Crust primordial

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

We can now write explicitly (t) in terms of the geochemical variable yc(t, t) through equation (7.2.38). Again, the condition that there is no primordial crust at t=0 requires that yc(t, t) and ym(t) are equal and therefore the constant is zero. Rearranging, we get... 

While it is relatively rare in the earth s crust, Mo is the most abundant transition metal in seawater. When we consider that the oceans are the closest we get today to the primordial soup in which life first arose, it is not surprising that Mo has been widely incorporated into biological systems. Indeed, the only organisms that do not require Mo, use W, which lies immediately below Mo in the Periodic Table, instead. The biological versatility of Mo and... 

As is true today, most phosphate in the primordial crust must have been sequestered in nearly insoluble calcium phosphates and carbonates or in basalts, and only dissolved monomeric phosphate was produced by weathering. [201] However, the volatile polyphosphate P4O10 is known to be a component of volcanic gases. [205] This material originates from the polymerization of phosphate minerals in mag-... 

Figure 11.23 shows the isochron obtained by Marshall and De Paolo (1982) for the granite batholith of Pikes Peak (Colorado). The effectiveness of the double-spike technique is evident, especially when we see that aliquot-spiked samples do not fall on the best-fit interpolant (York s algorithm York, 1969). The obtained age (1041 32 Ma) is consistent with that previously obtained with Rb-Sr whole rock analyses (1008 13 Ma see Marshall and De Paolo, 1982, for references). The initial ratio ( Ca/ Ca)o of 151.0 is identical, within the range of uncertainty, to upper mantle values, indicating negligible contamination by old crust components the relative K/Ca abundance in the earth s mantle is about 0.01, a value too low to alter the primordial (" Ca/" Ca)o composition. 

Garrels and Mackenzie (1974) report that as the crust solidified and magmatic minerals crystallized, volatile gases were liberated, forming the atmosphere. Below 100°C all the H2O would have condensed and all acid gases would have reacted with the primordial crust. In the first version, at 600°C the atmosphere would have had a Ph o 300 atm (it corresponds to the mass of water on the present oceans), Pqq 45 atm (it corresponds to... 

Assuming that the sedimentary rocks were derived from rocks with an original value of +1.0%c (original basalts, meteorites), and that the total amount of oxygen in the ocean is 1.2 10 " g and in sedimentary rocks 0.9 10 " g, the value found for the of the water of the primordial ocean is + 12.0%o (for the water of the present ocean, was taken as 0.0%o — SMOW standard). If granitized rocks are taken into account, then the of the ancient hydrosphere increases to + 16.2%o. These calculations cannot be considered reliable, as Silverman himself remarks, inasmuch as the composition and amount of primary rocks of the Earth s crust remain uncertain nor was juvenile water introduced from the mantle taken into account. 

With the exception of Davies, who favored whole-mantle convection all along, the above authors concluded that it was only the upper mantle above the 660 km seismic discontinuity that was needed to balance the continental crust. The corollary conclusion was that the deeper mantle must be in an essentially primitive, nearly undepleted state, and consequently convection in the mantle had to occur in two layers with only little exchange between these layers. These conclusions were strongly reinforced by noble gas data, especially He/ He ratios and, more recently, neon isotope data. These indicated that hotspots such as Hawaii are derived from a deep-mantle source with a more primordial, high He/" He ratio, whereas MORBs are derived from a more degassed, upper-mantle reservoir with lower He/ He ratios. The noble-gas aspects are treated in Chapter 2.06. In the present context, two points must be mentioned. Essentially all quantitative evolution models dealing with the noble gas evidence concluded that, although plumes carry... 

Crystallization of primordial crust episodic magma oceans( ) owing to large impacts... 

FIGURE 3.30 The Kramers and Tolstikhin (1997) Pb-isotopic evolution curve for the Earth s mantle, given an initial /x(1 8U/204Pb ratio) of 11.2, relative to the field of initial Pb-isotope compositions, the MORB field, the 4.55 Ga Geochron and the primordial Pb-isotopic composition. The numbered marks on the mantle evolution curve indicate Ga before the present. The cross marks the composition of the average continental crust. 

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