Evolved Lavas from the Snake River Plain: Craters of the Moon National Monument, Idaho

Leeman, William; Vitaliano, Charles; Prinz, Martin

Chemical analysis and composition of lava and volcanic rocks; evolution and origin of CRMO (Craters of the Moon National Monument) lava fields discussed. Holocene lavas from Craters of the Moon National Monument are representative of “differentiated” lavas which occur around the margins of the Snake River Plains (SRP) and they range serially in composition from alkali- and phosphorous-rich ferrobasalts to ferrolatites. Petrographic study indicates that these lavas evolved primarily by cotectic crystallization of olivine, plagioclase, magnetite and apatite in the mafic members of the suite (ferrobasalts), and by cotectic crystallization of plagioclase, magnetite, clinopyroxene and minor olivine in the salic members. Quantitative phase relations in the CRMO lavas, calculated by means of a least-squares mixing program, indicate that the observed range in composition among these lavas corresponds to at least 70 percent crystallization of a magma similar to the most mafic CRMO lavas. Anhydrous one-atmosphere experimental crystallization studies fail to reproduce exactly the inferred phase relations; the discrepancy is attributed to the presence of water in the naturally crystallized magmas. The origin of CRMO parental magma cannot be unequivocally resolved. Available evidence suggests that DRMO lavas do not represent melts derived directly from the mantle: (1) high SR87/SR86 ratios (0.708 to 0.712), (2) relatively high Fe/(Fe + MG) and excluded-element content in even the most mafic CRMO lavas, (3) occurrence of megacrysts of inferred high-pressure origin in the Lava Creek flow, Megacrysts occur in the Lava Creek flow as clusters of labradorite, aluminous clinopyroxene, and olivine. Analogy with the experiments of Thompson [1] and least squares mixing calculations indicate that intermediate (ea. 8 to 10 Kbar) pressure fractionation of such megcrysts from olivine tholeiite magma may yield derivative CRMO-type liquids.

Evolved Lavas from the Snake River Plain: Craters of the Moon National Monument, Idaho

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