Electronic Articles
in the American Mineralogist

3-dimensional imaging of garnet porphyroblast sizes and chemical zoning: Nucleation and growth history in the garnet zone
Frank S. Spear, Christopher G. Daniel
(Published 30 October 1998, 17 pages, 3 tables, 19 figures, 4 movies)

Keywords: kinetics, nucleation, growth, garnet

Three-dimensional imaging of garnet porphyroblast sizes and chemical zoning: Nucleation and growth history in the garnet zone

Frank S. Spear and Christopher G. Daniel

Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180
(Received July 1, 1998; Published October 30, 1998)

Abstract

The 3-dimensional growth history of two garnet zone samples (Grt + Chl + Bt + Ms + Pl + Qtz + Ilm) from southwestern Maine was examined by serial sectioning and 3-D reconstructions of compositional zoning from backscatter images and X-ray maps.

Mn, Fe, Mg and Ca zoning is broadly concentric. The concentration of Mn in garnet cores generally correlates with size (d = 50 to 750 microns), indicating progressive nucleation. In detail, all elements show irregular, patchy zoning in the cores. Assuming constancy of Mn on the rims of all garnets in a rock volume plus no subsequent diffusional modification, Mn concentration can be used as a “time line” for garnet growth. Examination of the evolution of individual garnets reveals that multiple nuclei formed simultaneously in the core regions and nuclei expanded by growth in amoeba-shape forms along preexisting mineral grain boundaries (primarily quartz and plagioclase), dissolving the interior grains until the grains were either gone or encapsulated, at which time dissolution ceased. Amoeba-shaped garnets coalesced as they grew and, simultaneously, new nuclei appeared in the nearby matrix. The net result was a single garnet porphyroblast that formed by the growth and coalescence of multiple nuclei.

Radius-rate plots, constructed by counting pixels in 2-D images, reveal that crystals grew at the same radial rate, regardless of size. The observation of continuous nucleation in the vicinity of preexisting crystals plus the radius-rate plots rule out diffusion over length scales on the order of the garnet radius or greater as the rate-limiting step and are consistent with either diffusion over shorter length scales or interface control as rate-limiting to garnet growth. The strong clustering of garnet nuclei requires clustering of favorable nucleation sites, which may have been caused by favorable orientation of garnet precursor minerals (i.e. chlorite + quartz).

Copyright ©1998 by the Mineralogical Society of America

Isotope geochemistry of Proterozoic talc occurrences in Archean marbles of the Ruby Mountains, southwest Montana, U.S.A.
John B. Brady, John T. Cheney, Amy Larson Rhodes, Angela Vasquez,
Chris Green, Mathieu Duvall, Ari Kogut, Lewis Kaufman, Dana Kovaric
(Published 31 December 1998, 16 pages, 3 tables, 19 figures)

Keywords: talc, oxygen, carbon, hydrogen, argon, isotopes, Proterozoic, metasomatic, dolomite, calcite, graphite, chlorite, Montana

Isotope geochemistry of Proterozoic talc occurrences in Archean marbles of the Ruby Mountains, southwest Montana, U.S.A.

John B. Brady¹, John T. Cheney², Amy Larson Rhodes¹
Angela Vasquez², Chris Green², Mathieu Duvall³, Ari Kogut⁴
Lewis Kaufman⁵, Dana Kovaric¹

Departments of Geology: ¹Smith College, Northampton, MA  01063  ²Amherst College, Amherst, MA, ³Colorado College, Colorado Springs, CO, ⁴Beloit College, Beloit, WI, ⁵College of Wooster, Wooster, OH
(Received July 15, 1998; Published December 31, 1998)

Abstract

Talc occurs as massive, economic deposits in upper amphibolite facies marbles of Archean age in southwestern Montana. Previous workers have demonstrated that the talc is a replacement of the marble that resulted from interaction with a large volume of fluid. d¹⁸O (SMOW) values for dolomite and calcite range from 20-25 per mil for the unaltered Archean marbles to as little as 8-10 per mil in the talc deposits, suggesting that the metasomatic fluids had low d¹⁸O values. In contrast, d¹³C values for calcite and dolomite are similar for all samples (-2 to +2 per mil PDB). Therefore, it is likely that the metasomatic fluids were oxygen-rich and carbon-poor, namely water-rich and CO₂-poor. A CO₂-poor fluid is also indicated by delta¹³C (calcite-graphite) values (3.6-5.3 per mil), which appear little altered from values expected for upper amphibolite facies marbles, and by the occurrence of the mineral assemblage talc+calcite. ⁴⁰Ar/³⁹Ar age spectra for hornblende, phlogopite, and biotite record cooling at 1.72 Ga from a regional thermal event. ⁴⁰Ar/³⁹Ar age spectra of fine-grained muscovite associated with the talc date talc formation at 1.36 Ga. The Ar data limit the temperature of talc crystallization to below ~350°C, the biotite closure temperature for Ar diffusion. If the metasomatic fluid was seawater (0 per mil), then the carbonate oxygen data require a minimum temperature of 270°C for talc formation. Oxygen (d¹⁸O = 4.7 to 8.8 per mil) and hydrogen (D/H = -49.9 to -57.6 SMOW) isotope data for the talc are consistent with a 200°-300°C metasomatic fluid derived from seawater, based on theoretical models of the fractionation of oxygen and hydrogen between talc and water. Regional, northwest-trending faults associated with the extension that formed the Belt Basin in the Middle Proterozoic may have provided channels for seawater to circulate in continental crust and to react with marble, forming talc at depths of 5-10 km.

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Copyright ©1998 by the Mineralogical Society of America

Real-time AFM diagrams on your Macintosh
Frank S. Spear
(Published 25 May 1999, 10 pages, 8 figures, 1 interactive figure, 3 movies)

Keywords: AFM diagrams, petrogenetic grid, pelites, pelitic schists, thermodynamics, computing, Gibbs method, education, metamorphic petrology

Real-time AFM diagrams on your Macintosh

Frank S. Spear

Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180
(Received December 15, 1998; Published May 25, 1999)

Abstract

An algorithm is presented for the calculation of stable AFM mineral assemblages in the KFMASH system based on an internally consistent thermodynamic data set and the petrogenetic grid derived from this data set. The P-T stability of each divariant (three-phase) AFM assemblage is determined from the bounding KFASH, KMASH and KFMASH reactions. Macintosh regions (enclosed areas defined by a sequence of x-y points) are created for each divariant region. The Macintosh toolbox routine “PtInRgn” (point-in-region) is used to determine whether a user-specified P and T falls within the stability limit of each assemblage, and the compositions of minerals in the stable assemblages are calculated and plotted. Implementation of the algorithm is coded in FORTRAN as a module for program Gibbs (Spear and Menard, 1989). Users can calculate individual AFM diagrams at any P-T condition within the limits of the P-T grid, and sequences of AFM diagrams along any P-T path. Diagrams can be saved as PICT images for creating animations. The internally consistent thermodynamic data sets of Holland and Powell (1998) and Spear and Cheney (unpublished) are supported.

The algorithm and its implementation provide a useful tool for researchers to explore the implications of a petrogenetic grid and to compare predictions of different thermodynamic data sets. Comparison of natural samples with predictions from the grid can be made if appropriate projections of the natural data into the KFMASH system are made. Results are also useful to students learning to understand petrogenetic grids and the progressive metamorphism of pelitic mineral assemblages.

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Copyright ©1999 by the Mineralogical Society of America

Three-dimensional model of heat flow in the aureole of the Marcy anorthosite, Adirondack Highlands, New York:
Implications for depth of emplacement
J. Alcock, Kevin Myer, P.D. Muller
(Published 22 September 1999, 11 pages, 1 table, 7 figures)

Keywords: Marcy anorthosite, Adirondack Highlands, contact metamorphism, conductive heating, finite-difference models.

Three-dimensional model of heat flow in the aureole of the Marcy anorthosite, Adirondack Highlands, New York: Implications for depth of emplacement

J. Alcock¹, Kevin Myer², and P. D. Muller³

¹Penn State, Abington, Abington, PA, 19001
²Penn State, University Park, PA, 16802
³SUNY Oneonta, Oneonta, NY, 13820
(Received April 1, 1999; Published September 22, 1999)

Abstract

The New Russia gneiss complex occurs within a broad, very high temperature (800-950 °C)
metamorphic aureole against the eastern margin of the Marcy anorthosite, northeastern
Adirondack Highlands of New York. Three-dimensional models of heat flow from the anorthosite indicate that an aureole like that preserved in the New Russia gneisses would form if the country rock were at high temperature (> 700 °C) and at depth prior to intrusion. These findings are consistent with geobarometric evidence indicating that partial melting in the aureole occurred at 700-1000 MPa and support the hypothesis that the anorthosite intruded at a depth of 25 to 35 km.

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Copyright ©1999 by the Mineralogical Society of America

Measuring Discrete Feature Dimensions in AFM Images with Image SXM
Barry R. Bickmore, Eric Rufe, Steve Barrett, and Michael F. Hochella, Jr.
(Published 23 November 1999, 10 pages, 9 figures, 1 movie)

Keywords: Atomic Force Microscopy, image analysis, perimeter, area, volume, Image SXM

Measuring Discrete Feature Dimensions in AFM Images with Image SXM

Barry R. Bickmore¹, Eric Rufe¹, Steve Barrett Michael F. Hochella, Jr.¹

¹Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 USA
²Surface Science Research Centre, University of Liverpool, Liverpool L69 3BX, United Kingdom
(Received August 2, 1999; Published November 23, 1999)

Abstract

A suite of macros for the freeware image analysis program, Image SXM, are described. These macros are designed to measure the perimeter, horizontal area, and volume of discrete features in AFM images, obtaining accurate and consistent estimates. Directions for using the software and example applications are also given. Such tools allow one to perform tasks which would otherwise be extremely tedious or next to impossible. Examples include calculating reaction rates from time-series images of reacting particles or etch pits with complex shapes, and classifying objects based on their dimensions.

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Yttrium zoning in garnet:
Coupling of major and accessory phases during metamorphic reactions
Joseph M. Pyle and Frank S. Spear
(Published 14 December 1999, 23 pages, 2 tables, 21 figures)

Keywords: garnet, pelite, xenotime, yttrium, trace element, zoning

Yttrium zoning in garnet:
Coupling of major and accessory phases during metamorphic reactions

Joseph M. Pyle and Frank S. Spear

Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute,
Troy, NY 12180, USA
(Received July 10, 1999; Published December 14, 1999)

Abstract

The concentration of yttrium in pelitic garnets as a function of metamorphic grade has been examined in relation to the distribution of xenotime (YPO₄) in samples from New England and British Columbia. Samples with xenotime present only as inclusions in garnet generally possess high-Y cores and concentrations that drop off discontinuously along zoning shoulders of variable width to low-Y outboard regions. Samples with matrix xenotime are restricted to the garnet zone; Y concentration of these garnets generally decreases smoothly from core to rim. Xenotime may also be present in reaction zones around garnet. In xenotime-bearing samples, [Y]_Grt is strongly temperature-dependent and ranges from ~5000 ppm in the garnet zone to ~150 ppm in the sillimanite zone. Measured yttrium zoning profiles in xenotime-absent samples are reproduced with both Rayleigh fractionation and diffusion models, but P-T histories of the samples examined favor the Rayleigh model, with garnet volume, bulk-rock yttrium, and mode of (Y,HREE) accessory phases controlling the profile shape. High-yttrium annuli in staurolite-zone samples may form by garnet overgrowth of proximal matrix enriched in yttrium due to garnet consumption during discontinuous staurolite-forming reactions. An increase in [Y]_Grt and [HREE]_Grt in garnet from anatectic samples is related to dissolution of phosphates in vapor-absent, peraluminous melt, with partitioning of highly compatible Y and HREE into garnet grown during anatexis; textural analysis reveals that phosphates are absent from regions of garnet grown in equilibrium with melt. A main result of this study is identification of an intimate coupling between major pelite phases and accessory phases during reaction progress. This coupling is of great advantage in that it may be used to: (1) calibrate sensitive geothermometers and geobarometers, (2) identify particular regions of garnet grown in different garnet-producing reactions over a range of grades, and (3) reveal portions of pelite reaction history invisible to major elements.

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Copyright ©1999 by the Mineralogical Society of America

AFTSolve: A program for multi-kinetic modeling of apatite fission-track data
Richard A. Ketcham, Raymond A. Donelick, and Margaret B. Donelick
(Published 15 March 2000, 18 pages, 2 tables, 12 figures)

Keywords: Fission track, apatite, geochronology, computer program, temperature-time history

AFTSolve: A program for multi-kinetic modeling of apatite fission-track data

Richard A. Ketcham¹,Raymond A. Donelick²
and Margaret B. Donelick³

¹Department of Geological Sciences, University of Texas, Austin, TX 78712, USA, ²Donelick Analytical, Inc., 1075 Matson Road, Viola, ID 83872, USA, ³Department of Geology and Geological Engineering, University of Idaho, Moscow, ID 83844, USA
(Received October 25, 1999; Published March 15, 2000)

Abstract

AFTSolve is a computer program for deriving thermal history information from apatite fission-track data. It implements a new fission-track annealing model that takes into account the known kinetic variability among different apatite species. To fully utilize this model, a fission-track worker must  obtain data that can be used to infer the kinetic characteristics of each apatite grain from which a measurement was taken. Such data can consist of etch figure lengths or chemical composition. The benefit of this overall approach is that it allows useful information to be derived from previously unusable analyses, extends the practical range of geological temperatures constrained by fission-track analyses, and increases overall confidence in model predictions. AFTSolve also incorporates the effects of fission-track orientation relative to the apatite crystallographic c-axis, variation in initial track length, and the biasing effect of ²⁵²Cf irradiation for enhancing confined horizontal track length detection. AFTSolve is written for Windows^® operating systems, and has a graphical interface that allows interactive input of thermal histories and real-time generation of estimates for fission-track length distributions and ages for up to six simultaneously modeled kinetic populations. It also includes procedures for estimating the range of time-temperature histories that are statistically consistent with a data set and constraints entered by the user.

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Copyright ©2000 by the Mineralogical Society of America

Multiple thermotectonic events in a continuous metamorphic sequence, Mica Creek area, southeastern Canadian Cordillera
J.L. Crowley, E.D. Ghent, S.D. Carr, P.S. Simony, and M.A. Hamilton
(Published 24 May 2000, 23 pages, 2 tables, 14 figures)

Keywords: Canadian Cordillera, metamorphism, monazite, U-Th-Pb geochronology

Multiple thermotectonic events in a continuous metamorphic sequence, Mica Creek area, southeastern Canadian Cordillera

J.L.Crowley¹, E.D.Ghent¹, S.D.Carr², P.S. Simony¹, and M.A.Hamilton³

¹Department of Geology and Geophysics,University of Calgary,Calgary,AB T2N 1N4,
Canada, ²Department of Earth Sciences and Ottawa-Carleton Geoscience Centre,Carleton
University,Ottawa,ON K1S 5B6,Canada, ³Geological Survey of Canada,601 Booth Street,
Ottawa,ON K1A 0E8,Canada, *now at Department of Earth Sciences, Memorial University of Newfoundland, St.Johns,NF A1B 3X5, Canada
(Received January 10, 2000; Published May 24, 2000)

Abstract

U-(Th)-Pb isotope dilution and SHRIMP dating indicates that multiple periods of metamorphism and deformation occurred between ~160 and 60 Ma in the Mica Creek area, southeastern Canadian Cordillera. The metamorphic sequence (Barrovian assemblages in the garnet zone through the sillimanite – K-feldspar zone), fold generations, and Neoproterozoic stratigraphy are apparently continuous across the area, yet it broadly lies between regions that were tectonized at different times, Middle Jurassic to the east and middle Cretaceous – Paleocene to the west. Our dating along a north (lowest metamorphic grade) to south (highest grade) transect shows that three, km-scale tectonic age domains exist. In garnet schist of domain 1, peak metamorphism occurred at ~163 Ma and a minor thermal overprint occurred at 70-60 Ma. In migmatitic kyanite schist of domain 2, monazite inclusions in garnet grew at 110 Ma, matrix monazite and monazite inclusions in kyanite grew at ~85-73 Ma, and rims of some grains grew at ~60 Ma. Kyanite growth during peak metamorphism must have occurred after 73 Ma, possibly during intrusion of 61 Ma leucosome. In migmatitic kyanite schist and sillimanite schist of domain 3, peak metamorphism occurred at 99-93 Ma, and lesser thermal events occurred at ~280, 160, 110, and 70 Ma. We thus conclude that the thermal peak of metamorphism took place at different times in each domain, with peak events coinciding with secondary events in other domains. Age constraints on S₁₊₂ are provided by dating variably deformed granitoid rocks.S₁₊₂ developed before 72 Ma in domain 1, at least partly at 61-58 Ma and 122-63 Ma in domains 2 and 3, respectively. Our findings show that the Mica Creek area is the only known region in the southeastern Canadian Cordillera that was affected by the five recognized major periods of tectonism (175-160, 140-120, 110, 100-90, and 75-50 Ma). The key to understanding the geologic history is determining why several metamorphic and deformation events spanning 100 Myr occurred within a single tectonometamorphic complex with apparent geologic continuity. If continuity is only apparent, the domains may have been tectonically assembled along cryptic, unrecognized shear zones. However, if continuity does indeed exist, the events must have been locally superimposed on the complex.

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Copyright ©2000 by the Mineralogical Society of America

An evaluation of spatial correlation functions in textural analysis of metamorphic rocks
David M. Hirsch, Richard A. Ketcham, and William D. Carlson
(Published 2 June 2000, 21 pages, 1 table, 20 figures, 5 3DMF figures, 3 appendices)

Keywords: Quantitative textural analysis, garnet, porphyroblast, correlation functions

An evaluation of spatial correlation functions in textural analysis of metamorphic rocks

David M. Hirsch, Richard A. Ketcham, and William D. Carlson

Department of Geological Sciences, The University of Texas at Austin, Austin, Texas, 78712, USA
(Received January 13, 2000; Published June 2, 2000)

Abstract

Spatial correlation functions, which quantify spatial relationships among porphyroblasts over a range of length scales, can be used in combination with other techniques of quantitative textural analysis to constrain crystallization mechanisms in metamorphic rocks. The utility, reliability, and robustness of these functions, however, depend critically upon correct methods of calculation and application to geological samples.

Application of the L’ -function, Pair Correlation Function, and Mark Correlation Function (Stoyan and Stoyan, 1994) to artificial arrangements of crystals yields results consistent with their predetermined ordering and clustering qualities. These results serve as a foundation for the interpretation of more complex simulated and natural crystal arrays. Analysis of artificial and simulated crystal arrays in which ordering signals are obscured in various ways (displacing crystals in an ordered array by increasing amounts, reducing the number of crystals, and increasing the sample’s aspect ratio) demonstrates that these scale-dependent functions are robust indicators of effects diagnostic of certain crystallization mechanisms, even in complex circumstances. The effects of clustering of nucleation sites, however, can strongly obscure any underlying signal that might reveal crystallization mechanisms.

The L’ -function and the Pair Correlation Function are sensitive to short-range ordering of crystals, which may reflect suppression of nucleation in the vicinity of growing porphyroblasts. The Mark Correlation Function is sensitive to size-isolation correlations, which may reflect retardation of growth among crystals competing for nutrients. Interpretation of these functions, however, requires careful attention to proper calculation of Monte Carlo simulations, which are used to identify values of the functions that constitute a null-hypothesis region for comparison to samples with unknown ordering and clustering characteristics. To yield functional values commensurate with those calculated for a particular natural rock specimen, each simulation must match as closely as possible several critical features of the natural rock, including the set of crystal radii, limitations on the observability of crystals, and the shape and size of the bounding surface of the sample.

Crystallization mechanisms in seven previously studied garnetiferous rocks from three localities (Carlson et al., 1995; Denison and Carlson, 1997) have been re-assessed using both scale-dependent correlation functions and single-valued spatial statistics, both evaluated by comparison to rigorously computed null-hypothesis regions. The results confirm previous inferences that the nucleation and growth rates of the garnet porphyroblasts in these specimens were governed by rates of diffusion through the intergranular medium.

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Copyright ©2000 by the Mineralogical Society of America

Metapelite phase equilibria modeling in MnNCKFMASH: The effect of variable Al2O3 and MgO/(MgO+FeO) on mineral stability
Douglas K. Tinkham, Carlos A. Zuluaga, and Harold H. Stowell
(Published 17 December 2001, 42 pages including 3 tables, 15 figures, 1 rollover figure)

Keywords: metapelite, phase equilibria, pseudosections, mineral assemblage, thermodynamics, bulk composition, Waterville Fm., metamorphic petrology, thermobarometry

Metapelite phase equilibria modeling in MnNCKFMASH: The effect of variable Al2O3 and MgO/(MgO+FeO) on mineral stability

Douglas K. Tinkham, Carlos A. Zuluaga, and Harold H. Stowell

Department of Geological Sciences, University of Alabama,
Tuscaloosa, AL 35487-0338, USA
(Received July 26, 2001; Published December 17, 2001)

Abstract

A series of MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (MnNCKFMASH) metapelite pseudosections highlights the dependence of predicted mineral assemblages on bulk rock Al2O3 and Mg# (MgO/(MgO+FeO)). T-XAl pseudosections portray the dependence of staurolite, biotite, and aluminum silicate on Al2O3 content, allowing the distinction between high-Al and low-Al pelite, as commonly portrayed with KFMASH modeling. The MnNCKFMASH system also shows the effect of Al2O3 on plagioclase and zoisite stability, which cannot be done in the KFMASH system. Comparison of MnNCKFMASH to KFMASH pseudosections highlights the consequence of ignoring the important rock constituents MnO, Na2O, and CaO when constructing pseudosections. KFMASH cannot model important phases such as plagioclase and zoisite, and there are significant differences in predicted garnet, biotite, and chloritoid stability in the two different systems. In particular, KFMASH does not model garnet stability appropriately at low pressures and temperatures because it cannot account for the stabilizing effect of Mn. The comparisons also show that the method of calculating a KFMASH bulk rock composition equivalent to a real rock composition is problematic and has significant implications for the predicted pseudosection assemblage stability.

Comparison of the MnNCKFMASH pseudosections to natural assemblages observed in the Waterville Fm., Maine, indicates that the MnNCKFMASH system comes very close to modeling naturally developed mineral assemblages successfully. The only major discrepancy between predicted and observed assemblages is the inability to predict the paragenesis staurolite + andalusite using an average or natural Waterville Fm. composition.

Garnet thermobarometric results from the Waterville Fm. are in poor agreement with pseudosection topology for an average Waterville Fm. composition. This suggests that if quantitative P-T path information is to be derived through a combination of pseudosections and thermobarometry, samples will have to be investigated on an individual basis in more detail than was done in this study.

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Copyright ©2001 by the Mineralogical Society of America

The origin of Mn and Y annuli in garnet and the thermal dependence of P in garnet and Y in apatite in calc-pelite and pelite, Gagnon terrane, western Labrador
Panseok Yang and Toby Rivers
(Published 11 March 2002, 35 pages including 2 tables, 20 figures)

Keywords: garnet, apatite, Y and Mn annuli, phosphorus zoning, REE peaks

The origin of Mn and Y annuli in garnet and the thermal dependence of P in garnet and Y in apatite in calc-pelite and pelite, Gagnon terrane, western Labrador

Panseok Yang and Toby Rivers

Department of Earth Sciences, Memorial University of Newfoundland St. John’s, NF, Canada A1B 3X5
(Received June 25, 2001; Published March 11, 2002)

Abstract

X-ray compositional maps and zoning profiles for major and trace elements have been measured in garnet, apatite and epidote-group minerals from calc-pelitic and pelitic schists from western Labrador, by electron microprobe and by laser ablation ICP-MS. High Y abundance in apatite adjacent to resorbed garnet and a decreasing modal abundance of apatite with garnet growth indicate that apatite participates in major rock-forming mineral reactions. Phosphorus concentration in garnet porphyroblasts coexisting with apatite decreases smoothly from core to rim and depends moderately on metamorphic grade. Apatite coexisting with xenotime shows high-Y cores and the concentration of Y in apatite increases with metamorphic grade.

Many of the analyzed garnet porphyroblasts exhibit Y annuli which provide information about different physicochemical processes operating during garnet growth. We describe criteria to fingerprint three of these processes in the analyzed garnets. (i) Yttrium annuli produced by garnet resorption and regrowth are characterized by an asymmetric shape with a steeper slope on the inner side of the annulus and by a decoupled variation between garnet-compatible and garnet-incompatible elements at the resorption margin. (ii) Y annuli produced by breakdown of Y- and REE-rich trace phases such as xenotime, monazite, apatite and epidote-group minerals are indicated by REE peaks in LREE (monazite, allanite), MREE (epidote), or HREE (xenotime, zircon). (iii) Y annuli associated with diffusion-controlled growth occur in garnet with oscillatory textural zoning defined by alternating inclusion-rich and inclusion-poor zones, with increases in concentrations of garnet-compatible elements such as Y and HREE correlating with inclusion-free zones. Mn annuli in some garnets were produced by sporadic local breakdown of Mn-rich epidote and in several cases indicate that thin-section scale equilibrium during annulus formation was not achieved, raising questions concerning the use of Mn concentration as a time marker in garnet growth modeling.

Copyright ©2002 by the Mineralogical Society of America

Compositional asymmetry in replacement tourmaline – An example from the Tauern Window, Eastern Alps
Darrell J. Henry, Barbara L. Dutrow, and Jane Selverstone
(Published 12 August 2002, 18 pages including 4 tables, 14 figures, 1 rollover figure)

Keywords: tourmaline, replacement, fluids, Tauern Window, Alps

Compositional asymmetry in replacement tourmaline – An example from the Tauern Window, Eastern Alps

Darrell J. Henry¹, Barbara L. Dutrow² and Jane Selverstone³

^{1, 2}Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70808, USA,  ³Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
(Received August 17, 2001; Published August 12, 2002)

Abstract

Tourmaline, partially replacing pre-existing tourmaline from a tectonically-dismembered tourmalinite vein, develops distinctive compositional asymmetry that reflects influx of reactive fluids. The tourmalinite clast, enclosed in a quartzite from the Tauern Window, Eastern Alps, experienced a clockwise P-T-t path with maximal burial depths of 35-40 km (10-11 kbar), peak temperatures of ~550°C and major deformation preceding peak thermal conditions. The primary tourmaline of the tourmalinite clast, generation-1, is texturally and compositionally heterogeneous, ranging from schorl to dravite [Mg / (Mg + Fe) = 0.27 – 0.61] with highly-variable Al consistent with combinations of the ^X[_]Al(NaR)_-1 and AlO(R(OH))_-1 exchange vectors, where ^X[_] represents X-site vacancy and R is Fe²⁺ + Mn + Mg. Generation-2 tourmaline is manifest as distinctive compositionally-asymmetric bands of colorless foitite (zone 1) and blue schorl (zone 2) replacing generation-1 tourmaline. Replacement takes place along a scalloped margin and advances preferentially towards the analogous pole (-c) of generation-1 tourmaline. The two zones of generation-2 range from foitite to schorl with a restricted ratio of Mg / (Mg + Fe) of 0.32 – 0.41, but with variable ^X[_], Al, Na and R predominantly reflecting ^X[_]Al(NaR)_-1. Post-deformational generation-3 tourmaline ranges from schorl to foitite and partially pseudomorphs generations-1 and -2 tourmaline. Deformation and fracturing of primary tourmaline (generation 1) from the tourmalinite clast provided access to reactive fluids with adequate chemical affinity to produce partial tourmaline replacement. It is likely that the reactive fluid was a neutral-to-alkaline aqueous fluid phase with relatively low Na contents.

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Copyright ©2002 by the Mineralogical Society of America

Diopside + F-rich phlogopite at high P and T: Systematics, crystal chemistry and the stability of KMgF₃, clinohumite and chondrodite
George E. Harlow
(Published 16 September 2002, 28 pages including 10 tables, 9 figures)

Keywords: potassic richterite, potassic clinopyroxene, KMgF₃, clinohumite, chondrodite, fluorine, experimental study, high pressure

Diopside + F-rich phlogopite at high P and T: Systematics, crystal chemistry and the stability of KMgF₃, clinohumite and chondrodite

George E. Harlow

Department of Earth and Planetary Sciences,
American Museum of Natural History, New York, NY 10024-5192, USA

Abstract

Multianvil experiments have been carried out from 3.0 to 11 GPa and from 1100 to 1500°C on an equimolar mixture of natural diopside (CaMgSi₂O₆) and F-rich phlogopite (K_1.7Na_0.24Mg_5.6Al_2.4Si_5.8O₂₀ [F_1.7OH_2.3]) to examine the behavior of K and F at high P and T in this system. As the first extensive experiments in the KCMASH composition space to include fluorine, the subsolidus crystallization of the perovskite KMgF₃ and F-bearing clinohumite and chondrodite at high pressures, up to 10 GPa and 1400ºC, is noteworthy. The compositions of crystalline phases produced in the experiments vary systematically with P and T. In clinopyroxene: Kcpx (KAlSi₂O₆) content in clinopyroxene increases with increasing pressure at pressures above 5 GPa without a noticeable temperature effect; CaTs (CaAl₂SiO₆) content is negatively correlated with P above 3 GPa and is positively correlated with T. In amphibole – an ^M4K-substituted potassic richterite, ideally K(KCa)Mg₅Si₈O₂₂(OH,F)₂: K content is positively correlated with P; Al and F content decrease with P; and F content is positively correlated with T and is lowered by coexisting KMgF₃. In garnet: ^[6]Si and Na content increases upon increasing P; Ca-Mg systematics are complicated by growth zoning of crystals. All of the above trends may merit further calibration in geobarometry above 6 GPa, where published Al-in-Opx and Opx-Cpx barometers fail for these experiments. Although fluorine content in hydrous minerals decreases upon increasing P, fluorine enlarges the high P-T stability field of the hydroxy-minerals, so F content, even at low levels, must be considered in modeling phase stabilities for mantle assemblages. However, the strictly subsolidus paragenesis of humites in these experiments adds to the evidence that mantle humites form via metasomatic rather than magmatic interactions.

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