Three zircon analyses from a granite plot below the concordia curve but define a linear discordia line with an upper intercept at 2.7 Ga and a lower intercept at 0.5 Ga. How are these intercepts interpreted?
AThe granite formed at 0.5 Ga and was contaminated at 2.7 Ga
BThe zircons crystallized at 2.7 Ga and experienced partial Pb loss during a thermal event at 0.5 Ga; the upper intercept gives crystallization age and the lower intercept dates the disturbance
CThe zircons are a mixture of 2.7 Ga and 0.5 Ga populations
DThe concordia diagram is unreliable for ages greater than 1 Ga
Undisturbed zircons plot on concordia. Partial Pb loss (during metamorphism, fluid interaction, or radiation damage) moves analyses off concordia toward the origin. A linear discordia array indicates a single episode of Pb loss from initially concordant grains. The upper intercept dates the original crystallization; the lower intercept dates the Pb-loss event. Grains that lost more Pb plot farther down the discordia line.
Question 2 True / False
Zircon is preferred for U-Pb dating primarily because it contains the highest uranium concentrations of any mineral.
TTrue
FFalse
Answer: False
While zircon does contain significant U (typically 100-1000 ppm), the primary reason it is preferred is its near-complete exclusion of initial Pb during crystallization -- meaning essentially all Pb measured is radiogenic (from U decay). This eliminates the need for large and uncertain common-Pb corrections. Additionally, zircon is extremely resistant to physical and chemical weathering, surviving multiple cycles of erosion, transport, and even metamorphism. Other minerals (uraninite, monazite) contain more U but are less robust or less common.
Question 3 Short Answer
Explain the advantage of having two independent U-Pb decay systems in a single mineral grain.
Think about your answer, then reveal below.
Model answer: The two decay chains (238U-206Pb and 235U-207Pb) provide an internal consistency check. In an undisturbed grain, both systems must yield the same age (concordant). If the grain has experienced Pb loss or U gain, the two ages will disagree (discordant), immediately flagging the disturbance. This self-checking property is unique to the U-Pb system and allows detection and even correction of open-system behavior through the concordia-discordia framework. No other radiometric system has this built-in redundancy.
Concordance between two independent chronometers in the same grain provides confidence that no other geochronometer can match -- discordance is its own quality control.