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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
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:
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.
- 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.
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
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
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