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Geochronology is the temporal study of earthly and planetary materials by

absolute or relative dating methods. More importantly, geochronology is concerned with understanding geologic time, when major event(s) occurred and the development and understanding of geochronometers. Absolute dating involves isotopic measurement of parent-daughter ratios of radiogenic isotopes to calculate the age of earth and planetary materials. Isotopes are atoms containing the same number of protons (Z) but different number of neutrons (N) making their atomic mass number (A) lighter or heavier.

Radiogenic isotopes decay due to insufficient binding energy in the atom nuclide because of an excess of protons or neutrons. Decaying nuclides emit partical and electromagnetic radiation in the form of three different types; alpha (α), beta (β) and gamma (γ or X-ray). The first observation of radioactivity by Becquerel (1896) led to thorium decay

1902). These experiments formed the law of radioactive decay based on half-lives and the fundamental half-life equation (Equation 3.2).

𝑁 = 𝑁𝜊𝑒−𝜆𝑡

Equation 3-2 Half-life equation derived from the Rutherford and Soddy experiments on thorium decay.

The half-life equation states the concentration of parent atoms (𝑁) is a function of the original concentration (𝑁𝜊) to the natural decay constant (λ) multiplied by time of decay (t). The decay constant is the rate of decay of a nuclide to half of its original concentration. The actual half-life (T1/2) of the nuclide is represented by Equation 3.3.

The half-life is the time it takes for half of the initial unstable isotope to decay. Different radiogenic nuclides have varying decay constants and as a result different half-lives. A list of half-lives for common radiogenic nuclides is listed in Table 3-5.

𝑇1/2 =

0.693 λ

Equation 3-3 Half-life equation where T1/2 is the time for half of the parent isotope to decay to its daughter.

Table 3-6 Table of radiogenic parent nuclides and daughter products compiled by Faure and Mensing (2005). Symbols α, β and e correspond to alpha, beta and electron capture decay.

Radiometric dating of geologic materials is concentrated around mineral and bulk rock isotopic concentrations. If the isotope decay system is closed, it represents the accumulation of stable daughter isotopes equal to the number of decayed parent atoms. The half-life equation is reworked to reflect these systematic changes (Equation 3.4)

Parent Isotope Decay Modes Halflives (109 Years) Daughter Isotope

40_{K e, β}+ _11.9 40_Ar 40_{K β}- _1.39 40_Ca 87_{Rb β}- _48.8 87_Sr 147_{Sm α 106} 143_Nd 176_{Lu β 36} 60_Hf 187 Re α, β- 41 187Os 232_{Th α, β}- ₁₄ 208_Pb 235_{U α, β}- _0.704 207_Pb 238 U α, β- 4.47 206Pb

where D is the final number of daughter atoms, D0 is the initial number of daughter atoms

and N is the number of parent nuclides. From this, time (t) can be calculated. A 𝐷 = 𝐷𝑜 + 𝑁(𝑒𝜆𝑡 _{− 1)} B 𝑡 =1 𝜆 ln ( 𝐷 − 𝐷𝑜 𝑁 + 1) C 𝑡 = 1 𝜆238_U ln ( 206_Pb 238_U + 1)

Equation 3-4 Half-life equations for radiometric decay of a parent isotope in a closed system (A). Equations B is the same but reformulated to find time (t). Equation C is the application of equation B to U-Pb decay chain of 238U to

206_Pb.

Dating geologic materials requires several assumptions outlined by Faure and Mensing (2005):

1. The system is closed and no daughter or parent has been lost.

2. Environmental conditions do not affect the decay constant of the parent nuclide.

3. The initial daughter values are known, or a relative D0 value can be applied

via knowledge of the isotopic reservoir within the hosting material or system. 4. Daughter and parent isotopic measurements are accurate and representative of

their respective materials and systems.

5. Secular equilibrium is reached indicating that daughter products measured are the result of the original parent and not an intermittent short lived decay component.

Crystallization, metamorphic and alteration ages are the focus for samples

collected within the MRG. Given that the MRG is Archean, the time scale is hundreds of millions of years making the U-Pb decay system applicable. The U-Pb isotope system is unique, having two decay chains resulting from the decay of 235U and 238U to 207Pb and

206

Pb. Isotopic data from both systems can be compared and plotted on a concordia plots: this is explained further in the data regression and plotting section. The system also allows for large spectrum of ages to be captured due to half-lives of 235U and 238U being 704 million and 4.47 billion years respectively. In addition, several silicate and

phosphate minerals incorporate uranium and lead into their crystal structure during crystallization. Their low common lead incorporation at the time of crystallization permits several silicate-phosphate mineral species to be used as geochronometers.

3.3.5.1

Datable Phases

3.3.5.1.1

Zircon

Zircon (ZrSiO4) is a colourless, green, grey, red or brown tetragonal orthosilicate

accessory mineral in igneous and metamorphic rocks and occurs as detrital mineral in sedimentary rocks. Zircon has specific gravity of 4.2 and a hardness of 7.5 (Lima-de- Faria, 1994) with 4/m 2/m2/m symmetry. The unit cell is an ATO4 structure formed by a

dodecahedron of one zirconium coordinated by eight oxygen within six SiO4 tetrahedra

(Finch and Hanchar, 2003). The structure forms alternating edge sharing chains at

dodecahedron margins parallel to the C axis (Speer, 1982). Theoretical elemental percent values of zirconium, silicon and oxygen are 49.77%, 15.32% and 34.91% respectively, however; the 4+ cation site generally allows the incorporation of hafnium (~1.71% ), REEs (<1.2%) (Speer, 1982), thorium (1-10,000ppm) and uranium (1 -12,000 ppm) (Belousova et al., 2002). These cation substitutions are reflected in the oscillatory crystal growth of igneous zircon (Speer, 1982) while metamorphic zircons appear as

overgrowths. Zircon’s solid state stability temperature (1525-1550° C )(Anseau et al., 1976) and low reactivity make it a robust geochronometer for various geological environments.

3.3.5.1.2

Monazite

Monazite is another accessory mineral primarily observed in igneous and metamorphic rocks. Detrital grains of monazite are identified in sedimentary rocks as well. Like zircon, monazite is an redish-brown to brown orthophosphate exhibiting a similar APO4 structure comprised of REEs (Ce, LREE, Th, Y) and phosphate (PO4) in a

monoclinic structure within a space group of P21/n (Lima-de-Faria, 1994; Boatner, 2002).

Monazite exhibits a stubby prismatic habit with a hardness of 5 to 5.5 (Lima-de-Faria, 1994). Unlike zircon, monazite contains an array of rare earth elements and is further classified into varieties based on 10% or more of the REE element in excess. Three varieties exist; Ce, La or Nd (Rosenblum and Fleischer, 1995). The cation slot hosting REEs is coordinated by nine oxygen forming REEO9 polyhedra (Ni et al., 1995). This

larger nine oxygen coordinated site hosts light rare earth elements from La to Eu (Williams et al., 2007). Monazite’s natural melting temperature by experimentation resulted in chemical stabilities up to 2057 + 40° C (Hikichi and Nomura, 1987). Monazite’s uranium concentrations range from 1,000 to 4,000 ppm in granitic rocks while thorium can be as high as 14% (Kato, 1958). About 1 ppm common lead is

incorporated into the monazite structure during crystallization. Monazite`s low common lead incorporation coupled with its high temperature stability and low reactivity makes it a robust geochronometer as well (Parrish, 1990).

3.3.5.1.3

Xenotime

Xenotime (Y,HREE)PO4 is a yellow, greenish brown to redish brown stubby to

prismatic-pyramidal orthophosphate and accessory mineral in igneous and metamorphic rocks. Like monazite, xenotime hosts a APO4 structure but is tetragonal and isostructural

with zircon, categorized in space group I4i/amd (Ni et al., 1995; Boatner, 2002). Unlike

monazite, xenotime has a REO8 cation site surrounding the PO4 tetrahedral units and

incorporates heavy rare earth elements with smaller ionic radii into its crystal structure due to lanthanide contraction (Lu-Tb and Y) (Ni et al., 1995). Theoretical xenotime endmembers are generally YPO4 and HREE-PO4 and are defined by molar percentages

common lead. Xenotime also has a high melting temperature of 1896-1995 + 20° C as reported by Hikichi and Nomura (1987).

3.3.5.2

LA-ICP-MS

Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) is the same analytical technique as described previously with exception of sample introduction to plasma. A laser controlled by the operator vaporizes sample material and introduces the vaporized material to the plasma excitation energy. A Thermo VG PQ EXCELL

quadrupole plasma mass spectrometer housed at the University of Toronto and operated by Dr. Don Davis was used in conjunction with an attached Nu-Wave UP-213 laser ablation microscope for U-Pb isotopic analyzes. A Nd:YAG deep UV (213nm) laser beam operating between 5 and 10 Hz ablated zones in zircon and monazite phases with spot sizes ranging between 8 and 12 microns in diameter depending on size and type of mineral phase. Mineral grains subjected to ablation incurred a 10-20 µm wide square raster reaching 1 µm deep prior to pitting to remove any surficial impurities. Laser ablation pitting commenced after rastering on selected mineral grains of zircon and monazite from polished thin section samples 6225 and 6220A. After three mineral grain analyses, a standard was analyzed and the sequence repeated.

3.3.5.3

Data Regression and Plotting

LA-ICP-MS analysis provided isotopic measurements in counts per second of

206

Pb, 207Pb, 238U and 232Th. The counts per second were compared to known age standards of the same mineral type and to a calibration standard prior to ablation. This technique is used to reduce the effects of fractionation during analysis. Ratios of

207

Pb/235U and 206Pb/238U were calculated by regression software developed by Dr. Davis. Regressed data provided the 207Pb/235U, 206Pb/238U and Th/U ratios in Excel spreadsheets. An addin Excel program called Isoplot was used to plot U-Pb isotopic ratios on concordia plots with 2 sigma error elipses. The program regressed a mean sum weighted deviation (MSWD) as well.

The concordia plots used are a comparison plot of isotopic ratios of 206Pb/238U to

207

representing equal ages of the two geochronometers. Analyses plotting below the concordant line are discordant and have lead loss while plots above are negatively discordant and have incurred uranium additions or mass fractionation.