EXPLORING A POSSIBLE LINK BETWEEN TRACKLESS ZIRCONS AND REGIONAL VOLCANISM IN NORTHERN AUSTRALIA

Spontaneous fission of uranium-238 in zircon grains produces fission tracks that accumulate over time and anneal when zircons are exposed to high temperatures. Trackless zircon grains are those that have very low U contents, recently experienced heating during km-scale burial or exposure to wildfire, or are simply very young (i.e. recently erupted from a volcano). We identified trackless zircon grains throughout a sediment core (K6E) extracted from a sinkhole in northern Australia (n = 57; 80–250 µm), which preserves ~100 kyr of landscape dynamics. The K6E sinkhole is ~20 m deep, too shallow for heating by geological burial. In this study, we assess the likelihood that trackless zircons from Core K6E originated from volcanic eruptions in the nearby Indonesian archipelago and subsequently transported through the atmosphere to Core K6E. We compiled dates of documented Quaternary volcanic eruptions from the Smithsonian Institution Global Volcanism Program, excluding those with a Volcanic Eruptive Index <5. We identified multiple volcanoes that erupted contemporaneously with the deposition of the youngest trackless zircons at Core K6E (~0.48 kyr), so a temporal link between the trackless zircon grains from Core K6E and volcanism seems possible. To assess the plausibility that the youngest trackless zircons in Core K6E are volcanic in origin, we determined the distance that zircon grains erupted from volcanoes could be transported atmospherically using an eolian sediment transport equation. Under standard atmospheric conditions and wind velocities, we find that zircons in the 80–250 µm grain-size range can only be transported ~100 km; however, the nearest volcano to Core K6E is ~815 km away. We conclude that although trackless zircons were deposited at Core K6E at the same time as regional volcanic eruptions, the transport distance between these eruptions and Core K6E is too great and thus it is highly unlikely that the trackless zircon grains extracted from Core K6E are volcanic in origin. Trackless zircons may simply have low uranium content, but in the absence of uranium data for trackless zircons from Core K6E, this remains untested. Based on the geological setting of Core K6E and the distance between Core K6E and nearby volcanoes, we suggest that trackless zircons were annealed by wildfire heating in northern Australia.