HELIUM DIFFUSION AND LOW-TEMPERATURE THERMOCHRONOMETRY OF APATITE

To investigate the potential of the (U-Th)/He system for low-temperatu re thermochronometry, we have studied helium diffusion and have measur ed helium ages on Durango fluorapatite and on apatites from a gabbro a nd two tonalites from the Peninsular Ranges Batholith. Diffusivity at moderate to very low temperatures (as low as 80 degrees C) was measure d to high analytical precision using long duration incremental outgass ing experiments. All four apatites displayed remarkably similar helium diffusion behavior. Helium loss apparently occurs via volume diffusio n from subgrain domains (<60 mu m) which are nearly identical in size in all samples. At temperatures below 290 degrees C, diffusivity obeys a highly linear Arrhenius relationship with an implied activation ene rgy of about 36 kcal/mol. Above this temperature, diffusivity deviates from Linearity toward lower activation energies. This transition does not arise from multiple diffusion domains, but rather from a reversib le change in the physical mechanism of helium diffusion. For thermochr onometric purposes the high-temperature diffusion behavior is largely irrelevant because essentially no helium is retained over geologic tim e at temperatures above 290 degrees C. Using the results from the low- temperature regime, all samples yield helium closure temperatures in t he range 75 +/- 7 degrees C. This value is independent of chemical com position and grain size of the apatites, suggesting that a single clos ure temperature may apply to a wide range of samples. The (U-Th)/He ag es of these apatites (17-120 Ma) range from a small fraction to nearly 100% of the crystallization age of their host rocks, and are consiste nt with a low-temperature thermochronometric interpretation. These res ults strongly support previous suggestions that (U-Th)/He dating of ap atite can provide high precision chronometry of very low temperature g eological events.