MELTING, FRACTIONATION, AND ERUPTION OF THE 5 KA CARRIZOZO LAVA FLOW, NEW MEXICO
Two transects across the flow were sampled: one at the distal end, where six units are apparent, and one at road cuts along Highway US 380, approximately 15 km from the vent, where 4 units are apparent. Analyses of 2-3 samples taken vertically at each lobe indicate less than 1% olivine fractionation within each lobe. Samples from the distal transect are more mafic (MgO=7.4-7.8%) than those from the proximal transect (MgO=5.9-7.3%), suggesting that proximal units represent magma that fractionated less than 5% olivine relative to distal units. However, the relationship between the proximal and distal units is complicated by lower concentrations of incompatible trace elements in the fractionated proximal lobes (i.e., Rb: 22-27 ppm proximal; 24-32 ppm distal).
Four separate partial melting events, each of subsequently higher degree of melting, are recorded in the incompatible trace element concentrations. The earliest flows have the lowest SiO2 but highest concentrations of incompatible elements; SiO2 is higher and incompatible element concentrations are lower in each subsequent flow unit. Basalt from the first and second melting events (Rb=28-32 ppm and 24-29 ppm, respectively) are found only at the distal end; no proximal samples (Rb=22-27 ppm) have higher Rb, despite their lower MgO. However, certain proximal units could be fractionates of the third melting event sampled at the distal end (Rb=24-28 ppm), which has similar Rb and lower Nb. The youngest proximal samples are the most fractionated (MgO=6.0%) yet have the lowest incompatible trace element concentrations (Rb=22-24), suggesting that they fractionated from a parental basalt produced by even greater degree of partial melting that has not yet been recognized or sampled at the distal end of the flow. These data suggest that individual mantle melting events produced flow units that fractionated slightly during eruption.