Pyroxene orientation within the upper mantle
Petrofabric analyses of orthopyroxene and olivine for a suite of rocks from the Bay of Islands ophiolite, Newfoundland, show systematic relationships between the minerals9 fabrics, the plane of the Mohorovicic discontinuity determined from the contact of the gabbro and ultramafic sections, and the spreading direction inferred from the attitude of the sheeted dikes. The orthopyroxene c crystallographic axes maxima, which are aligned subparallel to the spreading direction, coincide with olivine a axes maxima. The orthopyroxene b axes and olivine c axes also lie approximately within the plane of the Mohorovicic discontinuity but parallel the ridge, whereas the orthopyroxene a axes maxima and olivine b axes maxima are approximately perpendicular to the Mohorovicic discontinuity. Using olivine and orthopyroxene elastic constants and their pressure derivatives, velocities have been calculated from composite petrofabric data and contoured for varying proportions of olivine and pyroxene. The results of these calculations show that within a vertical direction compressional wave velocities change only slightly with increasing pyroxene content, whereas parallel to the Mohorovicic discontinuity velocities decrease with increasing pyroxene content. Anisotropy decreases from 4% for a rock with 100% olivine to 2.6% for a rock with 50% olivine and 50% orthopyroxene. Also within the plane of Mohorovicic discontinuity, the maximum and minumum velocities occur at approximately the same azimuth for differing percentages of orthopyroxene. A comparison of the calculated anisotropy and the observed seismic anisotropy off the coast of California shows a striking agreement. From this comparison, it is estimated that the percentage of orthopyroxene within the upper mantle of this region varies between 0% and 30%.