Dissection of a monogenetic volcano: Motukorea Lucy McGee, Marc-Alban Millet (VUW), Ian Smith, Károly Németh (Massey), Jan Lindsay
Photo: Bruce Hayward Acknowledgments: DoC boat crew, Auckland geoclub, Javier Agustin-Flores, Gabor Kereszturi, Ola Zawalna-Geer, Lucas Hogan, John Wilmshurst, Charlotte Allen (ANU) Motukorea volcano
Photo: Lloyd Homer Eruptive sequence at Motukorea: 1) Phreatomagmatic phase Eruptive sequence at Motukorea: 2) Magmatic phase
Scoria cone
*
Rafted scoria Lava flows Gratuitous fieldwork photos
Motukorea = monogenetic?
ü Formed in one continuous episode ü Small volume
The geochemistry tells a more complex story
Eruptive sequence ranges from early nephelinite (tuff ring) through basanite (upper scoriaceous unit) to late alkalic basalt (lava flows) D Fractional crystallisation D Deep seated fractionation á la Crater Hill (Smith et al., 2008) C Large systematic compositional variation C Two-fold decrease in incompatible elements from tuff à lava C Linear trends Chemistry with stratigraphic height
Note the transitional nature of the upper scoriaceous unit Pb-isotopes show three component mixing Two magma batches – mixing
Eclogite veins Two magma batches – mixing
Eclogite veins Two magma batches – mixing
Eclogite veins Two magma batches – mixing
Eclogite veins Two magma batches – mixing
~ x2 degree of melting Two magma batches – mixing Major conclusions • Superficially monogenetic, but compositionally polygenetic • High resolution sampling reveals complexity of volcanic sequences • 3 source components visible on the scale of a single eruption • Mixing of melts with no hiatus in eruption deposits suggests that the conduit remains open between melt batches • Despite at first seeming similar to Crater Hill, a different melting model has been constructed – little pattern in the AVF
Photo: Hugh Grenville Other, more far-reaching conclusions?
• AVF-style volcanoes begin with the eruption of small, nephelinitic, tuff- dominated melts • Eclogite veins are the locus of melting? • Size dependency on magmatic conditions in the mantle Melting and mixing model
Pb-isotopes with stratigraphic height