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 “plume” or perisphere? Concave up DM continent

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

carbonatites Iceland DM Natland 2004

In all variants of the standard model, OIB gases are considered to be the residue of massive degassing Degassed MORB has more 3 He than OIB Loihi al OIB W. Greenland Baffin Bay MORB idu ell p Ex s es rr a g d do mixing e 3 He/22 Ne with additions ine ta Re s ga [3 He] Gonnermann & Mukhopadhyay Seawater, air, trapped exsolved gases, shallow mantle Popping rock MORB degassing “the gap” paradox 3 He In the Canonical Model OIB contains residual gas. In the perisphere model OIB picks up old exsolved gases plus air-like gas

Nucleogenic ingrowth or addition radiogenic Seawater addition Louisville Air seawater

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

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

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 3 He for high 3 He/4 He & “solar” components

air Definitions Exsolved gas OIB Low 3 He/22 Ne Expelled gas Low 4 He/21 Ne Trapped gas Secondary gas Low 4 He/40 Ar Orphaned He & Ne Ancient 3 He/4 He 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 3 He/22 Ne etc.

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

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 [3 He] 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