IF SO, THEN CONSIDER TEACHING STABLE ISOTOPE GEOCHEMISTRY · TEACHING STABLE ISOTOPE GEOCHEMISTRY...
1
TEACHING STABLE ISOTOPE GEOCHEMISTRY IN AN UNDERGRADUATE PETROLOGY COURSE DUNN, Steven R., Earth and Environment, Mount Holyoke College, South Hadley, MA 01075, and BOWMAN, John R., Dept. of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112 ABSTRACT Stable isotope geochemistry has become commonplace in petrologic literature and stable isotope data strongly influence interpretations of many petrologic processes. In spite of this, undergraduate petrology textbooks tend to present limited background on stable isotope fundamentals and applications. In addition, stable isotopes provide an opportunity for quantitative problem solving that is mathematically within reach of students and involves a variety of chemical and geological processes. We present a lesson plan that introduces students to the fundamentals of stable isotope geochemistry with particular application to contact metamorphism of carbonate rocks. The lesson plan consists of (1) an introductory reading assignment with embedded problems and suggestions for classroom activities, (2) a reading assignment covering coupled C-O isotopic trends due to Batch and Rayleigh volatilization and fluid infiltration exchange fronts with problems, and (3) a reading assignment introducing the Alta aureole, Utah, with stable isotope data from Bowman et al. (1994). These are in Excel spreadsheets that allow students to explore the effects of varying parameters on volatilization and one-dimensional fluid flow. Variables that can be manipulated include fractionation factors and the relation between F-carbon and F-oxygen for Rayleigh and batch volatilization, and porosity, Darcy flux, effective diffusion coefficient, and oxygen content of fluid and rock for one-dimensional flow models. The lesson plan can be incorporated entirely or partly into a petrology course, depending on time available and desired depth of coverage. The level of sophistication can be tailored to the ability of the students and would be useful for graduate students as well. The Alta aureole is a classic locality, ideal for this lesson because the chemistry of the rocks is relatively simple and much has been published about them. Problem assignments start out very structured and become more open-ended and inquiry based. Assignments lend themselves well to group projects or cooperative learning methods. See http://www. mtholyoke.edu/courses/sdunn/geol201/index.html for spreadsheets and readings. GOAL: To introduce students in the use of stable isotope geochemistry in petrology by providing a framework and lesson plan to assist instructors. RATIONALE: w Stable isotope geochemistry has become commonplace in petrologic literature. w Stable isotope data strongly influence interpretations of many petrologic processes. w Yet, undergraduate petrology textbooks present limited background on stable isotope fundamentals and applications. w Stable isotopes provide an opportunity for quantitative problem solving that is mathematically within reach of students. w Stable isotope geochemistry involves a variety of interesting geochemical and petrological processes. w Stable isotope geochemistry connects petrology to other fields, e.g., chemistry, ecology, the hydrologic cycle, paleoclimatology, etc. w With the background provided by this lesson plan, students can begin reading primary literature that addresses a wide range of topics. WOULD YOU LIKE TO: What are your teaching goals? gEncourage more quantitative skills and practice? FProvide additional opportunities for critical thinking? CEnhance student access to primary literature? ?Connect petrology to other geological fields via geochemistry and physiochemical processes? IF SO, THEN CONSIDER INCORPORATING STABLE ISOTOPES INTO YOUR PETROLOGY OR GEOCHEM- ISTRY COURSE! Lesson plan, reading assignments, and problem sets available on-line at http://www.mtholyoke.edu/courses/sdunn/geol201/index.html influx } } } fluid dominated rock dominated isotopic front 10 3 ln α Cc-H2O for oxygen -5 0 5 10 15 20 25 30 35 40 0 100 200 300 400 500 600 700 800 900 Temperature ( o C) 10 3 ln α Figure 1 . Fractionation of oxygen isotopes between calcite and water as a function of temperature (0- 500°C). From O’Neil et al. (1969), updated by Friedman and O’Neil (1977), 10 3 lna Cc-H 2 O = 2.78(10 6 T -2 )-2.89, T in Kelvin in this equation. PART 1, START WITH THE BASICS Student Reading Assignment Addresses: Table 1. Light Stable Isotope Systems. abundance† notation standard used H†† D 99.985 0.015 dD SMOW, Standard Mean Ocean Water 12 C 13 C 98.892 1.108 d 13 C PDB, Peedee formation belemnite, S.C. 14 N 15 N 99.635 0.365 d 15 N N 2 -atm, Air nitrogen 16 O 17 O 18 O 99.759 0.037 0.204 d 18 O SMOW, Standard Mean Ocean Water or PDB, Peedee formation belemnite, S.C. 32 S 33 S 34 S 36 S 95.018 0.750 4.215 0.017 d 34 S CDT, Canyon Diablo Troilite (FeS), an iron meteorite † Approximate abundances, small variations are the basis for the use of stable isotopes (VG Instruments Fact Sheet). †† Note: hydrogen is the only element for which its isotopes have a separate name, i.e., deuterium and tritium for 2 H and 3 H. 1000 ) C / C ( ) C / C ( ) C / C ( C 12 13 12 13 12 13 13 · - = PDB PDB sample δ 1000 O) O/ ( O) O/ ( O) O/ ( O 16 18 16 18 16 18 18 · - = SMOW SMOW sample δ What are stable isotopes? Why do they fractionate? Definitions of standard delta notation, examples of the temperature dependence of fractionation factors, all are explained, no prior knowledge expected. Definitions are provided and explained. Calcite-water oxygen isotope fractionations are used for examples. B A B A δ δ α + + = - 1000 1000 ) ( D A-B = d A - d B 10 3 lna A-B » D A-B Abundances and standards for the light stable isotopes HERE'S WHAT WE HAVE: Reading assignments and problems for students in 3 Parts. PART 1: Introduction to stable isotope geochemistry including definitions, some background, and problem set. Problems use only simple algebra and worked examples are included. Problems with detailed worked examples are embedded in Part 1, for example: Problem 1. Suppose the d 18 O value of a calcite sample is 16.2‰ and fluid inclusion data indicates that this calcite formed at 260°C. What must the d 18 O value of the water have been? Problem 4. 2 moles of calcite (d 18 O = 25.0‰) equilibrates with 1 mole of water (d 18 O = 5‰) at 320°C. What is the final d 18 O value of both the calcite and the water? -40 -30 -20 -10 0 10 20 30 40 Earth's mantle Extraterrestrial material δ 18 O(‰) MORB Andesites Other basalts Dacites-rhyolites Granites-tonalites Metamorphic rocks Sedimentary rocks Seawater Meteoric waters Magmatic waters Metamorphic waters 5.7 OXYGEN -40 -30 -20 -10 0 10 20 30 40 δ 13 C(‰) Carbonatites, diamonds Marine carbonates Air CO 2 Freshwater carbonates Terrestrial plants Bacteria Methanogenic bacteria Eukaryotic algae Extraterrestrial material C3 C4 -5.0 Earth's mantle Natural gas Dissolved seawater C CARBON Figure 2. Typical oxygen isotopic composition of selected natural materials. Dashed line represents the earth's mantle. Modified from Hoefs (1997) and Best (2003). Igneous rock values exclude hydrothermally altered rocks. Figure 3. Typical carbon isotopic composition of selected natural materials. Dashed line represents the earth's mantle. Modified from Hoefs (1997). Terrestrial plants include ranges for C3 and C4 plants that use different photosynthetic pathways. PART 3: Presents a geologic overview of the Alta Stock contact aureole, including mineral isograds, T-X CO 2 interpretations, and the stable isotope data constraints. Problems are generally broad and rather open-ended. PART 2: Covers the concepts of volatilization and fluid infiltration, the isotopic effects resulting from these processes, the assumptions inherent in modeling these processes, and problems that explore these models. PART 2 PROVIDES USEFUL TOOLS Student Reading Assignment Explains: Volatilization by both "batch" and Rayleigh models, appropriate fractionation factors for both carbon and oxygen, fluid fluxes for one-dimensional fluid flow models and concept of isotopic "fronts." Coupled C-O Volatilization trends -10 -8 -6 -4 -2 0 10 15 20 25 α = 1.012 α = 1.006 Batch volatilization Rayleigh volatilization 0.01 0.5 δ 18 O 0.4 0.6 0.2 0.8 0.05 Initial Rock δ 13 C F-carbon values Figure 4. d 18 O versus d 13 C for volatilization (CO 2 -loss) of a rock with initial d 18 O = 22‰ and d 13 C = 0‰. Two values of a for oxygen (CO 2 - rock) are shown for both Rayleigh (solid lines) and batch (dashed lines) processes, a carbon = 1.0022. F values shown are for carbon. The F values for oxygen are related by Foxygen = 0.4F carbon + 0.6, which is the calc- silicate limit discussed in the text. (modified from Valley, 1986) Figure 7. Illustration of a column of rock undergoing infiltration and exchange as described in the text. The light region is "fluid-dominated" and the purple region is "rock-dominated." The "isotopic front" (or exchange front) moves in the direction of fluid flow and is a measure of the cumulative fluid flux. 0 200 400 600 meters 10 15 20 25 30 d 18 O (permil) Flow Direction 10 4 10 5 5 x 10 4 Fluid fluxes (cm 3 /cm 2 ) 10 3 Figure 8. d 18 O value of model rock as a function of distance along the flow path. Isotopic fronts are displaced in the direction of fluid flow with increasing fluid flux. In this model the initial rock is 25‰, the fluid enters from the left at 10‰, and D rock-fluid = 6‰. References Cited Berman, R.G. 1988. Internally-consistent thermodynamic data for minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3- SiO2-TiO2-H2O-CO2. Journal of Petrology, 29: 445-522. Berman, R.G. and Perkins, E.H. 1987. GEO-CALC: software for calculation and display of pressure-temperature-composition phase diagrams. American Mineralogist, 72: 861-862. Best, M.G., 2003. Igneous and Metamorphic Petrology, 2nd Edition. Blackwell Publ. 729 pp. Bickle, M. and Baker, J., 1990. Migration of reaction and isotopic fronts in infiltration zones: assessments of fluid flux in metamorphic terrains. Earth & Planetary Science Letters, 98: 1-13. Bowman, J.R., Willett, S.D., and Cook, S.J., 1994. Oxygen isotope transport and exchange during fluid flow: one-dimensional models and applications. American Journal of Science, 294: 1-55. Chacko, T., Mayeda, T.K., Clayton, R.N., and Goldsmith, J.R., 1991. Oxygen and carbon isotope fractionations between CO2 and calcite. Geochimica et Cosmochimica Acta, 55: 2867-288. Chacko, T., Cole, D.R., and Horita, J., 2001. Equilibrium oxygen, hydrogen, and carbon isotope fractionation factors applicable to geological systems. In Stable Isotope Geochemistry (J.W. Valley and D.R. Cole, eds.), Reviews in Mineralogy and Geochemistry, Vol 23: 1-81. Cook, S.J. and Bowman, J.R., 1994. Contact metamorphism surrounding the Alta stock: thermal constraints and evidence of advective heat transport from calcite + dolomite thermometry. American Mineralogist, 79: 513-525. Cook, S.J. and Bowman, J.R., 2000. Mineralogical evidence for fluid-rock interaction accompanying prograde contact metamorphism of siliceous dolomites: Alta stock aureole, Utah. Journal of Petrology 41: 739-757. Cook, S.J., Bowman, J.R., and Forster, C.B., 1997. Contact metamorphism surrounding the Alta stock: finite element model simulation of heat and 18O/16O mass transport during prograde metamorphism. American Journal of Science 297: 1-55. Criss, R.E. and Taylor, H.P., Jr., 1986. Meteoric hydrothermal systems. In Stable Isotopes in High Temperature Processes (J.W. Valley, H.P. Taylor, Jr., and J.R. O'Neil, eds.), Mineralogical Society of America, Reviews in Mineralogy, Vol. 16: 373- 424. Dipple, G.M., 1998. Reactive dispersion of stable isotope by mineral reaction during metamorphism. Geochimica et Cosmochimica Acta 62: 3745-3752. Friedman, I. and O'Neil, J.R., 1977. Compilation of stable isotope fractionation factors of geochemical interest. Chapter KK, Geological Survey Professional Paper 440-KK, Data of Geochemistry, 6th Edition (M. Fleischer, Tech. Ed.). Gerdes, M.L, Baumgartner, L.P., and Person, M., 1995. Stochastic permeability models of fluid flow during contact metamorphism. Geology 23:945-948. Hoefs, J., 1997. Stable Isotope Geochemistry. Springer-Verlag, New York. 201 pp. Kitchen, N.E. and Valley, J.W., 1995. Carbon isotope thermometry in marbles of the Adirondack Mountains, New York. Journal of Metamorphic Geology, 13: 577-594. Moore, J.N. and Kerrick, D.M., 1976. Equilibria in siliceous dolomites of the Alta aureole, Utah. American Journal of Science 276: 502-524. O'Neil, J.R., Clayton, R.N., and Mayeda, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. Journal of Chemical Physics, 51: 5547-5558. Shanks, W.C. III, 2001. Stable isotopes in seafloor hydrothermal systems: vent fluids, hydrothermal deposits, hydrothermal alteration, and microbial processes. In Stable Isotope Geochemistry (J.W. Valley and D.R. Cole, eds.), Reviews in Mineralogy and Geochemistry, Vol 23: 469-525. Sheppard, S.M.F. and Schwarz, H.P., 1970. Fractionation of carbon and oxygen isotopes and magnesium between coexisting metamorphic calcite and dolomite. Contributions to Mineralogy and Petrology, 26: 161-198. Valley, J.W., 1986. Stable isotope geochemistry of metamorphic rocks. In Stable Isotopes in High Temperature Processes (J.W. Valley, H.P. Taylor, Jr., and J.R. O'Neil, eds.), Mineralogical Society of America, Reviews in Mineralogy, Vol. 16: 445- 489. Valley, J.W., 2001. Stable isotope thermometry at high temperatures. In Stable Isotope Geochemistry (J.W. Valley and D.R. Cole, eds.), Reviews in Mineralogy and Geochemistry, Vol 23: 365-413. PART 3: APPLICATION, THE ALTA AUREOLE Student Reading Assignment Presents: Evidence for H 2 O infiltration of inner aureole from mineral reaction isograds and T-XCO 2 relations and affirmation from stable isotope data. Figure 9. Geologic map of the Alta area, Utah (from Moore and Kerrick, 1976). Mineral reaction isograds surround the Alta stock within siliceous dolomites of the Mississippian Deseret and Gardison formations. The study area within the southern contact aureole is from Bowman et al. (1994). 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 350 400 450 500 550 600 650 700 750 800 Do P = 1000 bars Do + Di Fo + Cc + CO 2 P e + C c + C O 2 F o + Di + CO 2 + H 2 O Cc + Tr Do + T r F o + C c + C O 2 + H 2 O T c + Cc + CO 2 D o + Qz + H 2 O Tc + Cc + Qz Tr + CO 2 + H 2 O T c + Cc T r + Do + C O 2 + H 2 O Tr + Cc + Qz Di + CO 2 + H 2 O Do + Qz + H 2 O Tr + Cc + CO 2 D o + T r F o + D i Tr + Cc D i + D o + C O 2 + H 2 O 5 2 1 3 4 5 1 3 2 4 Do + Qz Tc + Cc Tr + Cc Fo + Cc Pe + Cc X (CO 2 ) T e m p e ra ture ( C o ) Significant Assemblages Figure 11. T-XCO 2 diagram for selected reactions with significant mineral assemblages highlighted. The sequence of mineral reactions seen approaching the Alta stock contact are consistent with increasing temperature. Geothermometry yields peak temperatures of 600° C in the periclase zone, which requires H 2 O-rich fluid conditions. -8 -6 -4 -2 0 2 4 6 8 5 10 15 20 25 30 Tc Zone Tr Zone Fo Zone Per Zone w/Dol Per Zone no Dol F-value (carbon) 0.05 0.2 0.4 0.6 0.01 δ 18 O δ 13 C Alta Contact Aureole Figure 12. C-O isotopic trend for carbonates from the southern Alta aureole. The Rayleigh volatilization trend starts at the average of the talc zone samples, 26‰ and 3‰, and uses the same fractionation factors as Fig. 4. Data from Bowman et al., 1996 and additional unpublished data from J. R. Bowman. Figure 13. d 18 O versus distance (Z, in kilometers) from the contact of the Alta stock Temperatures are from Cook and Bowman (1994). Taken from Bowman et al. (1996). By making various assumptions about the fluid flow parameters, the oxygen isotope data can be explained. EXCEL ® SPREADSHEETS ARE AVAILABLE IN THAT CALCULATE THE EFFECTS OF VOLATILIZATION AND ADVECTIVE FLUID FLOW (following the methods of Valley, 1986, Bickle and Baker, 1990, and Bowman et al., 1994.) Volatilization cannot explain the oxygen isotope data, but down- temperature fluid flow can. Problems for Part 3 are more open-ended and subject to debate: 3.) The raw data is provided in a spreadsheet. Make a plot similar to Figure 13, except for the d 13 C values. 4.) Compare your plot of the d 13 C values versus distance. How is it similar and how does it differ from the d 18 O data? Can the carbon data be explained by a fluid flow exchange front? If you like, there are several papers students can read and discuss (see below). Problems for Part 2 are more sophisticated. For example, one question asks students to use the figure below to calculate the effect of dehydration (loss of 5% H 2 O) on the d 18 O value of a rock with a given mode. THE BOTTOM LINE: TEACHING STABLE ISOTOPES IN PETROLOGY IS CHALLENGING, BUT WORTHWHILE. ONE CAN COVER LOTS OF REALLY COOL TOPICS AND PROCESSES, AND THE STUDENTS GET PRACTICE MANIPULATING DATA IN USEFUL, PRACTICAL WAYS. IT DOES MEAN THAT OTHER THINGS MIGHT HAVE TO GO TO MAKE ROOM, BUT THE TRADE-OFFS CAN BE ADVANTAGEOUS. TRY IT AND SEE! Oxygen and carbon isotopic reservoirs. -20 -15 -10 -5 10 Rutile Forsterite Grossular Diopside H 2 O H 2 O Phlogopite Muscovite Albite Quartz Dolomite Magnetite 0 5 0 2 10 6 T -2 (K) 10 3 lnα(mineral-calcite) 250 300 400 500 800 Temperature ( o C) CO 2 Anorthite 1 3 OXYGEN Figure 5. Fractionation of oxygen isotopes between selected minerals (and volatiles) and calcite. 10 3 lnα between solids is normally assumed to be linear with 1/T 2 as shown. Dashed lines are projected outside their experimental temperature range. Fractionation factors used are from various studies summarized in Chacko et al., 2001. H 2 O-calcite is from O'Neil et al. (1969), updated by Friedman and O'Neil (1977), dolomite- calcite is from Sheppard & Schwarz (1970). Spreadsheets allow one to easily see the effects of modifying: initial fluid or rock isotopic value, F(carbon to F(oxygen) ratio ratio of oxygen in rock to oxygen in fluid fractionation factor, Darcy flux, porosity, diffusivity.
Transcript of IF SO, THEN CONSIDER TEACHING STABLE ISOTOPE GEOCHEMISTRY · TEACHING STABLE ISOTOPE GEOCHEMISTRY...
TEACHING STABLE ISOTOPE GEOCHEMISTRY IN AN UNDERGRADUATE PETROLOGY COURSEDUNN, Steven R., Earth and Environment, Mount Holyoke College, South Hadley, MA 01075, and BOWMAN, John R., Dept. of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112
ABSTRACTStable isotope geochemistry has become commonplace in petrologic literature and stable isotope data strongly influence interpretations of many petrologic processes. In spite of this, undergraduate petrology textbooks tend to present limited background on stable isotope fundamentals and applications. In addition, stable isotopes provide an opportunity for quantitative problem solving that is mathematically within reach of students and involves a variety of chemical and geological processes. We present a lesson plan that introduces students to the fundamentals of stable isotope geochemistry with particular application to contact metamorphism of carbonate rocks. The lesson plan consists of (1) an introductory reading assignment with embedded problems and suggestions for classroom activities, (2) a reading assignment covering coupled C-O isotopic trends due to Batch and Rayleigh volatilization and fluid infiltration exchange fronts with problems, and (3) a reading assignment introducing the Alta aureole, Utah, with stable isotope data from Bowman et al. (1994). These are in Excel spreadsheets that allow students to explore the effects of varying parameters on volatilization and one-dimensional fluid flow. Variables that can be manipulated include fractionation factors and the relation between F-carbon and F-oxygen for Rayleigh and batch volatilization, and porosity, Darcy flux, effective diffusion coefficient, and oxygen content of fluid and rock for one-dimensional flow models. The lesson plan can be incorporated entirely or partly into a petrology course, depending on time available and desired depth of coverage. The level of sophistication can be tailored to the ability of the students and would be useful for graduate students as well. The Alta aureole is a classic locality, ideal for this lesson because the chemistry of the rocks is relatively simple and much has been published about them. Problem assignments start out very structured and become more open-ended and inquiry based. Assignments lend themselves well to group projects or cooperative learning methods. See http://www.mtholyoke.edu/courses/sdunn/geol201/index.html for spreadsheets and readings.
GOAL: To introduce students in the use of stable isotope geochemistry in petrology by providing a framework and lesson plan to assist instructors.
RATIONALE: w Stable isotope geochemistry has become commonplace in petrologic literature. w Stable isotope data strongly influence interpretations of many petrologic processes. w Yet, undergraduate petrology textbooks present limited background on stable isotope fundamentals and applications. w Stable isotopes provide an opportunity for quantitative problem solving that is mathematically within reach of students.w Stable isotope geochemistry involves a variety of interesting geochemical and petrological processes.w Stable isotope geochemistry connects petrology to other fields, e.g., chemistry, ecology, the hydrologic cycle, paleoclimatology, etc.w With the background provided by this lesson plan, students can begin reading primary literature that addresses a wide range of topics.
WOULD YOU LIKE TO:
What are your teaching goals?
gEncourage more quantitative skills and practice?FProvide additional opportunities for critical thinking?CEnhance student access to primary literature??Connect petrology to other geological fields viageochemistry and physiochemical processes?
IF SO, THEN CONSIDERINCORPORATING STABLEISOTOPES INTO YOURPETROLOGY OR GEOCHEM-ISTRY COURSE!
Lesson plan, readingassignments, and problem
sets available on-line athttp://www.mtholyoke.edu/courses/sdunn/geol201/index.html
influx }} }
fluid dominated rock dominatedisotopic front
103lnαCc-H2O for oxygen
-5
0
5
10
15
20
25
30
35
40
0 100 200 300 400 500 600 700 800 900
Temperature (oC)
103 ln
α
Figure 1. Fractionation of oxygen isotopes between calcite and water as a function of temperature (0-500°C). From O’Neil et al. (1969), updated by Friedman and O’Neil (1977), 103lnαCc-H2O =2.78(106T−2)−2.89, T in Kelvin in this equation.
PART 1, START WITH THE BASICSStudent Reading Assignment Addresses:
Table 1. Light Stable Isotope Systems.abundance† notation standard used
H††D
99.9850.015 δD SMOW, Standard Mean Ocean Water
12C13C
98.8921.108 δ13C PDB, Peedee formation belemnite, S.C.
14N15N
99.6350.365 δ15N N2-atm, Air nitrogen
16O17O18O
99.7590.0370.204
δ18OSMOW, Standard Mean Ocean Water
orPDB, Peedee formation belemnite, S.C.
32S33S34S36S
95.0180.7504.2150.017
δ34SCDT, Canyon Diablo Troilite (FeS),an iron meteorite
† Approximate abundances, small variations are the basis for the use of stableisotopes (VG Instruments Fact Sheet).†† Note: hydrogen is the only element for which its isotopes have a separate name,i.e., deuterium and tritium for 2H and 3H.
1000)C/C(
)C/C()C/C(C
1213
1213121313 ×
−=
PDB
PDBsampleδ
1000O)O/(
O)O/(O)O/(O
1618
1618161818 ×
−=
SMOW
SMOWsampleδ
What are stable isotopes? Why do they fractionate?Definitions of standard delta notation, examples ofthe temperature dependence of fractionation factors,all are explained, no prior knowledge expected.
Definitions areprovided andexplained.
Calcite-water oxygen isotopefractionations are used forexamples.
B
ABA
δδα
++
=−1000
1000)(
∆A-B = δ A − δB
103lnαA-B ≈ ∆A-B
Abundances andstandards for thelight stable isotopes
HERE'S WHAT WE HAVE:Reading assignments and problems for
students in 3 Parts.
PART 1: Introduction to stable isotope geochemistry including definitions, some background, and problem set. Problems use only simple algebra and worked examples are included.
Problems with detailed worked examples are embeddedin Part 1, for example:
Problem 1. Suppose the δ18O value of a calcite sample is 16.2‰ and fluid inclusion data indicates that this calcite formed at 260°C. What must the δ18O value of the water have been?Problem 4. 2 moles of calcite (δ18O = 25.0‰) equilibrateswith 1 mole of water (δ18O = 5‰) at 320°C. What is the final δ18O value of both the calcite and the water?
-40 -30 -20 -10 0 10 20 30 40
Earth's mantle
Extraterrestrial material
δ18O(‰)
MORB
Andesites
Other basalts
Dacites-rhyolites
Granites-tonalites
Metamorphic rocks
Sedimentary rocks
Seawater
Meteoric waters
Magmatic waters
Metamorphic waters
5.7 OXYGEN
-40 -30 -20 -10 0 10 20 30 40
δ13C(‰)
Carbonatites, diamonds
Marine carbonates
Air CO2
Freshwater carbonates
Terrestrial plants
Bacteria
Methanogenic bacteria
Eukaryotic algae
Extraterrestrial material
C3 C4
-5.0Earth's mantle
Natural gas
Dissolved seawater C
CARBON
Figure 2. Typical oxygen isotopic composition of selected natural materials. Dashed line represents the earth's mantle. Modified from Hoefs (1997) and Best (2003). Igneous rock values exclude hydrothermally altered rocks.
Figure 3. Typical carbon isotopic composition of selected natural materials. Dashed line represents the earth's mantle. Modified from Hoefs (1997). Terrestrial plants include ranges for C3 and C4 plants that use different photosynthetic pathways.
PART 3: Presents a geologic overview of the Alta Stock contact aureole, including mineral isograds, T-XCO2 interpretations, and the stable isotope data constraints. Problems are generally broad and rather open-ended.
PART 2: Covers the concepts of volatilization and fluid infiltration, the isotopic effects resulting from these processes, the assumptions inherent in modeling these processes, and problems that explore these models.
PART 2 PROVIDES USEFUL TOOLSStudent Reading Assignment Explains:Volatilization by both "batch" and Rayleigh models,appropriate fractionation factors for both carbonand oxygen, fluid fluxes for one-dimensional fluidflow models and concept of isotopic "fronts."
Coupled C-O Volatilization trends
-10
-8
-6
-4
-2
0
10 15 20 25
α = 1.012 α = 1.006
Batch volatilization
Rayleigh volatilization
0.010.5
δ18O
0.40.6
0.2
0.8
0.05
Initial Rock
δ13C
F-carbonvalues
Figure 4. δ18O versus δ13C for volatilization (CO2-loss) of a rock with initial δ18O = 22‰ and δ13C = 0‰. Two values of a for oxygen (CO2-rock) are shown for both Rayleigh (solid lines) and batch (dashed lines) processes, αcarbon = 1.0022. F values shown are for carbon. The F values for oxygen are related by Foxygen = 0.4Fcarbon + 0.6, which is the calc-silicate limit discussed in the text. (modified from Valley, 1986)
Figure 7. Illustration of a column of rock undergoing infiltration and exchange as described in the text. The light region is "fluid-dominated" and the purple region is "rock-dominated." The "isotopic front" (or exchange front) moves in the direction of fluid flow and is a measure of the cumulative fluid flux.
0 200 400 600meters
10
15
20
25
30
δ18 O
(pe
rmil)
Flow Direction
104 1055 x 104
Fluid fluxes (cm3/cm2)103
Figure 8. δ18O value of model rock as a function of distance along the flow path. Isotopic fronts are displaced in the direction of fluid flow with increasing fluid flux. In this model the initial rock is 25‰, the fluid enters from the left at 10‰, and ∆rock-fluid = 6‰.
References CitedBerman, R.G. 1988. Internally-consistent thermodynamic data for minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-
SiO2-TiO2-H2O-CO2. Journal of Petrology, 29: 445-522.Berman, R.G. and Perkins, E.H. 1987. GEO-CALC: software for calculation and display of pressure-temperature-composition
phase diagrams. American Mineralogist, 72: 861-862.Best, M.G., 2003. Igneous and Metamorphic Petrology, 2nd Edition. Blackwell Publ. 729 pp.Bickle, M. and Baker, J., 1990. Migration of reaction and isotopic fronts in infiltration zones: assessments of fluid flux in
metamorphic terrains. Earth & Planetary Science Letters, 98: 1-13.Bowman, J.R., Willett, S.D., and Cook, S.J., 1994. Oxygen isotope transport and exchange during fluid flow: one-dimensional
models and applications. American Journal of Science, 294: 1-55.Chacko, T., Mayeda, T.K., Clayton, R.N., and Goldsmith, J.R., 1991. Oxygen and carbon isotope fractionations between CO2
and calcite. Geochimica et Cosmochimica Acta, 55: 2867-288.Chacko, T., Cole, D.R., and Horita, J., 2001. Equilibrium oxygen, hydrogen, and carbon isotope fractionation factors
applicable to geological systems. In Stable Isotope Geochemistry (J.W. Valley and D.R. Cole, eds.), Reviews in Mineralogy and Geochemistry, Vol 23: 1-81.
Cook, S.J. and Bowman, J.R., 1994. Contact metamorphism surrounding the Alta stock: thermal constraints and evidence of advective heat transport from calcite + dolomite thermometry. American Mineralogist, 79: 513-525.
Cook, S.J. and Bowman, J.R., 2000. Mineralogical evidence for fluid-rock interaction accompanying prograde contact metamorphism of siliceous dolomites: Alta stock aureole, Utah. Journal of Petrology 41: 739-757.
Cook, S.J., Bowman, J.R., and Forster, C.B., 1997. Contact metamorphism surrounding the Alta stock: finite element model simulation of heat and 18O/16O mass transport during prograde metamorphism. American Journal of Science 297: 1-55.
Criss, R.E. and Taylor, H.P., Jr., 1986. Meteoric hydrothermal systems. In Stable Isotopes in High Temperature Processes (J.W. Valley, H.P. Taylor, Jr., and J.R. O'Neil, eds.), Mineralogical Society of America, Reviews in Mineralogy, Vol. 16: 373-424.
Dipple, G.M., 1998. Reactive dispersion of stable isotope by mineral reaction during metamorphism. Geochimica et Cosmochimica Acta 62: 3745-3752.
Friedman, I. and O'Neil, J.R., 1977. Compilation of stable isotope fractionation factors of geochemical interest. Chapter KK, Geological Survey Professional Paper 440-KK, Data of Geochemistry, 6th Edition (M. Fleischer, Tech. Ed.).
Gerdes, M.L, Baumgartner, L.P., and Person, M., 1995. Stochastic permeability models of fluid flow during contact metamorphism. Geology 23:945-948.
Hoefs, J., 1997. Stable Isotope Geochemistry. Springer-Verlag, New York. 201 pp.Kitchen, N.E. and Valley, J.W., 1995. Carbon isotope thermometry in marbles of the Adirondack Mountains, New York.
Journal of Metamorphic Geology, 13: 577-594.Moore, J.N. and Kerrick, D.M., 1976. Equilibria in siliceous dolomites of the Alta aureole, Utah. American Journal of
Science 276: 502-524.O'Neil, J.R., Clayton, R.N., and Mayeda, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. Journal of
Chemical Physics, 51: 5547-5558.Shanks, W.C. III, 2001. Stable isotopes in seafloor hydrothermal systems: vent fluids, hydrothermal deposits, hydrothermal
alteration, and microbial processes. In Stable Isotope Geochemistry (J.W. Valley and D.R. Cole, eds.), Reviews in Mineralogy and Geochemistry, Vol 23: 469-525.
Sheppard, S.M.F. and Schwarz, H.P., 1970. Fractionation of carbon and oxygen isotopes and magnesium between coexisting metamorphic calcite and dolomite. Contributions to Mineralogy and Petrology, 26: 161-198.
Valley, J.W., 1986. Stable isotope geochemistry of metamorphic rocks. In Stable Isotopes in High Temperature Processes (J.W. Valley, H.P. Taylor, Jr., and J.R. O'Neil, eds.), Mineralogical Society of America, Reviews in Mineralogy, Vol. 16: 445-489.
Valley, J.W., 2001. Stable isotope thermometry at high temperatures. In Stable Isotope Geochemistry (J.W. Valley and D.R. Cole, eds.), Reviews in Mineralogy and Geochemistry, Vol 23: 365-413.
PART 3: APPLICATION, THE ALTA AUREOLEStudent Reading Assignment Presents:Evidence for H2O infiltration of inner aureole frommineral reaction isograds and T-XCO2 relations andaffirmation from stable isotope data.
Figure 9. Geologic map of the Alta area, Utah (from Moore and Kerrick, 1976). Mineral reaction isograds surround the Alta stock within siliceous dolomites of the Mississippian Deseret and Gardison formations. The study area within the southern contact aureole is from Bowman et al. (1994).
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9350
400
450
500
550
600
650
700
750
800
Do
P = 1000 bars
Do + Di
Fo + Cc + CO2
Pe +
Cc
+ CO 2
Fo + Di + CO2 + H2O
Cc + Tr
Do + Tr
Fo + Cc + CO2 + H2O
Tc + Cc + CO2
Do + Qz + H2O
Tc + Cc + QzTr + CO2 + H2O
Tc + CcTr +
Do + CO2 + H2O
Tr + Cc + QzDi + CO2 + H2O
Do + Qz + H2OTr + Cc + CO2
Do + Tr
Fo + Di
Tr + Cc
Di + Do +CO2 +H
2O
5
21
3
4
5
1
3
2
4
Do + Qz
Tc + Cc
Tr + Cc
Fo + Cc
Pe + Cc
X(CO2)
Tem
per
atu
re (C
o)
SignificantAssemblages
Figure 11. T-XCO2 diagram for selected reactions with significant mineral assemblages highlighted. The sequence of mineral reactions seen approaching the Alta stock contact are consistent with increasing temperature. Geothermometry yields peak temperatures of 600°C in the periclase zone, which requires H2O-rich fluid conditions.
-8
-6
-4
-2
0
2
4
6
8
5 10 15 20 25 30
Tc Zone
Tr Zone
Fo Zone
Per Zone w/Dol
Per Zone no Dol
F-value (carbon)
0.05
0.20.4
0.6
0.01
δ18O
δ13C
Alta Contact Aureole
Figure 12. C-O isotopic trend for carbonates from the southern Alta aureole. The Rayleigh volatilization trend starts at the average of the talc zone samples, 26‰ and 3‰, and uses the same fractionation factors as Fig. 4. Data from Bowman et al., 1996 and additional unpublished data from J. R. Bowman.
Figure 13. δ18O versus distance (Z, in kilometers) from the contact of the Alta stock Temperatures are from Cook and Bowman (1994). Taken from Bowman et al. (1996). By making various assumptions about the fluid flow parameters, the oxygen isotope data can be explained.
EXCEL® SPREADSHEETS ARE AVAILABLE IN THAT CALCULATE THE EFFECTS OF VOLATILIZATION AND ADVECTIVE FLUID FLOW (following the methods of Valley, 1986, Bickle and Baker, 1990, and Bowman et al., 1994.)
Volatilization cannot explain the oxygen isotope data, but down-temperature fluid flow can.
Problems for Part 3 are more open-ended and subject to debate:3.) The raw data is provided in a spreadsheet. Make a plot similar to Figure 13, except for the δ13C values.4.) Compare your plot of the δ13C values versus distance. How is it similar and how does it differ from the δ18O data? Can the carbon data be explained by a fluid flow exchange front? If you like, there are several papers students can read and discuss (see below).
Problems for Part 2 are more sophisticated. For example, one question asks students to use the figure below to calculate the effect of dehydration (loss of 5% H2O) on the δ18O value of a rock with a given mode.
THE BOTTOM LINE: TEACHING STABLE ISOTOPES IN PETROLOGY IS CHALLENGING, BUT WORTHWHILE. ONE CAN COVER LOTS OF REALLY COOL TOPICS AND PROCESSES, AND THE STUDENTS GET PRACTICE MANIPULATING DATA IN USEFUL, PRACTICAL WAYS. IT DOES MEAN THAT OTHER THINGS MIGHT HAVE TO GO TO MAKE ROOM, BUT THE TRADE-OFFS CAN BE ADVANTAGEOUS. TRY IT AND SEE!
Oxygen and carbon isotopic reservoirs.
-20
-15
-10
-5
10
Rutile
Forsterite
GrossularDiopside
H2OH2O
Phlogopite
MuscoviteAlbite
QuartzDolomite
Magnetite
0
5
0 2
106T-2(K)
103 ln
α(m
iner
al-c
alci
te)
250300400500800
Temperature (oC)
CO2
Anorthite
1 3
OXYGEN
Figure 5. Fractionation of oxygen isotopes between selected minerals (and volatiles) and calcite. 103lnα between solids is normally assumed to be linear with 1/T2 as shown. Dashed lines are projected outside their experimental temperature range. Fractionation factors used are from various studies summarized in Chacko et al., 2001. H2O-calcite is from O'Neil et al. (1969), updated by Friedman and O'Neil (1977), dolomite-calcite is from Sheppard & Schwarz (1970).
Spreadsheets allow one to easily see the effects of modifying:initial fluid or rock isotopic value, F(carbon to F(oxygen) ratioratio of oxygen in rock to oxygen in fluidfractionation factor,Darcy flux,porosity,diffusivity.
