New Mexico Geological Society Annual Spring Meeting — Abstracts

Manganese oxide minerals (MnOx), specifically cryptomelane [K_1-1.5(Mn⁴⁺,Mn²⁺)O₁₆] but including others, have been dated by the ⁴⁰Ar/³⁹Ar method with variable degrees of success. In most occurrences, MnOx minerals are very fine grained and often banded. Almost all MnOx minerals display significant solid solution behavior and vacancies are common in their structures. In addition, a number of the minerals (esp. cryptomelane) have high and low temperature forms. MnOx minerals are often intimately intermixed from the macro to micron scale within a single sample. MnOx minerals are typically contaminated with other mineral phases, especially from the weathering environment. All of these characteristics lead to the potential for argon/potassium loss, excess argon, recoil, contamination, and low radiogenic yields that affect the final age determination.

The majority of published dating studies utilizing MnOx involved those formed during the supergene (weathering) process. Utilization of manganese minerals of hydrothermal origin has produced more precise results probably because of larger grain sizes and the ability to produce purer mineral separates. Dating of pure hydrothermal cryptomelane separates at the MCA mine in the Luis Lopez mining district resulted in plateau ages of 6.31 ± 0.08 Ma. Additional dating in the Magdalena district manganese deposits has revealed additional complexities with respect to multiple mineral phases and their intergrowths. The dating of supergene MnOx minerals (mixed cryptomelane, todorokite, and an unidentified silicate) from a manganocrete in the Red River drainage produced an age of 64.6 ±1.1 Ma in a debris flow dated at 4220 ± 40 yrs by ¹⁴C analysis of charcoal. The variability of age dating results, even in coarse-grained hydrothermal MnOx minerals, illustrates the importance of sample selection and characterization in any successful dating project.