Core formation accretion decreasing with

Figure 4 Lead isotopic modeling of the composition of the silicate Earth using continuous core formation. The principles behind the modeling are as in Halliday (2000). See text for explanation. The Held for the BSE encompasses all of the estimates in Galer and Goldstein (1996). The values suggested by Kramers and Tolstikhin (1997) and Murphy et al. (2003) also are shown. The mean life (t) is the time required to achieve 63% of the growth of the Earth with exponentially decreasing rates of accretion. The p, values are the 2 U/2°4pb of the BSE. It is assumed that the p of the total Earth is 0.7 (Allegre et ah, 1995a). It can be seen that the lead isotopic composition of the BSE is consistent with protracted accretion over periods of 102-10 yr.

The accretion of a late veneer. Whether or not a late veneer was added to the Earth towards the end of accretion is not clear. The principal evidence comes from the elevated siderophile element chemistry of the mantle. If there was a late veneer, it had to happen after core formation. At present the evidence from the trace element chemistry is ambiguous, because these data can also be explained by the formation of the core at high pressures and temperatures in a magma ocean, or by continuous core formation with decreasing metal input. Currently, the best evidence for a late veneer comes from Os-isotope evidence, where there is a clear mismatch between the composition of the PUM and chondrite. However, even this is uncertain, as is discussed in the next chapter (Chapter 3, Section 3.2.3.4), for... 

Exponentially Decreasing Accretion with Concomitant Core Formation... 

The W isotope evolution of Earth s mantle during protracted accretion with continuous core formation can be calculated using equations 6 and 12 and assuming an accretion rate for the growth of Earth. A reasonable assumption is that the accretion of Earth took place at an exponentially decreasing accretion rate, such that... 

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