Olivine, Pyroxene, Feldspar, and Spinel in Ultramafic Nodules from Auckland, New Zealand

K. A. Rodgers and R. N. Brothers
Geology Department, University of Auckland, New Zealand

Summary: Chemical, optical, and X-ray data are presented for forsteritic olivine, bronzite-enstatite, diopsidic augite, anorthite, and spinel from ultramafic nodules at seven localities in Auckland province. Olivine has a composition range from Fo79–93 for large primary crystals to Fo80–100 for smaller interstitial grains. Orthopyroxenes (En83–96) carry exsolution lamellae of clinopyroxene parallel to (100); clinopyroxenes are mainly diopsidic augites with exsolved bronzite. The plagioclase (An97) has high-temperature characteristics and a lattice structure probably transitional between primitive and body-centred anorthite. Spinels include chromite, picotite, ceylonite, hercynite, and magnetite. Element distribution indicates that the ultramafic assemblages attained equilibrium at temperatures near 1250°C and at moderate pressures, corresponding to the melting interval of anhydrous basaltic magmas within the uppermost mantle.

Mineralogical Magazine; September 1969 v. 37; no. 287; p. 375-390; DOI: 10.1180/minmag.1969.037.287.11
© 1969, The Mineralogical Society
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