Geochronology 2007 08 Model Ages

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GEOCHRONOLOGY HONOURS 2008

Lecture 08

Model Ages and Crustal Evolution


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Crustal Evolution

What is the oldest material on the planet?

The Isua greenstone belt is an Archean greenstone belt insouthwestern Greenland dated at 3.8-3.7 Ga and contains theoldest known, well preserved, metavolcanic (metamorphosed

mafic volcanic), metasedimentary and sedimentary rocks onEarth.

The Acasta Gneiss is a rock outcrop of Archaean tonalitegneiss in the Slave Craton in Northwest Territories,Canada and is the oldest known rock outcrop in the world,

dated at 4.03 Ga (zircon SHRIMP). And the Jack Hills Australia..


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Jack Hills Zirconb

Detrital zircons with ages >4000 Ma have been found inthese rocks and a 4,404 +/- 8 Myr zircon was found atEranondoo Hill, the oldest dated material on Earth

They were found within part of the 3.6-3.8Ga supracrustal

sequence. These zircons are considered most likely to havebeen placed into these rocks by erosion of older material.

The importance of this interpretation is that, in order forthe rocks of the Jack Hills to contain detrital zircons, theEarth must have firstly been cool enough to support liquid

water on the surface, if not a water ocean; that there musthave been some kind of temporary continental-type crust,most likely very thin, on the surface of the Earth, and not aas postulated for the earliest phase of the Earth's history.


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The Early Earth


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d18O(Zr)


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Composition of the Primitive Earth

Comparison to meteorites???

Chondrite?


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Epsilon Notation

Archean plutons have initial 143Nd/144Nd ratiosthat are very similar to that of the ChondriticUniform Reservoir (CHUR) predicted frommeteorites.

Because of the similar chemical behaviour of Smand Nd, departures in 143Nd/144Nd isotopic ratiosfrom the CHUR evolution line are very small incomparison to the slope of the line.

Therefore Epsilon notation for Sm/Nd system is: eNd(t) = ((143Nd/144Nd)sample (t)/(143Nd/144Nd)CHUR(t) 1) x

104


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Model Ages

The isotopic evolution of Nd in the Earth is describedin terms of a model called CHUR, which stands forChondritic Uniform Reservoir.

CHUR was defined by DePaolo and Wasserburg in 1976.

The initial (or primordial) 143Nd/144Nd ratio andpresent 147Sm/144Nd ratio and the age of the Earthhave been determined by dating achondrite andchondrite meteorites

The model assumes that terrestrial Nd has evolved in auniform reservoir whose Sm/Nd ratio is equal to thatof chondritic meteorites.


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CHUR and the Isotopic Evolution of Nd We can calculate the value of CHUR at any time, t, in the

past using the following equation and values


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Implications

Partial melting of CHUR gives rise to magmashaving lower Sm/Nd ratios than CHUR

Igneous rocks that form from such a melt

therefore have lower present day143

Nd/144

Ndratios than CHUR

The residual solids that remain behind thereforehave higher Sm/Nd ratios than CHUR

Consequently, these regions (referred to asdepleted regions of the reservoir) have higher143Nd/144Nd ratios than CHUR at the present time


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Nd-Isotope Evolution of Earth


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Model Dates

CHUR can be used to calculate the date at which the Nd in acrustal rock separated from the chondritic reservoir.

This is done by determining the time in the past when the143Nd/144Nd ratio of the rock equaled that of CHUR

Skipping lots of in between steps the equation becomes


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Model Dates Relative to Depleted Mantle

Model dates have also been calculated relative to a depletedreservoir (DM) whose Sm/Nd ratio was increased by formation of apartial melt in a previous episode.

Because the DM has elevated Sm/Nd values relative to CHUR the

equation for calculating a model date is as follows:


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Model Dates

Dates calculated in the above manner make onevery big assumption

The Sm/Nd ratio of the rock has not changed since thetime of separation of Nd from the Chondritic Reservoir

If there was a disturbance in the Sm/Nd ratiothen the date calculated would not have anygeological meaning.

This criteria is better met by Sm/Nd than byRb/Sr because of the similar behaviour of Sm/Nd.


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Model Dates and Sr-Isotope Evolution

The isotopic evolution of Nd and Sr in the mantleare strongly correlated.

This correlation gives rise to the mantle array

The mantle array (defined from uncontaminatedbasalts in oceanic basins) arises through thenegative correlation of 143Nd/144Nd and 87Sr/86Srratios

This indicates that oceanic basalts are derivedfrom rocks whose Rb/Sr ratios were lowered butwhose Sm/Nd ratios were increased in the past


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Sr-Isotope Evolution of Earth


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Epsilon Sr Calculations

Geochronology 2007 08 Model Ages

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