From dome to dust: shallow crystallization and fragmentation of conduit magma during the 2004-2006 dome extrusion of Mount St. Helens, Washington

Abstract

An unusual feature of the 2004-6 eruptive activity of Mount St. Helens has been the continuous growth of successive spines that are mantled by thick fault gouge. Fault gouge formation requires, first, solidification of ascending magma within the conduit, then brittle fragmentation and cataclastic flow. We document these processes through field relations, hand samples, and thin-section textures. Field observations show that the gouge zone is typically 1-3 m thick and that it includes cataclasite and, locally, breccia in addition to unconsolidated (true) gouge. The gouge contains multiple slickenside sets oriented subparallel to each other and to the striation direction, as well as surface striations parallel to extrusion direction. Hand specimens show the cataclasite and gouge to be composed of a wide size range of broken dome and wallrock fragments. This grain-size heterogeneity is even more pronounced in thin section, where individual samples contain fragments that span more than four orders of magnitude in size (from more than 10 to less than 10 -3 mm). Textures preserved within the gouge zone provide evidence of different processes operating in time and space. Most individual fragments are holocrystalline, suggesting that crystallization of the ascending magma preceded gouge formation. Cataclasite samples preserve a wide range of clast sizes; pronounced rounding of many clasts indicates extensive abrasion during transport. Within the gouge, crystals and lava fragments adjacent to finely comminuted shear zones (slickensides) are shattered into small, angular fragments that are either preserved in place, with little disruption, or incorporated into shear trains, creating a well-developed foliation. Together, evidence of initial grain shattering, followed by shear, grinding, and wear, suggests extensive transport distances (large strains). Textural transitions are often abrupt, indicating extreme shear localization during transport. Comparison of groundmass textures from dome lavas and fault gouge further suggests that brittle fracture was confined to the upper 400-500 m of the conduit. Observed magma extrusion (ascent) rates of ~7 m/d (8×10 -5 m/s) permit several weeks for magma ascent from ~1,000 m (where groundmass crystallization becomes important) to ~500 m (where solidification nears completion). Brittle fracture, cataclastic flow, and shear localization (slickenside formation) probably dominated in the upper 500 m of the conduit. Comparison of eruptive conditions during the 2004-6 activity at Mount St. Helens with those of other spine-forming eruptions suggests that magma ascent rates of about 10 -4 m/s or less allow sufficient degassing and crystallization within the conduit to form large volcanic spines of intermediate composition (andesite to dacite). Solidification deep within the conduit, in turn, requires transport of the solid plug over long distances (hundreds of meters); resultant large strains are responsible for extensive brittle breakage and development of thick gouge zones. Moreover, similarities between gouge textures and those of ash emitted by explosions from spine margins indicate that fault gouge is the origin for the ash. As the comminution and generation of ash-sized particles was clearly a multistep process, this observation suggests that fragmentation preceded, rather than accompanied, these explosions.