Petrological variability of recent magmatism at Axial Seamount summit, Juan de Fuca Ridge

Abstract

A combined study of mapping, observational, age constraint, and geochemical data at the summit of Axial Seamount, Juan de Fuca Ridge, has revealed its recent petrological history. Multiple basalt types erupted at the summit in a time sequence. At least three different magma batches have been present beneath the Axial Summit caldera during the last millennium, each with a range in differentiation. The first, prior to 1100 CE, was compositionally diverse, dominantly aphyric T‐MORB. The second, from ∼1220 to 1300 CE, was dominantly plagioclase‐phyric, more mafic N‐MORB erupted mostly in the central portion of the caldera. Since ∼1400 CE, lavas have been more differentiated, and nearly aphyric T‐MORB mostly erupted in the caldera's rift zones. Parental magmas vary subtly due to small coupled differences in the degree of melting and sources, but all share a uniform differentiation trend indicating pooling at similar depths. Thus, melts percolate through melt‐rich lenses that remain partially isolated in space and/or time. Centennial magmatic timescales at Axial Seamount are similar to those for fast spreading ridge segments. The fluctuation between aphyric and plagioclase‐phyric lava likely reflects different pathways or velocities of melt migration.

Voluminous eruption from a zoned magma body after an increase in supply rate at Axial Seamount

“…We interpret that this trend reflects that the 2015 dike tapped different parts of the chemically zoned subcaldera magma reservoir along its length. The southernmost eruptive fissures were rooted in the MCS‐imaged high‐melt zone of the magma reservoir beneath the SE caldera (Figure ), where the hottest and most mafic magma is stored [ Clague et al ., ; Dreyer et al ., ]. This interpretation is consistent with the coeruption seismicity that showed dike propagation southward into the high‐melt zone (south of the 2015 eruptive fissures) early in the sequence, before stalling [ Wilcock et al ., ].…”

Contrasting styles of deep-marine pyroclastic eruptions revealed from Axial Seamount push core records

“…A lithic interpretation is consistent with a high proportion of block-sliver shaped shards compared to fluidal components, the latter being more juvenile in origin. Moreover, individual lava flows on Axial Seamount do not exhibit chemical heterogeneity Dreyer et al, 2013) to the extent of vitriclasts in HMT, arguing against a chemically heterogeneous source magma. Such chemical heterogeneity in HMT is likely not from surface processes (cf.…”

“…The common presence of bioturbation and lamination, and overall heterogeneous geochemistry (also see Dreyer et al, 2013) of shards in TM lithofacies imply that it represents a mixed deposit of ash-laminated mud. Sediment redistribution is supported by broad modal grain size peaks (poor sorting) and tendency to bimodal distributions (Fig.…”

“…It is the youngest seamount formed by the Cobb hotspot, which has created a chain of submarine volcanoes that stretches for 1800 km to the northwest of the JdFR to the Aleutian Trench (Desonie and Duncan, 1990). Axial Seamount produces normal to transitional MORB that is slightly more enriched and primitive than basalts generated immediately to the north and south along the spreading ridge axis (Chadwick et al, 2005;Dreyer et al, 2013). Glass inclusions hosted by plagioclase crystals Fig.…”

“…2A). These clastic deposits are typically much thicker (∼60 to >150 cm) on the rims of the caldera than on the caldera floor (<25 cm), where younger flows are concentrated Dreyer et al, 2013).…”