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We used a wavelength dispersive microprobe to describe patterns in otolith microchemistry of sockeye salmon (Oncorhyncbus nerka). Patterns in Sr/Ca ratios in transects across otoliths were consistent with changes in environmental chemistry associated with life history. The patterns we observed were consistent with those described for other salmonids. We found that Sr/Ca ratios in otolith primordia of fish of known anadromous origin were significantly higher than those in otolith primordia of fish of known resident origin. However, variation among samples was associated with differences in chemistry of the freshwater environments. These differences could confound attempts to discriminate sympatric resident and anadromous forms in lakes where Sr/Ca ratios are high. Samples used to discriminate origin of emigrants from Redfish Lake, Idaho, confirmed the presence of both resident and anadromous fish, but the two groups were not clearly resolved. Otolith microchemistry has the potential to determine the parental origin of O. nerka and to describe life history patterns, but may not clearly identify all individuals in all lakes. More work is needed to determine the inherent variability among individuals and among populations and to determine the influence that migration, spawning, and incubation environments have on the chemistry of otoliths.

[1] 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.

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