Noble Gas Constraints on Mantle Structure

Noble Gas Constraints on Mantle Structure He isotope geochemistry • Two isotopes of helium: 3He and 4He 3He is primordial 4He produced by radioactive decay of U and Th • He isotopes are a measure of time-integrated (U+Th)/3He ratio: ( ) ( ) ( ) 232 235 238 He  4 He  U λ 238t U λ 235t Th λ 232t =  3  + 8 3 e −1 + 7 3 e −1 + 6 3 e −1 3 He  He  o He He He 4 • Helium behaves as an incompatible element during mantle melting • Helium expected to be more incompatible than U and Th during mantle melting If so high 3He/4He ratios reflect less degassed mantle material Helium isotopic evolution in a two layer mantle evolution of helium isotopes He isotope geochemistry • Two isotopes of helium: 3He and 4He 3He is primordial 4He produced by radioactive decay of U and Th • He isotopes are a measure of time-integrated (U+Th)/3He ratio: ( ) ( ) ( ) 232 235 238 He  4 He  U λ 238t U λ 235t Th λ 232t =  3  + 8 3 e −1 + 7 3 e −1 + 6 3 e −1 3 He  He  o He He He 4 • Helium behaves as an incompatible element during mantle melting • Helium expected to be more incompatible than U and Th during mantle melting If so high 3He/4He ratios reflect less degassed mantle material Histogram of He isotope ratios in mid-ocean ridge basalts (MORBs) • • 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 Image removed due to copyright considerations. Please see: Graham, David W. "Noble Gas Isotope Geochemistry of Mid-Ocean Ridge and Ocean Island Basalts: Characterization of Mantle Source Reservoirs." In Noble Gases in Geochemistry and Cosmochemistry. Edited by D. Porcelli, C. J. Ballentine and R. Wieler. Reviews in Mineralogy and Geochemistry. Vol. 47. Washington, DC: Mineralogiocal Society of America, pp. 247-319, 2002. Comparison of He isotope ratios from selected MORs, OIBs, and continental hotspots • The mean 3He/4He ratio from different ridge segments are nearly identical although the variance is different • OIBs are much more variable • 3He/4He ratios less than MORBs are frequently associated with radiogenic Pb (HIMU) and reflects recycled components in the mantle After Barford, 1999 He isotope ratios in ocean island basalts (OIBs) • OIBs display a very large range in He isotopic composition • He isotopic distribution has a double-peak; maxima at 8 RA and 13 RA • The first maxima is identical to the mean from MORBs - Clear indication of the involvement of depleted mantle in ocean island volcanism Image removed due to copyright considerations. Please see: Farley, K. A., and E. Neroda. "Noble gases in the Earth's mantle." Ann Rev Earth Planet Sci 26 (1998): 189-218. • The 2nd peak is somewhat surprising and its meaning is unclear MORBs: sample well-mixed degassed mantle with low 3He/U+Th OIBs: sample heterogeneous, less degassed mantle with high 3He/U+Th Ne isotopic composition of mantle derived rocks Image removed due to copyright considerations. Please see: Graham, David W. "Noble Gas Isotope Geochemistry of Mid-Ocean Ridge and Ocean Island Basalts: Characterization of Mantle Source Reservoirs." In Noble Gases in Geochemistry and Cosmochemistry. Edited by D. Porcelli, C. J. Ballentine and R. Wieler. Reviews in Mineralogy and Geochemistry. Vol. 47. Washington, DC: Mineralogiocal Society of America, pp. 247-319, 2002. • • Mantle 20Ne/22Ne ratio is fixed; 21Ne/22Ne varies because of radiogenic ingrowth and varying degrees of degassing Different ocean islands have distinct 21Ne/22Ne 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 Geochemistry of Ar Image removed due to copyright considerations. Please see: Graham, David W. "Noble Gas Isotope Geochemistry of Mid-Ocean Ridge and Ocean Island Basalts: Characterization of Mantle Source Reservoirs." In Noble Gases in Geochemistry and Cosmochemistry. Edited by D. Porcelli, C. J. Ballentine and R. Wieler. Reviews in Mineralogy and Geochemistry. Vol. 47. Washington, DC: Mineralogiocal Society of America, pp. 247-319, 2002. • • • 1% Ar in the atmosphere Significant air contamination for Ar Even when 3He/4He ratios are as high as 30 RA, 40Ar/36Ar ratios can be atmospheric Image removed due to copyright considerations. • • 20Ne/22Ne ratio in the mantle does Please see: Moreira, M., J. Kunz, and C. J. Allegre. "Rare gas systematics not vary in popping rock: isotopic and elemental compositions in the upper Ar isotopic ratios in mantle mantle." Science 279 (1998): 1178-81. derived rocks can be corrected for air contamination by extrapolating the 40Ar/36Ar ratio to the upper mantle 20Ne/22Ne value Geochemistry of Ar Image removed due to copyright considerations. Please see: Graham, David W. "Noble Gas Isotope Geochemistry of Mid-Ocean Ridge and Ocean Island Basalts: Characterization of Mantle Source Reservoirs." In Noble Gases in Geochemistry and Cosmochemistry. Edited by D. Porcelli, C. J. Ballentine and R. Wieler. Reviews in Mineralogy and Geochemistry. Vol. 47. Washington, DC: Mineralogiocal Society of America, pp. 247-319, 2002. • • MORB mantle 40Ar/36Ar values are ~ 40000 OIBs have lower 40Ar/36Ar ratios; reasonable limit is 8000 • A value of 8000 does not represent pristine mantle material; must indicate some processing, although significantly less degassed than the mantle source sampled by MORBs The picture that emerges so far…… 1. MORBs are more homogenous compared to OIBs 2. MORBs dominantly sample a mantle source that is more processed and degassed compared to most OIBs Evidence for a layered structure: The Missing Argon problem • K content of Earth derived from the K/U ratio of 12700 in MORBs and U content of 20-22.5 ppb • Implied K content of bulk Earth is 250-285 ppm • Total 40Ar produced over Earth history = 140-156 x 1018 g • 40Ar in the atmosphere = 66 x 1018 g (~50%) • 40Ar in the crust = 9-12 x 1018 g 63-80 x 1018 of 40Ar has to be in the mantle Evidence for a layered structure: The Missing Argon problem I) Constraints from 40Ar flux • • 4He flux at ridge = 9.46 x 107 moles/yr 4He/40Ar ratio in MORBs 2-15 => 40Ar flux 0.63-5 x 107 moles/yr • Mass of oceanic lithosphere passing through ridges = 5.76 x 1017 g/yr If MORB mantle representative of entire mantle and if lithosphere completely degassed, 40Ar content in mantle 1.8-14 1018g Lower than the 63-81 x 1018g estimated (Allegre et al., 1996) and requires a hidden reservoir for 40Ar Evidence for a layered structure: The Missing Argon problem II) Constraints from Potassium content • K content of MORB source is 40-50 ppm; if representative of entire mantle produces 22-28 x 1018 g of 40Ar -significantly less than the 63-80 x 1018 g of 40Ar calculated to be in the mantle Bottom line: The constraints from 40Ar require some sort of layering or a hidden reservoir in the mantle Any wiggle room? Maybe we do not know the K/U ratio of the mantle as well as we think (e.g., Lassiter 2004) Image removed due to copyright considerations. Please see: Lassiter, J.C. "The role of encycled oceanic crust in the potassium and argon budget of the Earth: Towards a resolution of the "Missing argon" problem. Geochemistry Geophysics Geosystems 5, no. Q11012, doi: 10.1029/2004GC000711 (2004): 16. Noble Gas Concentrations • He concentrations higher in MORBs than OIBs • Maybe not too surprising since most OIBs are erupted at shallower water depths than MORBs; so would be degassed more • Turns out that such an explanation is not really tenable… Image removed due to copyright considerations. Please see: Honda, Masahiko, and Desmond B Patterson. "Systematic elemental fractionation of mantle-derived helium, neon, and argon in mid-oceanic ridge glasses." Geochimica et Cosmochimica Acta 63, no. 18 (September 1999): 2863-2874. Noble gas elemental ratios Ple Image removed due to copyright considerations. Please see: Honda, Masahiko, and Desmond B Patterson. "Systematic elemental fractionation of mantle-derived helium, neon, and argon in mid-oceanic ridge glasses." Geochimica et Cosmochimica Acta 63, no. 18 (September 1999): 2863-2874. Solubility controlled degassing does not explain the differences in gas concentration between MORBs and OIBs. Noble gas elemental ratios Image removed due to copyright considerations. Please see: Honda, Masahiko, and Desmond B Patterson. "Systematic elemental fractionation of mantle-derived helium, neon, and argon in mid-oceanic ridge glasses." Geochimica et Cosmochimica Acta 63, no. 18 (September 1999): 2863-2874. • Fractionation has to be recent, otherwise the slope would not be 1 and the 4He/21Ne ratio would have evolved back to the production value of ~2 x 10-7 Noble gas elemental ratios Image removed due to copyright considerations. Please see: Burnard, Pete. "Diffusive fractionation of noble gases and helium isotopes during mantle melting." Earth and Planetary Science Letters 220, no. 3-4 (April 2004): 287-295. Elsevier. • The Ar concentration decreases with degassing as expected • For some MORBs suites, as 4He/40Ar ratio increases (more degassed), He concentration increases as well!! So the problem appears to be with He Partition coefficient of the noble gases Are the noble gases (4He, 21Ne, 40Ar) really more incompatible than their radiogenic parents (e.g., U, Th, K)? Image removed due to copyright considerations. Please see: Brooker, R. A., V. Heber, S. P. Kelley, and B. J. Wood. "Noble Gas Partitioning Behaviour During Mantle Melting: A Possible Explanation for "The He Paradox"?" Nature 423 (2003): 738-741. • For clinopyroxene, Ar slightly more incompatible than K • Experimental data still not good enough to show conclusively how He behaves with respect to U and Th • Time integrated ratios however provide some insights; for example high 3He/4He ratios are never associated with the most depleted isotopic signatures of Sr and Nd, => seems to imply that He is more incompatible than U and Th Proving that He is more or less incompatible than U and Th will be a major challenge but will have tremendous implications for mantle geodynamics Can nature tell us something about of partition coefficients? Maybe…… Time integrated ratios however provide some insights; for example high 3He/4He ratios are never associated with the most depleted isotopic signatures of Sr and Nd -> seems to imply that He is more incompatible than U and Th After Barford, 1999 Global relationship between He and other lithophile tracers: The wormograms Image removed due to copyright considerations. Please see: Graham, David W. "Noble Gas Isotope Geochemistry of Mid-Ocean Ridge and Ocean Island Basalts: Characterization of Mantle Source Reservoirs." In Noble Gases in Geochemistry and Cosmochemistry. Edited by D. Porcelli, C. J. Ballentine and R. Wieler. Reviews in Mineralogy and Geochemistry. Vol. 47. Washington, DC: Mineralogiocal Society of America, pp. 247-319, 2002. Inferences: High 3He/4He ratios from a single, relatively undegassed mantle source that is characterized by well defined Sr, Nd, and Pb isotopic composition Primitive mantle may not exist but a reservoir that is less degassed than the MORB mantle almost certainly does. Relationship between He and other lithophile tracers • • If high 3He/4He ratios are due to an ancient melt depletion event, high 3He/4He ratios should be associated with very depleted Srisotopic composition. Higher 3He/4He ratios are associated with less depleted 87Sr/86Sr isotopic signal ⇒ high 3He/4He ratios are indicative of less degassed mantle More depleted Less depleted Image removed due to copyright considerations. Please see: Kurz, M. D. "Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project." Geochemistry Geophysics Geosystems 5, no. 4 (2004): Q04G14.