Fission-track and K-Ar ages of “macusanite” obsidian glasses, (SE Peru): Geodynamic implications

Macusani obsidian from southern Peru: A characterization of its elemental composition with a demonstration of its ancient use

“…Plateau ages for Macusani obsidian pebbles from the Caluyo Mayo area are 5.36-6.06 Ma (Poupeau et al, 1993b). These results are in agreement with prior FT age estimates of 5.52 AE 0.26 Ma and K-Ar estimates age estimates of 5.67 AE 0.1 Ma for these samples (Poupeau et al, 1992).…”

“…Fission-track plateau ages for Macusani obsidian pebbles range from 4.3 to 7.9 Ma (Miller et al, 1990;Miller and Wagner, 1981;Naeser et al, 1980;Poupeau et al, 1993aPoupeau et al, ,b, 1992. K-Ar ages from 4.2 AE 1.5 to 5.67 Ma have been reported (Barnes et al, 1970;Poupeau et al, 1992).…”

“…FT plateau ages indicate that the Macusani obsidian producing volcanism occurred 4.8-6.9 Ma, possibly in the form of discrete events (Poupeau et al, 1992:295). Macusani obsidians may have resulted from different volcanic eruptions (Poupeau et al, 1992), and this could explain the differences in ages (Osorio et al, 2003:411). Thus, though Macusani obsidian may have formed during a single event that occurred around 5.5 AE 1 Ma, it is also possible that multiple deposits formed in discrete events during this time (Poupeau et al, 1993a:303).…”

New Constraints on Ages of Glasses Proposed as Reference Materials for Fission‐Track Dating

“…The latter did not consider the ages obtained to be reliable due to argon loss, and assign an age of about 7 Ma to the last in situ volcanic episode recognised in the region on the basis of sanidine ages. It is worth noting that the percentage of atmospheric 40 Ar content of the samples analysed in this paper is far lower (1-8%) than the contents reported by Poupeau et al (1992), 59-80%, and Cheilletz et al (1992), 85-94%. This parameter, when high, is responsible for the major part of the analytical error; its size is obviously time-dependent because an increase in the proportion of radiogenic 40 Ar will decrease the proportion of atmospheric 40 Ar as they are complementary values.…”

“…Macusanite is an obsidian found as pebbles of various sizes in river banks and fluvio-glacial deposits around and in the Macusani volcanic field (southeastern Peru; Barnes et al 1970). This glass was proposed as a reference material for FT dating by McCorkell and Naeser (1988), and FT studies were performed by Poupeau et al (1992Poupeau et al ( , 1993. This glass is an obsidian with a high spontaneous track density (~30,000 cm -2 ).…”