Thermochronology

1
Thermochronology
2
History

• Early quest for the time scale
• Revelation in the late 70s there wasnt
  anything bad about discordant K-Ar
  datesactually more information than just a
  simple date
• ANDmainly came about from fission track dating
  clearly a low-T system people began to
  understand the implications

3
Modern Thermochronology

• Uses multiple chronometers multiple dates, track
  a cooling history
• Stillusually cant record a prograde metamorphic
  history (unless youre lucky)
• Cooling historyvery informative

4
Firstsome common terminology

• Uplift (rock uplift)vertical motion of rock
  toward the earths surface, relative to a datum
  (like sea level)
• Exhumationunroofing path of rock towards the
  earths surface, regardless of process
• Denudationremoval of rock cover by tectonics or
  surface processes
• Erosionremoval of rock cover exclusively by
  surface processes

5
Some common geochronometers used in
thermochronology

• U-Pb system (U decaying to lead through decay
  chain)
• Minerals zircon, titanite (aka sphene), apatite
• Rb-Sr (isochron method)
• Common w.r. mu /- plag w.r. bi \-plag
  w.r. k-spar \-plag
• K-Ar (including modern Ar/Ar method)
• Hb, bio, musc, k-spar (orth vs microcline vs
  sanadine)
• Fission track
• Apatite, zircon, titanite, glass
• U-Th-He
• Apatite, zircon, titanite
• Sm-Nd (isochron method)
• w.r. garnet amphibole w.r. other special
  cases

6
Most common today (cost)

• Ar/Ar
• U-Pb zircon
• U-Th-He
• Fission track

7
Critical Observation, led to thermochronology
typical pattern of age discordance

• Recallconcordant ages, are ages in which
  several geochronometers yield exactly the same
  age these were considered good when you
  wanted to know the age of the rock
• General observation instead typical discordance
  pattern was oldest ages to youngestU-Pb zircon,
  U-Pb monazite, hornblende Ar, muscovite Ar,
  biotite Ar, zircon f.t., titanite f.t., apatite
  f.t.
• Key insightyounging ages represent cooling time
  when the mineral system closes to gain/loss of
  parent and daughter.

8
Simple concept of a cooling age by exhumation
9
Concept of discordant agescould be elevation
discordance Orsame rock, different materials,
give cooling rate
10
Critical in this conceptclosure temperature

• Key features
• Closure temperature is not a fixed number
  depends on cooling rate (more on this in second)
• Different minerals have different closure
  temperatures (hence discordance issue) AND
  closure temperature can vary with mineral
  composition and crystal structure e.g. feldspars

11
General theorysame theory behind diffusion creep
D D0 exp -(Ea PVa)/RT
Theoretical expression for a diffusion constant,
where D goes in the diffusion equation analagous
to thermal diffusivity in thermal problems
dC/dtD del(chem pot element)
12
Detailsnot important here

• Keyexperimentally determine diffusion rates,
  back extrapolate to time
• ORfor low-T systems, the problem can be
  calibrated using materials extracted from drill
  holes (e.g. well known for Apatite f.t.)

13
Out of thisconcept of partial retention zone,
partial annealing zone NOTEactual temperatures!
Reiners and Brandon, 2006
14
(No Transcript)
15
Key conceptallows a time to be attached to the
temperature numerous applications

• Best knownlow-T thermochronometers applied to
  exhumation
• Higher Tcombine time with temperature in a PT
  path
• Example 1 exhumation of high-P rocks determine
  when events happened, and tie the exhumation to
  regional events
• Example 2 date deformationknow T deformation
  in a mylonite, use the right chronometers,
  bracket age of deformation
• Example 3 combine with igneous chronology
  (zircon dating), recognize chronology of
  overprints in metamorphic rocks

16
(No Transcript)
17
(No Transcript)
18
(No Transcript)