Jeff Taylor Ages of Highland Rocks1 Ages of Pristine Highlands Rocks Ages of lunar rocks informative about: –Timing of magma ocean crystallization –Timescales of planetary accretion –Time scale of lunar differentiation –Models of planetary formation and evolution of the solar system

Jeff Taylor Ages of Highland Rocks2 Ages of Pristine Highlands Rocks Very short course on isotopic dating Ferroan anorthosite suite Mg-suite and Alkali suite rocks Duration of magma ocean crystallization

Jeff Taylor Ages of Highland Rocks3 Introduction to Isotopic Dating Isotopes: –Nuclei of the same element with different masses, such as 86 Sr, 87 Sr, 88 Sr –Some are unstable and decay radioactively through time (radioactive parent nuclides) –Some are stable and produced by decay of a radioactive parent (radiogenic daughter) –Some are stable and not produced by radioactive decay (nonradiogenic)

Jeff Taylor Ages of Highland Rocks4 Introduction to Isotopic Dating Unstable parent isotopes evolve to daughter isotopes at a constant rate (half life, mean life, decay constant) On diagram N is number of parent isotope, D* is number of daughter isotope

Jeff Taylor Ages of Highland Rocks5 Introduction to Isotopic Dating Basic decay equation: D r = number of atoms of stable daughter isotopes produced by decay from parent nuclide P P o = number of atoms of parent nuclide at zero age (t = 0) t = time  = decay constant = (0.693)/t 1/2 where t 1/2 is the half life of radioactive parent nuclide (P) Decay constant sometimes expressed as = 1/ , where  is mean life of P (average life expectancy of a single radioactive atom of P)

Jeff Taylor Ages of Highland Rocks6 Introduction to Isotopic Dating More conveniently: Where P m = number of parent nuclide atoms remaining, hence measurable (m = measurable) For general case in nature where some number of stable daughter atoms already existed at t = 0,

Jeff Taylor Ages of Highland Rocks7 Introduction to Isotopic Dating Much easier to measure relative amounts of material (ratios), so we can modify the previous equation by dividing each side by the number of atoms of a stable nonradiogenic isotope: D m /D s and P m /D s can be measured directly, providing an equation of the form y = b + mx b = intercept = initial ratio (I = D i /D s ) m = slope = (e - t -1) t = date = [ln(m+1)]/

Jeff Taylor Ages of Highland Rocks8 Introduction to Isotopic Dating Requires: –Sufficiently different P o /D s were created in different samples to allow resolvably different D m /D s and P m /D s to develop –Sufficient time elapses between t=0 to t=present –The system has remained closed (no loss of parent or daughter isotopes) from t=0 to t=present –The initial ratio (I=D i /D s of the system was homogeneous at t=0 –The value of is known

Jeff Taylor Ages of Highland Rocks9 Introduction to Isotopic Dating Measurements: –Prepare rock samples or mineral separates –Dissolve the sample in strong acid –Isolate and purify the element of interest by cation exchange chromatography –Load element onto metal filament, load in mass spectrometer where a strong magnetic field separates ions by mass. Intensity of each ion beam (each individual mass) measured electronically –Useful data from tiny amounts of stuff: mg of sample, ng to pg of element –Extreme cleanliness required—filtered air, distilled reagents

Jeff Taylor Ages of Highland Rocks10 Introduction to Isotopic Dating Parent Nuclide Decay Constant (yr -1 ) Half-Life yr Daughter Nuclide 238 U x x 10 9 yr 206 Pb 235 U x x 10 8 yr 207 Pb 232 Th x x yr 208 Pb 147 Sm6.54 x x yr 143 Nd 87 Rb1.42 x x yr 87 Sr 187 Re1.52 x x yr 187 Os 176 Lu1.94 x x yr 176 Hf 182 Hf7.7 x x 10 6 yr 182 W 146 Sm1.02 x x 10 8 yr 142 Nd

Jeff Taylor Ages of Highland Rocks11 Introduction to Isotopic Dating Important information comes from different geochemical behavior of the parent and daughter isotopes (they have different distribution coefficients in different minerals) Applies to all isotopic systems, though differences in some are much smaller (e.g., Sm- Nd)

Jeff Taylor Ages of Highland Rocks12 Rb-Sr Dating

Jeff Taylor Ages of Highland Rocks13 a bc toto 86 Sr 87 Sr o () 86 Sr 87 Sr 86 Sr 87 Rb Begin with a rock with 3 minerals plotting at a b c at time t o

Jeff Taylor Ages of Highland Rocks14 After some time increment (t 0  t 1 ) each sample loses some 87 Rb and gains an equivalent amount of 87 Sr a bc a1a1 b1b1 c1c1 t1t1 toto 86 Sr 87 Sr 86 Sr 87 Rb 86 Sr 87 Sr o ()

Jeff Taylor Ages of Highland Rocks15 At time t 2 each rock system has evolved  new line Again still linear and steeper line a bc a1a1 b1b1 c1c1 a2a2 b2b2 c2c2 t1t1 toto t2t2 86 Sr 87 Sr 86 Sr 87 Sr o () 86 Sr 87 Rb

Jeff Taylor Ages of Highland Rocks16 Rb-Sr Dating Rise in 87 Sr/ 86 Sr depends on Rb/Sr in rock –In this example, all rocks formed 800 My ago, with same initial 87 Sr/ 86 Sr ratio –The higher the Rb/Sr, the faster the growth of 87 Sr/ 86 Sr. –Applies to whole rocks and minerals in rocks

Jeff Taylor Ages of Highland Rocks17 Other Isotope Systems Except for U-Th-Pb, all similar to Rb-Sr in their systematics Norman et al. (2003)

Jeff Taylor Ages of Highland Rocks18 Sm-Nd Identical in form to Rb-Sr:

Jeff Taylor Sm-Nd Ages of Highland Rocks19

Jeff Taylor Ages of Highland Rocks20 Sm-Nd One difference is the addition of two parameters, CHUR and epsilon (  ); both started with Sm-Nd: –CHUR = chondritic uniform reservoir –Assumes Nd evolved in a uniform reservoir whose Sm/Nd is equal to that of chondritic meteorites –Sm and Nd are refractory elements, so they do not fractionate in the solar nebula. Sm/Nd is uniform among chondrite groups –But they do fractionate during partial melting and crystallization

Jeff Taylor Ages of Highland Rocks21 CHUR and Epsilon Present chondritic 143 Nd/ 144 Nd = * Present chondritic 147 Sm/ 144 Nd = * Knowing these allows us to calculate 143 Nd/ 144 Nd of CHUR at any time (t) in the past: I t CHUR = 143 Nd/ 144 Nd in CHUR at any time in the past I 0 CHUR = 143 Nd/ 144 Nd in CHUR now = ( 147 Sm/ 144 Nd) 0 CHUR = this ratio in CHUR now (0.1967) *Wasserburg et al, 1981

Jeff Taylor Ages of Highland Rocks22 CHUR and Epsilon CHUR concept is important as it allows us to calculate the date at which Nd and Sm separated from a chondritic reservoir –Remember that they have difference distribution coefficients, so any event such as partial melting fractionates them –Crucially important in understanding planetary differentiation

Jeff Taylor Ages of Highland Rocks23 CHUR and Epsilon

Jeff Taylor Ages of Highland Rocks24 CHUR and Epsilon

Jeff Taylor Ages of Highland Rocks25 CHUR and Epsilon We want to know whether the initial 143 Nd/ 144 Nd ratios of different rocks are higher or lower than those of CHUR. But differences are small, so define a parameter epsilon (  ): This is the initial value of  at the time (t) the rock crystallized, as computed from the isochron. I t CHUR = 143 Nd/ 144 Nd in CHUR at time t ( 143 Nd/ 144 Nd) I is the initial ratio at the time of crystallization, as determined from the intercept of the isochron.

Jeff Taylor Ages of Highland Rocks26 CHUR and Epsilon Positive  Nd indicates that the rocks were derived from residual solids in a (mantle or crustal) reservoir after magma had been withdrawn at an earlier time. –“Depleted” part of the reservoir because of removal of LIL elements, accompanied by fractionation of Sm from Nd, giving higher Sm/Nd Negative  Nd indicates derivation from sources that had lower Sm/Nd. –These are “enriched” sources (higher in LIL elements), even though they have lower Sm/Nd