Lecture 8b: Absolute Dating + Hadean-Archean Time Scales

Absolute Dating: Methods and Core Concepts

• Absolute dating determines numerical ages for rocks, minerals, and events.
• Main dating methods discussed:
  • Dendrochronology (tree rings)
  • Paleomagnetics
  • Index fossils
  • Radiometric dating: uses decay of unstable radioactive isotopes in minerals
• Absolute Age Dating: key idea is measuring how much of a parent isotope remains as it decays to a daughter isotope over time.
• Context note: Time periods referenced in the lecture span the Hadean (≈4.6–4.0 Ga) and Archean (≈4.0–2.5 Ga).

Radiometric Dating: Isotopes, Half-Lives, and Decay Concepts

• Isotopes: same element with different numbers of neutrons; some are unstable and decay to other isotopes until reaching a stable form.

• Zircon (ZrSiO4) is a common mineral used in radiometric dating because it can incorporate trace amounts of Uranium and exclude Lead during formation.

• Key terms:

  • Parent isotope → Daughter isotope via radioactive decay.
  • Half-life (t1/2): time required for half of the parent to decay.
  • “Max ~5 half-lives”: practical dating window often cited as a limit for reliable radiometric ages.

• Quick algebra/check:

  • After n half-lives, the remaining fraction is rac{N}{N_0} = iggl(rac{1}{2}iggr)^n.
  • After 2 half-lives, rac{N}{N_0} = rac{1}{4}. → Answer: after 2 half-lives only ¼ of the parent remains.

• General radiometric decay equations:

  • N(t) = N0 iggl(rac{1}{2}iggr)^{rac{t}{t{1/2}}}
  • N(t) = N0 e^{- abla t}, ext{ where } abla = rac{ ext{ln}(2)}{t{1/2}}.
  • Relationship between decay constant and half-life:
    abla = rac{ ext{ln}(2)}{t_{1/2}}.

• Important context about dating windows: different isotope systems are useful over different time spans, depending on their half-lives.

Common Radioisotopes Used in Dating: Parent ⇄ Daughter, Half-life, and Useful Range

• Carbon-14 → Nitrogen-14
  • Half-life: t_{1/2} = 5730 ext{ a} (years)
  • Useful range: 100 ext{ a} - 50{,}000 ext{ a}
  • Common in: Biological material, CO2 in the atmosphere.
• Potassium-40 → Argon-40
  • Half-life: t_{1/2} = 1.3 ext{ Ga}
  • Useful range: 100{,}000 ext{ a} - 4.6 ext{ Ga}
  • Common in: Micas, hornblende.
• Rubidium-87 → Strontium-87
  • Half-life: t_{1/2} = 47 ext{ Ga}
  • Useful range: 10 ext{ Ma} - 4.6 ext{ Ga}
  • Common in: Micas, K-feldspar, biotite, glauconite.
• Uranium-238 → Lead-206
  • Half-life: t_{1/2} = 4.5 ext{ Ga}
  • Useful range: 10 ext{ Ma} - 4.6 ext{ Ga}
  • Common in: Zircon, uraninite.
• Uranium-235 → Lead-207
  • Half-life: t_{1/2} = 710 ext{ Ma}
  • Useful range: 10 ext{ Ma} - 4.6 ext{ Ga}
  • Common in: Zircon, uraninite.
• Units in the chart:
  • a = year
  • Ma = million years
  • Ga = billion years

International Chronostratigraphic Chart (ICS): Structure and Purpose

• The ICS chart is the official chronostratigraphic framework used to define time intervals.
• Top-level divisions include:
  • Eonothem / Eon
  • Erathem / Era
  • System / Period
  • Series / Epoch
  • Stage / Age
• GSSP: Global Boundary Stratotype Section and Points – the “golden spike” for lower bounds of divisions.
• Numerical ages (Ma) accompany many boundaries to anchor the divisions in absolute time.
• Example note: The chart includes a modern outline of the Phanerozoic, Proterozoic, Archean, and Hadean boundaries with representative ages (e.g., Cambrian at ~541 Ma). The chart is periodically updated by the ICS; the 2016 version is cited here with an updated PDF available at the ICS website.
• Useful link: ICS International Chronostratigraphic Chart (for visuals and exact GSSP ages) — http://www.stratigraphy.org/ICSchart/ChronostratChart2016-04.jpg

Geological Time Scale: Major Divisions and Key Boundary Ages

• The Geological Time Scale is organized from largest to smallest units: Eons → Eras → Periods → Epochs → Ages.
• Major Eons relevant here: Hadean, Archean, Proterozoic, Phanerozoic.
• Key numerical anchors mentioned:
  • Formation of the Solar System / Earth: ~4.54–4.56 Ga (Earth formed around 4.54 Ga).
  • Hadean-Archean boundary around ~4.0 Ga.
  • The PhanerozoicEon begins around ~541 Ma with the Cambrian explosion boundary.
  • Major mass-extinction and boundary boundaries are often cited at End-Permian (~252 Ma) and End-Cretaceous (~66 Ma).
• A compact timeline often used in study materials includes markers such as 541 Ma (start of the Cambrian) and 66 Ma (end of the Cretaceous), among others.
• The slide set includes a full color chart showing the internal structure of time divisions with numerical ages (Ma) for many baselines and stages.

Hadean–Archean Earth: Origins, Differentiation, and Early Plate Tectonics

• Timeline framing:
  • Origin of the Solar System and Earth’s accretion to differentiation occurs in the Hadean (~4.6 Ga onward).
  • The Hadean is characterized by a very hot Earth with a magma ocean and rapid differentiation into core and mantle.
  • Moon formation: ~4.5 Ga, resulting from a giant impact (Theia) that partially melted Earth and contributed material to form the Moon.
• Key processes and features in the Hadean-Archean:
  • Differentiation: separation into core, mantle, and crust following partial melting.
  • Decrease in internal heat over time; crust formation begins as the planet cools.
  • Decreasing bombardment and atmospheric evolution leading toward oceans and a more stable crust.
  • Beginning of internal magnetic field and early atmosphere & water.
  • Plate tectonics: initial stage of plate tectonics developing during the Archean (4.4–3.8 Ga) as surface cooled enough for ultramafic “plates” to form and begin moving.
  • Early subduction zones proposed as ultramafic oceanic plates cool and become denser than the descending slab.
  • Ocean-ocean collision may create volcanic island arcs and Greenstone belts; ocean-continent collisions contribute to continental crust growth via plutons and volcanism; continent-continent collisions fuse micro-continents into larger crust by the end of the Archean.
• Overall: Early plate tectonics involved rapid, high-volume volcanism and heat loss with rising light materials and formation of early crust; later evolution led to more mature plate tectonics.

Early Oceanic and Continental Crust: Types and Formation

• Earth’s early crust types:
  • Oceanic crust: ultramafic to mafic compositions; Komatiite-basalt compositions; very rapid recycling due to high mantle temperatures.
  • Continental crust: more felsic/intermediate compositions; tonalite-granite plutonic rocks later contribute to continents.
• Komatiite-Basalt: ultramafic volcanic rocks characteristic of the Hadean–Paleoproterozoic; rare today due to cooling of mantle.
  • Features: olivine-rich; Spinifex texture; high melting temperature around ~1600°C.
• Formation timing:
  • First appearance of oceanic and continental crust around ~4.5 Ga for crustal pieces, ~4.4 Ga for more distinct continental crust, with continued growth thereafter.
• Crustal evolution highlights:
  • Early ultramafic plates form and recycle quickly.
  • Partial melting of ultramafic rocks in the mantle produces new crust; subduction-related processes contribute to different rock types (tonalite-granite plutons in continental crust).
  • Crustal growth is initially rapid via magmatic intrusions and magmatic differentiation, then accelerates with ongoing subduction and collision processes.

Oldest Rocks and Minerals: Zircons, Gneisses, and Paleosols

• Oldest minerals: Zircon crystals in Jack Hills, Australia dating to about ~4.4 Ga via U-Pb radiometric dating.
• Oldest rocks in Canada: Acasta Gneiss around ~4.04 Ga.
• Oldest felsic crust in Antarctica: ~3.9 Ga.
• Oldest mid-ocean-ridge-type volcanic and sedimentary rocks: Isua, Greenland ~3.8 Ga.
• Oldest Amitsoq Gneiss in Greenland: ~3.8 Ga.
• Oldest paleosol (fossil soil): Pilbara, Australia ~3.46 Ga.

Oldest Known Continental Material: Implications of Jack Hills Zircons

• Jack Hills zircons (~4.4 Ga) indicate early continental crust or crustal components existed very early in Earth history.
• Interpretation: zircons formed from metamorphosed sandstone/conglomerate, suggesting formation of first sediments on a continent or proto-continent by ~4.4 Ga.

The First Supercontinent and Craton Concepts

• Ur: proposed first supercontinent around ~3.1 Ga; smaller than present-day Australia.
• Readings suggest ongoing continental accretion and assembly through time; supports a model of multiple smaller landmasses combining to build larger cratons by later eras.
• Reference article: Geoscientist feature on crustal assembly and subduction-related processes (linked in course materials).

Global Shield vs. Platform: Cratons, Shield, and Platform Concepts

• Definitions:
  • Shield: Large, ancient, Precambrian crystalline rock exposures (older than ~541 Ma) that form the core of continents; igneous and metamorphic rock basement.
  • Platform: Regions where Precambrian shield rocks are overlain by younger sedimentary covers; represents the geologically younger, but still ancient, outer portions of continents.
• Modern continents consist of a shield core surrounded by platform regions; many platforms were added to the margins during collisions (Wilson cycles).
• Example: North American Cratons show assembled pieces of shield and platform rocks, with exposure and shield/core rocks forming the nucleus of North American crust.
• Wilson cycles: cycles of supercontinent assembly and breakup through time, including magmatic intrusions and tectonic activity that grow cratons around margins.

How to Connect Time Periods to Real Geology (Exam-Oriented Tips)

• Study plan guidance (from the course):
  • Fill out Time Period Summary Sheets (link provided in course materials).
  • Know rough time spans of the Eons, Eras, and Periods and their order (oldest to youngest).
  • Identify the most important events discussed for each Period/Era.
  • Understand what rock types indicate specific events (e.g., evidence for plate tectonics, orogeny, supercontinent formation).
  • Connect conditions through time to broad geologic processes (plate tectonics, crust formation, mantle convection, atmospheric evolution).

Practical Timeline Anchors and Study Aids

• Rough epoch markers (from the slide set):
  • Hadean: ~4.6 Ga to ~4.0 Ga
  • Archean: ~4.0 Ga to ~2.5 Ga
  • Phanerozoic: starts at ~541 Ma
  • Major boundaries mentioned: End-Permian (~252 Ma), End-Cre taceous (~66 Ma), Cambrian boundary (~541 Ma)
  • Earth’s age and pivotal events such as Moon formation (~4.5 Ga) and crust formation (~4.4–4.0 Ga) are central anchors.
• Symbols and abbreviations used:
  • Ma = million years ago
  • Ga = billion years ago
  • a = year (annum)

Helpful Resources and Links Mentioned

• ICS Chronostratigraphic Chart (PDF): https://www.stratigraphy.org/ICSchart/ChronostratChart2016-04.pdf
• Nebular hypothesis and origin of the Solar System resources (video links in the slides)
• Formation of the Moon animation and related NASA materials (for conceptual understanding of the Giant Impact hypothesis)
• Time-period study sheets and course materials (link provided in the course portal)

Summary of Key Takeaways for the Exam

• Absolute dating relies on radioactive decay and half-lives to determine numerical ages, with equations such as N(t) = N0 igl(rac{1}{2}igr)^{rac{t}{t{1/2}}} and N(t) = N0 e^{- abla t}, abla=rac{ ext{ln}(2)}{t{1/2}}.
• Common isototope systems and their practical dating windows:
  • $^{14}$C–$^{14}$N: $t_{1/2}=5730$ a; useful range $10^2$–$5 imes10^4$ a; limitations for older samples.
  • $^{40}$K–$^{40}$Ar: $t_{1/2}=1.3$ Ga; useful for apx $10^5$–$4.6$ Ga samples.
  • $^{87}$Rb–$^{87}$Sr: $t_{1/2}=47$ Ga; useful for $10^7$–$4.6$ Ga ranges.
  • $^{238}$U–$^{206}$Pb: $t_{1/2}=4.5$ Ga; useful from $10^7$–$4.6$ Ga; zircon/uraninite are key minerals.
  • $^{235}$U–$^{207}$Pb: $t_{1/2}=710$ Ma; useful for $10^7$–$4.6$ Ga; zircon/uraninite.
• The Hadean–Archean Earth features early crust formation, Moon formation by a giant impact, differentiation, and the onset of plate tectonics in the Archean.
• Early Earth crust-types include ultramafic Komatiite-dominated oceanic crust and the later growth of continental crust through magmatism and subduction processes.
• The oldest known materials (zircons and gneisses) push back to about 4.4–4.0 Ga, indicating crustal formation very early in Earth history.
• Shield vs Platform concept helps explain the distribution of Precambrian rocks on continents and how cratons grow via collisions and magmatic intrusions. Cratons accumulate material around margins during Wilson cycles.
• Exam strategy emphasizes knowing the rough chronologies, key foundational events, and the evidence rocks provide for those events (rock types, structural features, metamorphism, isotopic ages).