Distinct Magma Differentiation Pathways for the Eastern Snake River Plain and Craters of the Moon, Idaho, from Zircon Geochronology and Geochemistry

The Snake River Plain plateau and the adjacent Columbia River and Yellowstone volcanic fields form one of the world largest flood basalt provinces. Eruptive centers migrated eastward since the late Cenozoic following the Yellowstone hotspot, erupting an archetypical bimodal volcanic suite with compositions ranging from olivine-tholeiite to rhyolite. Up to 80% of the ~1600 km² eastern portion of the Snake River Plain (ESRP) is covered by mafic lava flows, from which four prominent Pleistocene rhyolite domes emerge. Closely associated with the ESPR is the Craters of the Moon (COM) volcanic field, which is considered the largest Holocene lava field in the ESRP and the United States, with more than 60 eruptions over the past ~15,000 years. Geochemically, COM lavas range from basalt and trachybasalt to trachyte.

To elucidate processes and timescales of evolved magma production, zircon from Quaternary rhyolites from the ESRP was investigated along with zircon from three of the most recent and evolved COM lava flows and those from a COM crustal xenolith. Approximately 300 individual zircon crystals were analyzed for U-Pb and U-Th geochronology, respectively, and paired with crystal-scale δ¹⁸O, ɛHf, and trace element analyses. Composite zircon separates from a COM lava flow were also analyzed by combined (U-Th)/He and U-Th dating to constrain its eruption age.

U-Pb zircon dating indicates crystallization ages between 1.54 ±0.01 Ma in the oldest and 0.335 ± 0.003 Ma in the youngest ESRP rhyolite dome, with ages increasing from west to east. For COM trachyte lavas, zircon U-Th disequilibrium corresponds to an age of  ka which along with an indistinguishable (U-Th)/He zircon age demonstrates that zircons crystallized in the evolved magma shortly before the eruption. With the exception of rare Precambrian basement-derived xenocrysts, evolved COM lavas have zircon δ¹⁸O values ranging from +5.33 to +6.02, consistent with fractionation from ESRP mantle-derived mafic magmas. On the other hand, δ¹⁸O(VSMOW) zircon composition between +1.73‰ to +4.51‰ for ESRP agree with low δ¹⁸O Yellowstone rhyolites with decreasing values from west to east. COM lavas display ɛHf zircon values between -12.0 to -8.9, whereas ESPR rhyolite ɛHf zircon ranges from -6.3 to -1.3. The crustal xenolith from COM yielded U-Pb zircon ages between 1.9 and 3.4 Ga, consistent with low ɛHf > -35. These results indicate different pathways of evolved magma production that differ between ESRP and COM.