Primitive mantle ratios

Figure 42 Covariation of Lu/Hf versus Sm/Nd for minerals from mantle peridotite xenoliths. Lines marked are the primitive mantle ratios.

Several lines of evidence indicate problems with strict application of the layered model. It is particularly difficult to satisfy the requirement for primitive composition of the lower mantle. For example, Zindler and Hart (1986) showed that the isotopic database for the oceanic mantle could be described by mixing between depleted MORE mantle and enriched components that have compositions quite unlike primitive mantle. Similarly, the constancy of Nb/U and Ce/Pb (Hofmann, 1986 Newsom et al., 1986) suggests that there is no current reservoir with primitive mantle ratios. Interestingly, there is a strong correlation between a depleted component, termed FOZO by Hart et al. (1992), and the high He source observed in many hot spots (Hart et al., 1992 Farley et al., 1992 Hanan and Graham, 1996). 

Major-element compositions (weight ratios of Mg/Si and Al/Si) for mantle rocks (peridotites) and estimates of the primitive mantle composition of the Earth compared with various groups of chondrites and the Sun. No mixture of chondrite types provides an exact match to the primitive mantle composition, although some carbonaceous chondrites provide the closest match. Modified from Righter et al. (2006). 

More controversial (although sometimes cited as proven fact) have been claims (e.g., Taylor and Jakes, 1974 Taylor, 1982) that the bulk Moon is enriched roughly twofold in the cosmochemically refractory lithophile elements (a class that includes the REEs, the heat sources thorium and uranium, and the major elements aluminum, calcium, and titanium), and that compared to Earth s primitive mantle, the Moon s silicate mg ratio is much lower, i.e., its EeO concentration is much higher. Neither of these claims has been confirmed by recent lunar science developments, which include the advent of global thorium and samarium maps (Lawrence et al., 2002a Prettyman et al., 2002), data from lunar meteorites, and some radically changed interpretations of the Apollo seismic database. 

The two elements calcium and aluminum are RLEs. The assumption is usually made that aU RLEs are present in the primitive mantle of the Earth in chondritic proportions. Chondritic (undifferentiated) meteorites show significant variations in the absolute abundances of refractory elements but have, with few exceptions discussed below, the same relative abundances of lithophile and siderophile refractory elements. By analogy, the Earth s mantle abundances of refractory lithophile elements are assumed to occur in chondritic relative proportions in the primitive mantle, which is thus characterized by a single RLE/Mg ratio. This ratio is often normalized to the Cl-chondrite ratio and the resulting ratio, written as (RLE/Mg)N, is a measure of the concentration level of the refractory component in the Earth. A single factor of (RLE/Mg) valid for all RLEs is a basic assumption in this procedure and will be calculated from mass balance considerations. 

The late veneer hypothesis has gained additional support from the analyses of the osmium isotopic composition of mantle rocks. Meisel et al. (1996) determined the Os/ Os ratios of a suite of mantle xenoliths. Since rhenium is more incompatible during mantle partial melting than osmium, the Re/Os ratio in the mantle residue is lower and in the melt higher than the PM ratio. By extrapolating observed trends of Os/ Os versus AI2O3 and lutetium, two proxies for rhenium, Meisel et al. (1996) determined a Os/ Os ratio of 0.1296 0.0008 for the primitive mantle. This ratio is 2.7% above that of carbonaceous... 

Figure 10 La/Sm ratios in MORE (not smoothed), normalized to primitive-mantle values, as a function of latitude...

Figure 23 High field strength element (HFSE) variations in mantle xenolith minerals. Primitive mantle values plotted are from McDonough and Sun (1995). Dashed lines extend from PM values at constant ratios, e.g., at Nb/Ta = 17.6, Zr/Hf = 37. Solid lines are constant elemental ratios (1-100 labeled) (sources Moore et ah, 1992 Witt-Eickschen and Harte, 1994 Vannucci etal, 1995 Ionov and Hoffman, 1995 Johnson et a/., 1996 Chazot era/., 1996 Vaselli et al., 1996 Ionov et ah, 1997 Shimizu et al, 1997 Norman, 1998 Glaser et ah, 1999 Pearson and Nowell, 2002 Gregoire et al., 2002 Kalfoun et al., 2002 Table 9).

Nb/U ratios do not show a clear pattern of temporal evolution with, for example. Late Archean basalts having a range of ratios from primitive mantle (—30) to modem basalt values... 

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