Helium isotopic variations in the mantle beneath the central North Atlantic Ocean

Details Helium isotopic variations in the mantle beneath the central North Atlantic Ocean Helium isotopic variations in the mantle beneath the central North Atlantic Ocean

Mar 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982e%26psl..58....1k&link_type=abstract

Earth and Planetary Science Letters, Volume 58, Issue 1, p. 1-14.

Computer Science

86

Scientific paper

We have determined the helium concentrations and helium isotopic ratios for a set of 35 basaltic glass samples dredged from the Mid-Atlantic Ridge between 28° and 53°N latitude. Total helium concentrations vary between < 1 × 10-8 cm3 STP/g and 1.4 × 10-5 cm3 STP/g. Most of this variation can be accounted for by gas loss on the sea floor via vesiculation, since the samples with the lowest concentrations have extremely high vesicularities. Analysis of the plagioclase phenocrysts from one of the samples suggests that helium behaves as an incompatible element (KD << 0.01). Results of in vacuo crushing and melting experiments confirm that there is no helium isotopic fractionation during melt-vesicle or melt-phenocryst partitioning. The variations in 3He/4He ratio along the ridge therefore must be interpreted to be a result of heterogeneity within the oceanic mantle. The 3He/4He ratios for this set of samples range from 6.5 to 11.1 times the atmospheric ratio. These data, coupled with strontium isotopic analyses performed on the same samples by White and Schilling (1978), show the existence of two distinct geochemical provinces: the regions between 50-52.5°N and 27-33°N, which are characterized by a trend toward high 3He/4He ratios and high 87Sr/86Sr ratios, and the region between 33° and 50°N, characterized by a trend toward low 3He/4He ratios and high 87Sr/86Sr ratios. Since primitive mantle should retain more primordial 3He, the low 3He/4He trend requires the presence of a mantle reservoir with lower 3He/(Th + U) and higher Rb/Sr. The generation of this component requires separation between He and Th + U, either by selective loss of He (degassing), or the addition of Th + U. We believe that the most reasonable source for this component is subducted oceanic crust that is recycled back into the mantle.

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