Figure 4.

(A) Degassing drives residual magmas toward high values of He/Ne. Reference values are production ratios for ⁴He/²¹Ne and solar values for ³He/²²Ne. Gas evolved from decompressing magmas, and atmosphere/seawater values, are low. OIB fall in the field between MORB and air and may represent degassed magmas contaminated by atmospheric, xenolithic, and shallow mantle gases. (B) Degassing reduces the helium content of magmas and increases ratios such as ⁴He/⁴⁰Ar and ³He/²²Ne. Contamination with air- and fluid-filled inclusions (vesicles or vugs) decreases the noble gas ratios. Thus, both OIB and MORB can have higher ⁴He/⁴⁰Ar and ³He/²²Ne ratios than production ratios or primordial (solar) ratios.