Tristan da Cunha: Constraining eruptive behavior using the 40Ar/39Ar dating technique

Abstract

The historically active volcanic ocean island of Tristan da Cunha exhibits a complex and dynamic history, with numerous, often compositionally distinct, parasitic centers punctuating the large edifice. To date, the temporal relationship between differing styles of activity has been unclear. We have applied high-precision 40Ar/39Ar dating to 15 carefully selected samples from Tristan da Cunha to ascertain spatio-temporal relationships of recent volcanism, explore episodicity, and establish if the most recent summit activity post-dated eruptions from parasitic centers lower on the flanks. This has yielded a new suite of reliable Holocene ages, with the youngest dated deposit at 3 ± 1 ka (1σ). A recent flow at the summit was constrained to 5 ± 1 ka (1σ), confirming that summit and parasitic activity on the volcano’s flanks overlap in time. The oldest dated deposits were 118 ± 4 ka (1σ) from a parasitic cone in the southern sector, and 81 ± 10 ka (1σ) from one of the lowest sub-aerial shield-forming lava flows in the northern sector. Large-scale sector collapse is bracketed between 34 ± 1 ka and 26 ± 5 ka (1σ) via dating of the youngest headwall lava flow and oldest sub-aerial scarp-filling deposits. No systematic relationship between the new temporal framework, vent location, and eruptive compositions was found. Although magmatic flux has been inferred to be relatively low, Tristan da Cunha is capable of relatively frequent eruptions from a wide variety of vent locations across a broad range of compositions

Palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the South Atlantic

“…88 Pigbite to the west of Big Point (Fig. 1), where well-stratified basanitic and tephritic flows are exposed 98 (Hicks et al, 2012). There were no folding or faulting tectonic features in the sampling area, with the only 99 alteration to the strata being igneous intrusions.…”

An interdisciplinary approach to volcanic risk reduction under conditions of uncertainty: a case study of Tristan da Cunha

“…During phase one, volcanological data gathering involved field mapping, geological sampling and analysis via 40 Ar/ 39 Ar geochronology, refining views about timing, style, location and likely patterns of activity (Hicks et al, 2012). Scientific uncertainty was quantified using an established structured elicitation technique 6 (Cooke and Goosens, 1999), using volcanological data collected in phase one.…”

“…Given that the summit slopes are very steep, and the channelising nature of the gulches, the consequences of a summit eruption for the Settlement are different than an eruption on lower-lying areas. Although the two most recent eruptions occurred on low-lying coastal strips (1961)(1962) dome and flows; Stony Hill dome and flows (∼ 200-300 years BP)), the new temporal framework revealed that future eruption on the flanks, or from the summit, cannot be discounted (Hicks et al, 2012).…”

“…Numerous parasitic cones, considered to be post-shield volcanism, are scattered across the flanks; many of them are breached by lava flows. The island is almost entirely volcanic, comprising mainly silica-undersaturated basanites to phonolites, and probably emplaced within the last ∼ 200 kyr (Hicks et al, 2012;Dunkley, 2002;McDougall and Ollier, 1982). Young, low-lying coastal strips flank the northwestern and southern margins of the island.…”

“…The 40 Ar/ 39 Ar method (Renne et al, 1997;Lanphere et al, 2007) was applied to 15 well-defined eruptions on Tristan (Hicks et al, 2012). The aim of the new geochronology was to ascertain spatio-temporal relationships of recent volcanism, explore relative timings and frequency of eruptions and to establish if the most recent summit activity post-dated eruptions from the parasitic centres lower on the flanks.…”