Variations in δ56Fe during magma differentiation at Cedar Butte volcano: signatures of thermal diffusion?
The Snake River Plain (SRP) is a volcanically active region lying along the Yellowstone hot spot track. Cedar Butte is a SRP volcanic complex which erupted a compositional progression of lavas from initial high silica rhyolite to final basaltic trachyandesite in the late pleistocene (~0.4 Ma). The lavas form a curved array on a MgO vs SiO2 diagram, ruling out a primary role for mixing in producing this compositional sequence. This curvilinear variation is consistent with forward models of fractional crystallization from a tholeiitic basaltic parent (McCurry et al., 2008) implying that rhyolites and thus continental crust may form by extended fractionation from basalt. To gain additional constraint on how magma differentiation from basalt to rhyolite might occur, I undertook an investigation of non-traditional stable isotope ratios in a suite of Cedar Butte lavas; I analyzed iron isotope ratios at UIUC by high resolution MC-ICPMS while the same samples were sent to University of Saskatchewan for Ca isotope analysis by thermal ionization mass spectrometry (TIMS). Analyzed Cedar Butte samples ranged from 58 wt% SiO2 to 75 wt% SiO2. Results show a smooth, upwardly curving progression of δ56FeIRMM-14 with increasing SiO2. The most silicic samples have δ56Fe of ~0.40 ‰ while the basaltic endmember has a δ56Fe of ~0.10 ‰. δ44Ca values show a less systematic but still analogous pattern as those of Fe isotopes. This trend is unlikely to simply reflect fractional crystallization for two reasons: first, the predicted magnitude of fractionation of iron isotopes at inferred magmatic temperatures is smaller than observed. Second, the predicted sense of δ56Fe fractionation occurring during magnetite removal is opposite to that observed; heavier isotopes should preferentially partition into magnetite based on theory. An alternative mechanism for producing the compositional zoning and isotopic signature is a top down crystallization-reaction process involving a downward moving temperature gradient zone as suggested by Lundstrom (2009). Such a process differentiates the magma by a moving thermal migration zone while thermal diffusion isotopic fractionation leads to heavy isotope enrichment at the cold end of the gradient (in the most silicic material). Ultimately, the compositionally zoned mush is heated by arriving basaltic magmas leading to eruption.
volcanic rocks, Fe isotopes, Ca isotopes, Cedar Butte
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign