Two eruptive episodes of Los Lunas Volcano: geochemistry and 40Ar/39Ar age determination using electron microprobe sample evaluation

Abstract

A study of 86 samples of trachyandesitic and dacitic lava and cinders from the Los Lunas volcano, central New Mexico, demonstrates the volcano consists of two overlapping but temporally and compositionally distinct eruptive edifices differing in age by 2.56 Ma. Lava compositions at Los Lunas volcano are consistent with other lavas found in the Rio Grande rift, but, based on major and trace element determinations, no simple model can be proposed to genetically link the two dominant magma compositions. Rather, the two magma batches appear to have been independently generated, consistent with the large age gap between eruptions of the two edifices. An experiment carried out as part of this study indicates that the accuracy and precision of 40Ar/39Ar dates for young mafic to intermediate lavas can be maximized by selecting samples that contain K-feldspar as a late-crystallized phase, and contain little or no glass or alteration phases. Characterization of samples was undertaken using the electron microprobe, where samples were ranked on a scale of 1 to 10 (with 10 being most suitable for dating) based largely on whether the K in the sample could be determined to be housed in extremely fine-grained feldspar interstitial to more abundant phenocryst phases, or was in residual volcanic glass. Sample rating was downgraded if the sample showed evidence of chemical alteration. The 40Ar/39Ar results from the most highly rated samples yield an age of 3.83±0.05 Ma for the Southwest Edifice of Los Lunas volcano. This volcanic center is physically overlapped by the 1.271±0.014 Ma Main Edifice. The absolute ages agree with relative ages determined from geological mapping. Within the Main Edifice, field relationships indicate a sequence of five eruptive events. The ages of the best samples from the Main Edifice all agree within analytical error, suggesting that the entire period of eruptive activity spanned less than 100,000 years. The precision and accuracy of data in this study were much enhanced by using microprobe observations of groundmass glass and K-feldspar to aid in the sample selection process. Sample selection based only on thin section observations would have yielded lower quality data, because the degree of crystallization of residual glass would have been incompletely assessed. 203