3He abundances; MORB vs OIB

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

[3He]

Degassed MORB has more 3He than OIB

In all variants of the standard model, OIB gases are considered to be the residue of massive degassing

3He/22Ne

3He

“the gap” paradox

MORB

W.Greenland

Baffin Bay

High 3He/4He & high 3He!

carbonatites

Note large (103-104) range

Carbonatites have 3He/4He ratios that extend from 0.04 to 39 Ra & average 9 +/- 14 Ra!

Tolstikhin

HIMU

[3He]

Loihi“plume-magmas”, (e.g. Djibouti)

air

MORB

Meibom

“High” R/Ra basalts extend down to MORB

“Low” R/Ra extend up to MORB

Very few 3He atoms

3He/4He

HIMU

FOZO

MORB

Meibom

“High” R/Ra basalts extend down to MORB

“Low” R/Ra extend up to MORB

Very few 3He atoms

3He/4He

HIMU

FOZO

THE VARIANCE PARADOX; high R/Ra are always associated with low R/Ra

Unfortunately, maximum R/Ra values in Greenland, Samoa, Afar etc. are usually compared with the average MORB values

Ozima & Igarashi 2000

10-9

10-8Hypothetical undegassed OIB

gap

High R/Ra

contamination

No degassing trends

There is no reason to suppose & there never has been that high 3He/4He means high 3He! OIB are from 3He 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 3He)

OIB

MORB

3He/4He

(R/Ra)8

2525Distance from Mature Ridge

6

R/Ra

Contribution from MORB

Contribution from ambient mantle

R/Ra of mixOIB

3He atoms

3He atoms

Gonnermann & Mukhopadhyay

with additions

MORB degassing

Seawater, air, trapped exsolved gases, shallow mantle

mixing

OIB Popping rock

MORB has higher 3He than OIB, even after multiple stages of degassing. This is why high 3He/4He is a midplate signature

MORB, if present, will dominate any mix

Loihi

High [3He] component (MORB) dominates

High 3He/4He, low [3He] 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/EM2Anderson 1993

100 50 0 % MORB

Change in perspective; “plume components” are not due to presence of plume but absence of ridge

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

[3He]

Degassed MORB has more 3He than OIB

In all variants of the standard model, OIB gases are considered to be the residue of massive degassing

3He/22Ne

3He

“the gap” paradox

MORB

Gonnermann & Mukhopadhyay

with additions

MORB degassing

Seawater, air, trapped exsolved gases, shallow mantle

mixing

OIB Popping rock

MORB has higher 3He than OIB, even after multiple stages of degassing. This is why high 3He/4He is a midplate signature

MORB, if present, will dominate any mix

GAP

MORB

Ambient or hotspot mantle

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?

R/Ra

24

0.07

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 3He

These are grab samples from the shallow mantle

Popping rock

Most OIB

Because of their high 3He concentrations, MORB & some carbonatites dominate any mixing

[3He]

MORB

Mantle xenoliths in carbonatites can have high R/Ra and [3He]

It is not true, as often asserted that all xenoliths are ~< 8 R/Ra

Most continental “plumes”

MORB is gas rich!

Highest 3He materials on Earth: popping rock, MORB, carbonatites, manganese nodules

OIB

MORBHypothetical undegassed reservoir

Highest 3He/4He

Highest 3He