EVALUATING THE ORIGIN OF COLUMBIA RIVER BASALTS: INSIGHTS FROM MINERAL SCALE ISOTOPE VARIATIONS

Mineral scale isotopic variations have been invaluable in identifying and constraining petrogenetic process affecting the Columbia River Basalt Group (CRBG). Plagioclase crystals in early-erupted Imnaha basalts retain a range of 87Sr/86Sr signatures (0.7038 to >0.7060) that suggest plume-related magma resided in crust located east of or along the eastern margin of the North American craton. Emplacement of Imnaha-related dikes heated local country rocks to the point where mineral components, such as individual plagioclase crystals, could crystallize in a range of magmas reflecting a continuum from uncontaminated to substantially contaminated compositions existing at fine-scales. These fine-scale magma variations allowed for a range of plagioclase 87Sr/86Sr ratios to be inherited while retaining relatively limited variations in An contents. Similar plagioclase 87Sr/86Sr variations are observed in later-erupted Grande Ronde basaltic andesite and Wanapum basalt. Crystal isotope profiles range from constant higher ratios (vs. host groundmass) seen from core to rim, which are consistent with crystal growth in fine-scale magma/crustal mixtures, to 87Sr/86Sr ratios which increase from core to rim in which rim 87Sr/86Sr signatures are in near equilibrium with host groundmass. In these cases however, magmas retain generally higher 87Sr/86Sr consistent with greater overall amounts of crustal assimilation affecting the magma as a whole. Crystals from lavas of the latest-erupted Saddle Mountains retain a dramatic range of isotope variations. 87Sr/86Sr profiles include those with constant ratios from core to rim with groundmass 87Sr/86Sr that is both higher and lower, ratios which range from low 87Sr/86Sr in cores to high 87Sr/86Sr in rims, which presumably result from progressive contamination by radiogenic crust during growth, and ratios in which 87Sr/86Sr decreases towards rims, consistent with probable recharge during crystal growth. Overall, in situ 87Sr/86Sr ratios are consistent with plagioclase growth in fine-scale magma/cratonic crust mixtures early in the history of CRGB eruptions (Imnaha) while continued heating of crust promoted greater, large-scale crustal involvement in products of later eruptions (Grande Ronde, Wanapum, and Saddle Mountains).