Uranium loss and aragonite–calcite age discordance in a calcitized aragonite stalagmite

• Published in: Quaternary geochronology 2012-12, Vol.14, p.26-37
• Main Authors: Lachniet, Matthew S., Bernal, Juan Pablo, Asmerom, Yemane, Polyak, Victor
• Format: Article
• Language: English
• Subjects: Aragonite; Laser ablation; MC-ICPMS; Recrystallization; Speleothem; Uranium series dating
• ISSN: 1871-1014; 1878-0350
• DOI: 10.1016/j.quageo.2012.08.003

We analyzed uranium-series concentrations and isotopic ratios in a mixed aragonite and calcite stalagmite from Juxtlahuaca Cave, from the Sierra Madre del Sur of Mexico. The U-series data for the aragonite layers return highly precise and stratigraphically correct ages over the past ca. 4300 years. In contrast, age determinations from calcite layers are too old by several hundred years relative to the precise aragonite ages, have analytical uncertainties an order of magnitude larger than aragonite ages, and yield ages that do not overlap the aragonite ages within analytical uncertainties. Based on geochemical and petrographic observations, we interpret the calcite layers to have formed from recrystallization of aragonite soon after primary aragonite deposition. Calcite occurs as discontinuous lenses on and off the growth axis, and laminae can be traced between aragonite and calcite layers, demonstrating that visible growth banding is not effaced in the recrystallization process. Paired aragonite-calcite U-series data from coeval stratigraphic layers demonstrate that uranium concentrations decrease by two orders of magnitude during calcitization, and result in decreased (234U/238U). Uranium loss during diagenesis mimics a need for an age correction using an initial 230Th/232Th ratio one to two orders of magnitude larger than the bulk Earth ratio of 4.4 ± 2.2 × 10−6. A need for apparent high initial 230Th/232Th ratios results from ingrowth of 230Th during 234U decay. ► Recrystallization of aragonite to calcite results in uranium loss. ► Calcitization results in calcite ages hundreds of years older than aragonite ages. ► Diagenesis of aragonite to calcite occurred rapidly after initial deposition.