Heads or tails?

The basalt floods draped over some great continental plateaux and considerable areas of the ocean floor, ocean islands far from plate boundaries and the volcanic provinces sitting at the ends of various oceanic island chains are with little doubt the product of plume-like masses rising from great depth in the mantle.  What is not so well agreed is just what bit of a plume underwent partial melting to make the magma, the depth at which that took place and the prevailing temperature.  There is some support for plumes that rise from a mantle transition zone about 700 km down, where there is an abrupt increase in temperature.  Such plumes form a hot head when they impact the lithosphere, and that should be the source for magma.  Plumes that rise from the core mantle boundary, should in theory have heads that are cooler than their tails, and which grow hugely by being stalled at the 700 km discontinuity.  The two combined might form little plumes that rise from a big head at 700 km that spreads laterally.  Nicholas Arndt gives a neat summary of these unseen ramifications in a recent issue of Nature (Arndt, N.  2000.  Hot heads and cold tails. Nature, 407, 458-461).

Arndt was moved to make his comments by evidence from Namibian flood basalts from the 128-138 Ma old Paraná-Etendeka large igneous province (Thompson, R.N. and Gibson, S.A. 2000.  Transient high temperatures in mantle plume heads inferred from magnesian olivines in Phanerozoic picrites. Nature, 407, 502-506).  Thompson and Gibson found highly magnesian olivine crystals, among more normal ones, in basaltic dykes that cut the Etendeka basalts.  The more Mg-rich an olivine is the more primitive (the more like the composition of the mantle) the magma from which it crystallized.  They calculate that these anomalous olivines equilibrated with a magma with 24% MgO (compared with the <10% of most basalts) – probably a komatiite.  But they are in much more evolved basalts, so they suggest that a primitive magma at the hot head of a plume that hit the lithosphere itself underwent fractional crystallization to produce plain basalt.  They draw from that the conclusion that the plume head was 300-400°C hotter than the surrounding mantle – as expected in the first plume model above.  Arndt is sceptical, partly because there are so many unknowns about the source region and partly because there are many other possible explanations.  He suggests more similar work and other kinds of geochemical research on large igneous provinces in general.  To that might be added looking for some of the possible mechanical consequences of hot or cool plume heads.

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