Gonnermann amp Mukhopadhyay with additions MORB degassing Seawater air trapped exsolved gases shallow mantle mixing OIB Popping rock Loihi Retained or residual gas Expelled gas ID: 495577
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Slide1
3He abundances; MORB vs OIBSlide2
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 BaySlide3
High 3He/4He & high 3He!
carbonatites
Note large (10
3
-10
4
) range
Carbonatites have
3
He/
4He ratios that extend from 0.04 to 39 Ra & average 9 +/- 14 Ra!
TolstikhinHIMU
[
3He]
Loihi
“
plume-magmas”, (e.g. Djibouti)
airSlide4
MORB
Meibom
“
High
”
R/Ra basalts extend down to MORB
“
Low
”
R/Ra extend up to MORB
Very few
3
He atoms
3He/4He
HIMU
FOZO
VARIANCE PARADOXSlide5
MORB
Meibom
“
High
”
R/Ra basalts extend down to MORB
“
Low
”
R/Ra extend up to MORB
Very few
3
He atoms
3
He/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
VARIANCE PARADOXSlide6
Ozima & Igarashi 2000
10
-9
10
-8
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 3He deficient sourcesHigh 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
MORBSlide7
3
He/
4
He
(R/Ra)
8
25
25
Distance from Mature Ridge
6
R/Ra
Contribution from MORB
Contribution from ambient mantle
R/Ra of mix
OIB
3
He atoms
3
He atomsSlide8
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
LoihiSlide9
High [
3
He] component (MORB) dominates
High
3
He/
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/EM2
Anderson 1993
100
50
0
% MORB
Change in perspective;
“
plume components
”
are not due to presence of plume but absence of ridgeSlide10
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
MORBSlide11
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 mixGAP
MORB
Ambient or hotspot mantleSlide12
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?Slide13
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
3
He
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 [
3He]
It is not true, as often asserted that all xenoliths are ~< 8 R/Ra
Most continental
“plumes”
MORB is gas rich!Slide14
Highest
3
He materials on Earth: popping rock,
MORB,
carbonatites, manganese nodules
OIB
MORB
Hypothetical undegassed reservoir
Highest 3He/4He
Highest 3He