Post on 11-Dec-2015
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
Expelle
d 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
Expelle
d 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