Crust formation in the early Earth prior to the onset of subduction an analogue study from

1. 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

2. Given that • The Earth’s earliest crust was mafic, and • The Earths early continental crust was • TTG in composition

3. 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

4. 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

5. Historically this idea had been neglected, because Ophiolitic trondhjemites thought to be the product of crystal fractionation and so of minor importance

6. 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

7. So what has changed …….? • New models • of oceanic • magma chambers • large long-lived • magma chambers • are out

8. 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)

9. So what has changed …….? 3. Field evidence for partial melting - the occurrence of hornblende gabbros - abundant agmatites

10. 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)

11. Geochemical similarities………… Low Sr; high Y Oman trondhjemites overlap Limpopo belt TTGs, and to a lesser extent the classical TTG trend (eg Lewisian)

12. 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)

13. 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

14. 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

15. 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

16. That’s it !