Helium: Problematic primordial signals

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 Mγ 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, requires that MORB is on average 70-90% outgassed in He. These observations suggest, as noted, 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 1x103 to 5x105 years. (orig.)