The age of Lathrop Wells volcanic center: An 40Ar/39Ar dating investigation

Abstract

The 40Ar/39Ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the Lathrop Wells volcanic center, Nevada, yield eruption ages of 77.3 ± 6.0 and 76.6 ± 4.9 ka (2σ) for the Ql1 and Ql2 flows, respectively. Basaltic samples were analyzed in four to six replicate runs and yield normally distributed plateau ages. Miocene ash flow tuff xenoliths were collected from the basaltic units with the majority of samples coming from the Ql2. The sanidines from the Ql2 xenoliths yielded apparent plateau ages (n = 10) ranging from 69 ± 17 to 309 ± 11 ka with a strong cluster (n = 6) at ∼77 ka. Because the diffusion dimension of sanidine is approximated by the physical crystal size, cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra. Experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and HF acid etching lead to anomalously old apparent ages. These new ages for the Ql1 and Ql2 flows are analytically indistinguishable and agree well with published eruption ages determined by 3He and 36Cl cosmogenic surface exposure dating methods. We conclude that the Ql1 and Ql2 flows are temporally equivalent. Additionally, the agreement between our eruption ages for these units and the published 76 ± 20 ka [Zreda et al., 1993] 36Cl exposure age for the Lathrop Wells scoria cone (Qs3) suggests that the cone may be temporally equivalent to the Ql1 and Ql2 flows. We believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic.