Columbia River volcanism: the question of mantle heterogeneity or crustal contamination

Basalts from the Columbia River flood basalt province of the northwestern U.S.A. show a large diversity in chemical and Nd and Sr isotopic compositions. {¹⁴³Nd}/{¹⁴⁴Nd} ranges from 0.51303 to 0.51208 and is strongly correlated with variations in {⁸⁷Sr}/{⁸⁶Sr}. This correlation suggests mixing between two end member compositions, one characterized by {¹⁴³Nd}/{¹⁴⁴Nd} > 0.51303 and {⁸⁷Sr}/{⁸⁶Sr} < 0.7035, and the other with {¹⁴³Nd}/{¹⁴⁴Nd} < 0.5120 and {⁸⁷Sr}/{⁸⁶Sr} > 0.715. The more radiogenic component could be mantle enriched in incompatible elements during the Precambrian, or Precambrian materials of the continental crust. A quartz-rich xenolith found in the Columbia lavas has Rb-Sr and Sm Nd model ages of ≈ 1.4Æ, implying the existence of old, isotopically evolved crustal basement which could serve as contaminant. Nevertheless, crustal contamination alone cannot explain the chemical variation of the samples studied, and other fractionation processes must have occurred simultaneously. A model involving combined assimilation and crystal fractionation reproduces the chemical and isotopic characteristics of the volumetrically dominant Grande Ronde unit for an assumed crystallizing component of plagioclase, low calcium pyroxene and minor olivine. The data are not consistent with the suggestion that a 'primordial' mantle is the source for this continental flood basalt province. Rather they suggest that the main volume of these lavas was originally derived from a mantle similar in isotopic composition to island arc and ocean island basalts of the north Pacific. The primary magma was modified chemically and isotopically by crystal fractionation and assimilation of sialic crustal materials during its transport through, or storage in the continental crust.