16 February, 2012 – Seminar

When: 4pm on Thursday, February 16th, 2012
Where: DIAS, Geophysics Section, 5 Merrion Square, DUblin 2, Library

Speaker: Prof. Balz Kamber, Chair of Geology and Mineralogy at Trinity College Dublin
Title: The decay of radioactive heat production rates: expressions in the Precambrian rock record

Abstract:

On billion-year time-scales, the geology of Earth is driven by gravity and loss of heat. Only two long term heat sources exist within the Earth: latent heat of crystallisation of the inner core and radioactive heat from decay of the ⁴⁰K, ²³²Th, ²³⁵U, and ²³⁸U chains. The former heat source is responsible for mantle upwellings originating at the core-mantle boundary and expressed at the surface as within-plate volcanism. The radioactive heat production rate has exponentially decreased with time in a precisely predictable manner.

In view of the predictably smooth decay of the heat production rate it could reasonably be expected that the geology of the Earth also changed gradually and adjusted to the diminishing need for heat loss. However, long before any of these concepts were developed, and indeed before radioactivity was even discovered, geologists had noticed a fundamental change in rock types and map patterns of Archaean (>2.5 Ga [giga anna]) vs. Proterozoic terrains (the A-P transition). The four most obvious attributes of Archaean terrains at surface are: i) the presence of the high-Mg volcanic rock komatiite with eruption T’s of up to 1700℃; ii) the absence of platform sediments; iii) the bi-modal nature of rocks at surface (basaltic and granodioritic); and the dome-and-keel arrangement of greenstones and granitoids. These differences extent to the third dimension where it is found that Archaean cratons have exceptionally deep lithospheric mantle roots, typically extending to 200-250 km with obvious implications for the presence of diamond. Collectively, these phenomena argue for a relatively abrupt change of parameters having governed the A-P transition. The radioactive heat production rate offers no explanation for such a change because of the gradual nature of decay. In this presentation, I will propose a working hypothesis according to which the change was brought about by the destruction of a mechanical boundary layer in the mantle transition zone at ca. 2.6 Ga.

The proposal is built on geochemical and isotopic data, which show that the Archaean depleted mantle source follows a trajectory that can be explained by extraction and storage of continental crust. By contrast, 1.9 Ga mantle-derived rocks plot off this trajectory despite strong evidence for further extraction of continental crust. This can only be explained if the mass of the depleted mantle increased at the beginning of the Proterozoic, possibly by as much as a factor of 2. The inventory of U and Th in the continents requires that roughly half the mantle is depleted, whereas the upper mantle only contains 27% of the total mantle mass. It thus appears that throughout the Archaean, only the upper mantle was depleted and that the sharp A-P transition represents the catastrophic collapse of a mechanical boundary in the mantle transition zone. This could have been a mono-mineralic layer of garnetite at ca. 600 km depth representing a cumulate zone left after crystallisation of the magma ocean. This mechanical boundary would have impinged the traverse of mantle plumes originating from the core-mantle boundary allowing for sufficient build-up of heat to produce komatiite. Once the boundary was removed, whole mantle convection became feasible, leading to the observed expansion of the depleted mantle and storage of recycled slabs that eventually became source regions of enriched basalts erupting today.