Mantle depletion

Poreda RJ, Craig H (1992) He and Sr isotopes in the Lau Basin mantle Depleted and primitive mantle components. Earth Planet Sci Lett 113 487-493... 

Note that this is similar to the approach adopted earlier with Sm- " Nd upon which many ideas of Hadean mantle depletion, melting processes and early cmst were based (Chase and Patchett, 1988 Galer and Goldstein, 1991). However, the difficulty with insuring closed-system behavior with bulk rock Sm-Nd in metamorphic rocks and achieving a robust age correction of long-lived " Sm over four billion years has meant that this approach is now viewed as suspect (Nagler and Kramers, 1998). The isotopic system... 

With these caveats, one can deduce the following. Early single grains appear to have recorded hafnium isotopic compositions that provide evidence for chondritic or enriched reservoirs. There is no evidence of depleted reservoirs in the earliest (Hadean) zircons dated thus far (Amelin et al., 1999). Use of alternative values for the decay constants or values for the primitive mantle parameters increases the proportion of hafnium with an enriched signature (Amelin et al., 2000), but does not provide evidence for early mantle depletion events. Therefore, there is little doubt that the Hadean mantle was extremely well mixed. Why this should be is unclear, but it probably relates in some way to the lack of preserved continental material from prior to 4.0 Ga. 

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

Kamber B. S. and Collerson K. D. (2000) Role of hidden deeply subducted slabs in mantle depletion. Chem. Geol. 166, 241-254. 

The large potential of the Lu-Hf system for resolving questions of the amount of mantle depletion in the early Earth will remain unfulfilled until both the discrepancies in the Lu decay constant as determined from terrestrial (e.g. Scherer et al., 2001) and meteoritic materials... 

Figure 6 Effect of basalt storage on the isotopic composition of the upper mantle (representing 50% of total mantle, by mass) as a function of reservoir mass, age, and Os depletion of the upper mantle. The results are presented in terms of mass of stored mafic cmst, as a percentage of total mass of the mantle. Mass balance calculations assume that the missing Re, complementing depletion in the upper mantle, resides in subducted basaltic cmst existing as a stored reservoir within the mantle. (Basaltic cmst assumed to have 0.900 ng g Re and 0.005 ng Os upper mantle has 0.280 ng g Re and 3.1 ng g Os). Depletion of Os is with reference to primitive upper mantle composition of Meisel et al. (2001) of Os/ Os = 0.1296 0.0010. The upper curve assumes that the entire upper mantle is depleted to the extent of average abyssal peridotites, with Os/ Os = 0.1247 (Snow and Reisberg, 1995). The other curves are for more moderate amounts of mantle depletion. PUM indicates primitive upper mantle estimates, (after Walker et al., 2002 Bennett et al., 2002).

Although not strictly related to mantle depletion and crust formation, Hf- W isotopic compositions do provide clear evidence for early planetary differentiation of the Earth, Moon, and Mars related to core formation. The results (Kleine et al., 2002 Yin et al., 2002 Schoenberg et al., 2002) from this short half-life t — 8 Myr) system provide convincing evidence that metal... 

Regardless of the role of basaltic cmst in creating early mantle depletions, there is increasing chemical and seismic evidence for the presence of long-lived chemical reservoirs in the deep mantle, perhaps at the D" layer at the base of the mantle. Some of the strongest chemical evidence includes the range of trace element and... 

Although not strictly related to mantle depletion and crust formation, isotopic com-... 

Beyond the broad major-element constraints afforded by seismic imaging, the abundance of many trace elements in the mantle clearly records the extraction of core (Chapters 2.01 and 2.15) and continental crust (Chapter 2.03). Estimates of the bulk composition of continental cmst (Volume 3) show it to be tremendously enriched compared to any estimate of the bulk Earth in certain elements that are incompatible in the minerals that make up the mantle. Because the crust contains more than its share of these elements, there must be complementary regions in the mantle depleted of these elements—and there are. The most voluminous magmatic system on Earth, the mid-ocean ridges, almost invariably erupt basalts that are depleted in the elements that are enriched in the continental crust (Chapter 2.03). Many attempts have been made to calculate the amount of mantle depleted by continent formation, but the result depends on which group of elements is used and the assumed composition of both the crust and the depleted mantle. If one uses the more enriched estimates of bulk-continent composition, the less depleted estimates for average depleted mantle, and the most incompatible elements, then the mass-balance calculations allow the whole mantle to have been depleted by continent formation. If one uses elements that are not so severely enriched in the continental cmst, for example, samarium and neodymium, then smaller volumes of depleted mantle are required in order to satisfy simultaneously the abundance of these elements in the continental cmst and the quite significant fractionation of these elements in the depleted mantle as indicated by neodymium isotope systematics. 

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