1. Evidence that eclogite-forming reactions may have been suppressed in Archean time
Marcia Bjornerud
Lawrence University
Appleton, Wisconsin

2. • Modern tectonics involves efficient crustal recycling driven by eclogite metamorphism (densification) of mafic crust
• Today, eclogite formation is related to progressive dehydration of a subducting slab and occurs in the presence of abundant water
However, in the absence of water as a medium for advective ion transport, the transformation to eclogite may not necessarily occur even if rocks reach - and remain - at eclogite P & T conditions.

3. Holsnøy, Bergen arcs: Eclogite metamorphism caught in the act

4. THE GRANULITE-ECLOGITE TRANSITION ON HOLSNØY
Protolith: Anorthosite, gabbro, minor peridotite (layered mafic intrusion?)
At ca. 945 Ma, the rocks experienced granulite conditions
of oC and 1.0 GPa. [Gt + Cpx + Plag + Opx]
 The granulite event left the rocks almost completely dehydrated
P (GPa)
T (oC)
Blue- schist
ECLOGITE
0.5 -
1.0 -
Hornfels
Green schist
P-P
Zeolite
Amphi-
bolite
400
800
Granu-
lite
1.5 -
Holsnøy
MODERN
 By ca.420 Ma, tectonic burial
during Caledonian mountain
building subjected rocks to
eclogite conditions:
1.5 GPa and oC
[Gt2 + Omp +Ky +Zo + Phe]
Based on work by Austrheim, Bjornerud, Boundy, Erambert, Fountain,Jamtveit, Klaper, Lund, et al.

5.  Eclogite metamorphism was patchy and heterogeneous -- nowhere entirely complete
 WATER was necessary to achieve the granulite-eclogite transition, as indicated by spatial association with fractures and presence of hydrous phases (zoisite, phengite)
 Volume decrease upon granulite-eclogite conversion was ca. 10%
Prior to conversion, the dry granulite was strong enough to fail seismically at 50 km depth.
 Eclogite metamorphism was intially self-perpetuating owing to microfracturing from volume decrease, but it was eventually self-limiting because of overall strength decrease associated with eclogitization (macroscopic fracturing was no longer possible; no new channels for water).

6. Eclogite locally developed adjacent to fractures
(Well-sampled) Eclogite ‘finger’
Eclogite locally developed adjacent to fractures
Garnet reequilibrated to eclogite conditions along microfractures

7. 10 cm
1 m
Eclogite shear zones

8. Granulite pods surrounded by eclogite Mature eclogite shear zone

9. Pseudotachylyte generation zone and injection veins
Massive pseudotachylyte at dilational jog
Pseudotachylyte generation zone parallel to granulite foliation

10. Dendritic garnet in pseudotachylyte nucleated around kyanite --> Earthquakes occurred at eclogite facies depths

11. SEQUENCE of EVENTS in ECLOGITE FORMATION on HOLSNØY
1) Dry granulite fractured in one or more major earthquakes
2) Water entered fractures, triggering eclogite reactions
3) Microcracks caused by shrinkage allow eclogite fronts to migrate
4) Eclogites became locus for shear strain
5) Shear zones eventually linked, and bulk strength of complex fell, inhibiting further macroscopic fracturing and infiltration of water
6) A half-eclogitized rock mass remained, still too buoyant to be subducted
Rheologically Critical Fraction
Threshold behavior observed in many types of systems:
Sediments + water Crystals + melt (e.g. Arzi 1978) Biminerallic rocks (Ross 1987) Mylonites (Gilotti 1992)
Strength
RCF
% weak phase in a two-component system

12. GRANULITES PERSISTED METASTABLY AT ECLOGITE FACIES CONDITIONS,
owing to their extreme dehydration
Water is critical in Albite ->Jadeite + Qtz reaction (Hacker 1996)
Addition of water triggered rapid recrystallization
but even then the process could not go to completion
What does this have to do with the beginning of plate tectonics?
The Holsnøy P-T-t path to eclogite conditions may have been common in Archean time

13. Early tectonics in the absence of eclogite
Internally thickened mafic crust
25 km-
Granulite P & T
Partial melting (TTG source)
Eclogite P & T
50 km-
Dehydrated mafic crust: Strong, buoyant and resistant to eclogitization

14. NEUTRAL BUOYANCY WITH RESPECT TO UPPER MANTLE
Contrasting density evolution of subducted and tectonically buried crust in modern vs. Archean time
Today, eclogitization begins when rocks reach the critical P-T conditions, occurs at constant rate until conversion is complete
Relative
density
NEUTRAL BUOYANCY WITH RESPECT TO UPPER MANTLE
In Archean time,mafic granulite may have persisted metastably at eclogite conditions, until/unless water became available, then would rapidly -- but incompletely --convert to eclogite
Time since reaching eclogite P-T conditions

15. Dehydration/rehydration ‘hysteresis’ loop
for mafic lower crust: Dehydration is forever
Increasing dehydration of rock
Rocks converted to very dry --and strong --granulite. Will remain dry until fractures open fluid pathways.
Fracturing allows fluid infiltration and partial conversion to eclogite
Fracturing ceases when critical % of rock is converted to weak eclogite. Rock cannot be further rehydrated even if external water is available
Drying of rocks via devolatilization reactions and TTG melt extraction
Increasing availability of water
(positive hydrous ‘field intensity’)
Decreasing availability of water
(negative hydrous ‘field intensity’)
Initial state prior to tectonic burial
Increasing hydration of rock

16. Lithospheric strength (MPa) 10 20 30 40 50
Lithospheric strength (MPa)
1000
2000
3000
Dashed line: Typical strength-depth curve for modern continental lithosphere (wet lower crust, dry upper mantle)
Solid line: Possible strength-depth curve for thickened Archean crust.
Brittle-plastic transition in upper crust is shallower than today owing to higher geothermal gradient…
But lower crust is much stronger than today as a result of profound dehydration by more thorough prograde devolatilization combined with extraction of TTG melts.
THE POSSIBLE PARADOXICAL STRENGTH of ARCHEAN LITHOSPHERE ?

17. Summary and speculations (part I)
In Archean time, rocks in areas of tectonically thickened crust would commonly have experienced granulite facies conditions before reaching eclogite P&T .
Once thoroughly dehydrated, a rock may be impossible to convert entirely to eclogite. Dry granulite crust would have been strong and buoyant and may have constituted the first true lithosphere.
Dehydration of tectonically buried mafic rock may have been assisted by partial melting in the lower crust. This is consistent with geochemical studies showing that Archean tonalite-trondhjemite-granodiorite suites were generated by partial melting of hydrous basalt in the lower crust, without involvement of mantle material.

18. Summary and speculations (part II)
The emergence of modern plate tectonics may reflect a change in eclogite reaction mechanisms and rates, from slow, diffusion-dominated processes in dry mafic crust to much faster, fluid-assisted recrystallization of still-hydrated gabbro.
Venus may still be in a pre-plate tectonics mode, with limited eclogite production and a lithosphere that is hot but strong because it is dry.
The onset of true subduction -- i.e., recycling of crustal material deep into the mantle -- would have been a global geochemical change of the first order (implicated somehow in the huge C isotope excursion at the Archean-Proterozoic boundary?)
 Plate tectonics requires a deeper connection between Earth’s hydrosphere and interior than was possible on a hot, young Earth.

19. I see green omphacite
Red garnets too
Eclogite, and schists of blue
And I say to myself,
what a wonderful world.

20. Stella model of eclogitization process