ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract The compositions of coexisting orthopyroxene, clinopyroxene and garnet in the CaO-MgO-Al2O3-SiO2 system are fixed at any givenP andT. Reversed hydrothermal experiments over theP/T range 15–40 kb/900°–l,100° C indicate that the garnet composition is nearly constant at Py86Gr14; the Alcontent and Ca/Ca+Mg values of the pyroxenes vary significantly, however: T P Wt. %Al2O3 $$\frac{{Ca}}{{Ca + Mg}} \times 100$$ °C kbar in opx in cpx in opx in cpx 900 15 3.55 2.92 0.72 47.05 900 30 1.62 0.92 0.81 47.30 1,100 20 1.11 0.92 1.22 45.20 1,100 40 1.26 0.88 0.81 45.64 The significant variation of A12O3 content with pressure supports its use as a geobarometer. The Ca/Ca +Mg values of coexisting orthoand clino-pyroxene have been used as a geothermometer. However, the present results show that the presence of A12O3 in the pyroxenes decreases significantly the mutual solubility of the pyroxenes, which may cause errors of up to 200° C if the A12O3 effect is ignored. Thermodynamic analyses of the experimental results for the reaction Mg3Al2Si3O12(gar) =Mg3Al2Si3O12(opx) yield average values ofΔH 0 andΔS 0 of 7.4 Kcal and 5.7 eu, respectively, which are in marginal agreement with calorimetric results. Similar analysis of our results for the reaction MgSiO3(cpx)=MgSiO3(opx) support earlier conclusions that MgSiO3 substitution in clinopyroxene behaves as a regular solution (Holland et al. 1979) and that Al substitution is quasi-ideal (Wood 1979). Application of the present results to garnet lherzolites from southern Africa yields a geotherm which is parallel to those previously proposed, although the indicated heat flow is greater. Similar rocks, from the Solomon Islands, show a much steeper geothermal gradient, as expected for an oceanic plate.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01166769
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