Intrinsic Geochemical Variability in Volcanic Rift Zones - Delineating Complex Lava Flows and Magma Evolution within Sill and Dike Networks

Field and laboratory analyses of lava flows along the Great Rift of Idaho, USA are being conducted in the FINESSE (Field Investigations to Enable Solar System Science and Exploration) program to improve methods of surface mapping and to understand magmatic complexities in basaltic fissures. The ultimate goal is to have a better understanding, via analog studies, of planetary processes related to lunar floor-fractured craters (FFCs), linear rilles, and other potentially complex volcanic systems such as the Hortensius Domes, Gruithuisen Domes, and the Marius Hills. Analyses of over 200 samples obtained from relatively young lavas ( 2.1 - 2.3 ka) indicate three compositional groups: basalts, hawaiites and latites, with at least two separately erupted flows representing each type. The multi-element signature of each flow unit is associated with unique surface morphology and tortuous flow margins recognized in the field and in remote sensing imagery. The unique compositional and morphological signature of each lava flow thus enables high-resolution (m-scale) geologic interpretations and flow-margin mapping. Geochemical analyses confirm that relatively primitive basalts have greater intrinsic compositional variability than either the evolved hawaiites or latites in the same rift system. Geochemical variations in basalts, although not readily evident in lithology, are interpreted as being due to the generation of multiple small magma bodies in relatively deep lithospheric sources, compounded by crystal fractionation and magma mixing in relatively shallow, complex sill and dike networks. Geochemical evidence further suggests that some basaltic eruptions tapped magmatic sources that had previously erupted along nearby fissures. Hawaiites and latites are interpreted as variants related to more extensive fractionation and crustal assimilation in shallow sills. Compositional uniformity within these evolved types indicates that magma bodies remained separate prior to eruption, without the interaction of multiple dikes and sills. We suggest that similar compositional variants and potentially the processes of reservoir mixing, crustal contamination and fractional crystallization of magma in shallow dikes and sills may be common features of magmatic systems on the Moon and other planetary bodies.