Garnet has a very high partition coefficient for heavy rare earth elements (HREE) like Yb (D ~ 4-7) but low D for light REE like La (D ~ 0.01). How does this affect the REE pattern of a melt produced by partial melting of garnet-bearing mantle?
AThe melt will have a flat REE pattern
BThe melt will be depleted in HREE (retained by garnet in the residue) and enriched in LREE (incompatible in garnet), producing a steep, LREE-enriched pattern diagnostic of deep melting in the garnet stability field
CThe melt will be enriched in HREE from the garnet
DGarnet does not affect REE patterns during melting
During partial melting, HREE are retained by residual garnet (high D means they stay in the solid), while LREE enter the melt (low D). The resulting melt is strongly LREE-enriched and HREE-depleted. This steep REE pattern is diagnostic of melting at depths >60-80 km where garnet is stable. Shallower melting (spinel peridotite, where no mineral strongly fractionates HREE from LREE) produces flatter REE patterns. REE patterns thus constrain the depth of melting.
Question 2 True / False
Partition coefficients are fixed physical constants for each element-mineral pair.
TTrue
FFalse
Answer: False
D values depend on temperature, pressure, melt composition, mineral composition, and crystal chemistry. D for REE in clinopyroxene increases with pressure and decreases with temperature. D values in silica-rich melts differ from those in mafic melts. The lattice strain model (Blundy and Wood) provides a physical framework for predicting how D varies: it depends on the elastic strain energy required to substitute a foreign ion into a crystal site, which varies with the size mismatch between the ion and the site. Published D values must be used with attention to the conditions under which they were determined.
Question 3 Short Answer
Explain the concept of bulk partition coefficient and why it matters more than individual mineral D values for modeling partial melting.
Think about your answer, then reveal below.
Model answer: The bulk D is the sum of each mineral's D weighted by its mass fraction in the source rock: D-bulk = sum(x_i * D_i), where x_i is the weight fraction and D_i is the mineral-melt partition coefficient. It matters more than individual D values because the partial melting equation uses D-bulk as the aggregate control on element behavior. A highly compatible element in one mineral (e.g., Ni in olivine, D=15) may have a moderate D-bulk (~3-4) if olivine is only 25% of the source. The mineralogy of the source (proportion of olivine, pyroxene, garnet, spinel) directly controls D-bulk and therefore the predicted melt composition.
Individual mineral D values determine the partitioning physics; bulk D integrates the mineralogical reality of the source into a single parameter used in melting models.