Skip to content

Prolonged post-emplacement cooling of andesitic-dacitic lava flows produces optimal groundmass material for Ar/Ar dating

Chris Conway 1, Andrew Calvert2, Graham Leonard3, Seiko Yamasaki1, Osamu Ishizuka1, Yumiko Harigane4

  • Affiliations: 1Earthquake and Volcano Geology, Geological Survey of Japan, Tsukuba, Japan; 2Volcano Science Center, United States Geological Survey, Moffett Field, USA; 3Earth Structure and Processes, GNS Science, Lower Hutt, New Zealand; 4Geology and Geoinformation, Geological Survey of Japan, Tsukuba, Japan

  • Presentation type: Talk

  • Presentation time: Tuesday 16:00 - 16:15, Room R380

  • Programme No: 1.3.6

  • Theme 1 > Session 3

Abstract

40Ar/39Ar ages for lava flows contribute greatly to organizing the geological maps, eruptive histories, and petrogenetic trends that underpin hazard assessments at active volcanoes. These age data are collected by analyzing the groundmass material of rock samples; therefore, it is important to understand how the cooling and crystallization of melt affects the distribution of K and Ar in lavas during their effusion and emplacement. We have undertaken a petrological and geochemical study of late Pleistocene to Holocene andesitic-dacitic lavas from Ruapehu volcano, Aotearoa New Zealand, for which 40Ar/39Ar ages were previously determined. Studied samples from the exposed interiors of lava flows yielded ages with relative 2σ precisions of 2--32% (e.g., 42.8 ± 1.0 ka; 8.8 ± 2.8 ka). Groundmass microlite phases in these lavas are plagioclase, orthopyroxene, and magnetite. New microanalytical data show that pre-existing melt was quenched to form rhyolitic glass at lava margins; however, melt fractionation during slow cooling in lava interiors formed subhedral sanidine (~11 wt.% K2O) and patchy tridymite. The contribution of sanidine to the K/Ca budget of samples peaked through the middle stages of step-heating experiments, which resulted in consistent gas release spectra and measurement of high-precision plateau ages. Crystallization of sanidine in many Ruapehu lavas was aided by their impoundment by valley-filling glaciers during the last glacial period, which formed abnormally thick units (>30 m) with interiors that underwent prolonged cooling behind quenched margins. Eruption ages for these lavas have aided reconstructions of the edifice growth history, paleo-glacier extents, and geomagnetic excursion dynamics.