New Mexico Geological Society Annual Spring Meeting — Abstracts

The 75 km long Carrizozo basalt is one of the longest subaerial lava flows in the United States. The flow has an estimated total volume of 4.3 km³, covering 330 km² with depths of 10-15 m (Allen, 1952).

Two transects across the flow were sampled, based on Renault’s (1970) early work: across the distal end (lower flow), where three lobes are present, and across the proximal region (upper flow), where two lobes are present. The first lobe has the lowest SiO₂ concentrations and the highest concentrations of incompatible elements, SiO₂ is higher and incompatible element concentrations are lower in each subsequent flow unit.

Geochemical correlation of the distal lobes with the proximal lobes can be achieved through the addition of crustal melts to mantle-derived basaltic melts. Our model incorporates 3% crustal melt of a gneissic xenolith from Kilbourne Hole (Beard et al., 1993), followed by 2-4% olivine fractional crystallization to produce the composition of the proximal lobes. Consequently, we propose that five batch-melting events occurred in the mantle. For the first three batches (lower flow), heating of the lower crust occurred, yet not significantly enough to cause crustal melting. When the final two batches (upper flow) passed through lower crust, contamination occurred because the crustal rocks had reached their solidi. Surprisingly, estimates of the duration of flow range between a few months (Zimbelman and Johnston, 2002) and 3 decades (Keszthelyi and Pieri, 1993), suggesting that melting and differentiation processes were geologically rapid.