Hostname: page-component-cd9895bd7-gbm5v Total loading time: 0 Render date: 2024-12-24T00:27:55.607Z Has data issue: false hasContentIssue false
  • English
  • Français

The garnet-orthopyroxene Al barometer: problematic application to natural garnet lherzolite assemblages

Published online by Cambridge University Press:  05 July 2018

D. A. Carswell
D. A. Carswell*
Affiliation:
Department of Geology, The University of Sheffield, Mappin Street, Sheffield S1 3JD
Get access Rights & Permissions [Opens in a new window]

Abstract

The garnet-orthopyroxene Al barometer specifically considers the Al content of orthopyroxene in equilibrium with garnet resulting from Mg-Tschermaks substitution. It is demonstrated that P-T calibrations of this barometer derived solely from experimental data for the MAS system, such as that favoured by Finnerty and Boyd (1984, 1987) based on the data of MacGregor (1974), cannot be expected to yield meaningful pressure estimates for natural garnet lherzolite assemblages. The presence of additional CaO, FeO and Cr2O3 components in natural garnet lherzolites can be expected to influence substantially the Al partitioning between orthopyroxene, garnet and/or spinel at any particular P and T. Thus a more comprehensive barometer formulation is required, such as the one provided by Nickel and Green (1985) that is based on experimental data for the CMAS and SMACCR systems with thermodynamic modelling and addition of an Fe correction term.

It is further emphasised that for orthopyroxenes in natural garnet lherzolites the amount of Al introduced as Mg-Tschermaks substitution cannot be assessed simply as the total Al cation content since such orthopyroxenes frequently contain Al cations linked to Na substitution in M2 sites or to Cr, Ti and possibly Fe3+ in M1 sites. Revised algorithms for the calculation of specific orthopyroxene contents are presented. Application to analytical data sets for garnet lherzolite zenolith suites in the Thaba Putsoa and Mothae kimberlites generates revised upper mantle P-T arrays which refute the widely accepted advocacy by Finnerty and Boyd (1984, 1987) and Finnerty (1989) of an upper-mantle palaeogeotherm beneath northern Lesotho that is markedly inflected to a higher thermal gradient at the depths of derivation of the more chemically fertile, porphyroclastic textured, xenoliths.

Keywords

garnetorthopyroxeneAl barometerlherzolite
Type
Research Article
Information
Mineralogical Magazine , Volume 55 , Issue 378 , March 1991 , pp. 19 - 31
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1991

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Adams, G. E. and Bishop, F. C. (1986) The olivineclinopyroxene geobarometer: experimental results in the CaO–FeO–MgO–SiO2 system. Contrib. Mineral. Petrol., 94, 230–7.CrossRefGoogle Scholar
Akella, J. (1976) Garnet-pyroxene equilibria in the system CaSiO3–MgSiO3–Al2O3 and in a natural mineral mixture. Am. Mineral., 61, 598–98.Google Scholar
Bertrand, P. and Mercier, J-C. C. (1985) The mutual solubility of coexisting ortho- and clinopyroxene: towards an absolute geothermometer for the natural system? Earth Planet. Sci. Lett., 76, 109–22.CrossRefGoogle Scholar
Bertrand, P., Sotin, C., Mercier, J-C. C., and Takahashi, E. (1986) From the simplest chemical system to the natural one: garnet peridotite barometry. Contrib. Mineral. Petrol., 93, 168–78.CrossRefGoogle Scholar
Bertrand, P., Sotin, C., Gaulier, J-M., and Mercier, J-C. C. (1987) La solubilité de l'aluminium dans l'orthopyroxene. Inversion globale des données expérimentales du système MgO–Al2O3–SiO2 . Bull. Geol. Soc. France, 8, t.lll, 821–32.CrossRefGoogle Scholar
Boyd, F. R. (1973) A pyroxene geotherm. Geochim. Cosmochim. Acta, 37, 2533–46.CrossRefGoogle Scholar
Boyd, F. R. and England, J. L. (1964) The system enstatitepyrope. Carnegie Inst. Washington Yearb., 63, 157–61.Google Scholar
Boyd, F. R. and Finger, L. W. (1975) Homogeneity of minerals in mantle rocks from Lesotho. Ibid., 74, 519–25.Google Scholar
Boyd, F. R. and Nixon, P. H. (1973) Structure of the upper mantle beneath Lesotho. Ibid., 72, 431–45.Google Scholar
Boyd, F. R. and Nixon, P. H. (1975) Origins of the ultramafic nodules from some kimberlites of northern Lesotho and the Monastery Mine, South Africa. Phys. Chem. Earth., 9, 431–54.CrossRefGoogle Scholar
Brey, G. P., Nickel, K. G., and Kogarko, L. (1986) Garnet-pyroxene equilibria in the system CaO–MgO–Al2O3–SiO2 (CMAS): prospects for simplified (’T-independent’) lherzolite barometry and an eclogite-barometer. Contrib. Mineral. Petrol., 92, 448–55.CrossRefGoogle Scholar
Bundy, F. R. (1980) The P,T phase and reaction diagram for elemental carbon, 1979. J. Geophys. Res., 85, 6930–6.CrossRefGoogle Scholar
Bundy, F. R., Bovenkerk, H. P., Strong, H. M., and Wentorf, R. H. Jr. (1961) Diamond-graphite equilibrium line from growth and graphitization of diamond. J. Chem. Phys., 35, 382–91.CrossRefGoogle Scholar
Carswell, D. A. and Gibb, F. G. F. (1980) Geothermometry of garnet lherzolite nodules with special reference to those from the kimberlites of northern Lesotho. Contrib. Mineral. Petrol., 74, 403–16.CrossRefGoogle Scholar
Carswell, D. A. and Gibb, F. G. F. (1987a) Evaluation of mineral thermometers and barometers applicable to garnet lherzolite assemblages. Ibid., 95, 499–511.Google Scholar
Carswell, D. A. and Gibb, F. G. F. (1987b) Garnet lherzolite xenoliths in the kimberlites of northern Lesotho: revised P-T equilibration conditions and upper mantle palaeogeotherm. Ibid., 97, 473–87.Google Scholar
Carswell, D. A. and Harley, S. L. (1990) Mineral barometry and thermometry. In Eclogite Facies Rocks (Carswell, D. A., ed.) p. 83–110. Blackie and Son, Glasgow.CrossRefGoogle Scholar
Danckwerth, P. A. and Newton, R. C. (1978) Experimental determination of the spinel periodotite to garnet peridotite reaction in the system MgO–Al2O3–SiO2 in the range 900-1100° and Al2O3 isopleths of enstatite in the spinel field. Contrib. Mineral. Petrol., 66, 189–201.CrossRefGoogle Scholar
Dawson, J. B. and Smith, J. V. (1975) Occurrence of diamond in a mica-garnet lherzolite xenolith from kimberlite. Nature, 254, 580–1.CrossRefGoogle Scholar
Finnerty, A. A. (1989) Xenolith-derived mantle geotherms: whither the infection? Contrib. Mineral. Petrol., 102, 367–75.CrossRefGoogle Scholar
Finnerty, A. A. and Boyd, F. R. (1978) Pressure-dependent solubility of calcium in forsterite coexisting with diopside and enstatite. Carnegie Inst. Washington Yearb., 77, 713–7.Google Scholar
Finnerty, A. A. and Boyd, F. R. (1984) Evaluation of thermobarometers for garnet peridotites. Geochim. Cosrnochim. Acta, 48, 15–27.CrossRefGoogle Scholar
Finnerty, A. A. and Boyd, F. R. (1987) Thermobarometry for garnet periodotite xenoliths: a basis for upper-mantle stratigraphy. In Mantle Xenoliths (Nixon, P. H., ed.), p. 381–402. J. Wiley and Sons, New York.Google Scholar
Gasparik, T. (1984) Two-pyroxene thermobarometry with new experimental data in the system CaO–MgO–Al2O3–SiO2 . Contrib. Mineral. Petrol., 87, 87–97.CrossRefGoogle Scholar
Gasparik, T. and Lindsley, D. H. (1980) Phase equilibria at high pressure of pyroxenes containing monovalent and trivalent ions. In Pyroxenes (Prewett, C. T., ed.), Mineral. Soc. Am. Reviews in Mineralogy, 7, 309–39.CrossRefGoogle Scholar
Green, D. H. and Ringwood, A. E. (1967) The stability fields of aluminous pyroxene peridotite and garnet peridotite and their relevance in upper-mantle structure. Earth Planet. Sci. Lett., 3, 151–60.CrossRefGoogle Scholar
Green, H. W. and Gueguen, Y. (1974) Origin of kimberlite pipes by diapiric upwelling in the upper mantle. Nature, 249, 617–20.CrossRefGoogle Scholar
Harley, S. L. (1984) The solubility of alumina in orthopyroxene coexisting with garnet in FeO–MgO–Al2O3–SiO2 and CaO–FeO–MgO–Al2O3–SiO2 . J. Petrol., 25, 665–96.CrossRefGoogle Scholar
Harley, S. L. and Green, D. H. (1982) Garnet-orthopyroxene barometry for granulites and peridotites. Nature, 300, 697–701.CrossRefGoogle Scholar
Harte, B. (1978) Kimberlite nodules, upper-mantle petrology and geotherms. Phil. Trans. Roy. Soc. London, A288, 487–500.Google Scholar
Herzberg, C. T. (1978) Pyroxene geothermometry and geobarometry: experimental and thermodynamic evaluation of some subsolidus phase relations involving pyroxenes in the system CaO–MgO–Al2O3–SiO2 . Geochim. Cosmochim. Acta, 42, 945–57.CrossRefGoogle Scholar
Howells, S. and O'Hara, M. J. (1978) Low solubility of alumina in enstatite and uncertainties in estimated palaeogeotherms. Phil. Trans. R. Soc. London, A288, 471–86.Google Scholar
Kawasaki, T. and Matsui, Y. (1983) Thermodynamic analysis of equilibria involving olivine, orthopyroxene and garnet. Geochim. Cosmochim. Acta, 47, 1661–79.CrossRefGoogle Scholar
Kennedy, C. S. and Kennedy, G. C. (1976) The equilibrium boundary between graphite and diamond. J. Geophys. Res., 81, 2467–70.CrossRefGoogle Scholar
Kushiro, I. and Aoki, K. (1968) Origin of some eclogite inclusions in kimberlite. Am. Mineral., 53, 1347–67.Google Scholar
Lane, D. L. and Ganguly, J. (1980) Al2O3 solubility in orthopyroxene in the system MgO–Al2O3–SiO2: reevaluation and mantle geotherm. J. Geophys. Res., 85, 6963–72.CrossRefGoogle Scholar
Luth, R. W., Virgo, D., Boyd, F. R., and Wood, B. J. (1988) Iron in mantle-derived garnets: valence and structural state. Carnegie Inst. Washington Yearb. 1987–1988, 13–18.Google Scholar
MacGregor, I. D. (1970) The effect of CaO, Cr2O3, Fe2O3 and Al2O3 on the stability of spinel and garnet periodotite. Phys. Earth Planet. Interiors, 3, 372–7.CrossRefGoogle Scholar
MacGregor, I. D. (1974) The system MgO–Al2O3–SiO2: solubility of Al2O3 in enstatite for spinel and garnet peridotite compositions. Am. Mineral., 59, 110–9.Google Scholar
Mackenzie, D. (1989) Some remarks on the movement of small melt fractions in the mantle. Earth Planet. Sci. Lett., 95, 53–72.CrossRefGoogle Scholar
Mercier, J-C. C., (1979) Peridotite xenoliths and the dynamics of kimberlite intrusion. In The Mantle Sample: Inclusions in Kimberlites and other Volcanics (Boyd, F. R. and Meyer, H. O. A., eds.). Am. Geophys, Union Washington Proc. 2nd Int. Kimberlite Conference, 2, 197–212.Google Scholar
Mercier, J-C. C. and Carter, N. L. (1975) Pyroxene geotherms. J. Geophys. Res., 80, 3349–62.CrossRefGoogle Scholar
Mysen, B. O. and Griffin, W. L. (1973) Pyroxene stoichiometry and the breakdown of omphacite. Am. Mineral., 58, 60–3.Google Scholar
Nickel, K. G. (1986) Phase equilibria in the system SiO2–MgO–Al2O3–CaO–Cr2O3 (SMACCR) and their bearing on spinel/garnet lherzolite relationships. Neues Jahrb. Mineral. Abh., 155, 259–87.Google Scholar
Nickel, K. G. and Green, D. H. (1985) Empirical geothermobarometry for garnet peridotites and implications for the nature of the lithosphere, kimberlites and diamonds. Earth Planet. Sci. Lett., 73, 158–70.CrossRefGoogle Scholar
Nickel, K. G., Brey, G. P. and Kogarko, L. (1985) Orthopyroxene-clinopyroxene equilibria in the system CaO–Al2O3–SiO2 (CMAS): new experimental results and implications for two-pyroxene thermometry. Contrib. Mineral. Petrol., 91, 44–53.CrossRefGoogle Scholar
Nixon, P. H. and Boyd, F. R. (1973) Petrogenesis of the granular and sheared ultrabasic nodule suite in kimberlites. In Lesotho Kimberlites (Nixon, P. H., ed.), p. 48–56. Lesotho Nat. Dev. Corp. Maseru.Google Scholar
O'Hara, M. J. (1967) Mineral parageneses in ultrabasic rocks. In Ultramafic and Related Rocks, (Wyllie, P. J., ed.). John Wiley and Sons, New York, 393–403.Google Scholar
O'Neill, H. St. C. (1981) The transition between spinel lherzolite and garnet lherzolite, and its use as a geobarometer. Contrib. Mineral. Petrol., 77, 185–94.CrossRefGoogle Scholar
Perkins, D. and Newton, R. C. (1980) The compositions of coexisting pyroxenes and garnet in the system CaO–MgO–Al2O3–SiO2 at 900–1100°C and high pressures. Ibid., 75, 291–300.Google Scholar
Holland, H. J. B. and Newton, R. C. (1981) The Al2O3 contents of enstatite in equilibrium with garnet in the system MgO–Al2O3–SiO2 at 15–40kbar and 900–1600°C. Ibid., 78, 99–109.Google Scholar
Pollack, H. N. and Chapman, D. S. (1977) On the regional variation of heat flow, geotherms and lithospheric thickness. Tectonophysics, 38, 279–96.CrossRefGoogle Scholar
Webb, S. A. C. and Wood, B. J. (1986) Spinel-pyroxene-garnet relationships and their dependence on Cr/Al ratio. Contrib. Mineral. Petrol., 29, 471–80.CrossRefGoogle Scholar
Wood, B. J. (1974) The solubility of alumina in orthopyroxene coexisting with garnet. Ibid. 46, 1–15.Google Scholar
Wood, B. J. and Banno, S. (1973) Garnet-orthopyroxene and orthopyroxene-clinopyroxene relationships in simple and complex systems. Ibid., 42, 109–24.Google Scholar
Wood, B. J. and Henderson, C. M. B. (1978) Composition and unit cell parameters of synthetic non-stoichiometric Tschermakitic clinopyroxenes. Am. Mineral., 63, 66–72.Google Scholar
Wood, B. J. and Holloway, J. R. (1984) A thermodynamic model for subsolidus equilibria in the system CaO–MgO–Al2O3–SiO3 . Geochim. Cosmochim. Acta, 48, 159–76.CrossRefGoogle Scholar
Yamada, H. and Takahashi, E. (1984) Subsolidus phase relations between coexisting garnet and two pyroxenes at 50 to 100 kbar in the system CaO–MgO–Al2O3–SiO2 . In Kimberlites H: The Mantle and Crust-Mantle Relationships (Kornprobst, K., ed.) Developments in Petrology. l1B, 247–55, Elsevier.Google Scholar