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Calculating rate info from ages (Mineral pairing example (Minerals (AFT:…
Calculating rate info from ages
Method 1:
Mineral pairing
a. Assume geothermal gradient = known
b. Assume closure temp
x Commonly
over-estimates exhumation
Method 2:
Relief-dependent age-elev profile
Assume relationship btwn sample @ bottom & top of profile
= Age-elev profile
e.g. 500 = 5 Ma
100m = 10 Ma
1500m = 15 Ma
Calculate slope of age-elevation profile
btwn spatially related samples
e.g. m = (y2-y1 / x2-x1)
= (1500 - 500 / 15 - 5)
= 1000/10 = 100 m/Ma
Assumption: multiple samples =
from same vertical profile
Mineral pairing: How
Date two minerals from same sample (usually apatite & zircon)
Assume geothermal gradient & closure temp for each mineral
∴ Can calculate exhumation rate
(rate @ which mineral travelled toward surface)
Mineral pairing example
Minerals
AFT: Closure temp ~120°C & Age ~30Ma
Ap He: Closure temp ~ 60°C & Age ~ 20 Ma
Subtract 2 ages
(time ∆ 30 - 20 = 10 Ma)
2.
Subtract closure temps
(temp ∆ =120 - 60 = 60 °C)
Work out ∆ in Z from temp Z
i.e. 60°C/30 = 2km ∆ in Z
∴ in 10 Ma, exhumed 2 km
Work out rate
(S=D/T --> 2/10 = 0.2km/Ma)
Exhumation
rate units
= km / Ma
Rqmt Mineral Pairing
Must date same sample
with multiple different techniques
∴ need big rock sample
Big assumption mineral pairing
Geothermal gradient
= known
in reality = difficult to constrain
Closure
temp = known
specific to that sample
subject to error - affected by erosion etc
Limitation age-elev profile method
Not possible everywhere
Need vertical profile ∴ cannot use everywhere
i.e. if sampling has horizontal spread