Polygenetic magmatism in a monogenetic field: an isotopic investigation from the Auckland Volcanic Field, New Zealand
The Auckland Volcanic Field is a Quaternary monogenetic basaltic field of about 50 eruption centres, which have erupted ~4 km3 of magma during the past ~250,000yrs. 2 km3 of this magma has been from Rangitoto Island - the most recent centre (~600yrs BP) - which erupted in two discrete stages ~50 years apart - an intriguing feature for a so-called monogenetic field. Interestingly, the two eruptions produced lavas of different compositions, alkali basalt as the first eruption, and a far more voluminous tholeiite eruption as the second. Trace elements, as well as new U-Th-Ra and Sr-Nd isotopes are used to model the processes which have led to the eruption of two distinct compositions within a small-volume centre, over a short time interval. Rare earth element patterns indicate that both the alkalic and tholeiitic basalts formed in the presence of residual garnet although the amount is higher for the alkali basalt. Differences in degree of melting are also inferred with the alkali basalt requiring a smaller degree than the tholeiite: 2% and 6% respectively. Major elements are used in modelling formation depths and pressures with the alkali basalts requiring a somewhat deeper source than the tholeiites, corresponding well with the larger amount of garnet in the alkali basalt source as modelled by trace elements. (87Sr/86Sr), (143Nd/144Nd) and (238U/232Th) from both eruptions showed the source to have something of a different ‘flavour’ for the two rock types, indicating that the mantle underlying Auckland may be heterogeneous and possibly layered. The constraints from major and trace element data are combined with the U-Th-Ra isotope data to provide an estimate of the melting rates, inferred upwelling rate and conduit length. Higher 230Th-excesses are found in the alkali basalts than the tholeiites, suggesting a longer conduit and/or slower upwelling for the former. Dynamic melting calculations and likely source parameters for the two lavas indicate that the alkali basalts came from a deeper, more slowly rising source whilst the tholeiites originated from a shallower, faster upwelling source. The idea that Rangitoto volcano experienced such spatial and temporal changes in its source region in such a small space of time, to the extent where a new magma type was produced has interesting implications for how the mantle yields its melts.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- 1033 GEOCHEMISTRY / Intra-plate processes;
- 3619 MINERALOGY AND PETROLOGY / Magma genesis and partial melting;
- 8415 VOLCANOLOGY / Intra-plate processes