Plagioclase ultraphyric basalts (PUBs) have been sampled along most mid-ocean ridges with
ultraslow to intermediate spreading rates. Over the past 40 years, the prevalent models for their origin
assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of
plagioclase phenocrysts by floatation. However, when the global population of PUB lavas is examined,
this hypothesis becomes less compelling. PUB host lavas demonstrate a large range of compositions and
densities, similar to aphyric glasses from the same ridge segments. Most importantly, the majority of
PUB host liquids are less dense than their phenocryst cargo, meaning that plagioclase floatation within a
magma chamber cannot be the driving force for phenocryst enrichment. Furthermore, PUB lavas have
never been sampled on axis at fast-spreading centers or from locations with noted contemporaneous axial
magma chambers, where PUBs should be abundant if plagioclase is buoyant in mid-ocean ridge basalt
(MORB). Instead, we argue that the high modal abundance of plagioclase results from interaction
between magma and preexisting zones of crystal cumulates within the lower crust, possibly followed by
loss of olivine during magma ascent. PUBs erupt when the magma maintains an ascent velocity greater
than the settling rate of the plagioclase phenocrysts, which precludes long crustal residence times for
these magmas. In addition to being a proxy for lower spreading rates, our findings also suggest that PUB
eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit
system that bypassed an axial chamber.Keywords: Mid-ocean ridges, Plagioclase, Magma transport, PU
