Transcript Helium - Adrian Jones - Deep Carbon Observatory

DCO Summer School:
He isotopes, diamonds and deep
carbon isotopes
and Yellowstone!
APJones
Helium isotopes in igneous rocks
•
3He
•
4He
•
•
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 socalled “cosmogenic” 3He may be important in rocks that have lain at the surface of the Earth for
long periods.
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 MORBsource 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 protoIceland 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 largevolume 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 a
geochemical 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 coremantle 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 mantle
R/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 Yellowstone
JB 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 product
Diagram from Mikhail et al (2014)
Noble gases, hydrocarbons in mantle diamond
Hydrocarbons have occur in mantle diamond
with fluid inclusions.
(eg Kopylova et al EPSL 2010
126-137)
Figure 6. Calculated isotopic
evolution of methane and Fe-carbide
relative to diamond as a function of
Rayleigh fractionation
Mikhail et al 2014
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)