NEW EVIDENCE FOR A GLOBAL MANTLE PLUME EVENT AT 2.7 GA AND POSSIBLE EFFECTS ON ARCHEAN PALEOCLIMATE AND BIOSPHERE

Of the four well-established episodes of juvenile continental crust production before 2 Ga (2.8, 2.7, 2.55, and 2.1 Ga), only the 2.7 Ga event appears to be global in extent. A change in rate of depletion of the mantle as recorded by Nd isotopes and Nb/Th ratio in basalts requires an event at 2.7 Ga in which net continental crustal growth rate exceeds crustal recycling rate into the mantle. A global mantle plume event may be the answer. Supporting the existence of a 2.7-Ga global mantle plume event are large igneous provinces on the Rae, Pilbara and Kaapvaal cratons (Nunavut 2730-2690 Ma, Fortescue 2766-2713 Ma, Ventersdorp 2714-2700 Ma) and at least 16 giant radiating dyke swarms of this age. Also Re-Os depletion ages of Archean mantle xenoliths indicate a major period of thickening of the mantle lithosphere at 2.7 Ga and MgO-FeO considerations indicate these xenoliths represent restite plume material.

Also consistent with a global mantle plume event at 2.7 Ga is a maximum in the Chemical Index of Alteration (CIA) of shales of this age (ranging as high as 90), suggesting enhanced chemical weathering in shale source areas. The enhanced weathering reflects a hotter global climate, which could result from injection of greenhouse gases (CO2 or/and CH4) into the atmosphere accompanying plume-induced volcanism at 2.7 Ga. A large negative excursion in ?13C organic (to –60 per mil) at about 2.7 Ga may also be related to a global mantle plume event. Nutrients, such as CH4, H2, SO2, P and Fe, may be injected into seawater by plume-induced submarine volcanism, which could lead to enhanced productivity of marine methanotrophic bacteria, and thus to strong fractionation of carbon isotopes in organic reservoirs. The fact, however, that an excursion in ?13C carbonate is not observed at 2.7 Ga would seem to indicate that the ratio of carbon buried in organic and carbonate reservoirs was about the same as at present.