Entrainment of country rock during basaltic eruptions of the Lucero volcanic field, New Mexico

As magma rises through the lithosphere it may entrain wall rock debris. The entrainment process depends on the local hydrodynamic regime of the magma (e.g., velocity, temperature, bulk density), the extent of interaction of magma with groundwater, and the mechanical properties of the wall rocks. Wall rock entrainment results in local flaring of dikes and conduits, which in turn affects the hydrodynamics of magma ascent and eruption. We studied upper-crustal xenoliths erupted from small-volume basaltic volcanoes of the Lucero volcanic field (west-central New Mexico) in order to assess the relative importance of various entrainment mechanisms during a range of eruptive styles, including strongly hydrovolcanic, Strombolian, and effusive processes. Total xenolith volume fractions ranged from 0.3-0.9 in hydrovolcanic facies to < 10(-4)-10(-2) in Strombolian facies. The volcanoes erupted through a thick, well-characterized sequence of Paleozoic and lower Mesozoic sedimentary rocks, so that erupted xenoliths can be correlated with sedimentary units and hence depth ranges. The abundance of xenoliths from a given subvolcanic unit was divided by that unit's thickness to obtain an average entrainment rate (xenolith volume fraction derived per unit depth in the conduit). Shallow (< 510 m) entrainment rates are very sensitive to the degree of hydrovolcanic activity. Deep entrainment is more sensitive to the mechanical properties of the wall rocks and, in the cases studied here, is thought to depend mainly on brittle failure related to offshoot dikes, pore pressure buildup, and thermal stresses. The entrainment rate can be used as a source term in multiphase numerical models of conduit flow. Based on our data, theoretical models of conduit flow and erosion are justified in neglecting the contribution of mass and momentum from entrained material during basaltic eruptions driven by magmatic volatiles, but not in eruptions driven by hydrovolcanic processes.