Palaeotemperature determinations for the 1.8-ka Taupo ignimbrite,New Zealand,and implications for the emplacement history of a high-velocity pyroclastic flow

Palaeomagnetic data from lithic clasts collected at 46 sites within layers 1 and 2 of the 1.8-ka Taupo ignimbrite, New Zealand, have been used to determine the palaeotemperatures and thermal structure of the deposit on its emplacement. Equilibrium temperatures from sites less than 30–40 km from vent are 150–300 °C, whereas at greater distances site equilibrium temperatures increase up to 400–500 °C. This variation is seen in both layer 1 and 2 deposits, with values for layer 1 being somewhat cooler, and with its increase in temperature occurring at a greater distance from vent. A temperature maximum at ~50 km from vent coincides with a zone of pink thermal-oxidation colouration of pumices previously inferred to reflect higher emplacement temperatures. Additional palaeomagnetic data collected by us and others from pumice clasts show comparable temperature variations, but these temperature estimates are shown here to be due to a chemical remanence and unreliable for accurate temperature estimates. Cooler temperatures in proximal parts of the ignimbrite are consistent with admixture of >20% cold lithic clasts at source and interaction with the pre-eruption Lake Taupo. The similar, but offset, increases in equilibrium temperatures for medial and distal layers 1 and 2 are consistent with both layers being deposited from the same flow. However, any proximal deposits left by the later, hotter material must have been subsequently eroded, or be so thin that our collection failed to sample them. Radial asymmetries in equilibrium temperatures as well as other physical parameters suggest that the deposit emplacement temperature is primarily determined at source, rather than by interaction with air during transport. These data support previous interpretations that a concentrated basal flow played a dominant role in emplacement and deposition of the Taupo ignimbrite.Editorial responsibility: T. Druitt