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The Helium, Neon & Carbon Footprints of the Mantle

The Helium, Neon & Carbon Footprints of the Mantle. Ratios > 9 or 12 are conventionally attributed to the deepest mantle…they could be the shallowest. W.Greenland. …such materials have very low 3 He. Concave up. “ plume ” or perisphere?. DM. continent.

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The Helium, Neon & Carbon Footprints of the Mantle

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  1. The Helium, Neon & Carbon Footprints of the Mantle

  2. Ratios > 9 or 12 are conventionally attributed to the deepest mantle…they could be the shallowest W.Greenland …such materials have very low 3He Concave up “plume” or perisphere? DM continent

  3. MORB has much higher helium contents than the “primitive” component High 3He/4He, low 3He FOZO sequence Mixing lines 4He(FOZO)/4He(MORB)~1 ~1/33 Juan Fernandez EM Variable LIL isotopes, converge on 8 Ra Haleakala Hanyu et al 2005

  4. carbonatites Iceland DM Natland 2004

  5. In all variants of the standard model, OIB gases are considered to be the residue of massive degassing Degassed MORB has more 3He than OIB Gonnermann & Mukhopadhyay with additions mixing Retained or residual gas [3He] MORB Expelled gas Loihi 3He/22Ne MORB degassing OIB Popping rock W.Greenland Baffin Bay “the gap” paradox Seawater, air, trapped exsolved gases, shallow mantle 3He In the Canonical Model OIB contains residual gas. In the perisphere model OIB picks up old exsolved gases plus air-like gas

  6. Nucleogenic ingrowth or addition Seawater addition Air seawater Louisville radiogenic

  7. Figure 5: Plot of 3He/22Ne vs. 4He/21Ne (from, Dixon et al., 2001).

  8. Louisville & most OIB have orders of magnitude less He & Ne than MORB Many MORB have values this high

  9. Highest R/Ra • 21/22 0.030-0.04 • 20/22 9.9-10.3 • 208/204 37.5-38.8 • 206/204 17.4-19.0 • 143/144 .5132-.5126 • 87/86 .703-.704 • Data defines curvilinear triangles between endmembers with curvature implying low 3He for high 3He/4He & “solar” components

  10. air Definitions Exsolved gas Expelled gas Trapped gas Secondary gas Orphaned He & Ne Low 3He/22Ne Low 4He/21Ne Low 4He/40Ar Ancient 3He/4He OIB This step is not involved in Canonical Models for LIL Separation of He & Ne from U & Th gas MORB magma Residual gas, dissolved gas, high 3He/22Ne etc.

  11. High 3He/4He (R/Ra) & low 21/22 simply reflect ancient separation of He & U,Th (4He & 21Ne producers). U,Th-poor lithologies are buoyant (perisphere) 350 300 250 200 150 100 50 0 300 Rapid decay in U,Th-rich and/or gas poor mantle Slow decay in gas-rich or U,Th-poor mantle R/Ra The perisphere has very little in it that is of interest to isotope geochemists Shallow perisphere MORB-mantle (popping rock based) 4 3 2 1 0 …except the solution to the Pb, Os & He paradoxes! Age (Ga) After Porcelli & Pepin 2008

  12. Mixing curves • OIB & MORB data define triangular regions with essentially no data beyond the linear sides • This implies that He/Sr, He/Ne (high R/Ra) etc. is = or < He/Sr (MORB & other OIB) • Observed [3He] concentrations in MORB are >> than in OIB • MORB-like R/Ra is the floor of the triangle & the terminus of OIB fields • Mixing components are DM, “PM”/solar, seawater/air & IDP in sediments

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