New Mexico Geological Society Annual Spring Meeting — Abstracts

Establishing methods for determining the eruption ages of young volcanic rocks (< 1 Ma) is critical for understanding basalt petrogenesis in the western U.S. We have measured U and Th isotopes (by TIMS) and U and Th concentrations (by ID-TIMS) in mineral separates from the Blue Water flow ofthe Zuni-Bandera volcanic field of western NM and have calculated an internal isochron age for the Blue Water flow of 79 ka + 20 ka, -15 ka (1σ). Recently, the age of the Blue Water flow has al.so been detennined by 3He surface exposure dating techniques, giving an age of 57 ka±3 ka·(1σ) [Laughlinet al., Geology, v.22, 1994]. While the two ages are . similar within two sigma uncertainties, the U-Th isochron age is older than the ³He age. Because the half-life of ²³⁰Th is of the same order of magnitude as the time scale of magma chamber processes and short relative to the evolution of most volcanoes, the isotopic patterns of a U-Th isOchron can be significantly influenced by a long chamber residence time, or the remixing of crystalline separates back into that magma prior to or during eruption. Analytical uncertainties in the phenocrystic olivine and plagioclase of the Blue Water flow do not allow the resolution necessary to distinguish the effects of such processes on this isochron. It is important to note however, that the slope of this isochron is heavily weighted by the whole rock and groundmass phases which show wide dispersion and have relatively small uncertainties. Therefore it is likely that the detemlined U-Th isochron age closely reflects the eruption age of the Blue Water flow.

In order to better understand the controls on U/Ib fractionation during basalt petrogenesis, we have also measured U, Th, Nd and Sr isotopes and [U], [Th], [Sm] and [Nd] concentrations in a series of young Hawaiian basalts which show a wide range of compositions and thus inferred
melt fractions. Among the historic samples, there are significant correlations between U/Th fractionation and 1) major element chemistry, and 2) Sm/Nd fractionation. These correlations have important implications for basalt petrogenesis as they indicate that both the major-and trace-element chemistry of the Hawaiian basalts can be modeled by small degrees (5% for , tholeiites to 0.1%for pasanites) of Simple equilibrium batch melting in the presence of garnet. Deviations from these correlations are also significant as they can be used to evaluate non-zero
residence times and the ages of non-historic samples.