DCO Summer School: He isotopes, diamonds and deep carbon isotopes and Yellowstone!. APJones. Helium isotopes in igneous rocks.
DCO Summer School:He isotopes, diamonds and deep carbon isotopes and Yellowstone!
mid-ocean ridges exhale more 3He than hotspots [Anderson, 1998a; Anderson, 1998b].
The extent to which the 3He/4He isotope ratio can be used as ageochemical tracer to localisethe source and confirm the existence of mantle plumes at hotspots. R Farla Utrecht 2004
…In the classic model, a high helium ratio is an indicator for mantle plumes that reach the core-mantle boundary. However, growing evidence suggest that there cannot exist elevated 3He concentrations in the lower mantle. Instead, critics believe that a high 3He/4He ratio is due to lower 4He concentrations. The location where these lower 4He concentrations exist has been proposed to be in the upper mantle. This alternative model effectively rules out the need for core-mantle boundary mantle plumes at hotspots….
….The model whereby high 3He/4He is attributed to a lower-mantle source, and is thus effectively an indicator of plumes from the lower mantle, is becoming increasingly untenable as evidence for a shallow origin for many high-3He/4He hotspots accumulates. Shallow, low-4He models for high-3He/4He are logically reasonable, cannot be ruled out, and need to be rigorously tested if we are to understand the full implications of this important geochemical tracer…
Helium is used as a critical tracer throughout the Earth sciences, where its relatively simple isotopic systematics is used to trace degassing from the mantle, to date groundwater and to time the rise of continents1. The hydrothermal system at Yellowstone National Park is famous for its high helium-3/helium-4 isotope ratio, commonly cited as evidence for a deep mantle source for the Yellowstone hotspot2. However, much of the helium emitted from this region is actually radiogenic helium-4 produced within the crust by α-decay of uranium and thorium. Here we show, by combining gas emission rates with chemistry and isotopic analyses, that crustal helium-4 emission rates from Yellowstone exceed (by orders of magnitude) any conceivable rate of generation within the crust. It seems that helium has accumulated for (at least) many hundreds of millions of years in Archaean (more than 2.5 billion years old) cratonic rocks beneath Yellowstone, only to be liberated over the past two million years by intense crustal metamorphism induced by the Yellowstone hotspot. Our results demonstrate the extremes in variability of crustal helium efflux on geologic timescales and imply crustal-scale open-system behaviour of helium in tectonically and magmatically active regions.
Diagram from Mikhail et al (2014)
Hydrocarbons have occur in mantle diamond
with fluid inclusions.
(egKopylova et al EPSL 2010
Helium isotope ratios in diamond exceed the
ranges observed in all known igneous rocks,
We are just starting to undertsand the
significance of noble gases in diamond.
Basu et al (2013) An overview of noble gas (He,
Ne,Ar, Xe) contents and isotope signals in
Earth Science Reviews, 126 . pp. 235-249.
Figure 6. Calculated isotopic evolution of methane and Fe-carbide relative to diamond as a function of Rayleigh fractionation
Mikhail et al 2014