GEOCHRONOLOGY HONOURS 2007 Lecture 05 The Samarium-Neodymium System

Behaviour of Rb and Sr in Rocks and Minerals Rb behaves like K  micas and alkali feldspar Sr behaves like Ca  plagioclase and apatite (but not clinopyroxene) Rock Type Rb ppm K ppm Sr ppm Ca ppm Ultrabasic 0.2 40 1 25,000 Basaltic 30 8,300 465 76,000 High Ca granite 110 25,200 440 25,300 Low Ca granite 170 42,000 100 5,100 Syenite 110 48,000 200 18,000 Shale 140 26,600 300 22,100 Sandstone 60 10,700 20 39,100 Carbonate 3 2,700 610 302,300 Deep sea carbonate 10 2,900 2000 312,400 Deep sea clay 110 25,000 180 29,000

Behaviour of Sm and Nd in Rocks and Minerals Both Sm and Nd are LREE Because Sm and Nd have very similar chemical properties that are not fractionated very much by igneous processes such as fractional crystallisation. Useful for looking at metamorphic processes not igneous processes Rock / Min Sm ppm Nd ppm Sm/Nd Olivine 0.07 0.36 0.19 Garnet 1.17 2.17 0.539 Apatite 223 718 0.311 Monazite 15,000 88,000 0.17 MORB Thol 3.30 10.3 0.320 Rhyolite 4.65 21.6 0.215 Eclogite 2.61 8.64 0.302 Granulite 4.96 31.8 0.156 Sandstone 8.93 39.4 0.227 Chondrites 0.199 0.620 0.320

Rb-Sr vs Sm-Nd Sm-Nd Rb-Sr Mafic and Ultramafic igneous rocks Metamorphic Events Rocks that have lost Rb-Sr Rb-Sr Acidic and Intermediate igneous rocks Rocks enriched in rubidium and depleted in strontiu,

Application of Sm-Nd Similar chemical properties of Sm and Nd Sm very long half life (106 Byr) Means that large variations in Sm/Nd ratios in natural rocks are rare Therefore difficulty in obtaining a wide range of Sm/Nd ratios from a single rock body Combined with greater technical demands of Nd-isotope work has limited applications

Rb-Sr VS Sm-Nd Isochrons

Applicability of Sm-Nd Generally applied to problems where Rb-Sr not appropriate Very old rocks with likely disturbance of the Rb/Sr ratio Rocks with very low Rb/Sr ratios, ie achondrites Mineral pairs that concentrate Sm or Nd

Applicability of Sm-Nd Mineral isochrons for Sm-Nd can often work quite successfully because variations in partition coefficients causes moderately large variations in Sm-Nd ratios unlike whole rock systems Garnet and Cpx have mirror image partition coefficients which therefore give rise to large variations in Sm/Nd ratios. Common occurrence of garnet + cpx is in eclogites where Sm-Nd has been used extensively to date the timing of metamorphism Sm-Nd as REE are relatively immobile and may therefore not fully re-equilibrate during metamorphism

Sm-Nd Isotope System Sm has seven naturally occurring isotopes – 144, 147, 148, 149, 150, 152, 154 as well as 146 which is an intermediate between 150Gd and 142Nd, and 151 which is extinct because its half life is only 93 years. Of these 147Sm, 148Sm and 149Sm are radioactive but only 147Sm has a half life that impacts on the abundance of 143Nd. Nd also has seven naturally occurring isotopes – 142, 143, 144, 145, 146, 148, 150. 147Nd is extinct because the half life is only 10.99 days. 143Nd is produced by alpha decay of 147Sm

Sm/Nd Decay Equation The decay equation for Sm/Nd is 143Nd/144Nd = (143Nd/144Nd)i + 147Sm/144Nd(elt – 1) The decay equation is ratio-ed to 144Nd because the number of atoms of 144Nd in a unit weight of rock or mineral remains unchanged so long as the system in which it resides remains closed to Nd exchange.

Problems with the applicability of Sm-Nd Garnet + Omphacite Eclogite

Mineral Transformations Transformation of igneous augite to metamorphic omphacite Relatively minor cation exchange (Ca,Mg,Fe,Al)2(Si,Al)2O6 -> (Na,Ca)(Mg,Fe,Al)Si2O6 Monoclinic -> Monoclinic Often does not completely re-equilibrate Transformation of plagioclase to garnet Major chemical exchange and structural re-organisation CaAl2Si2O8 -> Ca3Al2Si3O12 Triclinic -> Isotropic Likely to completely reset Sm-Nd systematics and give the metamorphic age

Sm-Nd Remobilisation and Re-equilibration

Inclusions in Mineral Phases Garnet Inclusions Monazite in particular a problem because it also concentrates Sm and Nd Other inclusions that concentrate heavy REE’s

eNd(t) = ((143Nd/144Nd)sample (t)/(143Nd/144Nd)CHUR(t) – 1) x 104 Epsilon Notation Archean plutons have initial 143Nd/144Nd ratios that are very similar to that of the Chondritic Uniform Reservoir (CHUR) predicted from meterorites. Because of the similar chemical behaviour of Sm and Nd, departures in 143Nd/144Nd 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: eNd(t) = ((143Nd/144Nd)sample (t)/(143Nd/144Nd)CHUR(t) – 1) x 104