American Mineralogist; August-September; v. 94; no. 8-9;
p. 1291-1292; DOI: 10.2138/am.2009.549
© 2009 Mineralogical Society of America
Laser Raman microspectrometry of metamorphic quartz: A simple method for comparison of metamorphic pressures—Corrigendum
William D. Carlson1,*,
Emily A. McDowell1,
Masaki Enami2,
Tadao Nishiyama3 and
Takashi Mouri2
1 Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712, U.S.A.
2 Department of Earth and Planetary Sciences, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
3 Department of Earth and Environment, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
Correspondence: * E-mail: wcarlson{at}mail.utexas.edu
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ABSTRACT
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A minor error in calculation of geobarometric estimates from laser Raman microspectrometry of quartz inclusions in garnet appears in Enami et al. (2007). Correction of this error eliminates anomalous results for grossular-rich garnet, inspiring greater confidence in the validity and applicability of this geobarometric technique.
Key Words: Geobarometry • Raman spectrometry • quartz • garnet • elastic model
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BACKGROUND
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Enami et al. (2007) presented a novel and promising technique for estimation and comparison of metamorphic pressures based upon measurements of Raman spectra of quartz inclusions in garnet. Frequency shifts in the Raman spectrum of quartz are quantitative indicators of residual pressure on the inclusion imposed by the enclosing garnet. For quartz grains completely enclosed in garnet and not affected by fracturing or other modes of stress release, the residual pressure on the inclusion can be related to the original pressure of entrapment by means of elastic models that account for partial relaxation in response to cooling from the temperature of entrapment. The elastic model of Van der Molen (1981) is employed; it calculates normal stresses on a spherical isotropic inclusion in an infinite isotropic medium subjected to an external stress applied at infinite distance from the inclusion. Using this model, the relationship between the residual pressure PQtz and the entrapment pressure PGrt is given by the following equation, which appears (unnumbered) on p. 1311 of Enami et al. (2007):
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In this expression, 
is bulk modulus (GPa), µ is shear modulus (GPa), 
T is the difference between the temperature TM at which the Raman spectrum is measured and the temperature TE of entrapment (K), and A is the difference in volumetric thermal-expansion coefficients between garnet and quartz (K–1), which is defined by A 
AGrt – AQtz. In discussions among the authors, an ambiguity in the definition of T came to light that we wish to clarify here: when Equation 1 is applied to Raman barometry of quartz inclusions in garnet, T is correctly computed as TM – TE, and thus both A and T are negative quantities.
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DISCUSSION
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Enami et al. (2007, their Table 3, p. 1312) compared observed residual pressures to those calculated, on the basis of the elastic model embodied in Equation 1, for five samples for which pressures and temperatures of entrapment were constrained independently within small ranges. For each sample, expected residual pressures were computed for two limiting entrapment pressures at each of two limiting entrapment temperatures, using in each case four different sets of elastic parameters and thermal-expansion coefficients, corresponding to values for each of the four principal garnet end-members, namely almandine, pyrope, grossular, and spessartine.
In Table 3 of Enami et al. (2007), residual pressures computed for the grossular end-member stand out as anomalous: they are roughly half of the values computed for other end-members, despite much less pronounced differences between grossular and other end-members for values of elastic and thermal-expansion parameters. This was found to be the result of a minor error in the calculations, in which the factor of 3 that appears in the term "PGrt (3 Grt + 4 µGrt)" was omitted. Recalculated values are shown here in Table 1
; they demonstrate that the laser Raman geobarometric technique is significantly less sensitive to the composition of the garnet host than previously thought. Previously, a large effect had been ascribed to composition: in analysis of results for the epidote-amphibolite-facies metapelite and metabasite, Enami et al. (2007, p. 1312) stated that "incorporation of the grossular component will decrease the [computed] internal pressure drastically," and for all rocks, interpretations of computed pressures (p. 1311–1312) were influenced by allowances for large potential imprecision due to the grossular content of host garnets. Recalculation shows that compositional effects are actually relatively minor in comparison to other factors affecting the precision of geobarometric estimates by this technique, which should encourage its application across the full range of garnet compositions.
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Footnotes
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MANUSCRIPT HANDLED BY DANA GRIFFEN
MANUSCRIPT ACCEPTED June 10, 2009
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REFERENCES CITED
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Enami, M., Nishiyama, T., and Mouri, T. (2007) Laser Raman microspectrometry of metamorphic quartz: A simple method for comparison of metamorphic pressures. American Mineralogist, 92, 1303–1315.[Abstract/Free Full Text][CrossRef][Web of Science][GeoRef]
Van der Molen, I. (1981) The shift of the 
-β transition temperature of quartz associated with the thermal expansion of granite at high pressure. Tectonophysics, 73, 323–342.[CrossRef][Web of Science][GeoRef]
Copyright © 2009 by Mineralogical Society of America
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