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Subject: Rb/Sr ratios in MORB and Hot Spot lavas

This question may be very simple; I haven't had an isotope class before.... In looking at the Th vs. Sr plots of MORB and hotspot basalts, I wondered why hotspot lavas are enriched in (87/86)Sr relative to MORB. Does this charac- teristic tell in any detail about hotspots' source depth? I know they originate at much greater depth, but can it yet be determined how deep? What kinds of xenoliths come up down there?

In the simplest terms, hotspot lavas have higher (87/86)Sr relative to MORB because they have had a higher 87Rb/86Sr ratio in their mantle source. 87Rb decays to produce 87Sr, whereas 86Sr is non-radiogenic. In all materials except those without ANY Rb at all, the (87/86)Sr ratio has been increasing with time since the formation of the earth, due to the decay of 87Rb. Materials with high Rb/Sr ratios will see a faster increase in their (87/86)Sr than materials with low Rb/Sr.
Mathematically speaking, there are two end-member scenarios (and then all possibilities in between) for having two materials on the Earth with different (87/86)Sr, provided both materials started with the same initial (87/86)Sr prior to additional production of 87Sr from 87Rb decay:

Same Rb/Sr ratio, but the material with higher (87/86)Sr is older.
Different Rb/Sr ratios but the same Age. (the one with higher (87/86)Sr came from the one with higher Rb/Sr.

In a very general sense, MORB has lower (87/86)Sr because it is from a "depleted" resevoir of material in the mantle. It is believed that this mantle produced other lavas in the geologic past, and in so doing had its Rb/Sr ratio lowered. The mantle then evolved its (87/86)Sr more slowly than it would have since then, due to there being less 87Rb left in it. Hot spots (at least some of them) are believed to have a component of "primitive", or less "processesed" mantle in their sources, so that they have a relatively higher Rb/Sr and (87/86)Sr than MORB.
This explanation is a gross simplification of the real explanation to your question, but to go into more detail would require a VERY lengthy letter. You see, the mantle is likely made up of multiple domains of varying ages and compositions, as they have experienced different things in the 4.55 billion years of earth history. This causes lots of heterogeneity in Rb/Sr ratios, as well as in other tracers such as Th/U, U/Pb, and Sm/Nd, to be found in mantle-derived lavas.
As far as the depth of the mantle source for hot spots goes, depth per se and chemical/isotopic composition are not necessarily related. It is generally (but not universally) accepted by geologists that much of the upper mantle is more depleted than the lower mantle. Hence, the supposition that hot spot lavas come from deeper regions of the Earth. In reality, the material may reside deeper, but initiate melting at the same depth as MORB. This would mean that the less depleted mantle would somehow have to be transported (by convection) to the upper reaches of the mantle BEFORE melting. The depth at which melting initiated can be modelled because certain mineralogical changes occur at various depths in the mantle, and their pressence or absence in the mantle source will have major effects for the chemical (but not isotopic) composition of the resulting lavas. There is little firm evidence that ALL hot spot melting initiates deeper than ALL MORB melting.
Xenoliths in MORB are rare, but occur in some hot spot localities. Only xenoliths of mantle rock are useful in the context of your question. They usualy occur in lavas of alkalic composition. Both deep (garnet bearing) and shallow (no garnet) xenoliths are common in hot spot localities, but it is rare to find any from depths below about 75 km.
One other thing to remember, "MORB" and "hot spot" are generic terms. Each locality erupting these lavas will have its own flavor of the two. Some places (such as Iceland) are ridge-centered hotspots, so the lavas erupted there can be classified as both MORB and hot spot lavas.

Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822