Flood Basalt - Geochemistry

Geochemistry

Geochemical analysis of the major oxides reveals a composition close to that of mid-ocean ridge basalts (MORB) but also close to that of ocean island basalts (OIB). These are in fact tholeiites with a silicon dioxide percentage close to 50%.

Two kinds of basaltic floods basalts can be distinguished :

• those poor in P₂O₅ and in TiO₂, called LPT (low phosphorus and titanium)
• those rich in P₂O₅ and in TiO₂, called HPT (high phosphorus and titanium)

The isotopic ratios 87Sr/86Sr and 206Pb/204Pb are different from that observed in general, which shows that the basalt flood magma was contaminated as it passed through the continental crust. It is this contamination that explains the difference between the two kinds of basalt mentioned above. The LPT type has an excess of elements from the crust such as potassium and strontium.

The content in incompatible elements of basaltic floods is lower than that of ocean island basalts, but higher than that of mid-ocean ridge basalts.