The Helium, Neon & Carbon Footprints of the Mantle

DM

continent

W.Greenland

…such materials have very low 3He

Concave up“plume” or perisphere?

Ratios > 9 or 12 are conventionally attributed to the deepest mantle…they could be the shallowest

Ratios > 9 or 12 are conventionally attributed to the deepest mantle…they could be the shallowest

Hanyu et al 2005

Haleakala

Juan Fernandez

FOZO

4He(FOZO)/4He(MORB)~1

~1/33

High 3He/4He, low 3He

Variable LIL isotopes, converge on 8 Ra

EM

sequenceMixing 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

gasExpelled 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

Airseawater

Nucleogenic ingrowth or addition

Louisvilleradiogenic

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

• 21/22 0.030-0.04• 20/22 9.9-10.3• 208/204 37.5-38.8• 206/204 17.4-19.0• 143/144 .5132-.5126• 87/86 .703-.704• Data defines curvilinear triangles between

endmembers with curvature implying low 3He for high 3He/4He & “solar” components

DefinitionsDefinitionsExsolved gas

Expelled gas

Trapped gas

Secondary gas

Orphaned He & Ne

Low 3He/22Ne

Low 4He/21Ne

Low 4He/40Ar

Ancient 3He/4He

magma

Residual gas, dissolved gas, high 3He/22Ne etc.

Separation of He & Ne from U & ThSeparation 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

4 3 2 1 0

350

300

250

200

150

100

50

0

300

R/Ra

Age (Ga)

High 3He/4He (R/Ra) & low 21/22 simply reflect ancient separation of He & U,Th (4He & 21Ne 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