Gabbroic xenoliths from the Northern Gorda Ridge: Implications for magma chamber processes under slow spreading centers

Abstract

Abundant gabbroic xenoliths in porphyritic pillow basalt were dredged form the northern Gorda Ridge. The host lava is a moderately fractionated, normal mid‐ocean ridge basalt with a heterogeneous glass rind (Mg numbers 56–60). Other lavas in the vicinity range from near primary (Mg number 69) to fractionated (Mg number 56). On the basis of textures and mineral compositions, the xenoliths are divided into five types: Type A xenoliths consist of large clinopyroxene oikocrysts, enclosing euhedral to subhedral plagioclase. Olivine and glass are present only in trace amounts. Type B xenoliths consist of olivine and plagioclase (±spinel) with abundant intersertal glass. Partially resorbed olivine with deeply embayed margins in places subophitically encloses small plagioclase crystals. Olivine and large plagioclase crystals often contain glass inclusions and/or spinel. Type C xenoliths are a composite of types A and B, with areas of large clinopyroxene oikocrysts adjoining areas of loosely joined clusters of olivine and plagioclase in glass. Type D is represented by a single large xenolith that, except for containing a larger proportion of crystals, appears identical to the host lava. Type E is represented by a single small xenolith that is mineralogically similar to type B but with a distinct, fine‐grained, tightly interlocking texture, formed by small olivine oikocrysts enclosing euhedral plagioclase, in a small amount of intersertal glass. Many single crystals and glomerocrysts in the host lava appear to be xenocrysts. Some forsteritic olivine and anorthitic plagioclase contain glass inclusions more primitive than the host lava, while rare glass inclusions in skeletal olivine suggest entrapment of melt more evolved than the host lava. The overabundance of plagioclase crystals and their textural features suggest accumulation. The xenoliths are not cognate to the host lava, but they are genetically related. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. Plagioclase‐rich leucogabbro layers in ophiolites attest to similar processes in magma chambers of past spreading centers. However, xenoliths in ferrobasalt from the southern Juan de Fuca Ridge differ significantly from the Gorda Ridge xenoliths and suggest that the magma chamber at the southern Juan de Fuca Ridge is located at shallower depth and in cooler crust than the one at the northern Gorda Ridge.