Abstract:
In this paper we present what is, to the best of our knowledge, the first comprehensive
study of clinopyroxenes and plagioclases contained in the flows of the Grande Ronde
Basalt member of the Columbia River Basalt Group (northwestern USA). The rocks
have MgO (wt%)<6, and trace amounts of Cr and Ni. About 56% of extracted solid
containing normative clinopyroxene and plagioclase explains the liquid line of descent
from the more mafic sample (MgO¼5.89 wt%) to the most evolved. The most
ubiquitous phases in the basalts are plagioclase and augite. Ilmenite and magnetite are
accessories in all rocks. Olivine is present in small amount only in one sample (RT
89-7). Based on principles of Ca–Na plagioclase–liquid exchange, estimates of preeruptive
magmatic water are <2.4 wt%. From clinopyroxene–liquid equilibria,
calculated pressures and temperatures of ascending magmas are between 1 atm and
0.617 GPa, and 1068 C and 1166 C, respectively. Compositions of magnetite–ilmenite
pairs and olivine–clinopyroxene–oxide assemblages yield post-eruptive oxygen
fugacities of NNO=-1.923, and one pre-eruptive value of NNO=-2.455. A
simple model of asthenospheric melting and magma ponding in the lower crust fits the
physical parameters.
