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. 1998 Aug 4;95(16):9087–9092. doi: 10.1073/pnas.95.16.9087

Figure 3.

Figure 3

Trade-off curves between U/4He (ppm g/cc) and time, which give high and low 3He/4He reservoirs [modified from Zindler and Hart (42)]. The ordinate is the present day U/4He ratio. Ranges for MORB, OIB, and near-ridge seamounts are shown to the left. The dashed horizontal line is based on a composition of the MORB-reservoir (DM) using U = 0.06 ppm and [He] = 8 × 10−6 std cc/g (23, 34). These values correspond to 20% melting of DM to provide values in the range of MORB observations. The absolute values do not matter for the growth of 4He/3He, only the 238U/3He or 238U/4He ratios and time are involved. For this value of U/4He, the 3He/4He ratio declines from an assumed initial value of 100 Ra to MORB-like values in ≈6 × 108 years. The range of observed U/4He in MORB, if typical of the DM source, give a range of aging times from ≈108 to >109 years, to evolve from a high R source to MORB-like values. Reservoirs with U/4He ratios <104 (ppm g/cc) can maintain nearly their initial values for times in excess of 50 million years. If high and low R basalts come from reservoirs that differ only in U/4He, then the difference must be 1 or 2 orders of magnitude to explain the observed spread in 3He/4He ratios. The triangular region to the lower left shows combinations of U/4He and time for which initial R values are essentially frozen in. The region to the upper right indicates conditions under which low R reservoirs can form. The large arrows to the right show the effects on U/4He of degassing and metasomatism with a gas-rich, low-U, fluid. Parameters used are 232Th/238U = 3.0 and Ro = 100 Ra (42).