Geochemical Evolution of the East Otago Mantle


Dalton, Hayden Boyd (Hayden)


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Harzburgitic and lherzolitic mantle xenoliths entrained in the Miocene by the alkali basalts of the Dunedin Volcanic Group and emplaced in East Otago, New Zealand have ‘locked-in’ the intricate history of the sub-continental lithospheric mantle (SCLM). Geochemical analysis of 36 peridotite samples from three neighbouring localities in East Otago reveals a remarkable degree of heterogeneity within the SCLM. Different realms of the SCLM have experienced diverse degrees of depletion by partial melt extraction and this has been followed by, in some cases significant, light rare-earth element (LREE) enrichment. Interrogation of clinopyroxene trace element geochemistry suggests mantle metasomatic processes are responsible for this enrichment with asthenosphere-derived carbonatite melts the most likely metasomatic agent, while the influence of silicate melts cannot be ruled out.
Radiogenic isotope systematics (Sr, Pb) indicate the East Otago SCLM has an enriched HIMU-like composition similar to the alkali basalts erupted during Cenozoic intraplate volcanism. Samples which have experienced the greatest degree of metasomatism have a narrow isotopic range compatible with HIMU-like compositions. It is suggested here that the HIMU-like signature observed in the East Otago SCLM is of metasomatic origin. Based on orthopyroxene-clinopyroxene trace element equilibrium diffusion modelling, it is thought that this metasomatic event occurred at least 100 Ma while Zealandia was contiguous with Gondwana. The presence of HIMU-like isotopic signatures in the mantle beneath West Antarctica and Eastern Australia may indicate a pervasive metasomatic event with a common HIMU-like metasomatic agent.
A ubiquitous core to rim decrease in Al2O3 concentrations and associated diffusion modelling is interpreted as recording the cooling of the East Otago SCLM following heating and thinning associated with the Cretaceous rifting of Zealandia from Gondwana. Following this geochemical evolution the peridotite xenoliths were entrained in alkali basalts with a HIMU-like signature. The source of these basalts remains uncertain with a HIMU-like signature now recognised in the East Otago SCLM and the convecting asthenosphere below.

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xiii, 162 pages A4


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POLYGON ((170.382536924890672 -45.589280107697633,170.174004957660856 -45.553942919358377,170.230662222582623 -45.487456153096801,170.406206246536101 -45.554096778894859,170.382536924890672 -45.589280107697633))




Dalton, Hayden Boyd (Hayden), “Geochemical Evolution of the East Otago Mantle,” Otago Geology Theses, accessed April 16, 2021,

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