A Tale of Two Olivines: Magma Ascent in the Auckland Volcanic Field, New Zealand
Abstract
The Auckland Volcanic Field (AVF) is a nephelinitic to subalkali basaltic monogenetic field centered on the city of Auckland, New Zealand. Lavas are olivine-phyric, and the deposits of several volcanoes in the field contain olivine crystals with chrome spinel (Cr-spinel) inclusions. Microprobe analyses show at least two populations of olivine, categorised by their Mg# and their spinel inclusion compositions: the first has olivines that are euhedral, have compositions slightly less forsteritic than expected for whole rock Mg#, and have Cr-spinel inclusions with relatively low Cr2O3 contents of ~20%. These are interpreted as antecrysts inherited from the mantle source that yielded their host magma. The second population is characterised by olivines that are sub- to euhedral, are significantly more forsteritic than expected from their host whole rock Mg#, and have Cr-spinel inclusons with relatively high Cr2O3 contents of ~50%. These are interpreted as xenocrysts. The composition of these high Cr2O3 spinels very closely resembles the composition of spinels within olivines in dunite sampled from the Dun Mountain Ophiolite on the South Island of New Zealand. The northward extension of the Dun Mountain complex beneath the North Island is defined by the Junction Magnetic Anomaly, marking a crustal terrane boundary that underlies the Auckland Volcanic Field. These data indicate that the magmas that have risen to produce the volcanoes of the Auckland Volcanic Field have carried crystals from an underlying ultramafic crust as well as from their asthenospheric source. Euhedral olivine crystals which do not contain Cr-spinel are also present in AVF lavas and these are interpreted as true phenocrysts that crystallised directly from their host magmas. The lack of reaction textures at crystal margins suggests rapid ascent rates. A crustal origin for the xenocrysts not only has large implications for ascent rate modelling of olivines, but also for the crustal structure of the Auckland area and possible magma ascent paths under the AVF.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V31A2674S
- Keywords:
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- 1043 GEOCHEMISTRY Fluid and melt inclusion geochemistry;
- 1033 GEOCHEMISTRY Intra-plate processes;
- 3620 MINERALOGY AND PETROLOGY Mineral and crystal chemistry;
- 8488 VOLCANOLOGY Volcanic hazards and risks