Presentation on theme: "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."— Presentation transcript:
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, 3 He and 4 He Ne: three stable isotopes, 20 Ne, 21 Ne, 22 Ne Ar: three stable isotopes, 36 Ar, 38 Ar, 40 Ar 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 222 Rn with half-life 3.8 days)
3 Noble Gases Atmospheric composition: Elemental –He: 5.24 ppm = 2 x x solar –Ne: ppm = 0.01x solar –Ar: 0.934% = ppm = 1000x solar –Kr: 1.14 ppm = 12x solar –Xe: ppm = 6x solar Isotopic – 4 He/ 3 He = ; 3 He/ 4 He = 1.3 x 10 –6 = 1 R A – 20 Ne/ 22 Ne = 9.8, 21 Ne/ 22 Ne = 0.03 – 40 Ar/ 36 Ar = 295.5
4 Geochemistry of He 3 He is not manufactured by any endogenous terrestrial process. –Essentially all 3 He in the Earth is either primordial or brought in by cosmic dust 4 He is radiogenic, produced by all alpha-decays –8 atoms of 4 He per 238 U, 7 for every 235 U, 6 for every 232 Th 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 3 He = 1.3 x 10 –6 * 5.24 ppm * 5.2 x g Flux of 3 He = 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/R a in HIMU OIB localities
7 Geochemistry of He MORB samples are relatively homogenous in 3 He/ 4 He, at 8±1 R A, except where hotspot contaminated (a circular argument?) OIB samples have a wide range, from R A. Continental samples, sediments, etc. have very low 3 He/ 4 He (~0.01 R A ), since they are totally degassed and contain only radiogenic 4 He.
8 Geochemistry of He What do these He isotope ratios mean? They are a measure of time-integrated (U+Th)/ 3 He ratio: How can we change (U+Th)/ 3 He 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 3 He. Depleted upper mantle has ~2 ppb U, Th/U = 2.5, and 3 He/ 4 He = 8 R A. 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 3 He.
9 Geochemistry of He So where do OIB with 3 He/ 4 He = 30 R A come from? If primitive mantle is involved, it has 21 ppb U and Th/U = 3.8; 3 He/ 4 He would be lower than depleted mantle, if 3 He content were the same. It follows that there must be an undegassed reservoir that retains much primordial 3 He…this is the most straightforward way to explain the low (U+Th)/ 3 He. –Don Anderson likes to suggest alternatives: Subduction of 3 He from cosmic dust…not practical given diffusion of 3 He in subduction zones. CO 2 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 4 He from the Earth 4 He is produced by radioactive decay and we know quite precisely how much heat is liberated per alpha particle for terrestrial U and Th abundances ( J/ 4 He) 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 4 He 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 TW This implies a boundary layer in the mantle that passes heat (by conduction) but mostly retains 4 He
11 Geochemistry of He So where does the 3 He 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. – 3 He is not subducted or recycled, as far as we know –Hence 3 He 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 3 He/ 4 He 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 3 He/ 4 He 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 20 Ne is primordial and abundant, an nuclide 21 Ne is manufactured in abundance by nucleogenic reactions in the mantle: – 18 O(, n) 21 Ne and 24 Mg(n, ) 21 Ne –U drives both reactions: -decay as well as neutrons from spontaneous fission of 238 U (and 244 Pu in the early days); O and Mg are the most abundant target atoms in the mantle. Production ratio of 21 Ne/ 4 He is ~10 –7. There may be a small nucleogenic production of 22 Ne, 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/ 22 Ne.
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 20 Ne/ 22 Ne twice as much as 21 Ne/ 22 Ne. –(atm contains a little nucleogenic 21 Ne) 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 H 2, 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 20 Ne/ 22 Ne, perhaps due to 22 Ne production, perhaps due to contamination of even the best sample. Best MORB data are on the famous gas richpopping rock sample, 2 D43, 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 20 Ne/ 22 Ne, but less radiogenic 21 Ne/ 22 Ne, 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 20 Ne/ 22 Ne gives 21 Ne/ 22 Ne extrap, 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 Correlation with Ne in popping rock data, where three-isotope systematics let us fix upper mantle value, shows that for upper mantle Ar/ 36 Ar (atmosphere is 295.5!). There are several aspects to Ar geochemistry, because 40 Ar radiogenic production is so large. Geochronology…we have already covered the K- Ar and 39 Ar- 40 Ar methods. Consider how difficult it is to get good mantle Ar signals, when the air is 1% Ar. But it is not hopeless, since 40 Ar/ 36 Ar contrast between reservoirs is so big.
20 Argon Geochemistry We can also use the Ar 3- isotope diagram (in the few cases where good 38 Ar data are available) or correlation with neon to get 40 Ar/ 36 Ar of plume component from Loihi data. Same story: undegassed, less radiogenic than MORB source. Possibly, air has higher 36 Ar/ 22 Ne ratio than solar, consistent with atmospheric mass fractionation by hydrodynamic escape.
21 Argon geochemistry Because we also have some idea of terrestrial 40 K budget, we can confirm the existence of an undegassed lower mantle not from 40 Ar/ 36 Ar ratios but from total 40 Ar abundance. Remember this box model?
22 Terrestrial Xenology Pepin s curse: Let every element have isotope anomalies at every mass number…like Xenon. Xe has many isotopes, but they fall into three groups: –The non-radiogenic isotopes 124 Xe, 126 Xe, 128 Xe, 130 Xe – 129 Xe, the daughter of extinct 129 I (half life 17 Ma) –The fission products 131 Xe, 132 Xe, 134 Xe, 136 Xe, produced in slightly different relative abundances by spontaneous fission of long-lived 238 U and extinct 244 Pu (half-life 82 Ma).
23 Terrestrial Xenology Why no yield of 124, 126, 128, and 130? Fission yields: a10 20 a
24 Terrestrial Xenology Fundamental observation: MORB data differ from atmosphere, and show mixing between air and a component with excesses of both 129 Xe (from 129 I decay) and fissiogenic Xe isotopes (from Pu and/or U decay) Continental samples (granites, etc.) show only fissiogenic Xe from U decay. CO 2 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 129 I, half-life 17 Ma, was still alive. –Present atmosphere formed by degassing from interior of Earth while 129 I was still alive, so that ingrowth of 129 Xe in degassed residual mantle could generate high 129 Xe/ 130 Xe 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 Ga. – Degassing was then initially very rapid, with 80% of the remaining Xe transferred from upper mantle to atmosphere within the next Ma (but not instantaneous, or mantle 129 Xe 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, 129 I and 244 Pu 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 244 Pu decay and how much from continuing 238 U 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 244 Pu decay based on slightly different production ratios of 131 Xe, 132 Xe, 134 Xe, and 136 Xe. Note air has only 244 Pu- generated fission Xe.
28 Terrestrial Xenology Even better data from Hadean zircons, published 2004: clear evidence of 244 Pu 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 40 Ar), and we see noble gases derived from that reservoir in OIBs (based on 3 He/ 4 He, 21 Ne/ 22 Ne extrap, 40 Ar/ 36 Ar). –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.