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Presnall (2008)

NoRidgePlumes.html

sampled over a distance of ~5,300 km, that neither Na8 nor Fe8 correlate with ridge depth. Thus, the

LKP modeling fails. However, the combined Red Sea and Carlsberg data show an inverse correlation

in which the Red Sea data are consistently displaced to higher Fe8 and lower Na8 values than the

Carlsberg Ridge values (Figure 1b). This is consistent with mantle heterogeneity in which the Red

Sea mantle is enriched in FeO/MgO and depleted in Na

O relative to the mantle beneath the

Carlsberg Ridge. Melt extraction over a large and varying P-T range from a chemically homogeneous

mantle is replaced by melt extraction at relatively constant P-T conditions from a heterogeneous

mantle.

Figure 1. Na8-Fe8-depth data for Red Sea Rift and Carlsberg Ridge (from Presnall &

Gudfinnsson, 2007).

Presnall & Gudfinnsson (2007) proposed that the very narrow range of P-T conditions for MORB

extraction is imposed by the maximum temperature for explosive escape of CO

from the mantle. This

maximum temperature is caused by the abrupt temperature drop of the solidus curve for carbonated

lherzolite (Wyllie & Huang, 1975; Eggler, 1976; Falloon & Green, 1989; Presnall & Gudfinnsson, 2005)

at about 1.8-1.9 GPa (Figure 2). These P-T conditions are the same as the narrow P-T range for

MORB extraction indicated by the CMASNF solidus phase relations at the plagioclase/spinel lherzolite

transition (Presnall et al., 2002; Presnall & Gudfinnsson, 2007).

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No Plumes Along Mid-Ocean Ridges Page 2