GEOCHRONOLOGY HONOURS 2008 Lecture 08 Model Ages and Crustal Evolution

Crustal Evolution  What is the oldest material on the planet? –The Isua greenstone belt is an Archean greenstone belt in southwestern Greenland dated at Ga and contains the oldest known, well preserved, metavolcanic (metamorphosed mafic volcanic), metasedimentary and sedimentary rocks on Earth. –The Acasta Gneiss is a rock outcrop of Archaean tonalite gneiss 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…..

Jack Hills Zirconb  Detrital zircons with ages >4000 Ma have been found in these rocks and a 4,404 +/- 8 Myr zircon was found at Eranondoo Hill, the oldest dated material on Earth  They were found within part of the Ga supracrustal sequence. These zircons are considered most likely to have been placed into these rocks by erosion of older material.  The importance of this interpretation is that, in order for the rocks of the Jack Hills to contain detrital zircons, the Earth must have firstly been cool enough to support liquid water on the surface, if not a water ocean; that there must have been some kind of temporary continental-type crust, most likely very thin, on the surface of the Earth, and not a as postulated for the earliest phase of the Earth's history.

Jack Hills Zircons 4.4 Ga Zircon spot

The Early Earth

 18 O (Zr)

Composition of the Primitive Earth  Comparison to meteorites…???  Chondrite…?

Epsilon Notation  Archean plutons have initial 143 Nd/ 144 Nd ratios that are very similar to that of the Chondritic Uniform Reservoir (CHUR) predicted from meteorites.  Because of the similar chemical behaviour of Sm and Nd, departures in 143 Nd/ 144 Nd isotopic ratios from the CHUR evolution line are very small in comparison to the slope of the line.  Therefore Epsilon notation for Sm/Nd system is: –  Nd(t) = (( 143 Nd/ 144 Nd) sample (t)/( 143 Nd/ 144 Nd) CHUR (t) – 1) x 10 4

Model Ages  The isotopic evolution of Nd in the Earth is described in terms of a model called CHUR, which stands for “Chondritic Uniform Reservoir”.  CHUR was defined by DePaolo and Wasserburg in  The initial (or primordial) 143 Nd/ 144 Nd ratio and present 147 Sm/ 144 Nd ratio and the age of the Earth have been determined by dating achondrite and chondrite meteorites  The model assumes that terrestrial Nd has evolved in a uniform reservoir whose Sm/Nd ratio is equal to that of chondritic meteorites.

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

Implications  Partial melting of CHUR gives rise to magmas having lower Sm/Nd ratios than CHUR  Igneous rocks that form from such a melt therefore have lower present day 143 Nd/ 144 Nd ratios than CHUR  The residual solids that remain behind therefore have higher Sm/Nd ratios than CHUR  Consequently, these regions (referred to as “depleted regions” of the reservoir) have higher 143 Nd/ 144 Nd ratios than CHUR at the present time

Nd-Isotope Evolution of Earth

Model Dates  CHUR can be used to calculate the date at which the Nd in a crustal rock separated from the chondritic reservoir.  This is done by determining the time in the past when the 143 Nd/ 144 Nd ratio of the rock equaled that of CHUR  Skipping lots of in between steps the equation becomes

Model Dates Relative to Depleted Mantle  Model dates have also been calculated relative to a depleted reservoir (DM) whose Sm/Nd ratio was increased by formation of a partial 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:

Model Dates  Dates calculated in the above manner make one very big assumption –The Sm/Nd ratio of the rock has not changed since the time of separation of Nd from the Chondritic Reservoir  If there was a disturbance in the Sm/Nd ratio then the date calculated would not have any geological meaning.  This criteria is better met by Sm/Nd than by Rb/Sr because of the similar behaviour of Sm/Nd.

Model Dates and Sr-Isotope Evolution  The isotopic evolution of Nd and Sr in the mantle are strongly correlated.  This correlation gives rise to the “mantle array”  The mantle array (defined from uncontaminated basalts in oceanic basins) arises through the negative correlation of 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios  This indicates that oceanic basalts are derived from rocks whose Rb/Sr ratios were lowered but whose Sm/Nd ratios were increased in the past

Sr-Isotope Evolution of Earth

Epsilon Sr Calculations