Questions: Crustal Evolution and Geochemistry

Question 1

Global compilations of Nd model ages show peaks at ~2.7, 1.9, and 1.1 Ga. What do these peaks most likely represent?

• A. Random fluctuations in magmatic activity
• B. Episodes of enhanced continental crust generation from the mantle, potentially associated with supercontinent assembly events when subduction rates and arc magmatism were elevated
• C. Periods when isotopic resetting was most common
• D. Artifacts of uneven sampling of the geological record

Answer: (B)

Nd model ages record when source material was extracted from the depleted mantle. Peaks in the age distribution indicate episodes of enhanced crustal generation. These peaks correlate with known supercontinent assembly events (Kenorland ~2.7 Ga, Columbia/Nuna ~1.9 Ga, Rodinia ~1.1 Ga), when extensive subduction and collision would have generated large volumes of new arc crust. However, preservation bias must be considered: crust generated during supercontinent assembly may be preferentially preserved rather than representing higher production rates.

Question 2

The continental crust has grown continuously and steadily throughout Earth history.

• T. True
• F. False

Answer: False

Multiple lines of evidence -- Nd model ages, detrital zircon age spectra, Sr isotope evolution of seawater -- indicate that crustal growth has been episodic, with major pulses separated by relative quiescence. Whether the peaks represent true production rate variations or preservation bias (crust generated during assembly is less likely to be subducted) remains debated. Additionally, crustal recycling by subduction, delamination, and erosion returns material to the mantle, so net crustal growth is less than gross production.

Question 3

Explain how the average composition of the upper continental crust is estimated using fine-grained sedimentary rocks.

Model answer: Fine-grained sedimentary rocks (shales, loess, glacial tills) naturally average large areas of continental crust through weathering, erosion, transport, and deposition. Shales sample drainage basins spanning thousands of km2, homogenizing the diverse lithologies of the source region into a single average composition. Post-Archean Australian Shale (PAAS) and similar composites provide estimates of average upper crustal composition for REE and other insoluble elements that are not significantly fractionated during weathering and transport. Elements that are mobile during weathering (Na, Ca, Sr) require correction, but immobile elements (REE, Th, Sc, Ti) in shales closely reflect the average exposed crust.

Sedimentary rocks are nature's averaging tool: the very processes that destroy source rock information (weathering, transport, mixing) also produce well-mixed samples representative of large crustal areas.