Abstract:
Studies along the Kermadec arc, SW Pacific, over the past two decades have shown the presence of numerous volcanoes with histories of caldera collapse, and abundant evidence of explosive eruptions involving evolved magmas (dacite to rhyodacite). However, as there are only two substantially emergent volcanoes in the arc, Raoul and Macauley, the history of this explosive volcanism is difficult to constrain from subaerial exposures. In contrast, the marine record offers the opportunity to document activity from these volcanoes over a longer time period. This thesis presents stratigraphic and chemical data from sediment cores collected along offshore transects west and east of Raoul and Macauley islands. Individual grain analysis of glass shards from the tephra layers was conducted using electron microprobe and laser ablation inductively coupled plasma mass spectrometry to analyse each shard for major and trace element compositions. These geochemical data allowed for discrimination between successive tephra deposits, and identification of 51 tephra layers in the cores, each of which is interpreted to represent a single eruptive event. The data also addressed the diversity or uniformity of individual eruptions and traced changes in the geochemistry of the eruption units through time. Oxygen isotope analyses of foraminifera were used to construct an age model for the cores, allowing model age estimates of individual tephra units and proving the marine core record extends back to ca. 170 ka. Silicic volcanism from these two volcanoes dates back to at least ca. 166 ka, indicating that it is not a recent development as the onshore record would suggest. Onshore exposures on Macauley Island record only one silicic eruption, the 6.3 ka Sandy Bay Tephra; however, the offshore cores show that Macauley has erupted silicic magmas since 166 ka, and is the dominant source for tephras recorded in the cores.
Construction of a tephrochronology of the cores provides a context as to the eruptive frequency of volcanoes along the arc, indicates apparent fluctuations in the magma supply rate, and also indicates that fractionation is the dominant process by which silicic magmas are generated in the arc. The average eruptive frequency of Raoul and Macauley volcanoes is 1 eruption/kyr. If extrapolated to the other volcanoes along the arc, there are ca. 40 eruptions/kyr of significant size that could be preserved in the marine record. The presence of periods of quiescence in the eruptive history implies that magma supply rates may have a primary control on the frequency of eruptions linked to compositions, such that when supply rates slow, the magmas stall in the crust and fractionate to form silicic magmas.
