Metaperidotites and Marbles. Marbles and Metaperidotites; Geothermobarometry. Low Grade Reactions in. Metaperidotites

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1 Marbles and Metaperidotites; GEOL Metamorphic Lecture 5 Metaperidotites and Marbles Typical Composition of Peridotites and Carbonate Rocks Peridotite Limestone Dolostone SiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Low Grade Reactions in Metaperidotites 1

2 Isograds in Marbles Figure 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, and (1972) Schweiz. Mineral. Petrogr. Mitt., 52, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Metamorphism of Calcareous Rocks Figure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure 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, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 2

3 Mixed Volatile Reactions Figure After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths Isograds in Marbles Internal vs External Buffering Mixed Volatile Reactions 3

4 Intersecting Isograds 1 Ca-Amph+Kf 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, 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 = 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, 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 = After Carmichael (1970) J. Petrol., 11, , 4

5 Figure 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

6 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 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 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 lnk The Garnet - Biotite Geothermometer Figure 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

7 The GASP Geobarometer Figure 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, Figure 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, has been added. The GASP Geobarometer Figure 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

8 PTt Paths PTt Paths Figure 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, Figure 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