Hawaiian basalts have higher 206Pb/204Pb ratios than mid-ocean ridge basalts. What does this indicate about the Hawaiian mantle source?
AHawaiian basalts have been contaminated by seawater Pb
BThe Hawaiian source has evolved with a higher time-integrated U/Pb ratio than the depleted MORB mantle, consistent with a contribution from recycled oceanic crust that concentrated U relative to Pb during seafloor alteration and subduction
CPb decays faster beneath Hawaii due to higher mantle temperatures
DHawaii is older than mid-ocean ridges
Higher 206Pb/204Pb requires higher time-integrated 238U/204Pb (mu). Recycled oceanic crust acquires elevated U/Pb during seafloor hydrothermal alteration (U is added from seawater; Pb is removed by sulfide precipitation). After subduction and long residence in the mantle, this elevated U/Pb evolves to high 206Pb/204Pb, which is then sampled by the Hawaiian hotspot. This is the basis of the HIMU mantle component.
Question 2 True / False
The depleted mantle is complementary to the continental crust, meaning their isotopic and trace element compositions are mirror images.
TTrue
FFalse
Answer: True
Continental crust was extracted from the mantle, preferentially removing incompatible elements (Rb, Ba, Th, U, LREE). The residual depleted mantle is the complementary reservoir with low incompatible-element concentrations, high Sm/Nd (evolving to high epsilon-Nd), and low Rb/Sr (evolving to low 87Sr/86Sr). Mass balance requires that the depleted mantle plus continental crust approximate the bulk silicate Earth (primitive mantle) composition. This complementary relationship is the cornerstone of crust-mantle geochemistry.
Question 3 Short Answer
Explain why MORB has remarkably uniform isotopic composition globally compared to the extreme isotopic diversity of OIB.
Think about your answer, then reveal below.
Model answer: MORB is produced by large-degree melting (~10-20%) beneath spreading ridges, which effectively averages out small-scale heterogeneities in the shallow, convectively stirred upper mantle. The depleted MORB mantle (DMM) is relatively well-mixed by plate-scale convection. OIB is produced by smaller-degree melting of plume sources that may tap compositionally distinct domains in the deeper mantle -- recycled oceanic crust, subducted sediment, recycled lithosphere -- that have been isolated from the convecting upper mantle for billions of years. These deep sources preserve their isotopic diversity because they reside in less vigorously mixed regions of the mantle.
Homogeneity reflects mixing efficiency: the shallow convecting mantle is well-mixed (MORB uniformity), while the deep mantle preserves ancient compositional heterogeneities (OIB diversity).