6. Explain why there are variations in the isotope ratios of noble gases in different Earth reservoirs. How do these variations constrain models of Earth differentiation? Noble gases, such as He, Ne, Ar, and Xe, are unique elements in that they have full outer electron shells and are thus characteristically unreactive. Nonetheless, they still undergo isotopic mass fractionation and so can be used to constrain key differentiation processes and track different reservoirs within the Earth - each of which has a unique noble gas isotopic signature. Helium He has two isotopes: 3He which has a primordial source and thus does not increase in Earth reservoirs, and 4He which is produced by the radioactive decay of U and Th. He behaves incompatibly during mantle melting (more so than U or Th). Therefore, differentiation events such as mantle degassing result in preferential He loss, whilst U and Th remain in the mantle residue, the decay of which over time leads to the ingrowth of 4He and low 3He/4He ratios in a degassed source. MORB show 3He/4He ratios of around 8Ra, (where Ra is the 3He/4He of the atmosphere) whilst OIB show distinctively higher but more variable 3He/4He ratios with maxima at 8 and 13Ra. The higher 13R, maxima is thought to reflect a more primordial, less degassed deep mantle reservoir whilst the lower 8Ra value reflects a recycled oceanic crust/sedimentary source (continental crust is rich in U/Th which decay to 4He). Plume localities such as Iceland and Hawaii show elevated 3He/4He ratios indicative of sampling this deep mantle source as well as high U and Th concentrations indicative of a recycled component. Overall, this supports a two-layered mantle convection model as shown in figure 1. MORB OIB ARC Place 1 Recycling . C Entrainment
Neon Neon has three isotopes: 20Ne, 21Ne, and 22Ne. 20Ne and 22Ne are both primordial, whereas 21Ne is produced via nucleogenic reactions in the mantle. Ne also behaves incompatibly and so upon differentiation events the mantle is degassed and left depleted in Ne but enriched in Mg and U (21Ne parents) and so over time in grows to high 21Ne/22Ne ratios. Plume sources such as Iceland show lower 21Ne/22Ne ratios than MORB. This is further indicative of plumes sampling a deep undegassed source, whilst MORB are derived from a shallower degassed reservoir. 21Ne/22Ne ratios have also been shown to correlate well with 4He/3He ratios (figure 2), where samples with the most primordial 3He also having lower 21Ne/22Ne ratios (Moreria, 2013). This confirms isotopic mass fractionation of the light noble gases likely occurred during the same degassing differentiation event. 21 Ne 221 Ne * MORE PLUME 4 He / 3H Argon Argon also has three isotopes: 36Ar, 38 Ar, 40 Ar. 36Ar and 38Ar are both primordial so have invariant ratios within the mantle, whilst 40Ar is produced by the radioactive decay of 40K. Ar is more incompatible than K and so, as with He and Ne, a high 40Ar/36Ar ratio reflects a degassed source. Ratios of 40,000 observed