An Ar-Ar step-heating experiment on hornblende produces a flat age plateau at 450 Ma across 80% of the total 39Ar released. What does this indicate?
AThe rock crystallized at 450 Ma from magma
BThe hornblende cooled through its closure temperature (~500 C) at 450 Ma and has not been significantly reheated since; the flat plateau indicates a single, undisturbed cooling event
CThe sample is contaminated with excess argon
DThe age has no geological meaning because Ar-Ar only works on volcanic rocks
A well-defined plateau (concordant step ages over a large fraction of gas release) indicates that argon is uniformly distributed in the mineral, consistent with undisturbed radiogenic accumulation since closure. The 450 Ma age records when hornblende cooled below ~500 C -- this could be primary cooling of an igneous rock or cooling after a metamorphic event. The key interpretive point is that Ar-Ar dates cooling, not necessarily crystallization.
Question 2 True / False
The Ar-Ar method dates the same event as U-Pb zircon geochronology.
TTrue
FFalse
Answer: False
U-Pb zircon dates crystallization (zircon has a very high closure temperature, >900 C, and incorporates U during crystal growth). Ar-Ar dates cooling through the analyzed mineral's closure temperature (300-500 C for common minerals). In a slowly cooled pluton, U-Pb might give 100 Ma (crystallization) while Ar-Ar hornblende gives 95 Ma (cooling through 500 C) and Ar-Ar biotite gives 90 Ma (cooling through 300 C). The difference reveals the cooling rate. In rapidly quenched volcanic rocks, both methods give effectively the same age.
Question 3 Short Answer
Explain what excess argon is and how step-heating can detect it.
Think about your answer, then reveal below.
Model answer: Excess argon is 40Ar that was incorporated into the mineral from external sources (magmatic fluids, older material) rather than produced by in-situ 40K decay. It causes anomalously old apparent ages. In step-heating, excess argon typically resides in loosely bound sites or fluid inclusions released at low temperatures, producing anomalously old ages in the first few heating steps. If high-temperature steps (from retentive crystallographic sites) define a plateau while low-temperature steps are discordantly old, excess argon is diagnosed. The plateau age is then interpreted as reliable while the low-T steps are discarded.
Step-heating acts as a quality filter: excess argon in loosely bound sites is released first at low temperature, separated from the radiogenic argon released at high temperature from the crystal lattice.