ORIGIN AND EVOLUTION OF BASALTIC MAGMA OF NW COLORADO'S DOTSERO VOLCANO

In the most northern portion of the Rio Grande Rift, in Northwestern Colorado an eruption occurred at Dotsero Volcano, about 4200 y B.P producing alkali basalts similar to ocean island basalts. However, although Dotsero's lava is relatively primitive (7.3 wt% MgO), it is different from other alkali basalts in that it is enriched in K₂O, Pb, Ba, Sr, Cs, and La. The high K₂O (2.7 wt%) and Ba/Nb (59) are significantly greater than in basalts from the central and southern Rio Grande Rift. Two potential mechanisms for creating this enrichment in fluid mobile elements may exist. The enrichment may be caused by crustal contamination of the magma as it rose to the surface or it may record a modification of mantle source caused by ancient subduction zones. When a slab is subducted it dehydrates and fluid soluble elements may infiltrate and freeze in the mantle, providing an arc-like mantle source.

In this project, samples were collected from Dotsero and analyzed for major and trace elements, both whole rock analyses and olivine-hosted melt inclusions. This study focuses on melt inclusions because, as olivine grains crystallize, melt is often trapped within the crystals, protecting it from further modification. Olivine crystallizes out of the magma early, making it more likely that inclusions are trapped before compositions are significantly changed due to assimilation and crystallization. The olivine-hosted melt inclusions may provide evidence to the magma source if the olivine crystallized before there was much change in the original magma composition. Comparisons of inclusion analyses to the whole rock compositions will aid in the understanding of the magma's evolution. If the compositions of the melt inclusions are similar to that of the whole rock then the lava may be recording a mantle source signature. However, if the melt inclusions have low-K, and lack the significant fluid mobile trace element enrichment measured in the whole rock analyses, then the geochemical variations may have resulted from crustal contamination. Identifying crustal and mantle components in the Dotsero lavas allows us to evaluate the relative contributions of both the mantle and the crust to the evolution of young basaltic lavas in the Rio Grande Rift and may have implications for the evolution and growth of continental crust.