Apparent He ages from natural calcite samples have been analyzed and compared to laboratory He diffusion data, evaluating the diffusion kinetics of the He measured in the laboratory to the lower temperatures and longer timescales relevant in nature. Coarse-grained secondary vein and pore filling calcites ranging in weight from 3 to 118 mg were collected from samples buried to varying depths (0.7 to 4.1 km) from 5 drill holes penetrating the Ellenburger Group (Ordovician) of Central and West Texas, and analyzed for He, U, Th, and Sm. From the Central Carbonate platform of West Texas, analyses of samples collected at depths of 1.3 to 1.6 km record apparent alpha ages of 78 to 111 Ma, consistent with measured diffusivity data for He in calcite and burial history of the Ellenburger of Kupecz et al. (1992), who suggested the Ellenburger has been at it's current depth since before the Cretaceous. In contrast, samples from the Llano uplift area of Central Texas, collected at depths of 0.7 to 0.9 km, record ages (5-14 Ma) which suggest these samples have been held at their current depths for no more than 25 m.y. These young apparent He ages suggest that the calcites analyzed from the Llano uplift area are the result of (1) precipitation during a Tertiary diagenetic event or (2) faulting, bringing samples from higher temperatures up to their current temperatures only recently. The deepest samples analyzed for this study (2.9 to 4.1 km), collected from the Midland and Val Verde Basins of West Texas, record ages that are too-old for samples which may have resided at these depths for even a short period of time; indeed the apparent age of this material is greater than the age of the host rock. These old apparent He ages are here interpreted to be a reflection of low concentrations of U ( ∼0.15 ppm), which has not allowed the production of sufficient radiogenic He to overwhelm the common He present within these samples.
展开▼
