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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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The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW‐SE, parallel with the general trend of the Society Islands hotspot track. Rocks from this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K‐Ar radiometric dating we have established the age of volcanic activity. The oldest lavas ( ∼1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and a late‐stage, valley‐filling phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42–49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene‐phyric alkali basalts (Mg# 60–64), while the later valley‐filling lavas are basanitic (Mg# 64–68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic compositions also change systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield‐building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K‐Ar ages and magnetostratigraphic boundaries allow high‐resolution age assignments to this ∼0.7‐km‐thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0.9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield‐building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25 to 50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognize a quasi‐periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25±10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield‐building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB‐like. We propose that the Society plume is radially zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising...