Helium isotope geochemistry of oceanic volcanic rocks : implications for mantle heterogeneity and degassing
KeywordHelium isotopes; Geochemistry; Rocks, Igneous; Basalt; Volcanism; Chain (Ship : 1958-) Cruise CH115; Atlantis II (Ship : 1963-) Cruise AII107-6
The concentrations and isotopic compositions of helium have been measured in a number of mantle derived oceanic basalts. The goal of this research is to use the helium isotopic systematics to constrain the nature and origin of mantle heterogeneity in the oceanic mantle. Studies of helium partitioning in mid-ocean ridge basalt (MORB) glass, performed by crushing and melting in vacuo, show that a significant fraction of the helium resides within vesicles. Measured concentrations are therefore a function of original helium content, magmatic history, vesicle size and quantity, and grain size analyzed. The helium solubility inferred from the results is 3.7 x 10-4 cc STP/g-atm), which is significantly higher (by a factor of 5) than the enstatite value (Kirsten, 1968) most often used in the literature. Concentrations obtained from basaltic phenocrysts and glasses suggest that helium behaves as an incompatible element with respect to olivine, clinopyroxene, and plagioclase. Diffusion rates for helium in basaltic glass (in the temperature range 125-400˚C), determined using the method of stepwise heating, yielded an activation energy of 19.9 ± 1 Kcal/mole and 1nDo = -2.7 ± .6 (cgs units). Extrapolation of these results to ocean floor temperatures (0˚C) gives a diffusivity of 1.0 ± .6 x 10-17 cm2/sec, indicating that diffusion is an insignificant mechanism for helium loss from fresh basaltic glasses. The diffusion and partitioning studies suggest that these processes will not alter the helium isotopic ratios in basaltic melts. Therefore, the isotopic composition of the oceanic mantle can be inferred by extracting the helium from basaltic glasses and phenocrysts. A survey of the helium isotopic ratios in MORB glasses from all over the mid-ocean ridge system shows that there is considerable variation; the 3He/4He ratios range from 6.5 to 14.2 x atmospheric. The results from a number of oceanic island basalts show even more variability, with the 3He/4He ratios ranging from 5.0 x atmospheric (for alkali islands such as Gough and Tristan da Cunha) to 31.9 x atmospheric (for Loihi Seamount). The regional variability, and the correlations with 87Sr/86Sr can best be explained by the presence of three distinct reservoirs in the mantle which mix with one another. The three mantle source regions are believed to be 1) the depleted source for normal MORB (with 3He/4He -8.4 x atmospheric), presumed to be in the upper mantle; 2) an undepleted mantle reservoir with 3He/4He > 8.4 x atmospheric; and 3) a recycled oceanic crust reservoir with 3He/4He < 8.4 x atmospheric. A model for mantle structure that is consistent with the observations is proposed and discussed in light of the geophysical data. 3He concentrations for the different mantle reservoirs are inferred from the measurements, and suggest that the present-day 3He flux, and the 3He in MORB glasses, is ultimately derived from the lower mantle. Consideration of the 3He flux, available 3He/36Ar measurements, and the atmospheric 36Ar inventory, shows that present-day degassing rates are insufficient to generate the atmospheric argon. It is suggested that an episode of more rapid mant1e outgassing occurred in the past.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1982
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