ORIGINS OF COMPOSITIONAL VARIATIONS IN SMALL VOLUME BASALTS: ELEMENTAL, ISOTOPIC AND MELT INCLUSION EVIDENCE FROM THE ICE SPRINGS BASALT FLOW, UTAH

Primitive basalt flows from small volume monogenetic craters are often compositionally heterogeneous, and a major question is resolving the role of crustal assimilation in producing this heterogeneity. A good example is the Ice Springs flow – a young, small volume, primitive (~8% MgO) basalt flow in the Black Rock Desert of Central Utah that contains abundant crustal xenocrysts/xenoliths. The flow has two distinct flow lobes – an early erupted high SiO₂ (~51%), high Rb/Nb (~1.5) lobe and a later low SiO₂ (~49%) low Rb/Nb (~1.0) lobe, both with the same MgO. REE data suggest similar degrees of melting in the spinel peridotite stability field for both flow lobes. Variations within each lobe are consistent with olivine fractionation and minor assimilation of the entrained crustal xenoliths. Although the major and trace element compositional differences between the flow lobes are broadly consistent with progressive assimilation of crust, the high silica lobe has a higher εHf and lower ⁸⁷Sr/⁸⁶Sr – inconsistent with assimilation of these crustal xenoliths. Assimilation could have occurred in the lower crust, although xenoliths with the required isotopic composition have not been found. A surprising observation is that olivine hosted melt inclusion compositions are the same for both flow lobes and are similar in composition to the low SiO₂ lobe. If the high SiO₂ composition of the early flow lobe is the result of greater lower crust assimilation, then this assimilation must have occurred after olivine crystallization or the olivines are xenocrystic/antecrystic. Alternatively the compositional variations between the two flow lobes are the result of melts derived from a heterogeneous mantle source. In this case, mixing between melts in the plumbing system could explain the presence of ‘low-Si’ melt inclusions in olivines in the ‘high-Si’ flow lobe. This study highlights the complexities in resolving how mafic magmas interact with the crust during ascent and suggests that AFC (Assimilation Fractionation Crystallization) processes may be subordinate to mantle processes in the generation of these small volume basalts.