What can isotopes tell us about mantle dynamics? Sujoy Mukhopadhyay. Harvard University

Transcription

1 What can isotopes tell us about mantle dynamics? Sujoy Mukhopadhyay Harvard University

2 The mantle zoo Hofmann, Os/ 188 Os EM1 Hawaii Pitcairn DMM peridotites Shield Basalts >45 ppt Os Samoa EM2 Canary Is. St. Helena Azores Societies FOZO Comores B HIMU Australs Ballentine et al 2002 Pb/ 204 Pb

3 One approach to understanding isotopic heterogeneity: Box models Kellogg et al., 2002

4 The MORB distribution is reproduced Model OIB distribution fails to capture the real distribution Kellogg et al., 2002

5 What is the effect of sampling lengthscale and stirring time? Kellogg et al., 2002

6 Effect of sampling and stirring time scale Kellogg et al., 2002

7 Isotopic differences between OIBs and MORBs An issue of sampling lengthscales?

8 He isotope geochemistry Two isotopes of helium: 3 He and 4 He 3 He is primordial 4 He produced by radioactive decay of U and Th He He ( ) 235 ( ) 232 U Th ( ) e λ 238t e λ t e λ t He U 232 = He + He He He o Helium behaves as an incompatible element during mantle melting (i.e. prefers melt over minerals) Helium not recycled back into the mantle fresh glass 3 He= at/g; Jurrasic age MORB = <2x10 6 at/g 11Ma gabbros 3 He = 8 x at/g; mean = 3 x10 7 at/g (Staudacher and Allegre, 1988; Moreira et al.,2003).

9 Parman et al.,2005 Partitioning of Helium during mantle melting

10 Fraction of Helium retained in the residue as a function of partition coefficient

11 Histogram of He isotope ratios in mid-ocean ridge basalts (MORBs) 3 He/ 4 He ratios reported relative to the atmospheric ratio of 1.39 x 10-6 No relation between isotopic composition and spreading rate but the variance is inversely related to spreading rate Either reflects - efficiency of mixing in the upper mantle - differences in degree of magma homogenization Graham 2002

12 Comparison of He isotope ratios from selected MORs, OIBs, and continental hotspots The mean 3 He/ 4 He ratio from different ridge segments are nearly identical although the variance is different OIBs are much more variable After Barford, 1999 MORBs: sample well-mixed degassed mantle with low 3 He/U+Th OIBs: sample heterogeneous, less degassed mantle with high 3 He/U+Th

13 Geochemistry of Ne Neon has three isotopes 20 Ne, 21 Ne, and 22 Ne 20 Ne is primordial 21 Ne is produced by nucleogenic reactions in the mantle: 18 O(α, n) 21 Ne and 24 Mg(n, α) 21 Ne α from U decay; neutrons from spontaneous fission Production ratio of 21 Ne/ 4 He is ~ Ne is primordial. There may be a small nucleogenic production of 22 Ne, [ 19 F(α, n) 22 Ne] but it is likely to be negligible 20 Ne/ 22 Ne does not vary in the mantle derived rocks; 21 Ne/ 22 Ne does Ne is expected to be more incompatible than U and Th during mantle melting => low 21 Ne/ 22 Ne ratios reflect less degassed mantle material

14 Ne isotopic composition of mantle derived rocks Less degassed More degassed Increasing air contamination Radiogenic ingrowth Figure from Graham 2002 Mantle 20 Ne/ 22 Ne ratio is fixed; 21 Ne/ 22 Ne varies because of radiogenic ingrowth and varying degrees of degassing Different ocean islands have distinct 21 Ne/ 22 Ne ratios; either reflects varying amounts of MORB mantle addition to the OIB source(s) or different parts of the mantle have been degassed and processed to different degrees

15 Geochemistry of Ar Three stable isotopes of Ar, 36 Ar, 38 Ar, 40 Ar 36 Ar and 38 Ar are primordial 40 Ar produced by radioactive decay of 40 K Ar is expected to be more incompatible than K during mantle melting If so high 40 Ar/ 36 Ar reflects degassed mantle material

16 Geochemistry of Ar From Graham, 2002 MORB mantle 40 Ar/ 36 Ar values are ~ 40,000 OIBs have lower 40 Ar/ 36 Ar ratios; reasonable limit is 8000 OIBs significantly less degassed than the mantle source sampled by MORBs.

17 5 R A 8 R A 3 He/ 4 He 50 R A He isotopic variations are coupled to variations in other lithophile tracers (Sr, Nd, Pb)

18 Relationship between He and other lithophile tracers He isotopic variations are coupled to variations in other lithophile tracers (Sr, Nd, Pb) More depleted Less depleted Higher 3 He/ 4 He ratios are associated with less depleted 87 Sr/ 86 Sr isotopic signal high 3 He/ 4 He ratios are indicative of less degassed mantle Above data is from the 3 km deep drill hole from Mauna Kea, Hawaii (Kurz et al., 2004)

19 5 R A 8 R A 3 He/ 4 He 50 R A Primordial noble gases in both high 3 He/ 4 He (Hawaii, Iceland, Samoa) and low 3 He/ 4 He OIBs (HIMU-OIBs) from the same reservoir.

20 So what s the story so far? MORBs are more homogeneous than OIBs All of the isotopic data are consistent with a more processed MORB source and a less processed and more variable OIB source. FOZO is everywhere.

21 Noble Gas Concentrations He concentrations might be higher in MORBs than OIBs OIBS MORBS Maybe not too surprising since most OIBs are erupted at shallower water depths than MORBs; so would be degassed more Is such an explanation tenable? Honda and Patterson, 1999

22 Parman et al.,2005 Partitioning of Helium during mantle melting

23 But why do OIB lavas have lower noble gas concentrations than MORBs? MORBs 3 He/ 22 Ne OIBs log 10 [ 3 He cc g -1 ] Cannot be explained by simply invoking different eruption depths for OIBs and MORBs

24 Effects of varying CO 2 content and disequilibrium for eruption at a constant water depth (Gonnermann and Mukhopadhyay, 2007) 3 He/ 22 Ne log 10 [ 3 He cc g -1 ] Both CO 2 and diffusive fractionation need to considered for accurate representation of magmatic degassing

25 Effect of varying eruption depth and disequilibrium

26 3 He/ 22 Ne log 10 [ 3 He cc g -1 ] MORBs 0.07 to 0.24 wt % CO 2 ; OIBs 0.11 to 0.63 wt % CO 2. Higher CO 2 contents lead to greater 3 He loss for OIBs. Also provides zeroth order explanation for the shape of the data cloud.

27 Take home points Compared to parental MORB magmas, parental OIB lavas are ~10 times enriched in 3 He. consistent with derivation from a gas rich source

28 5 R A 8 R A 3 He/ 4 He 50 R A So what is the volatile rich 3 He/ 4 He reservoir? not recycled lithosphere Is it a reservoir that has largely escaped melt extraction and is relatively undegassed? Well, what is large and what does by one by relatively?

29 But first, an additional reason for a volatile rich reservoir: The 40 Ar budget (Allegre et al.,1996) Assuming ppm of K, 40 Ar produced over Earth history = x g 40 Ar in the atmosphere = 66 x g (~50%) 40 Ar in the crust = 9-12 x g x of 40 Ar has to be in the mantle MORB 40 Ar concentration too low to account for this

30 You could have a How can you preserve a reservoir with high 3 He,High 3 He/ 4 He and high 40 Ar? Primitive mantle In this view you either need a convectively isolated reservoir, or have some fraction of the primitive reservoir survive until present time.

31 Maybe different processing rates can explain OIB vs MORB difference Class and Goldstein (2005) A very odd inflection. Did not considering degassing as a function of gas content in the source (see for e.g., Porcelli and Elliot,2008) So where does the 3 He come from? Core, Enriched hidden reservoir (Porcelli and Elliot, 2008)

32 You could have a How can you preserve a reservoir with high 3 He,High 3 He/ 4 He and high 40 Ar? Primitive mantle

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34 from Fukao et al. (2001) At least some fraction of the slabs get subducted into the lower mantle. Let s impose the solution that both lower mantle and the upper mantle has been processed through partial melting but maybe at different rates. Lets assume the slabs gets mixed back into the mantle

35 Gonnermann and Mukhopadhyay (In review) dc dt C( t) = = C(1 C o e f ) q (1 f ) qt C o e qt C is concentration f is the fraction being recycled back q is no. of reservoir volumes processed per 4.5 Gy t is time (units of 4.5 Gy) For q =1 and for t = 1, ~ 40% of primordial He still left even after a mass equivalent to the mass of the reservoir is processed through melting.

36 What happens if mixing is not instantaneous? Here τ is the characteristic mixing time of the slab

37 What happens if mixing is not instantaneous? Bottom line: Substantial primordial noble gases can be preserved in a mantle reservoir that has been processed by partial melting. Even after a mass equivalent to 3-4 reservoir masses have been melted, a few percent of the original volatile inventory still left.

38 A little more complicated approach: geochemical reservoir modeling Each box will consist of a well mixed region and the unmixed slabs

39 Geochemical reservoir modeling: Some model parameters He, Ar outgassed Mass transfer between UM and LM not explicitly modeled, account for it through effective values of q and τ. Initial U, Th, K are equal to Bulk Silicate Earth values in both reservoirs. Initial 3 He/ 4 He is 120 R A in both reservoirs.

40 Some more parameters: U, Th, K extraction out of mantle Processing rates: Upper mantle and lower mantle processing rate tied to present day MOR processing rate and plume flux respectively. Let it increase exponentially back through time to let be constant over time.

41 Evolution of 3 He/ 4 He over time Prescribe q, τ and loss rate of radioactive elements then find initial 3 He that results in present day 3 He/ 4 He ratios. For UM only solutions that result in the same initial conditions as LM. For LM only solutions that satisfy both He and Ar constraints.

42 What does the solutions space look like for 3 He? UM: 3-5 reservoir masses processed and 0 τ 0.5 Gy LM: reservoir masses processed and 0.25 τ 4 Gy

43 The solutions space captures the requirement for a large amount of 40 Ar in the lower mantle UM: 3-5 reservoir masses processed and 0 τ 0.5 Gy LM: reservoir masses processed and 0.25 τ 4 Gy

44 The effect of recycling on the solution space for 40 Ar No. of reservoirs masses processed All of the extracted K recycled back to LM K has a one way ticket out of LM τ (Gy)

45 Unmixed slab (fraction of reservoir) UM: 3-5 reservoir masses processed and 0 τ 0.5 Gy LM: reservoir masses processed and 0.25 τ 4 Gy What does this imply for the compositional diversity of MORBs vs. OIBs?

46 Summary High 3 He concentration, high 3 He/ 4 He ratios and high 40 Ar concentrations can be preserved in a convecting mantle reservoir. no need to be hidden in the core, D, or some other hidden enriched layer. To first order, the range of He isotopic compositions sampled at OIBs results from sampling variable proportions of the well mixed reservoir and the unmixed slabs. The primitive noble gas signatures would be associated with depleted Sr and Nd (FOZO); FOZO does not have to be ancient.

47 So which is it? Layered or whole mantle convection? May depend on your definition. Take home points: The noble gas signatures suggest a more processed upper mantle and less processed lower mantle. The primitive noble gas signatures can be preserved even if there is substantial mass flux across the transition zone throughout Earth history; on order lower mantle mass over 4.5 Gy.