An Elemental and Isotopic Investigation of Quaternary Silicic Taupo Volcanic Zone Tephras from ODP Site 1123: Chronostratigraphic and Petrogenetic Applications
This thesis presents a chemical and isotopic investigation of well-dated silicic tephra layers sourced from the Taupo Volcanic Zone (TVZ), central North Island, New Zealand, that were recovered from deep ocean sediment cores at Ocean Drilling Program Site 1123 (41 degrees 47.16' S, 171 degrees 29.94' W; 3290 m water depth), located approximately 1000 km east of the TVZ. The relative quiescence of the deep ocean sedimentary setting, the continuous supply of biogenic and terrigenous sediment and the favourable location of Site 1123 close to the main TVZ ash dispersal path have resulted in an extensive TVZ tephra record (70 Quaternary tephra layers preserved in 3 sediment cores) at Site 1123. This record extends and compliments the onshore record of silicic TVZ volcanism which has been obscured by erosion of non-consolidated volcanic material and burial of older units by younger volcanic deposits. The Site 1123 cores comprise an important paleo-oceanographic record for the Southwest Pacific Ocean and as a result of previous paleo-environmental studies, the Site 1123 tephras have been assigned orbitally tuned stable isotope ages that are more precise than is currently possible by any radiometric dating techniques. These features of the Site 1123 tephra record highlight its potential to be established as a type section for Quaternary tephrochronological studies in the New Zealand region. In addition, the continuous stratigraphy and precise age control of these tephras enables the Site 1123 record to be used as a petrogenetic archive to investigate changes in chemical and isotopic composition of these tephras that may be related to changes in the petrogenesis of TVZ silicic magmas during the last ~ 1.65 Ma. This thesis establishes major and trace element chemical 'fingerprints' for the Site 1123 tephras using traditional (electron probe microanalysis) and novel (laser ablation inductively coupled plasma mass spectrometry) in situ geochemical techniques. Trace element fingerprints are demonstrated to provide a more precise means of correlating and distinguishing between tephras with essentially identical major element chemistries. These fingerprints are used to refine the original Site 1123 composite stratigraphy and age model and identify a section of repeated sediments in the Site 1123 cores that have introduced a significant error into the original composite stratigraphy and age model for the interval ~1.1 to 1.4 Ma. Correlation of the tephra layers between the 3 sediment cores (1123A, B and C) establishes that ~37-38 individual tephra units are recorded with ages ranging from 1.655 Ma to 27.1 ka. Approximately 50% of the eruptive units and cumulative tephra thickness at the site were recorded during the first ~ 150 ka of silicic TVZ volcanism (1.65 to 1.50 Ma). The fragmentary onshore record does not preserve clear evidence for this early period of hyperactivity. Four broad silicic melt types are identified on the basis of chemistry and eruptive age. Trace element indices of fractional crystallisation suggests the origin of the four melt types is primarily due to differential degrees of fractional crystallisation of accessory zircon, hydrous mineral phases and Fe-Ti oxides. Sr-Nd-Pb isotopic compositions of 13 representative Site 1123 tephras cannot be generated using traditional models in which Torlesse meta-sedimentary rocks are the sole contaminant of mafic magmas. Instead the data support a model in which ascending TVZ basalts assimilate crustal rocks of both meta-greywacke terranes: firstly up to 15% of Waipapa crust is assimilated at depth, followed by assimilation of between 20 and 45% Torlesse crust at shallower levels. In this model the majority of Site 1123 tephras indicate a remarkably uniform amount of crust (~ 35%) with the most evolved sample requiring 45% crustal contribution. However, extensive fractional crystallisation (55-85%) is required to have accompanied crustal assimilation in order to drive the relatively low SiO2 compositions of these contaminated mafic magmas (SiO2 = 53-58 wt% after crustal contamination) to the high SiO2 rhyolite (74-78 wt%) compositions of the Site 1123 tephras. The large crustal contributions to TVZ silicic magmas (35-45%) implied by these data are high compared to large volume silicic magmas from different settings (e.g. Yemen-Ethiopia; Long Valley, USA), a feature that likely reflects the thin crust and high thermal flux into the continental crust beneath the TVZ.