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Lecture 19: Noble Gas Geochemistry

Lecture 19: Noble Gas Geochemistry. Questions What are the isotopic and elemental distributions of the noble gases He, Ne, Ar, Kr, Xe, and Rn in the Earth? What do the noble gases tell us about the present geodynamic structure of the Earth: Is the mantle layered?

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Lecture 19: Noble Gas Geochemistry

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  1. Lecture 19: Noble Gas Geochemistry • Questions • What are the isotopic and elemental distributions of the noble gases He, Ne, Ar, Kr, Xe, and Rn in the Earth? • What do the noble gases tell us about the present geodynamic structure of the Earth: • Is the mantle layered? • Is there a primordial component left in the mantle? • What do the noble gases tell us about the history of the Earth system: • Where did the atmosphere come from? • When did the earth form? • When did the atmosphere form? • Reading • A few things in Chapter 8 of White but there is also a good book by Ozima and Podosek, Noble Gas Geochemistry.

  2. Noble Gases • He: two stable isotopes, 3He and 4He • Ne: three stable isotopes, 20Ne, 21Ne, 22Ne • Ar: three stable isotopes, 36Ar, 38Ar, 40Ar • Kr: six stable isotopes, 78, 80, 82, 83, 84, 86, (but no interesting anomalies) • Xe: nine stable isotopes: 124, 126, 128, 129, 130, 131, 132, 134, 136 (and numerous interesting anomalies) • Rn: no stable isotopes (longest lived is 222Rn with half-life 3.8 days)

  3. Noble Gases • Atmospheric composition: • Elemental • He: 5.24 ppm = 2 x 10-5x solar • Ne: 18.18 ppm = 0.01x solar • Ar: 0.934% = 93400 ppm = 1000x solar • Kr: 1.14 ppm = 12x solar • Xe: 0.087 ppm = 6x solar • Isotopic • 4He/3He = 750000; 3He/4He = 1.3 x 10–6 = 1 RA • 20Ne/22Ne = 9.8, 21Ne/22Ne = 0.03 • 40Ar/36Ar = 295.5

  4. Geochemistry of He • 3He is not manufactured by any endogenous terrestrial process. • Essentially all 3He in the Earth is either primordial or brought in by cosmic dust • 4He is radiogenic, produced by all alpha-decays • 8 atoms of 4He per 238U, 7 for every 235U, 6 for every 232Th • He is the only noble gas that is able to escape from the top of the present atmosphere. • Residence time of He in atmosphere: • Mass of 3He = 1.3 x 10–6 * 5.24 ppm * 5.2 x 1021 g • Flux of 3He = 1000 moles/yr = 3000 g/yr • Residence time = 10 Ma • This makes He geochemistry pretty easy, because atmospheric contamination is usually negligible.

  5. Geochemistry of He • Just how bad is atmospheric contamination for other noble gases?

  6. Geochemistry of He • Even for He, OIB samples are often contaminated, but MORB are usually deep enough to give magmatic He Note systematically low R/Ra in HIMU OIB localities

  7. Geochemistry of He • MORB samples are relatively homogenous in 3He/4He, at 8±1 RA, except where hotspot contaminated (a circular argument?) • OIB samples have a wide range, from 1 - 50 RA. • Continental samples, sediments, etc. have very low 3He/4He (~0.01 RA), since they are totally degassed and contain only radiogenic 4He.

  8. Geochemistry of He • What do these He isotope ratios mean? They are a measure of time-integrated (U+Th)/3He ratio: • How can we change (U+Th)/3He ratio, so that with time we will develop reservoirs with different He isotope ratios? Two ways: • Add or subtract U and Th, add or subtract 3He. • Depleted upper mantle has ~2 ppb U, Th/U = 2.5, and 3He/4He = 8 RA. Presumably it has been degassed by cycling through mid-ocean ridges. • Nonvolatile elements are cycled between upper mantle and oceanic crust, but only leave the system if accreted to continents; whereas He that reaches ridges is lost to space. • All recycled materials are very low in 3He.

  9. Geochemistry of He • So where do OIB with 3He/4He = 30 RA come from? • If primitive mantle is involved, it has 21 ppb U and Th/U = 3.8; 3He/4He would be lower than depleted mantle, if 3He content were the same. • It follows that there must be an undegassed reservoir that retains much primordial 3He…this is the most straightforward way to explain the low (U+Th)/3He. • Don Anderson likes to suggest alternatives: • Subduction of 3He from cosmic dust…not practical given diffusion of 3He in subduction zones. • CO2 fluid inclusions in upper mantle with very low U content? • To remain undegassed, the reservoir must never be circulated past a mid-ocean ridge. Geochemists therefore presume it resides in the lower mantle and that convection is either layered or sluggish. • Increasingly popular heretical view: He more compatible than U during melting??

  10. Heat/Helium imbalance • A further geochemical argument for some layering in mantle convection considers the flow of heat and of 4He from the Earth • 4He is produced by radioactive decay and we know quite precisely how much heat is liberated per alpha particle for terrestrial U and Th abundances (10-12 J/4He) • Heat alone suggests layering: BSE U, Th and K produce 19.2 TW; crustal heat production is 5-10 TW, and a uniformly depleted mantle produces 7 TW. Another 2-7 TW is elsewhere. • The 4He flux from the mantle corresponds to only 2.4 TW of heat production • Terrestrial heat flux of 44 TW includes 5-10 from crust and 3-7 from core; mantle heat flux is 27–36 TW of which only 18–22 is primordial heat; radiogenic heat, then should be 9-14 TW • This implies a boundary layer in the mantle that passes heat (by conduction) but mostlyretains4He

  11. Geochemistry of He • So where does the 3He in the upper mantle come from? • Cycling time of upper mantle through ridge system is relatively short, about 500 Ma, so it should be thoroughly degassed. • 3He is not subducted or recycled, as far as we know • Hence 3He must be added to upper mantle from the undegassed part of the mantle. • Two basic mechanisms: diffusion across 670 km discontinuity, or transport by plumes (any part of plume that does not melt or reach surface adds its He to the upper mantle system). • So which of the mantle zoo species is the carrier of high 3He/4He ratios? • None of the end members DMM, HIMU, EMI, or EMII! • DMM is degassed and the others are probably all recycled crust of one kind or another. • But trace element ratios show no sign of a primordial component in OIB…is He decoupled from lithophiles? • Probably the abundance of He in the undegassed reservoir is so high that it can dominate He budget without showing up in other tracers

  12. Geochemistry of He • 3He/4He is one of the reasons people make up “internal components” like FOZO, C, and PREMA. • Do mixing hyperbolas curve the wrong way? Undegassed reservoir should be high in [He].

  13. Geochemistry of Ne • Neon can tell a very similar story to He, about degassing and radiogenic ingrowth • Advantage is that Ne has three isotopes • Disadvantage is atmospheric contamination • 20Ne is primordial and abundant, an a nuclide • 21Ne is manufactured in abundance by nucleogenic reactions in the mantle: • 18O(a, n)21Ne and 24Mg(n, a)21Ne • U drives both reactions: a-decay as well as neutrons from spontaneous fission of 238U (and 244Pu in the early days); O and Mg are the most abundant target atoms in the mantle. Production ratio of 21Ne/4He is ~10–7. • There may be a small nucleogenic production of 22Ne, but it is probably negligible.

  14. The Ne 3-isotope diagram • Nucleogenic production moves degassed mantle to the right; undegassed mantle moves less because of low U/22Ne.

  15. The Ne 3-isotope diagram • Note that atmospheric composition and solar composition lie (approximately) on a mass fractionation line • Any physical or chemical process, thermodynamic or kinetic, fractionates isotopes by mass and will change 20Ne/22Ne twice as much as 21Ne/22Ne. • (atm contains a little nucleogenic21Ne) • But if Earth started out with Solar Ne isotope composition, why is atmosphere heavier in isotopic composition? • Preferential degassing of light isotopes would make atmosphere lighter than mantle. • Best story is hydrodynamic escape: earliest earth atmosphere was dominated by H2, whose escape flux under influence of early solar wind was so big it could carry away other atoms along with it. Light Ne was preferentially lost from the earth, leaving an isotopically heavy atmosphere.

  16. The Ne 3-isotope diagram • Highest quality MORB data lie on a mixing line through atmosphere, even though erupted under several km of water! The least contaminated samples so far measured do not quite extend up to solar 20Ne/22Ne, perhaps due to 22Ne production, perhaps due to contamination of even the best sample. • Best MORB data are on the famous gas rich “popping rock” sample, 2PD43, from the Atlantic.

  17. The Ne 3-isotope diagram • Highest quality OIB data also lie on a mixing line through atmosphere, but with a clearly steeper slope than MORB. This mantle component apparently has solar 20Ne/22Ne, but less radiogenic 21Ne/22Ne, consistent with being less degassed. Quality OIB data are extremely hard to get for heavy noble gases, since subaerial eruptions are totally degassed and contaminated by air. Loihi is an exception, because it is submarine, and so less degassed on eruption.

  18. Coupled He-Ne systematics • Extrapolating each Ne measurement along a line through atmosphere to solar 20Ne/22Ne gives 21Ne/22Neextrap, which varies coherently with He isotopes and indicates two component mixing along southern mid-Atlantic ridge between MORB source and undegassed OIB reservoir.

  19. Argon Geochemistry • There are several aspects to Ar geochemistry, because 40Ar radiogenic production is so large. • Geochronology…we have already covered the K-Ar and 39Ar-40Ar methods. • Consider how difficult it is to get good mantle Ar signals, when the air is 1% Ar. • But it is not hopeless, since 40Ar/36Ar contrast between reservoirs is so big. • Correlation with Ne in popping rock data, where three-isotope systematics let us fix upper mantle value, shows that for upper mantle 25000 ≤ 40Ar/36Ar ≤ 45000 (atmosphere is 295.5!).

  20. Argon Geochemistry • We can also use the Ar 3-isotope diagram (in the few cases where good 38Ar data are available) or correlation with neon to get 40Ar/36Ar of plume component from Loihi data. Same story: undegassed, less radiogenic than MORB source. Possibly, air has higher 36Ar/22Ne ratio than solar, consistent with atmospheric mass fractionation by hydrodynamic escape.

  21. Argon geochemistry • Because we also have some idea of terrestrial 40K budget, we can confirm the existence of an undegassed lower mantle not from 40Ar/36Ar ratios but from total 40Ar abundance. Remember this box model?

  22. Terrestrial Xenology • Xe has many isotopes, but they fall into three groups: • The non-radiogenic isotopes 124Xe, 126Xe, 128Xe, 130Xe • 129Xe, the daughter of extinct 129I (half life 17 Ma) • The fission products 131Xe, 132Xe, 134Xe, 136Xe, produced in slightly different relative abundances by spontaneous fission of long-lived 238U and extinct 244Pu (half-life 82 Ma). • Pepin’s curse: “Let every element have isotope anomalies at every mass number…like Xenon.”

  23. Terrestrial Xenology • Fission yields: • Why no yield of 124, 126, 128, and 130? 1024 a 1020 a

  24. Terrestrial Xenology • Fundamental observation: MORB data differ from atmosphere, and show mixing between air and a component with excesses of both 129Xe (from 129I decay) and fissiogenic Xe isotopes (from Pu and/or U decay) Continental samples (granites, etc.) show only fissiogenic Xe from U decay. CO2 well gases from continents show mixing between continental and mantle components. No OIB has ever shown Xe isotopes different from air -- either totally contaminated or lower mantle has atmospheric composition, we do not know which!

  25. Terrestrial Xenology • Facts proven by the MORB Xe data: • Earth formed while 129I, half-life 17 Ma, was still alive. • Present atmosphere formed by degassing from interior of Earth while 129I was still alive, so that ingrowth of 129Xe in degassed residual mantle could generate high 129Xe/130Xe mantle. Having three clocks (I, Pu, and U), it is possible to constrain several times: – Atmosphere began to be retained (i.e. accretion was complete) just after moon-forming impact, 50 to 70 Ma after solar system formation at 4.556 Ga. – Degassing was then initially very rapid, with 80% of the remaining Xe transferred from upper mantle to atmosphere within the next 20-30 Ma (but not instantaneous, or mantle 129Xe would be much bigger). – Degassing since has been slow, but 99% of upper mantle Xe is now in the atmosphere. Possibly this was onset of layered convection and lower mantle retains 20% of original Xe.

  26. Terrestrial Xenology • Here is a simple degassing and gas-loss model: total loss of anything degassed until closure, total retention of everything since. • With suitable choice of initial Xe composition, 129I and 244Pu abundances, you can make both I and Pu clocks give same age, 90 Ma after origin of solar system

  27. Terrestrial Xenology • It is hard to tell how much of the fissiogenic Xe in the MORB data comes from early 244Pu decay and how much from continuing 238U decay over the whole age of the Earth. • However, very high precision data in 1998 showed that ~30% of the fissiogenic Xe in MORB is from 244Pu decay based on slightly different production ratios of 131Xe, 132Xe, 134Xe, and 136Xe. • Note air has only 244Pu-generated fission Xe.

  28. Terrestrial Xenology • Even better data from Hadean zircons, published 2004: clear evidence of 244Pu in the fission Xe.

  29. Terrestrial Xenology • Unlike Ne and Ar, the mantle composition for the nonradiogenic Xe isotopes is similar to atmospheric, not to solar. Hence either • (1) whatever fractionated the atmosphere in Ne and Ar was unable to separate Xe isotopes, presumably because Xe is too heavy to escape, or • (2) the upper mantle Xe is mostly recycled atmosphere because Xe is retentive enough to be subducted.

  30. Xe and the Open-system upper mantle • Recall the argument based on Pb and Th isotopes that there must be a leak from lower mantle to upper mantle of Pb. We made the same case for He. Likewise, you can construct a model that makes Xe isotopes work if upper mantle is an open system. • The open-system model is too hard to solve without the steady-state assumption, but even with this limit it is much more powerful than the residual (or He-leak only) model Sorry: transposed

  31. Attempts at Synthesis • So we have a problem: • We know there exists an undegassed reservoir in the Earth (from 40Ar), and we see noble gases derived from that reservoir in OIBs (based on 3He/4He, 21Ne/22Neextrap, 40Ar/36Ar). • But we do not see obvious evidence for a primordial reservoir in radiogenic lithophile isotope ratios (Sr, Nd, Pb), and the trace element ratios (Nb/U, Ce/Pb, etc.) in OIB sources are clearly not primordial. • Yet we know of no way to differentiate a reservoir without degassing it. • Perhaps OIBs sample only recycled material in lithophile elements, but noble gases somehow diffuse into their sources either as they traverse the lower mantle or across the boundary layer at 670 km. • Perhaps it is all in the mixing ratios…recycled crust is high in incompatible lithophile elements but very low in noble gases, so a small lower mantle component might only be seen in effect on noble gas isotopes. • Perhaps it can be done with different residence times for each system. • Perhaps noble gases are compatible at high pressure, so that early differentiation in a deep magma ocean could alter trace element ratios without removing noble gases. • Or something else: core pumping, non-chondritic Earth, various Andersonian ideas, etc.

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