Ferro-andesites in the Grande Ronde Basalt: their composition and significance in studies of the origin of the Columbia River Basalt Group

Abstract

Flows from the Grande Ronde Basalt of the Columbia River Basalt Group include several with the chemical composition of ferro-andesites. These flows have SiO, > 55%, MgO < 4 % , and also have higher FeIMg than the average value for Grande Ronde Basalt. They are also distinctly richer in Cs, Rb, K, Ba, La, Ce, Th, and U than the remainder of the Grande Ronde Basalt flows, and possess small negative Eu anomalies. Their Pb isotopic compositions define a mixing line with a negative slope on a 206Pb/204Pb versus 207Pb/204Pb plot. Their Nd isotopic compositions lie between 143Nd/'44Nd = 0.51252 and 0.51264, and their Sr isotopic compositions lie between 87Sr/86Sr = 0.7055 and 0.7060. These values define the enriched end of the Columbia River Basalt Group spectrum on a Sr-Nd epsilon diagram (excluding the Saddle Mountains Basalt). These ferro-andesite flows also form a compositional end member of the Columbia River Basalt Group and are sufficiently distinctive to warrant special consideration. We compare them with lavas from other tholeiitic provinces. Petrogenetically, they can be related to the Grande Ronde Basalt low-Mg basalts by plagioclase fractionation, or by clinopyroxene fractionation in partially melted eclogites. However, the situation may not be that simple, as their unique isotopic compositions are closely related to those of parental mantle materials, perhaps thus requiring separate reservoirs and (or) sources. Extending this argument to other parts of the Columbia River Basalt Group suggests that the origin of these basalts may be in a system of comparatively small magma chambers in the uppermost mantle, frequently replenished from a variety of sources.RburnC : Plusieurs coulkes du Basalte de la Grande Ronde, qui est affilit au Groupe de Basalte de la Columbia River, ont la composition chimique des ferro-andksites. Ces coulkes ont des teneurs de Si02 > 55 %, MgO < 4 % , et des rapports Fe/Mg plus klevks que le rapport moyen fourni par le Basalte de la Grande Ronde. En outre, elles sont nettement plus riches en Cs, Rb, K, Ba, La, Ce, Th et U que tout le reste des coul6es de Basalte de la Grande Ronde, et elles affichent de faibles anomalies nkgatives en Eu. Leurs compositions isotopiques du Pb dtfinissent une droite de mklange avec une pente nkgative sur le diagramme 206Pb/2"Pb versus 207~b/204Pb. Leurs compositions isotopiques du Nd sont comprises entre 143Nd/'44Nd = 0,51252 et 0,51264, et leurs compositions isotopiques du Sr varient de 87Sr/86Sr = 0,7055 B 0,7060. Ces donnkes dkterminent le p61e enrichi de la courbe du Groupe de Basalte de la Columbia River dans le diagramrne epsilon Sr-Nd (excluant le Basalte de Saddle Mountains). Ces coulkes de ferro-andksite dtfinissent aussi un pBle chimique du Groupe de Basalte de la Columbia River, et elles sont suffisamment distinctes pour Ctre prises en considkration. Nous les comparons avec les laves de d'autres provinces tholtiitiques. Pktrogknktiquement, elles peuvent &tre relikes aux coulkes appauvries en Mg du Basalte de la Grande Ronde, soit par le fractionn...

The yield and isotopic composition of radiolytic H2, a potential energy source for the deep subsurface biosphere

“…The concentrations of radioactive elements along a depth profile were assumed constant and were derived from a spontaneous γ log of a Taylorsville basin borehole (Onstott et al, 1998). In scenario 2, several natural settings with dramatically different concentrations of radioactive elements, including carbon leaders in the Witwatersrand basin (South Africa) (Zumberge et al, 1978), granites in the Fennoscandian Shield (Haapala, 1997), basalts in the Columbia River Basalt province (Lambert et al, 1995), quartzites in the Witwatersrand Supergroup (Nicolaysen et al, 1981), and sediments in the Taylorsville basin (Onstott et al, 1998) were chosen to estimate the radiolytic H 2 production rates ( Table 3).…”