[1] This paper presents new magmatic helium isotopic compositions in a suite of lavas from phase II of the Hawaiian Scientific Drilling Project (HSDP2) core, which sampled Mauna Kea volcano to a maximum depth of 3098 m below sea level. Most of the measurements were performed by in vacuo crushing of olivine phenocrysts, but include submarine pillow glasses from the 2200 to 2500 meter depth interval, and orthopyroxene phenocrysts from an intrusive at 1880 m. The magmatic 3 He/ 4 He ratios range from 6 to 24.7 times atmospheric (Ra), which significantly extends the range of values for Mauna Kea volcano. The 3 He/ 4 He ratios are lowest (i.e., close to MORB values of $8 Ra) near the top of the Mauna Kea section and rise slowly, to 10-12 Ra, at 1000 m below sea level, consistent with results from the HSDP1 core. At depths greater than 1000 m in the core, primarily in the submarine lavas, there are brief periods when the 3 He/ 4 He ratios are higher than 14.5 Ra, always returning to a baseline value. Twelve such excursions were identified in the core; all but one are in the submarine section, and most (7) Nd, Zr/Nb, and SiO 2 . The correlations with major elements, trace elements and isotopes demonstrate that helium is coupled to the other geochemical variations, and that the Mauna Kea isotopic variability is caused by heterogeneities within the upwelling plume.
a b s t r a c tWe present the distributions of helium isotopes (in the form of helium isotope ratio anomaly relative to the atmospheric ratio) and tritium along two sections occupied in the subtropical North Atlantic as part of the first U.S. GEOTRACES survey (GEOTRACES GA03). The general distributions of these isotopes are consistent with the continuing penetration and evolution of bomb-produced tritium and its daughter isotope 3 He in the main thermocline and along the western boundary current system. We combine these two distributions to compute a tritium-3 He age, which is related to the elapsed time since the water was at the ocean surface. Although it is an indicator biased by the effects of mixing and influenced by the time history and spatial distribution of bomb tritium delivery to the ocean surface, it still remains a useful measure of ventilation time-scales. Aside from the continued propagation of the tritium-3 He transient into the ocean interior, there are three notable features of interest in these distributions. The first is the clear signature of upwelling in the water column near the coast of Mauritania, characterized by the upward bowing of isochrones in the thermocline and discernable 3 He excess at the ocean surface. A simple 3 He mass balance calculation suggests an upwelling flux of order 1.8 Â 10 6 m 3 s À 1 (1.8 Sv) along the Mauritanian coast. The second is a mid-depth ($ 1500-2000 m) core of ventilated waters centered over the Mid-Atlantic Ridge, an anticyclonic circulation of waters likely originating in the Labrador Sea. The third notable feature is a volcanic 3 He plume at about 3500 m depth emanating from the TAG Hydrothermal Area that is detectable as much as 500 km away on each side of the Mid-Atlantic Ridge. We estimate a 3 He:heat ratio of $ 7 Â 10 -18 mol J À 1 and a 3 He flux from the TAG site of $ 15 mmol y À 1 . Since 3 He is a conserved tracer in the absence of measureable tritium, the correlation of volcanic 3 He with other hydrothermally influenced TEIs (e.g., Fe) can be used as a dilution tracer as probe of non-conservative behavior in the water column. Also, since the regional and global fluxes of volcanic 3 He are known, the correlations can be used as a regional/global flux gauge for hydrothermal input of those TEIs.
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