Two Archaean birds with one stone

There are two major issues concerning the Archaean mantle: was the mantle hotter than it is now; was it in a reduced or oxidised state? The first has implications for Archaean plate tectonics. If loss of the higher radioactive heat produced in the mantle was accomplished by processes similar to those today, i.e. dominantly by mid-ocean volcanism the Archaean geotherm would have been similar to today’s and plate tectonics would have been similar. If this means of heat loss could not cope, then temperatures would increase more rapidly with depth, with implications for the style of plate tectonics, especially subduction. A mantle with reducing conditions would be expected to emit reduced gases, such as methane, as well as carbon dioxide, to produce a reducing atmosphere. If oxidising conditions prevailed, then CO2 would be a dominant emission to the atmosphere. There have been arguments over these two aspects of the Archaean for decade, but now they may have been resolved (Berry, A.J. et al. 2008. Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle. Nature, v. 455, p. 960-963). One factor alone allowed the arguments to damp down: A 2.7 Ga ultramafic lava flow from Zimbabwe preserved a pristine sample of the original magma in the form of small glass blobs trapped in olivine. Measured proportions of Fe(II) and Fe(III) in a melt indicate those in its source, and hence the redox state of the source, mantle peridotite. The Zimbabwe melt inclusions are similar in this respect to those found in modern mid-ocean ridge basalts; they show a high degree of reduction. In turn that suggests that the melting that formed them was almost anhydrous, otherwise dissociation of water would have added oxygen that would have upped the content of Fe(III) in the melt. Experiments show that the degree of anhydrous partial melting of peridotite needed to form ultramafic magma is compatible only with temperatures around 1700ºC, about 400 degrees hotter than those that form modern basalt magma. Significant volumes of the late Archaean mantle, and by extension that of earlier times, had to have been a great deal hotter than it is today.

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