PETROLOGY AND PETROGENESIS OF ANDESITES FROM AXIAL SEAMOUNT, JUAN DE FUCA RIDGE

Although published studies provide a range of possible models that attempt to explain the origins of high-silica rocks in mid-ocean ridge (MOR) settings, surprisingly little data exist on the phase chemical variations in MOR andesites and dacites. Greater understanding of the petrogenesis of these high-SiO₂ lavas have been obtained through detailed phase chemical and trace element studies of andesites from Axial Seamount on the Juan de Fuca Ridge. Recently active Axial Seamount is largely comprised of relatively evolved and incompatible element enriched basalts likely fed from a large seismically-imaged magma chamber underlying a summit caldera. Recent eruptions along rift zones to the north and south have been associated with diking events emanating from below the caldera. Numerous ridges up to 15 km long and 150 m high parallel the modern rift zone and may reflect past diking events.

A 200 m high elongate ridge which may extend up to 5 km located about 3 km east of Axial’s North Rift neovolcanic zone, located at 46° 4’ N and 129° 57’ W was investigated with the ROV “Doc Ricketts” during a cruise on the R/V Western Flyer. Twenty-six andesitic pillow lavas were collected from the southernmost section of this steep-sided, off-axis volcanic ridge. Their SiO₂ contents range from 52 to 57 wt. % and MgO contents are less than 3 wt. %. Relative to other MOR andesites the samples have lower K₂O wt. %, and significantly higher FeO wt. %. Primitive mantle-normalized trace element abundances exhibit enrichments in U, Hf, and Zr and depletions in Sr and Eu relative to the rare-earth elements.

Initial petrography reveals complex zoning in clino- and orthopyroxene, and resorption of olivine in many samples. Preliminary phase chemistry shows Mg# in clinopyroxenes within crystals clots range from 60-85, Mg#’s in olivine from 45-68, and An content in plagioclase crystals from 55-70. The Mg# of the glasses are considerably lower – ranging from 16-26, indicating the crystals are not in equilibrium with the melt. Possible petrogenetic models for this large volume of evolved rocks include assimilation of altered oceanic crust by dikes coming from the main axial chamber, magma mixing, perhaps in conjunction with fractional crystallization, as fractional crystallization models require greater than 80% crystallization to reach the evolved compositions.