suggest the formation of basaltic crust as early as 4.2 Ga, and on the other hand basaltic crust is not "processed" on a large enough scale to form continental crust until 3.0 Ga. This seems to suggest that at 3.0 Ga the nature of mantle processes changed from "ancient" to modern (Fig. 3.33). This time interval may mark the inception of the subduction process, and even large-scale mantle convection. Prior to this time, it is possible that basaltic crust was returned to the mantle by gravitational processes but in a more localized manner. 

In this present version of the model the D" layer is thought to have originated very early in Earth history, as an early, incompatible element- and metal-rich basaltic crust, enriched during late accretion (4,540-4,000 Ma) with chondritic material. There is support from Nd-and Hf-isotopes for the existence of this very early differentiate of the mantle (see Sections 3.2.3.1 and 3.2.3.2). This crust, when subducted, had a bulk density which exceeded that of the mantle and numerical modeling experiments confirm that it would have stabilized at the core-mantle boundary (Davies, 2006). 

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. 

Table 4.3 below summarizes the different reservoirs thought to be present in the modern silicate Earth. However, of these, only the depleted mantle and buried eclogitic slabs have compositions which make them directly complementary to the composition of the continental crust. It is suggested here that Archaean SCLM is not closely related in a geochemical sense to Archaean felsic crust but is the product of basalt extraction, basalt which now is emplaced within the Archaean crust. In contrast, Phanerozoic SCLM may be the restite-complement of the basaltic precursor to modern crust. The proposal that there was an early-formed (pre 3.7 Ga), enriched, basaltic crust (Section 4.5.1.2.2), if confirmed, has important implications for the balance between the major Earth reservoirs, not least because the primitive mantle and the bulk silicate Earth can no longer be regarded as com-positionally identical. 

As noted earlier, lunar meteorites are mostly breccias of ferroan anorthosite and related early crustal rocks, although a few mare basalt meteorites are known. The lunar meteorites likely sample the whole Moon. The absence of KREEP-rich breccias so common among Apollo samples collected from the nearside in the lunar meteorite collection implies that KREEP-rich rocks cover only a small area on the Moon. In fact, the lunar highlands meteorites appear to provide a closer match to the average lunar crust than do the Apollo highlands samples (Fig. 13.5), as measured by geochemical mapping (see below). 

The compositions of the crusts of the Moon and Mars are distinct - one is dominated by feldspathic cumulates from an early magma ocean, and the other by basaltic lavas. Regional patterns reflect differences in subjacent mantle compositions. The compositions of the mantles and cores of these bodies can be constrained by chemical analyses of mantle-derived basalts. The interiors of both bodies have remained geochemically isolated, because of the absence of plate tectonics. 

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