Abstract

Consideration of phase diagrams, mineralogical and chemical data, and textural evidence suggests that ultramafic and mafic nodules found in basaltic rocks at Kilbourne Hole, New Mexico, were formed by the interaction of partial fusion-partial crystallization processes. The olivine compositions of the nodules vary from 7 to 32 mol percent fayalite. Very few nodules were found with olivine compositions in the Fa 12 percent to Fa 14 percent interval, and significant mineralogical and chemical differences are found on either side of this compositional range. The model proposes that the original undepleted upper mantle composition falls in this range, and that it may be calculated from the modal and chemical data.

Comparison of the chemical composition of the Earth's mantle by models based on extraterrestrial data to the Earth's unaltered upper mantle by Ringwood's pyrolite model and Nicholl's model, and to the partial fusion-partial crystallization model reveals convergence toward a similar over-all composition for the Earth's undepleted upper mantle. The averaged undepleted upper mantle mineralogical composition for the major phases in volume percent under Kilbourne Hole, New Mexico, is: olivine, 50.9 ± 10.9; orthopyroxene, 21.7 ± 6.2; clinopyroxene, 23.0 ± 6.7; and spinel, 4.3 ± 1.2. The averaged undepleted, vapor-free, upper mantle chemical composition in weight percent under Kilbourne Hole, New Mexico, is: SiO2, 42.86 ± 0.32; TiO2, 0.33 ± 0.07; A12O3, 6.99 ± 0.28; Cr2O3, 0.18 ± 0.04; total iron as Fe, 7.22 ± 0.17; MnO, 0.14 ± 0.01; NiO, 0.20 ± 0.01; MgO, 35.07 ± 0.24; CaO, 4.37 ± 0.11; Na2O, 0.45 ± 0.08; K2O, 0.003 ± 0.002; CoO, 0.014 ± 0.001; CuO, 0.0012 ± 0.0005; and ZnO, 0.0087 ± 0.0008.

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