Linking Zircon Crystallization to Magmatic Processes in Basalt-Dominated Lava Fields: The Eastern Snake River Plain–Craters of the Moon Testbed

Abstract Throughout the Quaternary, the Eastern Snake River Plain (ESRP) has been the focal zone of bimodal volcanism which filled an elongated downwarp within the northwestern North American continental lithosphere. In the wake of the southwest–northeast trending Yellowstone hotspot track and its silicic caldera volcanism, massive outpourings of olivine tholeiite lava occurred within the ESRP, but several prominent rhyolite domes and intermediate lava flows also erupted in its center. Along its northern margin, the Craters of the Moon volcanic field (COM) is also dominated by Middle Pleistocene–Holocene mafic lavas, but they comprise comparatively Fe-rich and Mg-poor trachybasalts and three evolved basaltic trachyandesite to trachydacite flows emplaced during its youngest eruptive episode. The occurrence of zircon crystals in these young lavas and rhyolite domes provides a unique opportunity to evaluate zircon crystallization in well-preserved and accessible basalt-dominated lava fields as analogs to large igneous provinces in the geologic past. Zircon crystallization ages determined by U–Pb and U–Th dating methods were paired with crystal-scale trace element and isotopic (δ18O, ɛHf) data. Additionally, the eruption age of the evolved COM Highway flow was determined by combined (U–Th)/He and U–Th dating. U–Pb data indicate crystallization ages between 1540 ± 10 ka in the oldest and 335 ± 3 ka (2se) in the youngest ESRP rhyolite domes which are consistent with reported K–Ar and 40Ar/39Ar eruption ages (1400 ± 28–327 ± 3.2 ka). Similarly, U–Th ages averaging ${6.2}_{-2.5}^{+2.6}$ ka compared to (U–Th)/He ages of 2.05 ± 0.56 ka (2se) for evolved COM lava also indicate crystallization with preeruptive residence over several millennia. Values for δ18O (ESRP: +1.89 to +4.32; COM: +5.13 to +6.03) and εHf (ESRP: −6.4 to −1.6; COM: −11.9 to −8.9) suggest that ESRP and COM magmas originated form a similar mantle source, but evolved via divergent assimilation and fractional crystallization paths. These paths for ESRP and COM magmas depend on the abundance of hydrothermally altered felsic rocks in the crust, which strongly influence ESRP magmas on the axis, but less so the COM magmas at the margins of the downwarp. Zircon crystallization postdates assimilation and is facilitated by progressive Zr enrichment due to extensive fractional crystallization. Parallel whole rock and zircon trace element (e.g. Zr/Hf, Eu/Eu*) fractional crystallization trends at nearly identical degrees of remaining liquid support autocrystic zircon crystallization and argue against crystal recycling from largely solidified crystal mushes. In a global comparison to volcanic rocks from large igneous provinces, such high-Zr mafic-intermediate lavas are scarce. Thus, autocrystic zircon in mafic lavas are clearly exceptional and should be scrutinized against potential contamination using trace element or isotopic data. Unless Zr abundances are exceptionally high, zircon saturation in mafic magmas only appears achievable in interstitial melt pockets in slowly crystallizing sills or intrusions.