LIMITATIONS OF THE U-PB METHOD WHEN DATING URANIFEROUS OPALS DEPOSITED IN OPEN CAVITIES: IMPLICATION FOR THE THERMOCHRONOLOGY OF YUCCA MOUNTAIN

Lead contained in secondary minerals deposited in open cavities comes from three "sources": (a) allogenic Pb dissolved in the mineral forming fluid; (b) Pb formed via in situ decay of U contained in the mineral; and (c) lead accumulated in water filling cavities due to the decay of U residing in the surrounding rock.

The decay chains of U contain Rn. Being a gas, the latter readily diffuses into the cavities, where after further decays it finally yields stable Pb. Mathematical modeling shows that concentrations of this Rn-derived radiogenic Pb in cavities will be substantially higher than in thin fractures or pores (<0.1 cm). Minerals depositing in such cavities may be enriched with radiogenic Pb, not accounted for by the “common Pb” correction.

The developed mathematical model describes: diffusion of Rn and the Rn-derived Pb in cavities filled with stagnant and flowing water; diffusion fluxes of Pb isotopes and adsorption of Pb on the surface of growing minerals; adsorption of Pb on the micelles of silica; and coagulation and sedimentation of micelles.

Diffusion-flux enrichment. Characteristic diffusion fluxes of Pb on the surface of growing opal (assuming typical for Yucca Mountain rhyolite [U]=5 ppm), could produce amounts of radiogenic Pb comparable to those measured in opals from Yucca Mountain over a period of time of ~1 Ma.

Colloid enrichment. Uraniferous opals (e.g., Yucca Mountain opals with [U]=200+ ppm) are thought to be formed from colloidal solutions. In such solutions the accumulation of the Rn-derived Pb occurs on the micelles, which possess a very large surface area. Upon coagulation and sedimentation, the micelles become incorporated in the opal. Concentrations [206Pb]=n·100 ppb and [207Pb]=n·10 ppb (typical of the Yucca Mountain opals) may be acquired by micelles over a period of several days. Controlled by the velocity of water exchange in the cavities, the absolute quantities of Pb contained in the Yucca Mountain opals could accumulate within n·100 to n·1000 years. For a typical opal with [U]=n·100 ppm this would translate into apparent (erroneous) U/Pb ages of several Ma.

The processes discussed above represent fundamental constraint of the U-Pb technique, making it unsuitable for dating geologically young materials formed in open cavities from colloidal solutions.