CRYSTAL-MUSH COMPACTION IN THE COHASSETT FLOW, HANFORD, WASHINGTON

Although the vertical chemical profile through the Cohassett flow beneath Hanford, Washington, is complicated and suggests that the flow was inflated by a central pulse of distinctly different composition magma, the profile through the central part has a simple shape that is consistent with compaction of crystal mush with upward expulsion of residual liquid.

As in most thick flood-basalt flows, the downward crystallizing roof zone (entablature) has a distinctly different texture from the upward accumulating floor zone (colonnade). The texture of the floor zone, where the compaction occurs, is most easily interpreted as resulting from recrystallization of material that sank from the roof zone as dense plumes of crystal mush. During recrystallization, the texture of the crystal mush becomes anistropic as a result of compaction. The network of plagioclase crystals surrounding granular pyroxene patches becomes horizontally flattened. The plagioclase crystals within the network, which are initially randomly oriented and separated by interstitial liquid, rotate into parallel alignment and wrap around the pyroxene patches in a manner resembling roof tiles. These tiled plagioclase laths have less residual liquid between them than do the more randomly oriented crystals in the less compacted rock. Plagioclase phenocrysts trapped in the compacting mush are rotated toward horizontal. The asymmetric distribution of patches of glass trapped on the lower side of horizontal plagioclase phenocrysts is evidence of upward migration of residual liquid during compaction.

Compaction profiles through the flow are determined using four independent, quantitative measures of textural anisotropy. These profiles agree well with the amount of compaction (maximum ~30%) indicated from the chemical profile. Quantitative modeling of crystallization of the central part of this flow indicates that the observed chemical profile can result from compaction as long as a touching framework of crystals forms early enough (~30% crystallized) for the mush to have a high permeability (~10^-9 m²). If crystal mush can undergo compaction in a flood-basalt flow, compaction is likely to be an important process of differentiation in intrusive bodies.