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Crust formation in the early Earth prior to the onset of subduction an analogue study from the Oman ophiolite Hugh Rollinson Sultan Qaboos University, Oman. Given that The Earth’s earliest crust was mafic, and The Earths early continental crust was TTG in composition. Given that

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Presentation Transcript
slide1

Crust formation in the early Earth

  • prior to the onset of subduction
  • an analogue study from
  • the Oman ophiolite
  • Hugh Rollinson
  • Sultan Qaboos University, Oman
slide2

Given that

  • The Earth’s earliest crust was mafic, and
  • The Earths early continental crust was
  • TTG in composition
slide3

Given that

  • The Earth’s earliest crust was mafic, and
  • The Earths early continental crust was
  • TTG in composition
  • It is proposed that the mechanisms whereby
  • felsic rocks (trondhjemites) form from mafic
  • crust in a geologically recent oceanic
  • environment is a powerful analogue for the
  • formation of felsic crust in the Hadean and
  • early Archaean
slide4

Given that

  • The Earth’s earliest crust was mafic, and
  • The Earths early continental crust was
  • TTG in composition
  • It is proposed that the mechanisms whereby
  • felsic rocks (trondhjemites) form from mafic
  • crust in a geologically recent oceanic
  • environment is a powerful analogue for the
  • formation of felsic crust in the Hadean and
  • early Archaean
  • A consequence of this proposal is that such
  • crust was not created in a subduction setting
slide5

Historically this idea had been neglected, because

Ophiolitic trondhjemites

thought to be the product

of crystal fractionation

and so of minor importance

slide6

Historically this idea had been neglected, because

Ophiolitic trondhjemites

thought to be the product

of crystal fractionation

and so of minor importance

LILE and LREE depleted

Ophiolitic and Archaean

trondhjemites have

different compositions

slide7

So what has changed …….?

  • New models
  • of oceanic
  • magma chambers
  • large long-lived
  • magma chambers
  • are out
slide8

So what has changed …….?

2. New experimental

evidence for water

saturated partial

melting of oceanic

gabbro at low

pressures

(data from Koepke et

al. 2004 @ 0.2 Gpa)

slide9

So what

has

changed …….?

3. Field evidence for

partial melting

- the occurrence

of hornblende gabbros

- abundant agmatites

slide10

So what

has changed?

Elevated

Sr-isotope

ratios

4. Geochemical

evidence for

high temperature

sea water

interaction

with mafic

and felsic melts

Elevated

O-isotope

ratios

Oman 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)

slide11

Geochemical similarities…………

Low Sr; high Y

Oman trondhjemites overlap

Limpopo belt TTGs, and to

a lesser extent the classical TTG trend (eg Lewisian)

slide12

Geochemical similarities…………

Low Sr; low MgO

Similar to early Archaean

but not late Archaean TTGs

Low Sr; high Y

Oman trondhjemites overlap

Limpopo belt TTGs, and to

a lesser extent the classical TTG trend (eg Lewisian)

slide13

Can this work for early Archaean felsic rocks at Isua ?

(Kamber et al. 2005)

Unit B1 – includes

felsic volcanic rocks

Abundant

mafic

rocks

Enriched in LILE, K, LREE & Pb

Low Sr, Ta, Ti

NE limb of Isua Greenstone belt, W Greenland

slide14

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
slide15

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
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