Introduction to Quantitative Geology Lesson 13.1 Basic concepts of thermochronology Lecturer: David Whipp david.whipp@helsinki.fi 4.12.17 Intro to Quantitative Geology www.helsinki.fi/yliopisto 3
Goals of this lecture • Introduce the basic concepts of thermochronology • Discuss the closure temperature concept and how closure temperatures are estimated Intro to Quantitative Geology www.helsinki.fi/yliopisto 4
Why thermochronology? • Popular dating technique for studying long-term tectonic and erosional processes (i.e., stuff we’ve been learning) Spanish Pyrenees Intro to Quantitative Geology www.helsinki.fi/yliopisto 5
Why thermochronology? • Inherently linked to crustal heat transfer processes (advection, diffusion, production, etc.) Intro to Quantitative Geology www.helsinki.fi/yliopisto 6
Why thermochronology? Predicted ages X ( O i − E i ) 2 χ 2 = σ 2 Measured age i Age • Incorporates many equations we’ve seen and many other concepts presented earlier in the course (hillslope processes, river erosion, heat conduction/advection, basic geostatistics) Intro to Quantitative Geology www.helsinki.fi/yliopisto 7
Geochronology versus thermochronology • Geochronology is the science of dating geological materials, and in many ways most radioisotopic chronometers are also thermochronometers • An important distinction lies in what the ages mean and their interpretation • Geochronological ages are generally interpreted as ages of the materials (crystallization ages) • Thermochronological ages are often interpreted as the time since the material cooled below a given temperature (cooling ages) Intro to Quantitative Geology www.helsinki.fi/yliopisto 8
General thermochronology terms Solid-State Diffusion • Thermochronometer Temperature History A radioisotopic system consisting of: Temperature • a radioactive parent Spontaneous Nuclear Reaction • a radiogenic daughter isotope or Time crystallographic feature • the mineral in which they are T 0 found Tectonics + Surface Processes = Exhumation = Cooling Fig 1.1, Braun et al., 2006 Intro to Quantitative Geology www.helsinki.fi/yliopisto 9
Geochronology versus thermochronology • Geochronology is the science of dating geological materials, and in many ways most radioisotopic chronometers are also thermochronometers • An important distinction lies in what the ages mean and their interpretation • Geochronological ages are generally interpreted as ages of the materials (crystallization ages) • Thermochronological ages are often interpreted as the time since the material cooled below a given temperature (cooling ages) Low-temperature thermochronology www.helsinki.fi/yliopisto December 4, 2017 10
General thermochronology terms Solid-State Diffusion • Thermochronometry Temperature History The analysis, practice, or application Temperature of a thermochronometer to Spontaneous Nuclear Reaction understand thermal histories of rocks or minerals Time • Thermochronology The thermal history of a rock, T 0 mineral, or geologic terrane. Tectonics + Surface Processes = Exhumation = Cooling Fig 1.1, Braun et al., 2006 Intro to Quantitative Geology www.helsinki.fi/yliopisto 11
General thermochronology terms Solid-State Diffusion • Thermochronometry Temperature History The analysis, practice, or application Temperature of a thermochronometer to Spontaneous Nuclear Reaction understand thermal histories of rocks or minerals Time • Thermochronology The thermal history of a rock, T 0 mineral, or geologic terrane. Tectonics + Surface Processes = Exhumation = Cooling Fig 1.1, Braun et al., 2006 Intro to Quantitative Geology www.helsinki.fi/yliopisto 12
The aim of thermochronology • In most modern applications of thermochronology, the goal is to use Solid-State Diffusion the recorded thermal history to provide insight into past tectonic or Temperature History erosional (surface) processes Temperature Spontaneous Nuclear Reaction • To do this, it is essential to link the Time temperature to which a thermochronometer is sensitive to a T 0 depth in the Earth Tectonics + Surface Processes = Exhumation = Cooling • This is not easy, and the field of quantitative thermochronology is Fig 1.1, Braun et al., 2006 growing rapidly as a result Intro to Quantitative Geology www.helsinki.fi/yliopisto 13
The essence of thermochronology Closed System Open System Parent decay Daughter Fig 1.3, Braun et al., 2006 • Daughter products are continually produced within a mineral as a result of radioactive decay • Daughter products may be lost due to thermally activated diffusion • The temperature below which the daughter product is retained depends on the daughter product and host mineral Intro to Quantitative Geology www.helsinki.fi/yliopisto 14
The essence of thermochronology Closed System Open System Parent decay Daughter Fig 1.3, Braun et al., 2006 Low T High T • Daughter products are continually produced within a mineral as a result of radioactive decay • Daughter products may be lost due to thermally activated diffusion • The temperature below which the daughter product is retained depends on the daughter product and host mineral Intro to Quantitative Geology www.helsinki.fi/yliopisto 15
The concept of a closure temperature Closed System Open System uplif Parent Te Sur decay aleo-surface vation a Daughter Partial retention/ Temperature annealing zone Fig 1.3, Braun et al., 2006 Depth / partial • retention The transition from an open to a closed system does zone not occur instantaneously at a given temperature, but rather over a temperature range known as the partial retention (or partial annealing ) zone t Daughter concentration / Apparent age Apparent age Fig 1.6a, Braun et al., 2006 Intro to Quantitative Geology www.helsinki.fi/yliopisto 16
The concept of a closure temperature Closed System Open System uplif Parent Te Sur decay aleo-surface vation a Daughter Partial retention/ Temperature annealing zone Fig 1.3, Braun et al., 2006 Depth / partial • The transition from an open to a closed system does retention zone not occur instantaneously at a given temperature, but rather over a temperature range known as the partial retention (or partial annealing ) zone t • The partial retention zone temperature range spans Daughter concentration / Apparent age from the point at which nearly all produced daughter Apparent age products are lost to diffusion to where they are nearly Fig 1.6a, Braun et al., 2006 all retained Intro to Quantitative Geology www.helsinki.fi/yliopisto 17
Effective closure temperature, defined • Defined by Dodson (1973), the closure temperature is the ‘temperature of a thermochronological system uplif Te Sur at the time corresponding to its apparent age’ aleo-surface vation a Partial retention/ • Temperature This concept is quite useful, as we can thus relate a annealing zone Depth / measured age to a temperature in the Earth partial retention zone T c • Effective closure Unfortunately, closure temperatures vary as a temperature function of the thermochronological system, mineral t size, chemical composition and cooling rate Daughter concentration / Apparent age Apparent age • This definition also only works when cooling is Fig 1.6a, Braun et al., 2006 monotonic (no reheating) Intro to Quantitative Geology www.helsinki.fi/yliopisto 18
Influence of cooling rate on effective T c Cooling rate ( ° C Myr -1 ) • C In general, the effective closure temperature 0.1 1 10 100 0 for a given thermochronometer system will increase with increasing cooling rate AHe 100 Effective closure temperature ( ° C) • AFT For the retention of 4 He in apatite, the ZHe effective closure temperature is ~40°C at a 200 KsAr THe cooling rate of 0.1 °C/Ma and ~80°C at a ZFT rate of 100°C/Ma 300 • BiAr The absolute difference in effective closure 400 MuAr temperature is also larger for higher temperature thermochronometers 500 • ~40°C for 4 He in apatite HbAr 600 • ~130°C for 40 Ar in hornblende Reiners and Brandon, 2006 Intro to Quantitative Geology www.helsinki.fi/yliopisto 19
What causes cooling? • With the idea of an effective closure temperature, we now have the main concept of thermochronology - a date will ideally reflect the time since the rock sample was at T c • But, what causes cooling? Intro to Quantitative Geology www.helsinki.fi/yliopisto 20
Erosional exhumation Erosion at surface Landslides, hillslope Rivers/glaciers diffusion, etc. Mountains Crustal block • Occurs as a result of erosion and removal of overlying rock bringing relatively warm rock to the surface • Can take place in convergent, extensional, strike-slip or inactive tectonic settings • Most common “cooling type” for thermochronology Intro to Quantitative Geology www.helsinki.fi/yliopisto 21 21
Erosional exhumation Erosion at surface Landslides, hillslope Rivers/glaciers diffusion, etc. Mountains Rock uplift Crustal block • Occurs as a result of erosion and removal of overlying rock bringing relatively warm rock to the surface • Can take place in convergent, extensional, strike-slip or inactive tectonic settings • Most common “cooling type” for thermochronology Intro to Quantitative Geology www.helsinki.fi/yliopisto 22 22
Recommend
More recommend
