An enthusiastic view of deep-Earth processes

In EPN of January 2004, there appeared a summary of Warren Hamilton’s sceptical view of recent ideas about what happens beneath the 660 km mantle discontinuity (Geoscience consensus challenged).  It is below that level that the dominant mantle mineral, olivine (MgSiO4), is thought to change to the more densely packed perovskite (MgSiO3).  Encouraged by an experiment which suggests that at the pressure and temperature just above the core-mantle boundary (CMB) perovskite itself undergoes a phase change to define the D” seismic discontinuity (Murakami, M. et al. 2004.  Post-perovskite phase transition in MgSiO3Science, v. 304, p. 855-858),  Edward Garnero of Arizona State University takes a very different view.  In his Science Perspectives review of the CMB region (Garnero, E.J. 2004.  A new paradigm for the Earth’s core-mantle boundary.  Science, v. 304, p. 834-836) he builds into a comprehensive, illustrated model everything that Hamilton finds dubious: whole-mantle plumes and slab descent; zones of ultra-low velocity close to the CMB; undulations on it; and massive bulges of low-velocity mantle above D”, such as that suggested to underlie the South Atlantic and southern Africa from which constellations of plumes rise.  He links this to a wealth of anisotropies which basalt-oriented geochemists have found and continue to relish.  His enthusiastic account makes fascinating reading, but makes no mention of Hamilton’s and others’ doubts about gilding the lily of only a few short years of seismic tomography.

Mesoproterozoic large igneous province and Rodinia

Flood basalt events in the Phanerozoic seem generally to have preceded the break-up of supercontinents, and many geoscientists believe that their formation is implicated in the mechanism of continental disaggregation.  So it comes as something of a surprise to learn that the assembly of most continental lithosphere to form the Rodinia supercontinent about 1100 Ma ago, which ranks in size with Pangaea, was probably accompanied by massive igneous activity (Hanson, R.E. et al. 2004.  Coeval large-scale magmatism in the Kalahari and Laurentian cratons during Rodinia assembly.  Science, v. 304, p. 1126-1129).  The Proterozoic sediments of southern Africa and once-adjacent Antarctica are intruded, wherever they occur, by basaltic sills up to hundreds of metres thick.  In a few places relics of flood basalts above the sedimentary groups have the same composition and age, around 1100 Ma.  Like Phanerozoic large igneous provinces, most of the magmatism occupied only a few million years, perhaps less than 1Ma.  The distribution of the probable feeder intrusions for the few relics of CFBs suggests that the province in the Kalahari craton formerly covered about 2 million km2, so it ranks in size with most Phanerozoic LIPs.  In North America, cored by the craton of Laurentia, there occurs the Keeweenawan dyke swarm and other mainly mafic intrusions, that probably fed another veneer of CFBs.  Dating them using the same single-crystal U-Pb method reveals ages that are within error of those from southern Africa.  Combined, the two LIPs are much larger than the biggest know LIP from the Phanerozoic – the Ontong-Java Plateau that formed on the floor of the West Pacific Ocean during the Cretaceous.  So, were there two massive, but short-lived igneous events while Rodinia was assembling, or one that unites both the Kalahari and Laurentian cratons?  In many models of Rodinia, stitched together using orogenic belts that formed in the late Mesoproterozoic between1150 and 950 Ma, the Kalahari craton has been placed against Laurentia; both LIPs could be a single super-province.  However, the same authors also measured palaeomagnetic pole positions from the southern African igneous rocks.  They are different from those revealed by the Laurentian LIP, and imply considerable separation of the two continental masses at the time of igneous activity.  That suggests either separate melting events in the mantle beneath both cratons at the same time, or that both are parts of an even larger magmatic upheaval that spanned about 1/5 of a hemisphere.  Whichever turns out to be the case, this ancient large-scale mantle event bucks the Phanerozoic trend of LIPs’ presaging or accompanying continental break-up.  Maybe the rare mantle upwellings thought to generate LIPs are really random in their positioning, and “just happened” to rise beneath Pangaea and its fragments from the Devonian onwards.


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