Figure 6.

Plot of ³He/⁴He (R/R_a) versus CH₄/³He for whole-rock samples from the Mid-Cayman Rise and Zambales ophiolite. Except for the gneissic metagabbro sample, all samples exhibit CH₄/³He ratios that are significantly higher than the average $\mathrm{\Sigma }{\text{CO}}_2{/}^3\text{He}$ ratio for mid-ocean ridge basalt (MORB). High CH₄/³He ratios may reflect inefficient helium extraction by crushing during the helium analyses or addition of He to the vent fluid from a source other than fluid inclusions. See text for discussion. (Online version in colour.)