CHEMICAL VARIABILITY WITHIN SCORIA CONES: NO MAGMA CHAMBERS?

Chemical heterogeneity is observed in the erupted products of fifteen southern Cascade quarried scoria cones. Variability can exist within the LILEs, HFSEs, radiogenic isotopes, or major elements. Simple modeling rules out fractional crystallization as the source of the chemical variations. Contamination and/or magma mixing may play a role in some of the variations, but cannot explain the most extreme variations we observe. The compositional heterogeneities occur on two scales. The largest differences are observed between the scoria and the late-stage lava flow. In some cases, the two magma types appear to be from different mantle sources and may have major isotopic differences. A smaller magnitude of variation is observed within the scoria itself, although up to a 2-fold variation of Ba/Nb has been measured within the scoria from a single cone. Here we present four different cones that are suggestive of three different sources of chemical variation. Two of the cones show contrasting magma types for the scoria vs. lava. A third cone shows variability of LILEs within the scoria (increasing over the eruption cycle), suggesting the tapping of a source heterogeneous in these components. A fourth, of LKOT composition, shows a 10% increase of REEs over the duration of the eruption, suggesting that changes in partial melting are recorded. The fact that these variations are observed at the surface questions the existence of magma chambers for these small-volume basaltic systems. Perhaps melts do not aggregate in a chamber but instead ascend to the surface in dike-like structures that prevent homogenization. The ability of one batch of magma to reach the surface may open up a pathway for other magma batches that may otherwise not reach the surface due to thermal constraints.