Contributions to Mineralogy and Petrology

, Volume 92, Issue 4, pp 448–455 | Cite as

Garnet-pyroxene equilibria in the system CaO-MgO-Al2O3-SiO2 (CMAS): prospects for simplified (‘T-independent’) lherzolite barometry and an eclogite-barometer

  • G. P. Brey
  • K. G. Nickel
  • L. Kogarko
Article
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Accepted:

Abstract

New experimental data on compositions of garnets in two-pyroxene — garnet assemblages in the system CaO −MgO −Al2O3 −SiO2 (CMAS) are presented for conditions between 1,100 and 1,570° C and 30 to 50 kb. Garnets in these assemblages become less calcic with increasing pressure. Garnet-orthopyroxene barometry (Al-solubility-barometry) pertinent to geobarometry for garnet lherzolites has been evaluated with a set of experimental data covering the range 900 to 1,570° C and 15 to 100 kb. Various formulations of this barometer work well to 75 kb. Phase equilibria are not sufficient to positively verify the thermodynamic validity of any of such models. Empirical garnet-orthopyroxene barometry at least in the system CMAS can be formulated to obtain a pressure estimate without previous temperature estimation (P(kb)=34.4-19.175 1n X Al M1 +17.702 1n X Ca M2 ). The potential application of an analogous garnetclinopyroxene equilibrium is limited because the amount of Ca-Tschermaks in natural clinopyroxenes is usually quite small in garnet lherzolites and many eclogites. The Ca-Mg exchange between garnet and clinopyroxene appears however sufficiently sensitive to pressure to allow calibration of a CMAS barometer. The reaction 3CaMgSi2O6+Mg3Al2Si3O12=3Mg2Si2O6+Ca3Al2Si3O12 has a ΔVo of 3.5 cm3. The total pressure dependency of this reaction is however closer to a theoretical ΔVo of about 5 cm3 when excess volume properties of the phases involved are taken into account. We have calibrated such a barometer (mean error of estimate 2.8 kb) for assemblages with pyrope-rich (py>80) garnets and orthopyroxenes. This may provide the basis for a geobarometer for eclogites from kimberlites.

Keywords

SiO2 Al2O3 Phase Equilibrium Mineral Resource Total Pressure 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Abbreviations Used in the Text

CaTs

Ca-tschermak's molecule, CaAl2SiO6

cpx

clinopyroxene

di

diopside, CaMgSi2O6

en

enstatite, Mg2Si2O6

gr

grossular, Ca3Al2Si3O12

gt

garnet

MgTs

Mg-Tschermak's molecule, MgAl2SiO6

opx

orthopyroxene

px

pyroxene

py

pyrope, Mg3Al2Si3O12

aij

activity of component i in phase j

γ

activity coefficient

ΔG(I)

molar Gibbs free energy difference of reaction (I) at standard state unless specified otherwise

H(I), (ΔHI)

molar enthalpy (difference) of phase (reaction) (I) at standard state unless specified otherwise

S (I), (ΔSI)

molar entropy (difference) of phase (reaction) (I) at standard state unless specified otherwise

Vo, (ΔVIo)

molar volume (difference) of phase (reaction) (I) at standard state

Xij

mole fraction of component i in phase j

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Copyright information

© Springer-Verlag 1986

Authors and Affiliations

  • G. P. Brey
    • 1
  • K. G. Nickel
    • 1
  • L. Kogarko
    • 2
  1. 1.KosmochemieMax-Planck-Institut für ChemieMainzFederal Republic of Germany
  2. 2.Akademia Nauk USSRVernadski InstitutMoscowUSSR

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