Questions: Thermochronology and Crustal Cooling Ages

Question 1

A granite sample gives an apatite (U–Th)/He age of 8 Ma. A colleague reports this as 'the age when the granite formed.' What is the most fundamental error in this statement?

• A. Apatite (U–Th)/He cannot be applied to granites — it only works on sedimentary rocks
• B. The age records when the granite cooled below the apatite closure temperature (~75°C), not when it crystallized. The granite may have formed hundreds of millions of years earlier and only recently been exhumed near the surface
• C. The error is the assumed closure temperature — apatite actually closes at ~350°C, not ~75°C
• D. 8 Ma is too young for a granite, since granites require at least 100 Ma to cool from magma

Answer: (B)

Thermochronology measures cooling ages, not formation ages. At temperatures above the closure temperature (~75°C for apatite (U–Th)/He), helium diffuses out of apatite grains as fast as it is produced — no clock accumulates. The clock only starts when the rock cools below that threshold. An 8 Ma apatite age means the rock passed through ~75°C at 8 Ma; it may have crystallized at 500 Ma and spent hundreds of millions of years at depth. Confusing a cooling age with a formation age is the most important conceptual error in thermochronology.

Question 2

A geologist has muscovite ⁴⁰Ar/³⁹Ar (closes ~350°C), biotite ⁴⁰Ar/³⁹Ar (~300°C), and apatite (U–Th)/He (~75°C) ages of 60, 55, and 5 Ma for the same rock. What is the most geologically significant implication?

• A. The three systems disagree, indicating the rock experienced three separate formation events at 60, 55, and 5 Ma
• B. The rock cooled relatively slowly from 350°C to 300°C between 60–55 Ma, then stagnated tectonically until rapid exhumation brought it through 75°C at 5 Ma — a two-phase cooling history
• C. All ages should be averaged (40 Ma) to determine the true crystallization age
• D. The apatite age is most reliable because it has the lowest closure temperature

Answer: (B)

Each age marks when the rock crossed a specific temperature threshold. Plotting these: the rock was at ~350°C at 60 Ma and ~300°C at 55 Ma — slow cooling of ~10°C/Ma. Then from 55 Ma to 5 Ma (50 Myr), the rock had not yet cooled to ~75°C — suggesting very slow exhumation or thermal stagnation at depth. Then at 5 Ma, rapid exhumation brought it through 75°C. This reveals a history of early tectonic activity followed by quiescence, then a recent acceleration — information no single geochronometer could provide.

Question 3

A thermochronological age records when the mineral originally crystallized from melt.

• T. True
• F. False

Answer: False

A cooling age records when the rock cooled below the closure temperature of the specific mineral-isotope system — NOT when it formed. Above the closure temperature, daughter isotopes diffuse out as fast as they are produced and no radiometric clock accumulates. A biotite ⁴⁰Ar/³⁹Ar age of 30 Ma means biotite cooled through ~300°C at 30 Ma; it may have crystallized during a metamorphic event at 200 Ma. The crystallization age would require a system with a much higher closure temperature, or a different geochronological method entirely.

Question 4

Applying multiple thermochronological systems with different closure temperatures to the same rock enables reconstruction of a temperature–time cooling path, not just a single date.

• T. True
• F. False

Answer: True

Each system acts as a geothermometer at a specific threshold. Muscovite closes at ~350°C, biotite at ~300°C, zircon fission-track at ~240°C, and apatite (U–Th)/He at ~75°C. Multiple ages from the same rock define multiple points on a temperature–time curve. The slope between those points is the cooling rate, which can be converted to exhumation rate using the geothermal gradient. This multi-system approach reveals the history of how fast the rock moved toward the surface — far more informative than any single date.

Question 5

What is the closure temperature, and why does it matter that different mineral-isotope pairs have different closure temperatures?

Model answer: The closure temperature is the temperature below which a mineral effectively stops losing radiogenic daughter products by diffusion — below this point, daughters accumulate and the radiometric clock runs. Each mineral-isotope system has a different closure temperature because diffusion rates depend on the crystal structure and the size of the diffusing species (muscovite ~350°C, biotite ~300°C, apatite (U–Th)/He ~75°C). Having multiple systems at different temperatures means you can sample the rock's cooling history at multiple points and reconstruct a temperature–time path, revealing exhumation rates and tectonic events across different depths and times.

If all systems had the same closure temperature, you would get one date — when the rock crossed that single threshold. Multiple closure temperatures act like thermometers positioned at different depths in the crust: each one 'turns on' as the rock cools through it, leaving a dated record. The time spacing between records constrains the cooling rate, and with an assumed geothermal gradient, that translates to an exhumation rate — how fast erosion or faulting brought the rock toward the surface.