Sebastien Pilet

Short-term metasomatic control of Nb/Th ratios in the mantle sources of intraplate basalts

Variations in the niobium/thorium ratio in basaltic rocks are thought to be related to the long-term extraction of continental crust from the mantle and the extent of sediment recycling and mixing in the mantle. However, basalts erupted between 13 and 3 Ma from a single volcanic center in the Cantal alkali massif (France) have Nb/Th of 10.5-18.7, a range encompassing nearly the entire basalt record. Cantal basalts are isotopically ho- mogeneous, ruling out variable sediment contamination of their mantle sources. Instead, the new data indicate a mineralogical control on Nb/Th in a veined mantle source. We postulate that the large Nb/Th variations are the result of a metasomatic process, called percolative fractional crystallization, that produced veins containing pyroxene and Nb- rich oxide within the upper mantle. Short-term metasomatic-induced variations in mantle Nb/Th may have occurred throughout the geologic record, and provide an alternative explanation to sediment recycling for Nb/Th heterogeneity in the upper mantle.

Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction.

The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.