Sm-Nd and Lu-Hf ages of ultramafic xenoliths from South African kimberlites and the secular cooling of the Earth

Details Sm-Nd and Lu-Hf ages of ultramafic xenoliths from South African kimberlites and the secular cooling of the Earth Sm-Nd and Lu-Hf ages of ultramafic xenoliths from South African kimberlites and the secular cooling of the Earth

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmmr71b..05a&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #MR71B-05

Physics

8120 Dynamics Of Lithosphere And Mantle: General, 8124 Earth'S Interior: Composition And State (Old 8105), 8125 Evolution Of The Earth, 8147 Planetary Interiors (5430, 5724)

Scientific paper

Among Francis Birch's major contributions, his insightful theory of heat flow and estimates of energy balance within the Earth had a strong impact on the younger generations. Here, we use the apparent Sm-Nd and Lu-Hf ages obtained on garnet-bearing peridotite inclusions by MC-ICP-MS to infer the temperature and cooling rate of the sub-continental lithosphere under South Africa, and by inference, the secular cooling rate of the Earth over the last 3 Gy. The Sm-Nd and Lu-Hf ages measured on garnet-clinopyroxene pairs are discordant, with Lu-Hf ages being systematically several hundred My older than Sm-Nd ages, and therefore do not reflect crystallization ages. Although the lithosphere probably formed some 3 Gy ago, garnet ages vary between 1800 Ma and the host kimberlite emplacement age (90 Ma). The temperatures inferred from (Fe,Mg) equilibrium between pyroxene pairs and the diffusion data of Van Orman et al. (2002) indicate that the thermal conditions of the sub-continental lithosphere lie near or above the closure temperature for REE diffusion in pyrope (1000-1100oC). We developed a new analytical solution for the accumulation of a radiogenic isotope in a cooling system subjected to loss by diffusion (leaky chronometer). In a spherical mineral of known radius, the knowledge of the activation energy, and the frequential term for diffusion of a particular element leaves the solution to depend on two variables, the cooling rate and the present temperature. The solution bears some similarity to Dodson's (1973) theory for closure temperature but more directly reveals cooling rates from age differences in a cooling leaky system. Once data on Hf diffusion in garnet become available, the combination of the two chronometers will unambiguously constrain the two parameters. We applied this theory to our data on South African xenoliths using the Sm diffusion data on pyrope from van Orman et al. (2002). Our best estimate of the lithospheric temperature depends on grain size but for the cm-sized minerals analyzed in this study, our best estimate is 1300-1500 K. In contrast, the cooling rate of the lithosphere, which can be translated into the cooling rate of the Earth, is relatively well constrained in the range of 10-30 K per Gy and is relatively insensitive to mineral grain size. Such a slow cooling rate requires that the Urey ratio is close to unity and that the terrestrial mantle is at near thermal steady-state. Abundant radioactive heat sources are concealed in the deep mantle or the core.

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