Volatiles, major oxide, trace element and isotope geochemistry in the Snake River Plain and Columbia River Flood Basalts: Implications for the evolution of a continental hotspot.

摘要

It is becoming increasingly clear that volatiles, such as H2O, CO2, S, Cl, and F, play a major role in both the formation and evolution of mantle melts, and therefore, also in the formation and evolution of the Earth's crust. Volatile availability defines where melting is most likely to occur. It also directly drives melting in volcanic arcs associated with subduction zones. Although the body of knowledge surrounding volatile budgets in arc and MORB lavas has grown significantly in recent years, there remains only limited data for hotspots, particularly those that penetrate continental crust. The first part of this study reports volatile data, in concert with major oxide and trace element data, from olivine-hosted melt inclusions in basaltic lavas of the Snake River Plain (SRP) and the Columbia River Basalts (CRB), which are believed to be the surface expressions of the Yellowstone hotspot. Almost all samples analyzed record minimum H2O concentrations in excess of 1 wt%, exceeding the largest values obtained for sub aerial eruptions in Hawaii of 0.8 wt%. The most H2O rich lava in the SRP had 3.3 wt%, and in the Columbia River Basalts (CRB) values reach 4.2 wt% H2O. Furthermore, these highest values are always found in the more primitive melt inclusions, based on major oxide and trace element abundances, indicating that the volatiles are of mantle origin, not artifacts of differentiation in the crust. The last part of this study presents new Sr, Nd and the first Hf isotope data for basalts of the CRB and SRP as well as new 40Ar/39Ar dates to further constrain petrogenesis and eruption history. These data strongly support the conclusion that CRB and SRP lavas have undergone significant interaction with the crust in the area. The result is a revelation that the surface expression of the Yellowstone hotspot may be strongly influenced by the availability of volatiles, particularly H2O, left over from previous subduction events in the region.