DCO Summer School:He isotopes, diamonds and deep carbon isotopes

and Yellowstone!

APJones

Helium isotopes in igneous rocks• 3He is a so-called primordial isotope. It was made in the Big Bang and incorporated into Earth

during its initial accretion and in the subsequent long-term acquisition of “late veneer” material. 3He is not produced in any large quantities by radiogenic decay, and is thus not being added to Earth’s inventory at a significant rate. Nevertheless, a small amount is constantly being added to the surface of the Earth by interplanetary dust particles [Anderson, 1993] and by cosmic rays. This so-called “cosmogenic” 3He may be important in rocks that have lain at the surface of the Earth for long periods.

• 4He is a product of alpha decay of U and Th, and accumulates over time. This accumulation is most rapid in rocks that are rich in U+Th, but may be very slow in rocks that contain little U+Th. The U+Th content of mantle rocks and recycled material varies by 3 or 4 orders of magnitude and the opportunity therefore arises to develop large variations in 3He/4He ratios.

• The Earth is constantly degassing, which transports helium from the crust and mantle into the oceans and atmosphere. Because it is such a light atom, helium escapes from Earth and is thereby continually lost from the atmosphere. The approximate lifetime of helium in the atmosphere is ~ 1 to 2 Myr.

• The absolute abundance of helium in rocks is difficult to interpret since helium is so mobile. Thus, the 3He/4He ratio (R) is usually used as a proxy for 3He content. R is generally expressed as a multiple of the present-day atmospheric 3He/4He ratio, Ra, which is 1.38 x 10-6.

Observed 3He/4He

• The observed value of 3He/4He varies in terrestrial rocks. Some typical values for R/Ra are:

• Continental rocks (high-U+Th) << 1• Commonly assumed for mid-ocean ridge basalt (MORB)

8 ± 2• Average spreading ridge basalt 9.1 ± 3.6• Ocean island basalt (OIB) (“hotspot” rocks) ~ 5 - 42• The highest, non-cosmogenic value for R/Ra reported

for “hotspot” rocks anywhere on Earth (42) is from Iceland [Breddam & Kurz, 2001].

3He/4He not lower mantle?• a) high 3He/4He is observed in Samoan xenoliths that are known to

be of upper mantle origin,• b) high 3He/4He has been measured in diamonds known to have

been mined from pipes. (High 3He/4He has been reported in diamonds of unknown origin, but in these cases it is has been suggested that they may be “detrital” diamonds, i.e., they may have lain on the surface for a long time and acquired “cosmogenic” 3He. Although this is conjecture, it is safer to use diamonds known to be from pipes.)

• c) high 3He/4He is observed at Yellowstone, where extensive work has provided a strong case that the magmatic system there is lithospheric only [see Yellowstone page & Christiansen et al., 2002].

OIBs are less degassed

mid-ocean ridges exhale more 3He than hotspots [Anderson, 1998a; Anderson, 1998b].

Extremely high He isotope ratios in MORB-source mantle from the proto-Iceland plume

• ABSTRACT The high 3He/4He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earth's accretion. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earth's mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic 3He/4He ratios yet recorded. A strong correlation between 3He/4He and 87Sr/86Sr, 143Nd/144Nd and trace element ratios demonstrate that the 3He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest 3He/4He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a 3He-recharged depleted mantle (HRDM) reservoir may be the principal source of high 3He/4He in mantle plumes, and may explain why the helium concentration of the 'plume' component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure.

• F Stuart et al Nature 2003

The extent to which the 3He/4He isotope ratio can be used as ageochemical tracer to localise the 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…

Lithospheric/upper mantleR/Ra snapshot 2013

• Scotland upper mantle xenoliths R/Ra 3-6 (Kirstein et al 2004 GeolSocLond 223)

• Spain upper mantle xenoliths R/Ra 1.4-6.5 (Martelli et al 2011 JVGR)

• S Africa Roberts Victor R/Ra 0.4-4.7 • Siberian craton R/Ra 2.7-3.8(Day et al 2012 AGU abst

#V53A-2796)

• Lithospheric average R/Ra 6.1 (Gautheron and Moreira 2002 EPSL)

3He/4He Yellowstone

Prodigious degassing of a billion years of accumulated radiogenic helium at YellowstoneJB Lowenstern et al Nature 2014 (February)

Abstract 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.

Mantle carbon updates- high pressure research from diamond

• Diamond mineral inclusions, DMGC*. Diamond provides the oldest and deepest materials of Earth.– *support provided by DCO

• Fe metal/ carbide– Mikhail et al (2014) – G3 publication

• Diamond as a C and He reservoir• Isotopic C fractionation at High

pressure and high temperature (HPHT)– Mikhail (2014)

• Core planetary model – Mikhail (2014)– Wood (2013) RiMG 75 Carbon in Earth

book, DCO productDiagram from Mikhail et al (2014)

Noble gases, hydrocarbons in mantle diamond

Figure 6. Calculated isotopic evolution of methane and Fe-carbide relative to diamond as a function of Rayleigh fractionationMikhail et al 2014

Hydrocarbons have occur in mantle diamond with fluid inclusions.

(eg Kopylova et al EPSL 2010126-137)

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 terrestrial diamond. Earth Science Reviews, 126 . pp. 235-249. ISSN 0012-8252 (doi:10.1016/j.earscirev.2013.08.010)