Crust formation in the early Earthprior to the onset of subduction

-an analogue study from the Oman ophiolite

Hugh RollinsonSultan Qaboos University, Oman

Given that(a)The Earth’s earliest crust was mafic, and(b) The Earths early continental crust was

TTG in composition

Given that(a)The Earth’s earliest crust was mafic, and(b) The Earths early continental crust was

TTG in composition

It is proposed that the mechanisms wherebyfelsic rocks (trondhjemites) form from mafic crust in a geologically recent oceanicenvironment is a powerful analogue for the formation of felsic crust in the Hadean and early Archaean

Given that(a)The Earth’s earliest crust was mafic, and(b) The Earths early continental crust was

TTG in composition

It is proposed that the mechanisms wherebyfelsic rocks (trondhjemites) form from mafic crust in a geologically recent oceanicenvironment is a powerful analogue for the formation of felsic crust in the Hadean and early Archaean

A consequence of this proposal is that suchcrust was not created in a subduction setting

Historically this idea had been neglected, because

Ophiolitic trondhjemitesthought to be the productof crystal fractionationand so of minor importance

Historically this idea had been neglected, because

Ophiolitic trondhjemitesthought to be the productof crystal fractionationand so of minor importance

Ophiolitic and Archaeantrondhjemites have different compositions

Rb Ba Th U K Ta La Ce Pb Sr Nd Hf Zr Sm Ti Y Yb

1000

100

10

1

0.1

Oman trondhjemites

Av CC

Av TTGLILE and LREE depleted

So what has changed …….?

1. New models of oceanic magma chambers

large long-lived magma chambers are out

So what has changed …….?

2. New experimental evidence for watersaturated partial melting of oceanicgabbro at lowpressures

(data from Koepke etal. 2004 @ 0.2 Gpa)

To G d

Tdh G r

experimentalmelt compositions

An

Ab Or

Albiteenrichment

So what has changed …….?

3. Field evidence for partial melting - the occurrenceof hornblende gabbros- abundant agmatites

So what has changed?

4. Geochemical evidence forhigh temperature sea waterinteraction with mafic and felsic melts

trondhjemite (WR)

trondhjemite (WR)

Elevated Sr-isotope ratios

Elevated O-isotope ratiosOman amphiboles have

elevated chlorine (Coogan 2003)

Plagioclase-amphibole temperatures 830-970 C (Bosch et al 2004)

Ophiolite data from Bosch et al. (J. Pet.,2004) with Trondhjemite data from Gregory and Taylor (JGR, 1981)

Geochemical similarities…………

100

NMZLIMPOPO

BELTLEWISIAN

Y (ppm)

Oman vs Archaean TTG0 .1

1

10

100

1000

0 .1 1 10

OMAN

Low Sr; high YOman trondhjemites overlapLimpopo belt TTGs, and toa lesser extent the classical TTG trend (eg Lewisian)

Geochemical similarities…………

100

NMZLIMPOPO

BELTLEWISIAN

Y (ppm)

Oman vs Archaean TTG0 .1

1

10

100

1000

0 .1 1 10

OMAN

Low Sr; high YOman trondhjemites overlapLimpopo belt TTGs, and toa lesser extent the classical TTG trend (eg Lewisian)

0

500

1000

1500

2000

0 5 10 15

Na O + CaO %2

0

2

4

6

50 60 70 80

t < 3.0 Gat >3.0 Gat < 3.5 Ga

t >3.5 Ga

t < 3.0 Ga t >3.0 Gat < 3.5 Ga

t >3.5 Ga

Oman

Oman

SiO (wt %)2

Low Sr; low MgOSimilar to early Archaeanbut not late Archaean TTGs

Can this work for early Archaean felsic rocks at Isua ?(Kamber et al. 2005)

NE limb of Isua Greenstone belt, W Greenland

Unit B1 – includes felsic volcanic rocks

Abundantmaficrocks Rb Ba Th U K Ta La Ce Pb Sr Nd Hf Zr Sm Ti Y Yb

1000

100

10

1

0.1

Oman trondhjemites

Enriched in LILE, K, LREE & PbLow Sr, Ta, Ti

So how does/did it work ?

Oman

• Partial melting of low K gabbro section of oceanic crust• Access of water (seawater)• Heat source – active magmatism of ridge• Melting in presence of plagioclase and amphibole absence of garnet

So how does/did it work ?

Oman

• Partial melting of low K gabbro section of oceanic crust• Access of water (seawater)• Heat source – active magmatism of ridge• Melting in presence of plagioclase and amphibole absence of garnet

Early Earth

• Partial melting of ?high K mafic crust• Access of water• Heat source – ? Radioactive heating• Melting in presence of plagioclase +/- garnet

That’s it !