Chemical and thermal evolution of the upper mantle; implications for Earth's climate and biosphere and the nature of mantle convection

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The chemistry of mid-ocean ridge basalts (MORB) can be used to infer the temperature of the upper mantle, and ancient seafloor lavas preserve a record of mantle temperature and crustal thickness variations since the Mesozoic. Existing chemical data for old seafloor suggest that before 80 Ma the upper mantle was 60°C hotter than today. Mantle temperature variations on this scale impact on global climate and the biosphere. It has been argued that the Cretaceous greenhouse climate and sea-level highstand was triggered by higher than normal rates of oceanic crust production, due to higher mantle temperatures. However, there are presently very few chemical data for older oceanic crust, and most existing analyses were carried out on highly altered, phenocryst-rich, whole-rock samples. We will determine the changes in MORB chemistry over the past 170 Ma in each of the major ocean basins, avoiding the effects of alteration by analysing fresh basaltic glass using microanalytical techniques. Suitable samples for this study are available from the numerous DSDP/ODP sites drilled on old oceanic crust in the Pacific, Atlantic and Indian Oceans. The results will be used to examine the implications for the scale of mantle convection, and effects on sea-level, seawater chemistry and climate on timescales of 1-10 Ma.