Australia: The Land Where Time Began

A biography of the Australian continent 

Superplumes – Global Circulation of Material and Petrogenesis of Superplume Rocks

Petrogenesis of hotspot magma depends on the physical processes of mantle convection, especially the variation in composition of the mantle. Immediately following the consolidation of the magma ocean at 4.6 Ga, there are 2 end-member states of chemical composition that are considered:

1.      A homogeneous mantle formed without any chemical stratification between the upper and lower mantle, if there was strong enough convective flow in the magma ocean;

2.      The lower mantle must be enriched in perovskite, though olivine is enriched in the upper mantle, if a magma ocean was consolidated under the static condition to produce a chemical gradient that was sufficiently steep by fractional crystallisation over time. In this case the lower mantle must be richer in SiO₂, and stratified chemically from the bottom to the top: wüstite, Mg-perovskite, Ca-perovskite, CF (Na-Al phase), and SiO₂ respectively. A magma ocean is assumed to be in a whole mantle scale in these models, jest as the Moon was formed by a giant impact.

The mantle must have evolved chemically over the past 4.5 Ga to extract the lighter (continental crust, ocean and atmosphere on the surface) and the heavier, anti-crust, on the bottom), fractions from the top to the bottom of the mantle. The whole mantle must have been partially molten once on the way, if MORB was produced by the mantle at a rate similar to that of the Earth during the Mesozoic-Cainozoic, 20 km³/yr, over the time back to 2.5 Ga, and from 2.5-4.0 Ga at a rate of 3 times more. The recycled MORB must have accumulated at the bottom to a thickness of 500 km, as it is heavier than the lower mantle that surrounds it. It is suggested by this that the Dʺ layer, 0-350 km thick, must be enriched in MORB component and have caused the chemical heterogeneity, as has been previously pointed out. Thick ant-crust may have been present on the bottom of the CMB.

Alternatively, it has long been believed by isotope geochemists that the presence of an ORB source mantle which has never partially melted remained the primordial source since the Earth’s genesis, this idea coming mainly from the constraints of Re-Os and He isotopes.

It has been argued by Kogiso that source mantle based on the geochemistry of HIMU basalts from the Pacific Superplume in French Polynesia and experiments on melting of sandwiched MORB peridotite. He also suggests the origin of HIMU basalts.

Komiya shows the chemical and thermal evolution of the mantle of Earth over 4.0 Gyr, in reviews on his own works on MORBs and OIBs from the Archaean to the Phanerozoic, as well the work of others. The mantle overturns that took place at 2.7 and 2.3 Ga were the most outstanding events. Influence on the compositional evolution of the mantle can also have been exerted by superplumes since 2.3 Ga.

Sources & Further reading

1. Yuen, D.A., Maruyama, S, Karato, Shun-ichiro & Windley, B., (Eds), 2007, Superplumes: Beyond Plate Tectonics, Springer.