ON THE SIGNIFICANCE OF CO2 INCLUSIONS IN PLAGIOCLASE MICROPHENOCRYSTSIN THOLEIITE FROM MOERAKI, NEW-ZEALAND

Liquid plus vapour inclusions Of CO2 are widespread in plagioclase mic rophenocrysts in small tholeiitic intrusions and tephra of the Moeraki and adjacent areas of northeast Otago, New Zealand. They imply the pr esence of immiscible CO2 droplets in the magma at depths of about 7-14 km. Their presence within 5 mum of the edges of microphenocrysts as l ittle as 35 mum thick and 118 mum long indicates minimal feldspar crys tal growth during the final ascent and quenching of the magma. Delicat e branching clusters of lathlike microphenocrysts escaped disruption d uring this ascent. Such CO2 inclusions are a potential source of 'exce ss argon' perturbing K-Ar age determinations.