A basalt has 300 ppm Ni and 500 ppm Cr, while a co-genetic rhyolite has 5 ppm Ni and 10 ppm Cr. What process explains this dramatic difference?
AThe rhyolite source contained less Ni and Cr
BFractional crystallization of olivine (which strongly incorporates Ni) and clinopyroxene/spinel (which incorporate Cr) from the basaltic parent magma removed these compatible elements from the melt, producing a highly depleted residual liquid that eventually became the rhyolite
CWeathering of the rhyolite removed Ni and Cr
DNi and Cr are radioactive and decayed during the rhyolite's longer cooling time
Ni has a very high partition coefficient in olivine (D ~10-30) and Cr is highly compatible in spinel and clinopyroxene. During fractional crystallization, each increment of olivine removal drastically depletes Ni in the remaining melt. After extensive fractionation, evolved melts (andesites, dacites, rhyolites) have extremely low compatible-element concentrations. This depletion pattern is diagnostic of fractionation from a mafic parent.
Question 2 True / False
An incompatible element with a partition coefficient of 0.01 will be enriched by a factor of 100 in a 1% partial melt relative to the source.
TTrue
FFalse
Answer: True
For batch melting at low melt fractions, the concentration in the melt approaches C_source/D (when F << D). With D = 0.01 and F = 0.01, the enrichment factor is C_melt/C_source = 1/(D + F(1-D)) = 1/(0.01 + 0.01*0.99) = ~50. For very small F approaching zero, the enrichment approaches 1/D = 100. This extreme enrichment of highly incompatible elements in small melt fractions is why alkalic basalts (small degree melts) are enriched in incompatible elements relative to tholeiites (larger degree melts).
Question 3 Short Answer
Explain what a primitive-mantle-normalized spider diagram reveals about a subduction zone basalt that a simple major-element analysis cannot.
Think about your answer, then reveal below.
Model answer: A spider diagram plots a suite of trace elements (ordered by incompatibility) normalized to primitive mantle values. Subduction zone basalts show a characteristic pattern: enrichment in large-ion lithophile elements (LILE: Rb, Ba, K, Sr) and light REE, but depletion in high-field-strength elements (HFSE: Nb, Ta, Ti, Zr). The LILE enrichment reflects addition of fluid-mobile elements from the dehydrating subducted slab. The HFSE depletion reflects the retention of these elements in residual rutile and other refractory minerals in the slab. This Nb-Ta 'trough' on the spider diagram is the definitive geochemical fingerprint of subduction-related magmatism, invisible in major-element data.
The spider diagram reveals the selective element transfer from slab to mantle wedge: fluid-mobile elements are transferred (LILE enrichment) while fluid-immobile elements are retained (HFSE depletion).