Geothermometry: Analysis of Black Hills Schist

Size: px

Start display at page:

Download "Geothermometry: Analysis of Black Hills Schist"

Shanon Miller
5 years ago
Views:

1 Geothermometry: Analysis of Black Hills Schist Presented By: Andy Gorz and Chad Cro;y Department of Geosciences North Dakota State University h"p://

2 Overview Background of Black Hills Methods Geothermobarometry Conclusion

3 Black Hills of South Dakota N

4 Geologic History Trans- Hudson Orogen ( Ga) deformaqon events Emplacement of Harney Peak Granites (1.7Ga) contact metamorphism deformaqon event Laramide Orogeny UpliV (80-35 Ma)

5 South Dakota Geological Survey 1965; Compiled by N. H. Darton 1951 USGS

6 (Helms and Labotka, 1991)

7 Hand Specimen Fine grained groundmass, displaying metamorphic schistosity, supporqng Garnet phenocrysts.

8 Thin SecQons PPL 1 mm 1 mm XP

9 Scanning Electron Microscopy (SEM) pt. 1 pt. 2 pt. 6 pt. 3 Image courtesy of NDSU Electron Microscopy Center (2010)

10 SEM cont.

11 SEM cont. pt. 1 pt. 3 pt. 2 Image courtesy of NDSU Electron Microscopy Center (2010)

12 SEM cont. Biotite K(Mg, Fe) 3 AlSi 3 O 10 (OH, F) 2 Almandine Fe 3 Al 2 (SiO 4 ) 3 Silica SiO 2

13 SEM Data Weight % O- K Mg- K Al- K Si- K K- K Ti- K Fe- K GARNET SCHIST(2)_pt GARNET SCHIST(2)_pt GARNET SCHIST(2)_pt Atom % O- K Mg- K Al- K Si- K K- K Ti- K Fe- K GARNET SCHIST(2)_pt GARNET SCHIST(2)_pt GARNET SCHIST(2)_pt

14 Geothermobarometry Geobarometry The evaluaqon of the pressure at which a rock was formed Geothermometry The evaluaqon of the temperature at which a rock formed (Winter, 2010)

15 Who is ready for EquaQons!

16 Geothermobarometry G = H TS G = Gibbs Free Energy (Measure of energy content of chemical systems) H = Enthalpy (Heat Content) T = Temperature (in Kelvins) S = Entropy (Most easily preceived as randomness) (Winter, 2010)

17 Geothermobarometry (Winter, 2010)

18 Geothermobarometry Combining and rearranging these equaqons lnk = - H 0 /RT + S 0 /R ( V/RT)dP V = Volume P = Pressure d = finite change in variables of state (Ex: T & P) If H 0, V, and S 0 are kept constant - Can use measurable K to find T (P almost insignificant) (Winter, 2010)

19 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 by measuring the raqo of Mg/Fe in both (Winter, 2010)

20 Geothermometry Ferry and Spear (1978) Performed Series of Experiments on Fe- Mg exchange between BioQte and Ca- free Garnet: Almandine Phlogopite Pyrope Annite Fe 3 Al 2 Si 3 O 12 + KMg 3 Si 3 AlO 10 (OH) 2 = Mg 3 Al 2 Si 3 O 12 + KFe 3 Si 3 AlO 10 (OH) 2 Constant P = Gpa T = C (Winter, 2010)

21 Ferry and Spear, 1978 X Grt = Alm 90 Prp 10 X Fe = molar Fe / (Fe + Mg) = 0.9 Two different X Bt Approach equilibrium from both sides Grt and Bt molar proporqons of 98:2 Bt would have to change far more to accommodate Grt composiqon in a"aining equilibrium K D value (Winter, 2010)

22 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)

Ferry and Spear, 1978 = (2/98) = (2/98) * (1.0 0.750) = 0.005 So final = 0.90 + 0.

23 Ferry and Spear, 1978 = (2/98) = (2/98) * ( ) = So final = = Table from (Winter, 2010) p. 588, originally from (Ferry and Spear, 1978)

24 Ferry and Spear, 1978 Can use these final composiqons to find K D Assuming an ideal soluqon: K = K D = (Mg/Fe) Grt (Mg/Fe) Bt lnk = lnk D = - H 0 /RT + S 0 /R ( V/RT)dP Measurable Trying to find

25 Ferry and Spear, 1978 Rearranging previous equaqons: lnk D = - H - P V 3R * (1/T) + S 3R Experimentally derived constants: S = J/K mol V = J/Mpa (Robbie and Hemingway, 1995) H = kj/mol

26 Ferry and Spear, 1978 Rerranging previous equaqon & subsqtuqng in constant values: 52, P(MPa) T C = lnk D P only variable lev

27 Spear, 1993 Change in P almost insignificant. KD values for the garnet- bioqte exchange. Diagram from (Winter, 2010) p. 590, originally from (Spear, 1993)

28 (0.65/13.71) Grt (8.34/19.72) Bt Our SEM Data!"#$%&%!!!"#$!!%&#$!!'(#$!!)*#$!!!$#$!!+*#$!!,-#$!.*/0*0-!!!!!"#$%!!!&'#()!!!$(#&(!!!''#))!!!!'#&)!!!'*#(&! '(1234*3-!!!!!+#),!!!,,#%(!!!$+#')!!!!!'$#('!! ~ K D = = Assuming an average P of 4 kilobars = 400 Mpa 52, *400 T C = = ln(0.112) C

29 Conclusion Diagram from (Winter, 2010) p. 588

30 (Helms and Labotka, 1991)

31 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 Helms, T.S., and Labotka, T. C., 1991, Petrogenesis of Early Precambrian peliqc schists of the southern Black Hills, South Dakota: Constraints on regional low- pressure metamorphism. Geological Society of America BulleQn. v. 103, p Holm, D.R., Dahl, P. S., and Lux, D.L., 1997, 40Ar/39Ar evidence for middle proterozoic ( Ma) slow cooling of the southern Black Hills, South Dakota, midconqnent, North America: ImplicaQons for Early Proterozoic P- T evoluqon and pos"ectonic magmaqsm. Tectonics. v. 16, no. 4, p 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

Geothermobarometry of the Garnet Schist near Keystone, South Dakota By Kristopher Ekart NDSU Geology Department Petrology /01/14

Geothermobarometry of the Garnet Schist near Keystone, South Dakota By Kristopher Ekart NDSU Geology Department Petrology 422 05/01/14 Geothermobarometry Geothermobarometry: is the calculafon of equilibrium