DIFFUSION OF 4HE IN CALCITE: IMPLICATIONS FOR VERY-LOW-TEMPERATURE THERMOCHRONOLGY

Most thermochronometers involve minerals with high concentrations of either U or K; we have begun a study of the feasibility of the use of the low-U mineral calcite as an alpha thermochronometer. For values of [U], [Th], and [Sm] typical of marine calcite, the production rate of ⁴He is 50 to 500 times lower than production rates typical for apatite and zircon. However, the lower production rate in calcite can be overcome if a sample is sufficiently large or old. For typical extraction systems a sphere of calcite with diameter= 2 mm, [U]=4 ppm will allow an analysis that is only 1% blank for an age of 525,000 years. In many rocks, such as limestones and marbles, finding sufficiently large calcites will not be difficult. Moreover, using only large calcite crystals would obviate the need for correcting (U+Th)/He ages for alpha ejection. Iceland spar cleavage fragments were experimentally saturated with tank helium using a conventional cold-seal pressure vessel of 3/8" bore inserted in a furnace at 725°C and 240 bars for 24.2 days; these conditions are sufficient to have saturated the calcite in He. Subsequent Step heating experiments indicate an activation energy of 30.8 kcal/mole and a closure temperature of ~70°C (dT/dt=10°C/m.y.) for helium in the experimental charge. Analyses of diffusion of He in natural untreated crinoid ossicles from the Thrifty Fm (Penn.) of central Texas and calcite from the Magnet Cove carbonatite (Cret.) in Arkansas yield consistent values for activation energy of ~30.5 kcal/mole with D₀/a² values ranging from ~24,000 to ~270,000 sec^-1; this variation is probably due to microstructural imperfections affecting a, suggesting multiple diffusion domain behavior. By etching samples and examining calcites with scanning-electron and atomic-force microscopy, we have imaged screw dislocations and other defects in some samples. Preliminary observations suggest an inverse correlation between defect density and diffusion domain size. Based on our Iceland spar data, we calculate the partial retention zone for He in calcite to lie between ~70 and 30 °C; ~5-10°C lower than He in apatite. This preliminary investigation of calcite as an alpha thermochronometer suggests that this ubiquitous sedimentary mineral can be a sensitive monitor of passage through the upper 3 km of the crust by either erosion or deposition.