Phenocryst Abundances and Compositions at the 9deg 03'N Overlapping Spreading Center: Implications for Magma Source Composition

Phenocryst abundances and compositions were analyzed in lavas recovered from the propagating east limb (EL) of the 9deg 03'N overlapping spreading center (OSC) and compared to associated glass compositions to explore the nature and variability of magmatism at the OSC and the phyric nature of some lavas. Samples were collected on- and off-axis along the propagating EL between 9deg 01'N to 9deg 10'N and include samples collected above a wide, off-axis melt sill in the northern portion of the overlap basin, as well as samples collected above the southern plunging off-axis melt sill. All of these lavas show regional variability in phenocryst abundance. Along-axis samples found north of approximately 9deg04'N are typically aphyric, while samples further south contain 3-7% phenocrysts on average. Off-axis samples collected within the north overlap basin are either aphyric or sparsely phyric. Plagioclase, clinopyroxene, and olivine are the dominant phenocrysts in these lavas, with minor amounts of FeTi oxides and pigeonite in some samples. Microprobe analyses show a comparable degree of evolution in cpx, plag, and olivine cores from all regions, despite the diversity of host lava compositions and variations in phenocryst rim compositions. This may indicate that most EL lavas originated from a common parental magma composition in terms of major elements, and subsequently experienced variable extents of mixing with more evolved magmas. Samples from the northern part of the EL, both on- and off-axis, likely erupted from the wide melt sill imaged beneath the axis and extending off-axis into the north overlap basin. Dikes feeding these eruptions may have traveled primarily vertically, and tapped the melt sill and magma pockets beneath them. Southern EL lavas may have instead erupted from lateral dikes that propagate along-axis southward from the more robust northern melt sill. These latter dikes may also incorporate some more crystal-rich magma from the low velocity zone beneath the melt sill; this may explain the phyric nature of samples collected from the southern end of the propagating EL.