Presentation on theme: "The Helium, Neon & Carbon Footprints of the Mantle."— Presentation transcript:
The Helium, Neon & Carbon Footprints of the Mantle
DM continent W.Greenland …such materials have very low 3 He Concave up “plume” or perisphere? Ratios > 9 or 12 are conventionally attributed to the deepest mantle…they could be the shallowest
Hanyu et al 2005 Haleakala Juan Fernandez FOZO 4 He(FOZO)/ 4 He(MORB)~1 ~1/33 High 3 He/ 4 He, low 3 He Variable LIL isotopes, converge on 8 Ra EM sequence Mixing lines MORB has much higher helium contents than the “primitive” component
Natland 2004 carbonatites Iceland DM
Gonnermann & Mukhopadhyay with additions MORB degassing Seawater, air, trapped exsolved gases, shallow mantle mixing OIB Popping rock Loihi Retained or residual gas Expelled gas In the Canonical Model OIB contains residual gas. In the perisphere model OIB picks up old exsolved gases plus air-like gas [ 3 He] Degassed MORB has more 3 He than OIB In all variants of the standard model, OIB gases are considered to be the residue of massive degassing 3 He/ 22 Ne 3 He “the gap” paradox MORB W.Greenland Baffin Bay
Air seawater Nucleogenic ingrowth or addition Louisville radiogenic Seawater addition
Figure 5: Plot of 3He/22Ne vs. 4He/21Ne (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/ / / / / / Data defines curvilinear triangles between endmembers with curvature implying low 3He for high 3He/4He & “solar” components
Definitions Exsolved gas Expelled gas Trapped gas Secondary gas Orphaned He & Ne Low 3 He/ 22 Ne Low 4 He/ 21 Ne Low 4 He/ 40 Ar Ancient 3 He/ 4 He magma Residual gas, dissolved gas, high 3 He/ 22 Ne etc. Separation of He & Ne from U & Th This step is not involved in Canonical Models for LIL air gas OIB MORB
After Porcelli & Pepin 2008 MORB-mantle (popping rock based) Rapid decay in U,Th-rich and/or gas poor mantle Slow decay in gas-rich or U,Th-poor mantle Shallow perisphere R/Ra Age (Ga) 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) The perisphere has very little in it that is of interest to isotope geochemists …except the solution to the Pb, Os & He paradoxes!
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