Radium isotopes, alkaline earth diagenesis, and age determination of travertine from Mammoth Hot Springs, Wyoming, U.S.A.

Abstract

Travertine from active springs, former vents, and drill core from Mammoth Hot Springs, Wyoming, was analyzed for Ra isotopes ( 226Ra, 228Ra), other alkaline earth elements (Mg, Sr, Ba), and mineralogical composition. Thermal water also was analyzed. Travertine, presently being deposited, contains 3.0–15.3 pCi/g 226Ra, and has a 228Ra/ 226Ra ratio identical to that in thermal water. Travertine precipitates mostly as aragonite and experiences a complete diagenetic transformation to calcite within 9 a (estimated from 228Ra/ 226Ra value of youngest completely calcite travertine). Systematic compositional changes associated with this diagenetic transformation are enrichment of Mg and depletion of Sr, Ba and Ra. Apparent mineral-water distribution coefficients for Mg, Sr and Ba in aragonite and calcite are within the range of those determined experimentally, implying near-equilibrium conditions and high water-rock ratios during travertine diagenesis. Impure travertine from near the base of the travertine section in the U.S. Geological Survey Y-10 drill hole (at 72.9 m depth) has a 230Th/ 234U isochron age of 7700± 440a. The content of 226Ra in normal, subhorizontally layered, porous travertine decreases with depth in Y-10. The observed 226 Ra vs depth relation is consistent with continuous deposition of travertine at the Y-10 site from 7700 a BP to near present at a mean rate of ∼1.0 cm/a, and indicates minimal exchange of Ra between travertine and pore water after the early diagenetic transformation of aragonite to calcite. These data imply that, under favorable conditions, 226Ra measurements may be useful in determining ages and deposition rates for other travertines.