Helium: problematic primordial signals

Details Helium: problematic primordial signals Helium: problematic primordial signals

Aug 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986e%26psl..79....1z&link_type=abstract

Earth and Planetary Science Letters, Volume 79, Issue 1-2, p. 1-8.

Physics

36

Scientific paper

Various lines of evidence suggest that pre-eruptive degassing of basalts is not only important but may dominate the He flux from the mantle to the oceans and atmosphere. These include: (1) correlations between U/4He ratios and Mg# in volcanic suites, which suggest that U/4He increases as differentiation proceeds; (2) the observation that U/4He ratios in MORB average about a factor of 400 lower than those from Loihi Seamount, although 3He/4He ratios suggest that Loihi is derived from a source with a lower U/4He ratio than is appropriate for the MORB source; and (3) the 3He budget of the oceans which, as defined by Craig and Lupton [6], requires that MORB is on average 70-90% outgassed in He. These observations suggest, as noted in [1], that mantle 3He/4He ratios may be subject to perturbation due to radiogenic accumulation of 4He in systems where U/4He has been increased by pre-eruptive degassing.
Models involving continuous diffusive loss (CDL) of He from a magma chamber and exhalative loss (EL) via solution of He in a CO2-rich gas or fluid phase are investigated. Low U/4He ratios in MORBs preclude significant pre-eruptive reduction of 3He/4He ratios; however, OIBs with U/4He > 106 may be subject to significant reduction of 3He/4He (>= 10%) with pre-eruptive aging over time periods ranging from 1 × 103 to 5 × 105 years.

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