Flood basalt research at the Geological museum

A four-year research project MARZ (Magmatism in the Africa-Antarctica Rift Zone), funded by the Academy of Finland, was launched at the Geological museum in September 2011. The project focuses on the Karoo continental flood basalt province that was emplaced on the juxtaposed land masses of Africa and Antarctica during the early stages of the breakup of the Gondwana supercontinent ~180 Ma ago (Fig. 1). About 2,000,000 km³ (~100 times the volume of the Baltic Sea) of basaltic magma was emplaced in and on to the crust in a time scale of a few million years (Fig. 2).

Figure 1. Remnants of the Mesozoic flood basalts on the reconstructed Gondwana supercontinent. In the case of Karoo province, intrusive rocks (formed when the feeding channels and magma chambers crystallized; cf. Fig. 3) also shown. Main research areas marked with red. © Jussi Heinonen

The generation of flood basalts is associated with opening of oceans, climate changes, and large-scale extinctions of terrestrial and marine life – for example, the Permian mass extinction of up to 96% of all marine species and 70% of terrestrial vertebrate species has been linked to gases released by the contemporaneous eruption of flood basalts in Siberia. Flood basalts are known to have erupted almost throughout the history of the Earth, but the causes of large magma eruptions in continental interiors is controversial, given that such phenomena are usually related to lithospheric plate boundaries.

Figure 2. Remnants of the Jurassic Karoo flood basalts in Antarctica. The wall is c. 300 m high (snowmobiles for scale). Individual lava flows can be distinguished as horizontal stripes. © Jussi Heinonen

One of key obstacles in resolving the origin of flood basalts stems from the fact that flood basalt magmas generally have been strongly modified by crystal fractionation and contamination during ascent. Specifically, strong interaction with crust adjacent to magma conduits hinders characterization of the original magma compositions derived from deep mantle sources (Fig. 3). Understanding of deep subcontinental mantle is fundamentally important for resolving the mysteries of flood basalt magmatism, however. Was the mantle source of flood basalts compositionally exceptional? Was the mantle exceptionally hot?

Figure 3. Schematic cross-section of the Karoo continental flood basalt province c. 180 million years ago. For some reason, the deep mantle melts extensively (1). These melts intrude the lithosphere (=crust + brittle upper mantle), where they form large magma chambers and mix with lithospheric materials (2). The melts further proceed upwards through the fracturing crust (3) and finally erupt from shield volcanoes or fissures (4). In some rare occasions, proceeding melts do not assimilate lithosphere and preserve their mantle-derived geochemical signatures (5).Our research team has found rocks crystallized from such melts from Antarctica. © Jussi Heinonen

Flood basalts of the Karoo province have been studied in the University of Helsinki since the early 1990’s and, for example, three PhD theses focus on this topic (Luttinen 2000, Vuori 2004, Heinonen 2011). The MARZ project is based on our recent results on Karoo magmatism in the Antarctic subprovince (e.g. Heinonen et al. 2010, Luttinen et al. 2010). The project concentrates on recently discovered ferropicrites and meimechites from Antarctica and relatively little-studied Karoo flood basalts from RSA and Mozambique (Fig. 1). The former represent rare, highly primitive magmas that were derived from deep sublithospheric mantle below Antarctica (Fig. 3; Riley et al. 2005; Heinonen 2011). Two expeditions to Mozambique will be carried out in 2012 and 2014 to collect new rock samples. The samples will be analysed for major and trace element compositions, mineral chemistry, and radiogenic and stable isotope (Sr, Nd, Pb, Os, Li, B, He) compositions. These analyses shed light on the origin and evolution of Karoo magmas and provide crucial information on the composition and temperature of the mantle sources involved (Fig. 3). Our results from Africa and Antarctica may also provide new general insights into the causes of catastrophic flood basalt eruptions that have shaped our planet.

Our collaborators are:

Department of Terrestrial Magnetism, Carnegie Institution of Washington (Washington DC, USA)
Woods Hole Oceanographic Institution (Woods Hole MA, USA)
Geological Survey of Finland (Espoo, Finland)
Eduardo Mondlane University (Maputo, Mozambique)
British Antarctic Survey (Cambridge, UK)
University of Cape Town (Cape Town, South Africa)

Ask for more information:

Arto Luttinen (FT), director of research
P.O. Box 17 (Arkadiankatu 7, room A416)
Phone: 09-191 28745
E-mail: arto.luttinen(at)helsinki.fi

Jussi Heinonen (FT), postdoctoral researcher
P.O. Box 17 (Arkadiankatu 7, room A409)
Phone: 09-191 28624
E-mail: jussi.s.heinonen(at)helsinki.fi

This study is an activity of the Evolving Continents – Research Community.

References:
Heinonen, J.S. 2011. Geochemistry and petrology of the ferropicrite dikes and associated rocks of Vestfjella, western Dronning Maud Land, Antarctica. Academic Dissertation, Department of Geosciences and Geography A9, Unigrafia, Helsinki, 50 s. https://helda.helsinki.fi/handle/10138/26267

Heinonen, J.S., Carlson, R.W. ja Luttinen, A.V. 2010. Isotopic (Sr, Nd, Pb, and Os) composition of highly magnesian dikes of Vestfjella, western Dronning Maud Land, Antarctica: A key to the origins of the Jurassic Karoo large igneous province? Chemical Geology 277:227–244.

Luttinen, A. 2000. Origin of Jurassic Vestfjella flood basalts, Dronning Maud Land, Antarctica. Academic Dissertation, Helsingin yliopisto, Yliopistopaino, Helsinki, 106 s.

Luttinen, A.V., Leat, P.T., Furnes, H. 2010. Björnnutane and Sembberget basalt lavas and the geochemical provinciality of Karoo magmatism in western Dronning Maud Land, Antarctica. Journal of Volcanology and Geothermal Research 198:1–18.

Riley, T.R., Leat, P.T., Curtis, M.L., Millar, I.L., Duncan, R.A. ja Fazel, A. 2005. Early-Middle Jurassic dolerite dykes from Western Dronning Maud Land (Antarctica): Identifying mantle sources in the Karoo Large Igneous Province. Journal of Petrology, 46:1489–1524.

Vuori, S.K. 2004. Petrogenesis of the Jurassic gabbroic intrusions of Vestfjella, Dronning Maud Land, Antarctica. Academic Dissertation, Helsingin yliopisto, Gummerus Kirjapaino Oy, Saarijärvi, 25 s.