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3He abundances; MORB vs OIB. Gonnermann & Mukhopadhyay with additions MORB degassing Seawater, air, trapped exsolved gases, shallow mantle mixing OIB.

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Presentation on theme: "3He abundances; MORB vs OIB. Gonnermann & Mukhopadhyay with additions MORB degassing Seawater, air, trapped exsolved gases, shallow mantle mixing OIB."— Presentation transcript:

1 3He abundances; MORB vs OIB

2 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

3 High 3He/4He & high 3He! carbonatites Note large ( ) range Carbonatites have 3 He/ 4 He ratios that extend from 0.04 to 39 Ra & average 9 +/- 14 Ra! Tolstikhin HIMU [ 3 He] Loihi “plume-magmas”, (e.g. Djibouti) air

4 MORB Meibom “High” R/Ra basalts extend down to MORB “Low” R/Ra extend up to MORB Very few 3 He atoms 3 He/ 4 He HIMU FOZO VARIANCE PARADOX

5 MORB Meibom “High” R/Ra basalts extend down to MORB “Low” R/Ra extend up to MORB Very few 3 He atoms 3 He/ 4 He HIMU FOZO THE VARIANCE PARADOX; high R/Ra are always associated with low R/Ra average Unfortunately, maximum R/Ra values in Greenland, Samoa, Afar etc. are usually compared with the average MORB values VARIANCE PARADOX

6 Ozima & Igarashi Hypothetical undegassed OIB gap High R/Ra contamination No degassing trends There is no reason to suppose & there never has been that high 3 He/ 4 He means high 3 He! OIB are from 3 He deficient sources High 3He/4He can & probably does mean low 4He, low U-Th (no evidence for) OIB are identical to MORB (with much lower 3 He) OIB MORB

7 3 He/ 4 He (R/Ra) 8 25 Distance from Mature Ridge 6 R/Ra Contribution from MORB Contribution from ambient mantle R/Ra of mix OIB 3 He atoms

8 Gonnermann & Mukhopadhyay with additions MORB degassing Seawater, air, trapped exsolved gases, shallow mantle mixing OIB Popping rock MORB has higher 3 He than OIB, even after multiple stages of degassing. This is why high 3 He/ 4 He is a midplate signature MORB, if present, will dominate any mix Loihi

9 High [ 3 He] component (MORB) dominates High 3 He/ 4 He, low [ 3 He] component only detectable away from MORB Reason why high R/Ra samples are midplate or away from mature rapidly spreading ridges EM2 FOZO or FOZO/EM2 Anderson % MORB Change in perspective; “plume components” are not due to presence of plume but absence of ridge

10 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

11 Gonnermann & Mukhopadhyay with additions MORB degassing Seawater, air, trapped exsolved gases, shallow mantle mixing OIB Popping rock MORB has higher 3 He than OIB, even after multiple stages of degassing. This is why high 3 He/ 4 He is a midplate signature MORB, if present, will dominate any mix GAP MORB Ambient or hotspot mantle

12 Atmospheric/seawater contamination Residual gases vesicles Degassed gases Secondary trapped He OIB MORB OIB is not extensively degassed; it is moderately contaminated Carbonatites: slab components?

13 R/Ra Kola peninsula kimberlites & carbonatites & inclusions These are attributed to plumes but they are part of a continuum 7 U=18 to 0.1 ppm Tolstikhin High levels of 3 He These are grab samples from the shallow mantle These are grab samples from the shallow mantle Popping rock Most OIB Because of their high 3 He concentrations, MORB & some carbonatites dominate any mixing [ 3 He] MORB Mantle xenoliths in carbonatites can have high R/Ra and [ 3 He] It is not true, as often asserted that all xenoliths are ~< 8 R/Ra Most continental “plumes” MORB is gas rich!

14 Highest 3 He materials on Earth: popping rock, MORB, carbonatites, manganese nodules OIB MORB Hypothetical undegassed reservoir Highest 3He/4He Highest 3He


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