Secular Changes in Mantle Composition as Recorded by Tholeiitic Basalts

Abstract:

Online geochemical databases have been used to study secular changes in mantle composition and temperature. Because samples in these databases are often miss-classified, altered, crustally contaminated, and/or poorly dated, we have re-evaluated secular changes with a carefully filtered mafic database of over 6000 analyses. We include only samples with 4-25% MgO and 44-57% SiO₂, and do not include komatiites, ultramafic rocks, cumulates, alkali basalts, or basalts from LIPS, giant dyke swarms or continental rifts. Our database is dominated by tholeiites from juvenile mantle sources. Unlike earlier studies, our results do not show large decreases in MgO or Ni at the end of the Archean and thus do not demand a rapidly cooling mantle at this time. Using relatively immobile incompatible element ratios that minimize the effects of degree of melting (Nb/Th, Zr/Nb, Th/Yb, Nb/Yb, La/Sm, Gd/Yb, La/Nb), four periods of time stand out in the secular evolution of tholeiitic basalts: 1) > 3.6 Ga, relatively few locations with large variations in ratios between locations; 2) 3.6-2.4 Ga, relatively constant ratios in widespread geographic locations; 3) 2.4-2.1 Ga, a period of rapid change in ratios; and 4) < 2.1 Ga, increase in variation of ratios in basalts between different tectonic settings. If the relatively few geographic locations in Period 1 are representative, they reflect very heterogeneous, poorly mixed mantle sources. Period 2 basalts come from mantle sources with depleted to primitive mantle characteristics, the depleted component possibly produced during rapid growth of continental crust. The rapid changes that occur over 200-300 myr beginning at 2.4 Ga in Period 3 reflect increasing amounts of recycled enriched components, perhaps related to a rapid global increase in plate tectonics. The sparsity of crust of this age may reflect extensive recycling as subduction propagated around the globe. During Period 4, increasing variation of element ratios reflects increasing heterogeneity of mantle sources perhaps due to continued recycling of oceanic and continental crust into the mantle.