One of the major question with respect to genesis of flood basalts is whether crustal mixing plays a significant role during formation. The specific chemical characteristics of the Re-Os isotopic system make it ideal for addressing this question. During mantle melting Re behaves incompatibly, whereas Os behaves as a compatible element, such that very high Re/Os ratios are produced in the melt (Heterogen et al., 1980). As a consequence, crustal rocks will evolve significantly more radiogenic Os isotopic compositions. These differences will result in distinct Os isotopic compositions over short periods of time as compared with Sr and Nd.
The young age, moderate size and accessibility of the Columbia River Flood Basalts (CRB) have allowed workers to generate a comprehensive stratigraphic framework based on detailed fieldwork, petrography, magnetic stratigraphy and chemistry (Hooper, 1988). Different investigations of the Columbia River Flood Basalt (CRB) terrain using the same chemical and isotopic (Rb-Sr, Sm-Nd and U-Pb) techniques have reached contradictory conclusions. The debate over formation of flood basalts in the western US hinges on the relative influences of mixing of heterogeneous mantle domains versus varying degrees of crustal assimilation. Different investigations of this well defined flood basalt sequence using similar chemical and isotopic (Rb-Sr, Sm-Nd and U-Pb) techniques have reached contradictory conclusions (for more thorough reviews see Carlson and Hart, 1988; Hooper and Hawkesworth, 1993).
Some workers contend that variations in basaltic compositions of the CRB can be explained by progressive crustal contamination of a single or multiple (Carlson and Hart, 1988) mantle sources. Carlson and Hart (1988) identified three distinct basalt source components using chemical and isotopic criteria. These source components combined with varying degrees of crustal assimilation and fractional crystallization are used to explain the different compositions of flows within the CRB.
Other researchers, believe that variation in chemical compositions of the basalts can be accounted for by differing degrees of fractional crystallization and mixing of different end members of a heterogeneous mantle. Hooper and Hawkesworth (1993) employ subcontinental mantle compositions enriched by earlier subduction events at three different periods and argue that there is little direct evidence of, or need for, the assimilation of crustal rocks with possible exception of the Saddle Mountains Basalt.
We are undertaking a systematic investigation of CRB terrain in the western United States utilizing the Re-Os isotopic system. Preliminary Re-Os isotopic results on the earliest Imnaha and Grande Ronde series yield 187Os/188Os initial ratios of ~0.149-0.220. These elevated 187Os/188Os isotopic ratios are significantly higher than any ratios reported for sub-continental lithospheric mantle or enriched heterogeneous mantle sources (Walker et al., 1994). This suggests that the Columbia River flood basalts most likely contain a significant crustal component.
Carlson, R. W. & Hart, W. K., In Continental Flood Basalts (Macdougall, J. D., ed.), 35-61 (1988).
Heterogen, J., Janssens, M-J. & Palme, H., Geochim. Cosmochim. Acta. 44, 2125-2143 (1980).
Hooper, P. R., In Continental Flood Basalts (Macdougall, J. D., ed.), 1-35 (1988).
Hooper, P. R. & Hawkesworth, C. J., J. Petrol. 34, 1203-1246 (1993).
Walker, R. J., Morgan, J.W., Horan, M.F., Czamanske, G.K., Krogstad, E.J., Fedorenko, V.A. & Kunilov, V.E., Geochim. Cosmochim. Acta. 58(19), 4179-4155 (1994).