Lecture 18: Chemical Geodynamics, or Mantle Blobology. Questions What can geochemistry tell us about the deep interior of the Earth? Is the mantle homogeneous and if not how many reservoirs are there? How long have they maintained their separate identities?
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Condensation and Accretion
(siderophile & chalcophile)
(melting; gravity and geochemical affinity)
(lost due to impacts)
(plate tectonics: partial melting, recycling)
Modern Ocean &
“Whole mantle convection model”
“Strongly Layered model”
“Lava lamp model”
MORB = mid-ocean ridge basalt
OIB = ocean island basalt
the “mantle array”
Hofmann and White model
So how many components do you need?
For Sr-Nd-Pb-Pb-Pb space, at least four:
If 206Pb, 207Pb, 208Pb are not really independent, then four end members to span data in 3-space (Sr-Nd-Pb) is trivial, but the same components also bound data in Hf and Os space.
How do the four bounding components mix with one another?
Is there evidence of an “internal” component, that everything mixes towards? If so, what is it?
Some authors see mixing towards particular locations, and argue that these represent common components with well-defined compositions: FOZO, C, PREMA
More on this when we talk about noble gas systems.
Getting back to geodynamics...
Nb/U and Ce/Pb in oceanic basalts do not correlate with [Nb] and [Ce]. This implies (1) that the ratio in basalt does not depend on extent of melting, and (2) that depleted and enriched sources are equal also, so the ratio in the residue does not get fractionated. Hence either the elements have
equal partition coefficients or are both incompatible enough to be totally extracted.
But the ratio in MORB and OIB is not chondritic! The continent-forming process did fractionate these element pairs (probably because arc processes involve oxidizing fluids), and crust and mantle are complementary reservoirs. But…OIB do not mix towards primitive value, so there is no evidence here of a primitive reservoir sampled by any basaltic magma!
Isotopic composition of mid-Atlantic ridge samples near the Azores hotspot…
Begs questions: How well mixed is DMM reservoir? Is even “pure DMM” recharged with a flux from somewhere? How does the upper mantle stay fertile over time?