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Poreda, R.J., 1983

Helium, neon, water, and carbon in volcanic rocks and gases

Bibliographic Reference

Poreda, R.J., 1983, Helium, neon, water, and carbon in volcanic rocks and gases: University of California, San Diego, Ph.D. dissertation, 215 p., illust.


This research has been primarily concerned with the applications of isotopic analysis of volatiles to the understanding of specific petrologic problems. Helium isotope results for a suite of samples from the Mid-Atlantic Ridge, 52°-73°N, confirm the existence of an enriched mantle source region (relative to MORB) centered on Iceland. The Reykjanes and Kolbeinsey Ridges, with 3He/4He ratios of 10.5 to 16.1 times the atmospheric ratio (10.5-16.1 RA), are unique among all mid-ocean ridge segments so far studied, in displaying high 3He/4He ratios relative to MORB (R/RA = 8.0) along their entire length. In contrast, the Mohns Ridge, north of Jan Mayen Island, which is highly enriched in alkali elements, has 3He/4He ratios lower than MORB, indicating that there are at least two distinct types of enriched mantle ('hot spot') environments. The D/H isotopic ratio of the water is also significantly higher for the enriched regions of the Mohns and Reykjanes Ridges ([delta]D = -44 to -50‰), relative to typical MORB ([delta]D = -77‰). A study of the helium isotope ratios in fumaroles from active arc volcanoes in the Aleutians, Cascades, Mexico, and Guatemala indicates that the dominant source of the helium is the mantle. The 3He/4He ratios within the Alaskan-Aleutian Arc are very uniform for five summit fumaroles at 7.6 to 8.0 RA. These ratios reflect a MORB helium isotopic signature. The influence of sediment or continental crust on the measured 3He/4He ratios appears to be minimal for these samples. From a compilation of all arc data, there is a suggestion that the age of the descending oceanic crust may influence the 3He/4He ratio. For the arcs in which the descending crust is > 100 m.y. old (New Zealand, Marianas, Japan and Kamchatka), the 3He/4He ratios are lower and range from 5.7 to 7.3 RA. Helium and hydrogen isotope results for Mariana Trough and Lau Basin basalts indicate that these rocks represent contrasting back arc regimes. For Mariana Trough basalts, the elevated water contents (to 2.1 wt.%) and distinct D/H ratios ((delta)D = -46 to -32‰) indicate the presence of a component from the descending slab, while the 3He/4He ratios reflect typical MORB values (7.8 to 8.4 RA). . . .(Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI

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