geothermometry: analysis of black hills schist
TRANSCRIPT
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Geothermometry: Analysis of Black HillsSchist
Presented By:
Andy Gorz and Chad Cro;y
Department of Geosciences
North Dakota State Universityh"p://www.sdrcd.org/Portals/SDRCD/Black%20Hills/images/Pic.SylvanLake.jpg
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Overview
• Background of Black Hills
• Methods
• Geothermobarometry
• Conclusion
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Black Hills of South Dakota
N
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Geologic History
• Trans-‐Hudson Orogen (2.5–2.0 Ga)– deformaQon events
• Emplacement of Harney Peak Granites (1.7Ga)– contact metamorphism
– deformaQon event
• Laramide Orogeny UpliV (80-‐35 Ma)
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©South Dakota Geological Survey 1965; Compiled by N. H. Darton 1951 USGS
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(Helms and Labotka, 1991)
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Hand Specimen
Fine grained groundmass, displaying metamorphicschistosity, supporQng Garnet phenocrysts.
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Thin SecQons
1 mm 1 mm
PPL XP
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Scanning Electron Microscopy (SEM)
Image courtesy of NDSU Electron Microscopy Center (2010)
pt. 1
pt. 2
pt. 3pt. 6
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SEM cont.
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SEM cont.
Image courtesy of NDSU Electron Microscopy Center (2010)
pt. 1
pt. 2
pt. 3
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SEM cont.BiotiteK(Mg, Fe)3AlSi3O10(OH, F)2
AlmandineFe3Al2(SiO4)3
SilicaSiO2
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SEM Data
O-‐K Mg-‐K Al-‐K Si-‐K K-‐K Ti-‐K Fe-‐K
102522 GARNET SCHIST(2)_pt1 5.87 17.00 30.31 13.20 1.74 31.88
102522 GARNET SCHIST(2)_pt2 0.51 47.89 27.11 24.50
102522 GARNET SCHIST(2)_pt3 37.34 62.66
O-‐K Mg-‐K Al-‐K Si-‐K K-‐K Ti-‐K Fe-‐K
102522 GARNET SCHIST(2)_pt1 5.87 17.00 30.31 13.20 1.74 31.88
102522 GARNET SCHIST(2)_pt2 0.51 47.89 27.11 24.50
102522 GARNET SCHIST(2)_pt3 37.34 62.66
Weight %
Atom %
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Geothermobarometry
• Geobarometry– The evaluaQon of the pressure at which a rockwas formed
• Geothermometry– The evaluaQon of the temperature at which a rockformed
(Winter, 2010)
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Who is ready forEquaQons!
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G = H – TS
G = Gibbs Free Energy (Measure of energycontent of chemical systems)
H = Enthalpy (Heat Content)
T = Temperature (in Kelvins)
S = Entropy (Most easily preceived as randomness)
Geothermobarometry
(Winter, 2010)
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Geothermobarometry
(Winter, 2010)
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• Combining and rearranging these equaQons…
lnK = -‐∆H0/RT + ∆S0/R – (∆V/RT)dP
V = VolumeP = Pressured = finite change in variables of state (Ex: T & P)
• If ∆H0, ∆V, and ∆S0 are kept constant-‐ Can use measurable K to find T (P almost
insignificant)
Geothermobarometry
(Winter, 2010)
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Geothermometry:The Garnet-‐BioQte Exchange
• Exchange Fe and Mg with changing temperatures and pressures
• Trying to reach equilibrium (Depends on T)
• Can infer temperature at formaQon of rock bymeasuring the raQo of Mg/Fe in both
(Winter, 2010)
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Geothermometry
Ferry and Spear (1978)
•Performed Series of Experiments on Fe-‐Mg exchange between BioQte and Ca-‐free Garnet:
Fe3Al2Si3O12 + KMg3Si3AlO10(OH)2 = Mg3Al2Si3O12 + KFe3Si3AlO10(OH)2
•Constant P = 0.207 Gpa
•T = 500-‐800 ̊C
(Winter, 2010)
Almandine Phlogopite AnnitePyrope
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• XGrt = Alm90Prp10
• XFe= molar Fe / (Fe + Mg) = 0.9
• Two different XBt
– Approach equilibrium from both sides
• Grt and Bt molar proporQons of 98:2– Bt would have to change far more to accommodateGrt composiQon in a"aining equilibrium KD value
Ferry and Spear, 1978
(Winter, 2010)
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Ferry and Spear, 1978
•Experiments lasted 13 to 56 days to give Qme for equilibraQon to be reached
Table from (Winter, 2010) p. 588, originally from (Ferry and Spear,1978)
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Ferry and Spear, 1978
∆ = (2/98)∆ = (2/98) * (1.0 – 0.750) = 0.005
So final = 0.90 + 0.005 = 0.905
Table from (Winter, 2010) p. 588, originally from (Ferry and Spear,1978)
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Ferry and Spear, 1978
• Can use these final composiQons to find KD
• Assuming an ideal soluQon:
K = KD=
lnK = lnKD = -‐∆H0/RT + ∆S0/R – (∆V/RT)dP
(Mg/Fe)Grt
(Mg/Fe)Bt
Measurable Trying to find
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• Rearranging previous equaQons:
Ferry and Spear, 1978
lnKD =-‐∆H -‐ P∆V
3R* (1/T) + ∆S
3R
Experimentally derived constants:
∆S = 19.506 J/K mol∆V = 2.494 J/Mpa (Robbie and Hemingway, 1995)∆H = 52.09 kJ/mol
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• Rerranging previous equaQon & subsQtuQng inconstant values:
52,090 + 2.494 P(MPa)
19.506 – 24.943 lnKD
Ferry and Spear, 1978
T ̊ C = -‐ 273
•P only variable leV…
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KD values for the garnet-‐bioQte exchange.Diagram from (Winter, 2010) p. 590, originally from (Spear,1993)
Spear, 1993
•Change in Palmostinsignificant.
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Our SEM Data
(0.65/13.71)Grt
(8.34/19.72)Bt
52,090 + 2.494*400
19.506 – 24.943 ln(0.112)
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KD = = 0.112
T ̊ C = -‐ 273 =
Assuming an average P of 4 kilobars = 400 Mpa…
443.31 ̊C
~
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Conclusion
Diagram from (Winter, 2010) p. 588
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(Helms and Labotka, 1991)
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References Cited• Ferry, J.M. and Spear, F.S., 1978, Experimental calibraQon of the parQQoning of Fe and
Mg between bioQte and garnet: Contrib. Mineral. Petrol., v.66, p. 113-‐117.
• Helms, T.S., and Labotka, T. C., 1991, Petrogenesis of Early Precambrian peliQc schistsof the southern Black Hills, South Dakota: Constraints on regional low-‐pressure
metamorphism. Geological Society of America BulleQn. v. 103, p. 1324-‐1334
• Holm, D.R., Dahl, P. S., and Lux, D.L., 1997, 40Ar/39Ar evidence for middle proterozoic(1300-‐1500 Ma) slow cooling of the southern Black Hills, South Dakota,
midconQnent, North America: ImplicaQons for Early Proterozoic P-‐T evoluQonand pos"ectonic magmaQsm. Tectonics. v. 16, no. 4, p. 609-‐622
• Spear, F. S., 1993, Metamorphic Phase Equilibria and pressure-‐temperature-‐Qme Paths.Monograph 1. Mineralogical Society of America. Washington, DC. Chapter 15
• Winter, J.D., 2010, Principles of Igneous and Metamorphic Petrology: Second EdiQon,Pearson EducaQon, Inc., p. 587-‐606