Assigning a rock to a metamorphic facies based on its mineralogy is a good first-order assessment of peak metamorphic P-T conditions, but there are also ways to use measured mineral compositions to quantitatively assess P-T conditions. One such method in metapelites is garnet-biotite geothermometry, which uses the Mg and Fe content of garnet and biotite to estimate peak metamorphic temperature The general equation for calculating temperature using this method is: T = (52112 + 0.238*P)/(19.51-24.942*ln(K)) Where T is temperature (in Kelvin) P is pressure (in bars) and K is the Mg/Fe ratio in garnet divided by the Mg/Fe ratio in biotite: K = (Mg/Fe)Gt/ (Mg/Fe)Bt Assuming a pressure of 5 kbars, calculate a peak metamorphic temperature (in °C) for rocks with the following garnet and biotite compositions A. (Mg/Fe)Gt = 0.110; (Mg/Fe)Bt = 0.420 B. (Mg/Fe)Gt = 0.114; (Mg/Fe)Bt = 0.695 C. (Mg/Fe)Gt = 0.115; (Mg/Fe)Bt = 0.513 What facies would you assign to these three rocks? How does this compare to what you would expect based on mineralogy List the 3 rocks in order of increasing metamorphic grade (lowest to highest)
Assigning a rock to a metamorphic facies based on its mineralogy is a good first-order assessment of peak metamorphic P-T conditions, but there are also ways to use measured mineral compositions to quantitatively assess P-T conditions. One such method in metapelites is garnet-biotite geothermometry, which uses the Mg and Fe content of garnet and biotite to estimate peak metamorphic temperature The general equation for calculating temperature using this method is: T = (52112 + 0.238*P)/(19.51-24.942*ln(K)) Where T is temperature (in Kelvin) P is pressure (in bars) and K is the Mg/Fe ratio in garnet divided by the Mg/Fe ratio in biotite: K = (Mg/Fe)Gt/ (Mg/Fe)Bt Assuming a pressure of 5 kbars, calculate a peak metamorphic temperature (in °C) for rocks with the following garnet and biotite compositions A. (Mg/Fe)Gt = 0.110; (Mg/Fe)Bt = 0.420 B. (Mg/Fe)Gt = 0.114; (Mg/Fe)Bt = 0.695 C. (Mg/Fe)Gt = 0.115; (Mg/Fe)Bt = 0.513 What facies would you assign to these three rocks? How does this compare to what you would expect based on mineralogy List the 3 rocks in order of increasing metamorphic grade (lowest to highest)
Applications and Investigations in Earth Science (9th Edition)
9th Edition
ISBN:9780134746241
Author:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa
Publisher:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa
Chapter1: The Study Of Minerals
Section: Chapter Questions
Problem 1LR
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Question
Assigning a rock to a metamorphic facies based on its mineralogy is a good first-order
assessment of peak metamorphic P-T conditions, but there are also ways to use measured
mineral compositions to quantitatively assess P-T conditions. One such method in
metapelites is garnet-biotite geothermometry, which uses the Mg and Fe content of garnet
and biotite to estimate peak metamorphic temperature
assessment of peak metamorphic P-T conditions, but there are also ways to use measured
mineral compositions to quantitatively assess P-T conditions. One such method in
metapelites is garnet-biotite geothermometry, which uses the Mg and Fe content of garnet
and biotite to estimate peak metamorphic temperature
The general equation for calculating temperature using this method is:
T = (52112 + 0.238*P)/(19.51-24.942*ln(K))
Where T is temperature (in Kelvin) P is pressure (in bars) and K is the Mg/Fe ratio in
garnet divided by the Mg/Fe ratio in biotite:
K = (Mg/Fe)Gt/ (Mg/Fe)Bt
Assuming a pressure of 5 kbars, calculate a peak metamorphic temperature (in °C) for
rocks with the following garnet and biotite compositions
A. (Mg/Fe)Gt = 0.110; (Mg/Fe)Bt = 0.420
B. (Mg/Fe)Gt = 0.114; (Mg/Fe)Bt = 0.695
C. (Mg/Fe)Gt = 0.115; (Mg/Fe)Bt = 0.513
What facies would you assign to these three rocks? How does this compare to what you
would expect based on mineralogy
List the 3 rocks in order of increasing metamorphic grade (lowest to highest)
Transcribed Image Text:File Preview
P
(kbar)
20
18
16
14
12
10
8
6
4
2
0
0
Blueschist
Prehnite-
Pumpellyite
Zeolite
Greenschist
100 200 300
400
Amphibolite
Hornfels
Eclogite
500
T (°C)
600
Granulite
Sanidinite
700 800 900 1000
Transcribed Image Text:Pressure (kbar)
14
12
10
2
Low-grade
Blueschist
Prehnite Pumpelvite
Chlorite
400
Greenschist
Biotite
Medium-grade High
Eclogite
Barrovian
Amphibolite
Sarnet
Staurolite
600
Temperature (°C)
Kyanite
Sillimanite
Granulite
40
30
20
800
10
Depth (km)
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