Download - Did the Plates Subduct in Hadean?

DID THE PLATES SUBDUCT IN HADEAN?

Rijumon Dasgupta2nd Semester; Applied Geology

University of Calcutta

Hadean - first geologic eon of Earth. The planet was very hot due to high volcanism, a partially molten surface and frequent collisions with other Solar System bodies. Post-Hadean earth began to cool. The Hadean eon remain poorly understood because lack of rock record dating from that time.

Introduction

Oldest terrestrial component detrital Zircons (age: ~ 4,276 Ma), from Jack Hills conglomerates and in metamorphosed sediments at Mt. Narreyer and Jack Hills Gneissic terrain in W. Australia, can potentially provide insights into the conditions existed on Earth at that time.

Ref: ‘Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago’ by Simon A. Wilde, John W. Valley, William H. Peck & Colin M. Graham ; Nature, Vol 409 January 2001

Clues from Geochemistry Negative Eu anomalies of the zircon grain

indicative of evolved Granitic melts. High Titanium concentrations for the zircons,

are characteristic of derivation from peraluminous granites containing Ti-bearing phases. Titanium concentrations range from 3 to 9 ppm, yielding apparent crystallization temperatures of 665–745°C

50 μm


δ18O of 5‰ at point 1 and 7.4 ‰ at point 2 indicative of involvement of supracrustal material which has undergone low-temperature interaction with a liquid hydrosphere.

Fig : Lighter circular areas are the SHRIMP analytical sites and the values record the 207Pb/206Pb age of each site

Fig: Rare earth element data for Jack Hills zircon W74-2/36 measured by ion

microprobe.

Observations from U-Pb analysison SHRIMP II

Fig: Combined concordia plot for grain W74/2-36. The inset shows the most concordant data points together with their analysis number

Large difference found in Pb : U for similar 207Pb / 206Pb ages No evidence of subsequent Pb loss during Precambrian, suggests that Zircon was not affected by later igneous or metamorphic processes before its deposition in conglomerate.


Clues from zoning within ZirconMelting

of existing

continental crust at

about 4,404 Ma

ago

Zircon first

crystallized in that magma

This crystal

was mixed with a more

primitive magma between

4,364 Ma & 4,289 Ma ago

and formed a magmatic overgrowt

h

Indicates that at 4.4Ga ago there were already intermediate to granitic, high δ18O continental rocks.


50 μm

Fig: The larger area showing light luminescence, the well defined cracks and the oscillatory zoning in both the light and dull portions of the crystal.

Qtz

inclusions Individual

muscovite inclusions Muscovite+quarz

+ FeldsparHornblende inclusions

Inclusions trapped during the growth of host phases from magmas - Record information about the conditions under which they formed

The crystal grew in SiO2

saturated environment.

Experimental results indicate formational P = 6.9 ± 0.8 kbar.

The limited stability field of muscovite+ quartz+feldspar in peraluminous granitoids alone restricts T<800°C and P > 4 kbar.

Hopkins et al. (’08) obtained a

formation pressure of ~7± 2 Kbar

Ref: ‘Low heat flow inferred from .4 Gyr zircons suggests Hadean plate boundary interactions’ by Michelle Hopkins, T. Mark Harrison & Craig E. Manning; Nature, Vol 456 November 2008

50 μm

Story behind the Inclusions

Story behind the InclusionsDiamond inclusions

P-T condition does not match with other inclusions

Absence of high P minerals except diamond, do not support formation of zircon at UHP metamorphic conditions. Diamond inclusions imply the presence of a relatively thick continental lithosphere and that crust–mantle interaction occurred on Earth as early as 4,250 Myr ago.

Inclusions have rounded to hexagonal, oval, angular, or needle-like shapes. Graphite surrounds diamond.

All the Clues At a GlanceHadean Jack Hill Zircons : 1. Suggest their origin from clay rich protoliths.2. Crystallized at low temperature (~700°C), in condition

close to water saturation.3. Contain inclusion assemblages predominantly of

magmatic Muscovite, Biotite, Quartz.4. Yielded thermobarometric results suggestive of their

formation in low heat flow ( ~ 40 – 80 mW/m2 ) environments.

5. Point towards their origin from hydrous, SiO2 saturated, meta and peraluminous melts similar to the convergent margin magmas observed today.

Controversy in Environment of Formation

Jack Hill Zircons formed from impact melts, assumed to be commonplace during Hadean.

- If this is true, then their crystallizing temperature would be >800°C (Darling et al. 2009), and does not support their formation close to H2O saturation (Watson & Harison, 2005 ).

1) A near surface rock mass downwells into mantle.2) Relatively high P achieved.3) Incipient melting while melting T is reached.4) low apparent geotherm arose via sagduction (Gorman et al. 1978)

- Doesn’t satisfy the characteristic assemblage of muscovite + quartz and formation in H2O saturated environment

Melting of a matured continental sediment during continuous, submarine under-thrusting beneath a stable upper plate.Melting in 2 ways:a) Fluxed melting of underthrust sedimentary materialb) fluxed melting of sediment in the upper plate due to H2O delivery to the melting site- Appears consistent with all of the relevant constraints

Ref: ‘Constraints on Hadean geodynamics from mineral inclusions in >4 Ga zircons’ by Michelle Hopkins, T. Mark Harrison & Craig E. Manning ; Earth and Planetary Science Letters, 2010

A lithospheric slab, travelled

3000 km from the spreading ridge.

It thrusted beneath a rigid

upper plate containing continental lithosphere

extending to 50 km depth.

Minimum melting

temperatures attained at depths of 20–30 km.

The Ultimate Tectonic Model Relevant With all Conditions

Additional assumptions: 25° dip of the downgoing slab, basal and surface continental lithosphere heat fluxes of 30 and 200 mW/m2 respectively, and a Moho depth of 15 km.

Heat flow in the upper plate in this model is entirely diffusive (i.e., no counter flow).

The Ultimate Tectonic Model Relevant With all Conditions

It assumes that : Convective type of the mantle is a simple function of its

temperature. High degrees of partial melting resulting from high interior

temperature. This altered the viscosity structure of the upper mantle as the

primitive silicate Earth dewatered. Resulting sluggish convection reduces heat loss such that a mantle

potential temperature (Tp) of 1600 °C corresponds to a globally averaged heat loss similar to present.

For the Tp = 1600 °C corresponding to ~ 4 Ga, a ~140 km thick oceanic lithosphere would become neutrally buoyant in ~120 Ma, having an average plate velocity of ~2 cm/yr (Korenaga 2006).

Ref: ‘Constraints on Hadean geodynamics from mineral inclusions in >4 Ga zircons’ by Michelle Hopkins, T. Mark Harrison & Craig E. Manning ; Earth and Planetary Science Letters, 2010

Traditional View – A Contrary to The New Model

High mantle temperatures would result in thick ( > 40 km), fast-spreading oceanic crust that resists subduction (McKenzie and Bickle, 1988; Davies, 1992). Thus Hadean plate-tectonic-like behavior has historically been viewed as unlikely.

Success of The New Model It is the simplest model that successfully incorporates all relevant constraints by hydrous melting in the hanging

wall of thrust faults, possibly in a manner similar to modern convergent margins.

Ref: ‘Constraints on Hadean geodynamics from mineral inclusions in >4 Ga zircons by Michelle Hopkins, T. Mark Harrison & Craig E. Manning ; Earth and Planetary Science Letters, 2010