Marbles and Metaperidotites; GEOL 13.53 Metamorphic Lecture 5 Metaperidotites and Marbles Typical Composition of Peridotites and Carbonate Rocks Peridotite Limestone Dolostone SiO 2 42.26 3.64 0.41 Al 2 O 3 4.23 0.98 0.13 FeO 11.19 1.15 0.60 MnO 0.41 0.07 0.05 MgO 31.24 1.59 21.88 CaO 5.05 53.06 32.87 Na 2 O 0.49 0.07 0.03 K 2 O 0.34 0.24 0.05 Low Grade Reactions in Metaperidotites 1
Isograds in Marbles Figure 29-5. Metamorphic zones developed in regionally metamorphosed dolomitic rocks of the Lepontine Alps, along the Swiss-Italian border. After Trommsdorff (1966) Schweiz. Mineral. Petrogr. Mitt., 46, 431-460 and (1972) Schweiz. Mineral. Petrogr. Mitt., 52, 567-571. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Metamorphism of Calcareous Rocks Figure 29-1. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure 29-2. A portion of the Alta aureole in Little Cottonwood Canyon, SE of Salt Lake City, UT, where talc, tremolite, forsterite, and periclase isograds were mapped in metacarbonates by Moore and Kerrick (1976) Amer. J. Sci., 276, 502-524. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 2
Mixed Volatile Reactions Figure 29-4. After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths Isograds in Marbles Internal vs External Buffering Mixed Volatile Reactions 3
Intersecting Isograds 1 Ca-Amph+Kf 2 3 4 Bt+Cc+Qz = Figure 29-7b. Isograds mapped in the field. Note that isograd (5) crosses the others in a manner similar to that in part (a). This behavior is attributed to infiltration of H 2 O from the syn-metamorphic pluton in the area, creating a gradient in X H2O across the area at a high angle to the regional temperature gradient, equivalent to the T-X diagram. After Carmichael (1970) J. Petrol., 11, 147-181. Intersecting Isograds Figure 29-7a. T-X H2O diagram illustrating the shapes and relative locations of the reactions for the isograds mapped in the Whetstone Lake area. Reactions 1, 2, and 4 are dehydration reactions and reaction 3 is the Ky = Sil transition, all in metapelites. Reaction 5 is a dehydration-decarbonation in calcic rocks with a temperature maximum at X H2O = 0.25. b. Isograds mapped in the field. Note that isograd (5) crosses the others in a manner similar to that in part (a). This behavior is attributed to infiltration of H 2 O from the syn-metamorphic pluton in the area, creating a gradient in X H2O across the area at a high angle to the regional temperature gradient, equivalent to the T-X diagram. After Carmichael (1970) J. Petrol., 11, 147-181. Intersecting Isograds Figure 29-7a. T-X H2O diagram illustrating the shapes and relative locations of the reactions for the isograds mapped in the Whetstone Lake area. Reactions 1, 2, and 4 are dehydration reactions and reaction 3 is the Ky = Sil transition, all in metapelites. Reaction 5 is a dehydration-decarbonation in calcic rocks with a temperature maximum at X H2O = 0.25. After Carmichael (1970) J. Petrol., 11, 147-181, 4
Figure 27-6. AFM projections showing the relative distribution of Fe and Mg in garnet vs. biotite at approximately 500 o C (a) and 800 o C (b). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Metamorphic facies and petrogenetic grids allow us to estimate PT conditions based on mineral assemblages Only requires field and/or petrographic data Ion Exchange Reactions Continuous Reactions Continuous and ionexchange reactions require chemical composition data from coexisting minerals Can further limit PT estimates 5
Distribution Coefficient (K D ) The distribution coefficient (K D ) is the ratio by which two elements are partitioned between two minerals at equilibrium For example, the distribution coefficient of Fe and Mg in garnet and biotite is: K D Gt-Bt = (Mg/Fe) Gt / (Mg/Fe) Bt The Garnet - Biotite Geothermometer Figure 27-6. AFM projections showing the relative distribution of Fe and Mg in garnet vs. biotite at approximately 500 o C (a) and 800 o C (b). From Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. The Garnet - Biotite Geothermometer Table 27-2. Experimental results of Ferry and Spear (1978) on a Garnet-Biotite Geothermometer T o C Initial X(Fe-Bt) Final X(Fe-Bt) Final X(Fe-Grt) Final (Mg/Fe)Grt Final (Mg/Fe)Bt K T Kelvins 1/T Kelvins 799 1.00 0.750 0.905 0.105 0.333 0.315 1072 0.00093-1.155 799 0.50 0.710 0.896 0.116 0.408 0.284 1072 0.00093-1.258 749 0.50 0.695 0.896 0.116 0.439 0.264 1022 0.00098-1.330 738 1.00 0.730 0.906 0.104 0.370 0.281 1011 0.00099-1.271 698 0.75 0.704 0.901 0.110 0.420 0.261 971 0.00103-1.342 698 0.50 0.690 0.896 0.116 0.449 0.258 971 0.00103-1.353 651 0.75 0.679 0.901 0.110 0.473 0.232 924 0.00108-1.459 651 0.50 0.661 0.897 0.115 0.513 0.224 924 0.00108-1.497 599 0.75 0.645 0.902 0.109 0.550 0.197 872 0.00115-1.623 599 0.50 0.610 0.898 0.114 0.639 0.178 872 0.00115-1.728 550 0.75 0.620 0.903 0.107 0.613 0.175 823 0.00122-1.741 550 0.50 0.590 0.898 0.114 0.695 0.163 823 0.00122-1.811 lnk 601 0.50 0.500 0.800 0.250 1.000 0.250 874 0.00114-1.386 601 0.25 0.392 0.797 0.255 1.551 0.164 874 0.00114-1.807 697 0.75 0.574 0.804 0.244 0.742 0.329 970 0.00103-1.111 697 0.25 0.468 0.796 0.257 1.137 0.226 970 0.00103-1.487 The Garnet - Biotite Geothermometer Figure 27-7. Pressure-temperature diagram similar to Figure 27-4 showing lines of constant K D plotted using equation (27-35) for the garnet-biotite exchange reaction. The Al 2 SiO 5 phase diagram is added. From Spear (1993) Metamorphic Phase Equilibria and Pressure- Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. 6
The GASP Geobarometer Figure 27-8. P-T phase diagram showing the experimental results of Koziol and Newton (1988), and the equilibrium curve for reaction (27-37). Open triangles indicate runs in which An grew, closed triangles indicate runs in which Grs + Ky + Qtz grew, and half-filled triangles indicate no significant reaction. The univariant equilibrium curve is a best-fit regression of the data brackets. The line at 650 o C is Koziol and Newton s estimate of the reaction location based on reactions involving zoisite. The shaded area is the uncertainty envelope. After Koziol and Newton (1988) Amer. Mineral., 73, 216-233 Figure 27-11. P-T phase diagram calculated by TQW 2.02 (Berman, 1988, 1990, 1991) showing the internally consistent reactions between garnet, muscovite, biotite, Al 2 SiO 5 and plagioclase, when applied to the mineral compositions for sample 90A, Mt. Moosilauke, NH. The garnet-biotite curve of Hodges and Spear (1982) Amer. Mineral., 67, 1118-1134 has been added. The GASP Geobarometer Figure 27-8. P-T diagram contoured for equilibrium curves of various values of K for the GASP geobarometer reaction: 3 An = Grs + 2 Ky + Qtz. From Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. PTt Paths 7
PTt Paths PTt Paths Figure 27-12. Chemically zoned plagioclase and poikiloblastic garnet from meta-pelitic sample 3, Wopmay Orogen, Canada. a. Chemical profiles across a garnet (rim rim). b. An-content of plagioclase inclusions in garnet and corresponding zonation in neighboring plagioclase. After St- Onge (1987) J. Petrol. 28, 1-22. Figure 27-13. The results of applying the garnetbiotite geothermometer of Hodges and Spear (1982) and the GASP geobarometer of Koziol (1988, in Spear 1993) to the core, interior, and rim composition data of St-Onge (1987). The three intersection points yield P-T estimates which define a P-T-t path for the growing minerals showing nearisothermal decompression. After Spear (1993). 8